libgo: update to go1.9

[official-gcc.git] / libgo / go / crypto / x509 / x509.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package x509 parses X.509-encoded keys and certificates.
//
// On UNIX systems the environment variables SSL_CERT_FILE and SSL_CERT_DIR
// can be used to override the system default locations for the SSL certificate
// file and SSL certificate files directory, respectively.
package x509

import (
        "bytes"
        "crypto"
        "crypto/dsa"
        "crypto/ecdsa"
        "crypto/elliptic"
        "crypto/rsa"
        _ "crypto/sha1"
        _ "crypto/sha256"
        _ "crypto/sha512"
        "crypto/x509/pkix"
        "encoding/asn1"
        "encoding/pem"
        "errors"
        "fmt"
        "io"
        "math/big"
        "net"
        "strconv"
        "time"
)

// pkixPublicKey reflects a PKIX public key structure. See SubjectPublicKeyInfo
// in RFC 3280.
type pkixPublicKey struct {
        Algo      pkix.AlgorithmIdentifier
        BitString asn1.BitString
}

// ParsePKIXPublicKey parses a DER encoded public key. These values are
// typically found in PEM blocks with "BEGIN PUBLIC KEY".
//
// Supported key types include RSA, DSA, and ECDSA. Unknown key
// types result in an error.
//
// On success, pub will be of type *rsa.PublicKey, *dsa.PublicKey,
// or *ecdsa.PublicKey.
func ParsePKIXPublicKey(derBytes []byte) (pub interface{}, err error) {
        var pki publicKeyInfo
        if rest, err := asn1.Unmarshal(derBytes, &pki); err != nil {
                return nil, err
        } else if len(rest) != 0 {
                return nil, errors.New("x509: trailing data after ASN.1 of public-key")
        }
        algo := getPublicKeyAlgorithmFromOID(pki.Algorithm.Algorithm)
        if algo == UnknownPublicKeyAlgorithm {
                return nil, errors.New("x509: unknown public key algorithm")
        }
        return parsePublicKey(algo, &pki)
}

func marshalPublicKey(pub interface{}) (publicKeyBytes []byte, publicKeyAlgorithm pkix.AlgorithmIdentifier, err error) {
        switch pub := pub.(type) {
        case *rsa.PublicKey:
                publicKeyBytes, err = asn1.Marshal(pkcs1PublicKey{
                        N: pub.N,
                        E: pub.E,
                })
                if err != nil {
                        return nil, pkix.AlgorithmIdentifier{}, err
                }
                publicKeyAlgorithm.Algorithm = oidPublicKeyRSA
                // This is a NULL parameters value which is required by
                // https://tools.ietf.org/html/rfc3279#section-2.3.1.
                publicKeyAlgorithm.Parameters = asn1.NullRawValue
        case *ecdsa.PublicKey:
                publicKeyBytes = elliptic.Marshal(pub.Curve, pub.X, pub.Y)
                oid, ok := oidFromNamedCurve(pub.Curve)
                if !ok {
                        return nil, pkix.AlgorithmIdentifier{}, errors.New("x509: unsupported elliptic curve")
                }
                publicKeyAlgorithm.Algorithm = oidPublicKeyECDSA
                var paramBytes []byte
                paramBytes, err = asn1.Marshal(oid)
                if err != nil {
                        return
                }
                publicKeyAlgorithm.Parameters.FullBytes = paramBytes
        default:
                return nil, pkix.AlgorithmIdentifier{}, errors.New("x509: only RSA and ECDSA public keys supported")
        }

        return publicKeyBytes, publicKeyAlgorithm, nil
}

// MarshalPKIXPublicKey serialises a public key to DER-encoded PKIX format.
func MarshalPKIXPublicKey(pub interface{}) ([]byte, error) {
        var publicKeyBytes []byte
        var publicKeyAlgorithm pkix.AlgorithmIdentifier
        var err error

        if publicKeyBytes, publicKeyAlgorithm, err = marshalPublicKey(pub); err != nil {
                return nil, err
        }

        pkix := pkixPublicKey{
                Algo: publicKeyAlgorithm,
                BitString: asn1.BitString{
                        Bytes:     publicKeyBytes,
                        BitLength: 8 * len(publicKeyBytes),
                },
        }

        ret, _ := asn1.Marshal(pkix)
        return ret, nil
}

// These structures reflect the ASN.1 structure of X.509 certificates.:

type certificate struct {
        Raw                asn1.RawContent
        TBSCertificate     tbsCertificate
        SignatureAlgorithm pkix.AlgorithmIdentifier
        SignatureValue     asn1.BitString
}

type tbsCertificate struct {
        Raw                asn1.RawContent
        Version            int `asn1:"optional,explicit,default:0,tag:0"`
        SerialNumber       *big.Int
        SignatureAlgorithm pkix.AlgorithmIdentifier
        Issuer             asn1.RawValue
        Validity           validity
        Subject            asn1.RawValue
        PublicKey          publicKeyInfo
        UniqueId           asn1.BitString   `asn1:"optional,tag:1"`
        SubjectUniqueId    asn1.BitString   `asn1:"optional,tag:2"`
        Extensions         []pkix.Extension `asn1:"optional,explicit,tag:3"`
}

type dsaAlgorithmParameters struct {
        P, Q, G *big.Int
}

type dsaSignature struct {
        R, S *big.Int
}

type ecdsaSignature dsaSignature

type validity struct {
        NotBefore, NotAfter time.Time
}

type publicKeyInfo struct {
        Raw       asn1.RawContent
        Algorithm pkix.AlgorithmIdentifier
        PublicKey asn1.BitString
}

// RFC 5280,  4.2.1.1
type authKeyId struct {
        Id []byte `asn1:"optional,tag:0"`
}

type SignatureAlgorithm int

const (
        UnknownSignatureAlgorithm SignatureAlgorithm = iota
        MD2WithRSA
        MD5WithRSA
        SHA1WithRSA
        SHA256WithRSA
        SHA384WithRSA
        SHA512WithRSA
        DSAWithSHA1
        DSAWithSHA256
        ECDSAWithSHA1
        ECDSAWithSHA256
        ECDSAWithSHA384
        ECDSAWithSHA512
        SHA256WithRSAPSS
        SHA384WithRSAPSS
        SHA512WithRSAPSS
)

func (algo SignatureAlgorithm) isRSAPSS() bool {
        switch algo {
        case SHA256WithRSAPSS, SHA384WithRSAPSS, SHA512WithRSAPSS:
                return true
        default:
                return false
        }
}

var algoName = [...]string{
        MD2WithRSA:       "MD2-RSA",
        MD5WithRSA:       "MD5-RSA",
        SHA1WithRSA:      "SHA1-RSA",
        SHA256WithRSA:    "SHA256-RSA",
        SHA384WithRSA:    "SHA384-RSA",
        SHA512WithRSA:    "SHA512-RSA",
        SHA256WithRSAPSS: "SHA256-RSAPSS",
        SHA384WithRSAPSS: "SHA384-RSAPSS",
        SHA512WithRSAPSS: "SHA512-RSAPSS",
        DSAWithSHA1:      "DSA-SHA1",
        DSAWithSHA256:    "DSA-SHA256",
        ECDSAWithSHA1:    "ECDSA-SHA1",
        ECDSAWithSHA256:  "ECDSA-SHA256",
        ECDSAWithSHA384:  "ECDSA-SHA384",
        ECDSAWithSHA512:  "ECDSA-SHA512",
}

func (algo SignatureAlgorithm) String() string {
        if 0 < algo && int(algo) < len(algoName) {
                return algoName[algo]
        }
        return strconv.Itoa(int(algo))
}

type PublicKeyAlgorithm int

const (
        UnknownPublicKeyAlgorithm PublicKeyAlgorithm = iota
        RSA
        DSA
        ECDSA
)

// OIDs for signature algorithms
//
// pkcs-1 OBJECT IDENTIFIER ::= {
//    iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) 1 }
//
//
// RFC 3279 2.2.1 RSA Signature Algorithms
//
// md2WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 2 }
//
// md5WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 4 }
//
// sha-1WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 5 }
//
// dsaWithSha1 OBJECT IDENTIFIER ::= {
//    iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 3 }
//
// RFC 3279 2.2.3 ECDSA Signature Algorithm
//
// ecdsa-with-SHA1 OBJECT IDENTIFIER ::= {
//        iso(1) member-body(2) us(840) ansi-x962(10045)
//    signatures(4) ecdsa-with-SHA1(1)}
//
//
// RFC 4055 5 PKCS #1 Version 1.5
//
// sha256WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 11 }
//
// sha384WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 12 }
//
// sha512WithRSAEncryption OBJECT IDENTIFIER ::= { pkcs-1 13 }
//
//
// RFC 5758 3.1 DSA Signature Algorithms
//
// dsaWithSha256 OBJECT IDENTIFIER ::= {
//    joint-iso-ccitt(2) country(16) us(840) organization(1) gov(101)
//    csor(3) algorithms(4) id-dsa-with-sha2(3) 2}
//
// RFC 5758 3.2 ECDSA Signature Algorithm
//
// ecdsa-with-SHA256 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
//    us(840) ansi-X9-62(10045) signatures(4) ecdsa-with-SHA2(3) 2 }
//
// ecdsa-with-SHA384 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
//    us(840) ansi-X9-62(10045) signatures(4) ecdsa-with-SHA2(3) 3 }
//
// ecdsa-with-SHA512 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
//    us(840) ansi-X9-62(10045) signatures(4) ecdsa-with-SHA2(3) 4 }

var (
        oidSignatureMD2WithRSA      = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 2}
        oidSignatureMD5WithRSA      = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 4}
        oidSignatureSHA1WithRSA     = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 5}
        oidSignatureSHA256WithRSA   = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 11}
        oidSignatureSHA384WithRSA   = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 12}
        oidSignatureSHA512WithRSA   = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 13}
        oidSignatureRSAPSS          = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 10}
        oidSignatureDSAWithSHA1     = asn1.ObjectIdentifier{1, 2, 840, 10040, 4, 3}
        oidSignatureDSAWithSHA256   = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 3, 2}
        oidSignatureECDSAWithSHA1   = asn1.ObjectIdentifier{1, 2, 840, 10045, 4, 1}
        oidSignatureECDSAWithSHA256 = asn1.ObjectIdentifier{1, 2, 840, 10045, 4, 3, 2}
        oidSignatureECDSAWithSHA384 = asn1.ObjectIdentifier{1, 2, 840, 10045, 4, 3, 3}
        oidSignatureECDSAWithSHA512 = asn1.ObjectIdentifier{1, 2, 840, 10045, 4, 3, 4}

        oidSHA256 = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 2, 1}
        oidSHA384 = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 2, 2}
        oidSHA512 = asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 2, 3}

        oidMGF1 = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 8}

        // oidISOSignatureSHA1WithRSA means the same as oidSignatureSHA1WithRSA
        // but it's specified by ISO. Microsoft's makecert.exe has been known
        // to produce certificates with this OID.
        oidISOSignatureSHA1WithRSA = asn1.ObjectIdentifier{1, 3, 14, 3, 2, 29}
)

var signatureAlgorithmDetails = []struct {
        algo       SignatureAlgorithm
        oid        asn1.ObjectIdentifier
        pubKeyAlgo PublicKeyAlgorithm
        hash       crypto.Hash
}{
        {MD2WithRSA, oidSignatureMD2WithRSA, RSA, crypto.Hash(0) /* no value for MD2 */},
        {MD5WithRSA, oidSignatureMD5WithRSA, RSA, crypto.MD5},
        {SHA1WithRSA, oidSignatureSHA1WithRSA, RSA, crypto.SHA1},
        {SHA1WithRSA, oidISOSignatureSHA1WithRSA, RSA, crypto.SHA1},
        {SHA256WithRSA, oidSignatureSHA256WithRSA, RSA, crypto.SHA256},
        {SHA384WithRSA, oidSignatureSHA384WithRSA, RSA, crypto.SHA384},
        {SHA512WithRSA, oidSignatureSHA512WithRSA, RSA, crypto.SHA512},
        {SHA256WithRSAPSS, oidSignatureRSAPSS, RSA, crypto.SHA256},
        {SHA384WithRSAPSS, oidSignatureRSAPSS, RSA, crypto.SHA384},
        {SHA512WithRSAPSS, oidSignatureRSAPSS, RSA, crypto.SHA512},
        {DSAWithSHA1, oidSignatureDSAWithSHA1, DSA, crypto.SHA1},
        {DSAWithSHA256, oidSignatureDSAWithSHA256, DSA, crypto.SHA256},
        {ECDSAWithSHA1, oidSignatureECDSAWithSHA1, ECDSA, crypto.SHA1},
        {ECDSAWithSHA256, oidSignatureECDSAWithSHA256, ECDSA, crypto.SHA256},
        {ECDSAWithSHA384, oidSignatureECDSAWithSHA384, ECDSA, crypto.SHA384},
        {ECDSAWithSHA512, oidSignatureECDSAWithSHA512, ECDSA, crypto.SHA512},
}

// pssParameters reflects the parameters in an AlgorithmIdentifier that
// specifies RSA PSS. See https://tools.ietf.org/html/rfc3447#appendix-A.2.3
type pssParameters struct {
        // The following three fields are not marked as
        // optional because the default values specify SHA-1,
        // which is no longer suitable for use in signatures.
        Hash         pkix.AlgorithmIdentifier `asn1:"explicit,tag:0"`
        MGF          pkix.AlgorithmIdentifier `asn1:"explicit,tag:1"`
        SaltLength   int                      `asn1:"explicit,tag:2"`
        TrailerField int                      `asn1:"optional,explicit,tag:3,default:1"`
}

// rsaPSSParameters returns an asn1.RawValue suitable for use as the Parameters
// in an AlgorithmIdentifier that specifies RSA PSS.
func rsaPSSParameters(hashFunc crypto.Hash) asn1.RawValue {
        var hashOID asn1.ObjectIdentifier

        switch hashFunc {
        case crypto.SHA256:
                hashOID = oidSHA256
        case crypto.SHA384:
                hashOID = oidSHA384
        case crypto.SHA512:
                hashOID = oidSHA512
        }

        params := pssParameters{
                Hash: pkix.AlgorithmIdentifier{
                        Algorithm:  hashOID,
                        Parameters: asn1.NullRawValue,
                },
                MGF: pkix.AlgorithmIdentifier{
                        Algorithm: oidMGF1,
                },
                SaltLength:   hashFunc.Size(),
                TrailerField: 1,
        }

        mgf1Params := pkix.AlgorithmIdentifier{
                Algorithm:  hashOID,
                Parameters: asn1.NullRawValue,
        }

        var err error
        params.MGF.Parameters.FullBytes, err = asn1.Marshal(mgf1Params)
        if err != nil {
                panic(err)
        }

        serialized, err := asn1.Marshal(params)
        if err != nil {
                panic(err)
        }

        return asn1.RawValue{FullBytes: serialized}
}

func getSignatureAlgorithmFromAI(ai pkix.AlgorithmIdentifier) SignatureAlgorithm {
        if !ai.Algorithm.Equal(oidSignatureRSAPSS) {
                for _, details := range signatureAlgorithmDetails {
                        if ai.Algorithm.Equal(details.oid) {
                                return details.algo
                        }
                }
                return UnknownSignatureAlgorithm
        }

        // RSA PSS is special because it encodes important parameters
        // in the Parameters.

        var params pssParameters
        if _, err := asn1.Unmarshal(ai.Parameters.FullBytes, &params); err != nil {
                return UnknownSignatureAlgorithm
        }

        var mgf1HashFunc pkix.AlgorithmIdentifier
        if _, err := asn1.Unmarshal(params.MGF.Parameters.FullBytes, &mgf1HashFunc); err != nil {
                return UnknownSignatureAlgorithm
        }

        // PSS is greatly overburdened with options. This code forces
        // them into three buckets by requiring that the MGF1 hash
        // function always match the message hash function (as
        // recommended in
        // https://tools.ietf.org/html/rfc3447#section-8.1), that the
        // salt length matches the hash length, and that the trailer
        // field has the default value.
        if !bytes.Equal(params.Hash.Parameters.FullBytes, asn1.NullBytes) ||
                !params.MGF.Algorithm.Equal(oidMGF1) ||
                !mgf1HashFunc.Algorithm.Equal(params.Hash.Algorithm) ||
                !bytes.Equal(mgf1HashFunc.Parameters.FullBytes, asn1.NullBytes) ||
                params.TrailerField != 1 {
                return UnknownSignatureAlgorithm
        }

        switch {
        case params.Hash.Algorithm.Equal(oidSHA256) && params.SaltLength == 32:
                return SHA256WithRSAPSS
        case params.Hash.Algorithm.Equal(oidSHA384) && params.SaltLength == 48:
                return SHA384WithRSAPSS
        case params.Hash.Algorithm.Equal(oidSHA512) && params.SaltLength == 64:
                return SHA512WithRSAPSS
        }

        return UnknownSignatureAlgorithm
}

// RFC 3279, 2.3 Public Key Algorithms
//
// pkcs-1 OBJECT IDENTIFIER ::== { iso(1) member-body(2) us(840)
//    rsadsi(113549) pkcs(1) 1 }
//
// rsaEncryption OBJECT IDENTIFIER ::== { pkcs1-1 1 }
//
// id-dsa OBJECT IDENTIFIER ::== { iso(1) member-body(2) us(840)
//    x9-57(10040) x9cm(4) 1 }
//
// RFC 5480, 2.1.1 Unrestricted Algorithm Identifier and Parameters
//
// id-ecPublicKey OBJECT IDENTIFIER ::= {
//       iso(1) member-body(2) us(840) ansi-X9-62(10045) keyType(2) 1 }
var (
        oidPublicKeyRSA   = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 1, 1}
        oidPublicKeyDSA   = asn1.ObjectIdentifier{1, 2, 840, 10040, 4, 1}
        oidPublicKeyECDSA = asn1.ObjectIdentifier{1, 2, 840, 10045, 2, 1}
)

func getPublicKeyAlgorithmFromOID(oid asn1.ObjectIdentifier) PublicKeyAlgorithm {
        switch {
        case oid.Equal(oidPublicKeyRSA):
                return RSA
        case oid.Equal(oidPublicKeyDSA):
                return DSA
        case oid.Equal(oidPublicKeyECDSA):
                return ECDSA
        }
        return UnknownPublicKeyAlgorithm
}

// RFC 5480, 2.1.1.1. Named Curve
//
// secp224r1 OBJECT IDENTIFIER ::= {
//   iso(1) identified-organization(3) certicom(132) curve(0) 33 }
//
// secp256r1 OBJECT IDENTIFIER ::= {
//   iso(1) member-body(2) us(840) ansi-X9-62(10045) curves(3)
//   prime(1) 7 }
//
// secp384r1 OBJECT IDENTIFIER ::= {
//   iso(1) identified-organization(3) certicom(132) curve(0) 34 }
//
// secp521r1 OBJECT IDENTIFIER ::= {
//   iso(1) identified-organization(3) certicom(132) curve(0) 35 }
//
// NB: secp256r1 is equivalent to prime256v1
var (
        oidNamedCurveP224 = asn1.ObjectIdentifier{1, 3, 132, 0, 33}
        oidNamedCurveP256 = asn1.ObjectIdentifier{1, 2, 840, 10045, 3, 1, 7}
        oidNamedCurveP384 = asn1.ObjectIdentifier{1, 3, 132, 0, 34}
        oidNamedCurveP521 = asn1.ObjectIdentifier{1, 3, 132, 0, 35}
)

func namedCurveFromOID(oid asn1.ObjectIdentifier) elliptic.Curve {
        switch {
        case oid.Equal(oidNamedCurveP224):
                return elliptic.P224()
        case oid.Equal(oidNamedCurveP256):
                return elliptic.P256()
        case oid.Equal(oidNamedCurveP384):
                return elliptic.P384()
        case oid.Equal(oidNamedCurveP521):
                return elliptic.P521()
        }
        return nil
}

func oidFromNamedCurve(curve elliptic.Curve) (asn1.ObjectIdentifier, bool) {
        switch curve {
        case elliptic.P224():
                return oidNamedCurveP224, true
        case elliptic.P256():
                return oidNamedCurveP256, true
        case elliptic.P384():
                return oidNamedCurveP384, true
        case elliptic.P521():
                return oidNamedCurveP521, true
        }

        return nil, false
}

// KeyUsage represents the set of actions that are valid for a given key. It's
// a bitmap of the KeyUsage* constants.
type KeyUsage int

const (
        KeyUsageDigitalSignature KeyUsage = 1 << iota
        KeyUsageContentCommitment
        KeyUsageKeyEncipherment
        KeyUsageDataEncipherment
        KeyUsageKeyAgreement
        KeyUsageCertSign
        KeyUsageCRLSign
        KeyUsageEncipherOnly
        KeyUsageDecipherOnly
)

// RFC 5280, 4.2.1.12  Extended Key Usage
//
// anyExtendedKeyUsage OBJECT IDENTIFIER ::= { id-ce-extKeyUsage 0 }
//
// id-kp OBJECT IDENTIFIER ::= { id-pkix 3 }
//
// id-kp-serverAuth             OBJECT IDENTIFIER ::= { id-kp 1 }
// id-kp-clientAuth             OBJECT IDENTIFIER ::= { id-kp 2 }
// id-kp-codeSigning            OBJECT IDENTIFIER ::= { id-kp 3 }
// id-kp-emailProtection        OBJECT IDENTIFIER ::= { id-kp 4 }
// id-kp-timeStamping           OBJECT IDENTIFIER ::= { id-kp 8 }
// id-kp-OCSPSigning            OBJECT IDENTIFIER ::= { id-kp 9 }
var (
        oidExtKeyUsageAny                        = asn1.ObjectIdentifier{2, 5, 29, 37, 0}
        oidExtKeyUsageServerAuth                 = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 1}
        oidExtKeyUsageClientAuth                 = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 2}
        oidExtKeyUsageCodeSigning                = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 3}
        oidExtKeyUsageEmailProtection            = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 4}
        oidExtKeyUsageIPSECEndSystem             = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 5}
        oidExtKeyUsageIPSECTunnel                = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 6}
        oidExtKeyUsageIPSECUser                  = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 7}
        oidExtKeyUsageTimeStamping               = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 8}
        oidExtKeyUsageOCSPSigning                = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 3, 9}
        oidExtKeyUsageMicrosoftServerGatedCrypto = asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 311, 10, 3, 3}
        oidExtKeyUsageNetscapeServerGatedCrypto  = asn1.ObjectIdentifier{2, 16, 840, 1, 113730, 4, 1}
)

// ExtKeyUsage represents an extended set of actions that are valid for a given key.
// Each of the ExtKeyUsage* constants define a unique action.
type ExtKeyUsage int

const (
        ExtKeyUsageAny ExtKeyUsage = iota
        ExtKeyUsageServerAuth
        ExtKeyUsageClientAuth
        ExtKeyUsageCodeSigning
        ExtKeyUsageEmailProtection
        ExtKeyUsageIPSECEndSystem
        ExtKeyUsageIPSECTunnel
        ExtKeyUsageIPSECUser
        ExtKeyUsageTimeStamping
        ExtKeyUsageOCSPSigning
        ExtKeyUsageMicrosoftServerGatedCrypto
        ExtKeyUsageNetscapeServerGatedCrypto
)

// extKeyUsageOIDs contains the mapping between an ExtKeyUsage and its OID.
var extKeyUsageOIDs = []struct {
        extKeyUsage ExtKeyUsage
        oid         asn1.ObjectIdentifier
}{
        {ExtKeyUsageAny, oidExtKeyUsageAny},
        {ExtKeyUsageServerAuth, oidExtKeyUsageServerAuth},
        {ExtKeyUsageClientAuth, oidExtKeyUsageClientAuth},
        {ExtKeyUsageCodeSigning, oidExtKeyUsageCodeSigning},
        {ExtKeyUsageEmailProtection, oidExtKeyUsageEmailProtection},
        {ExtKeyUsageIPSECEndSystem, oidExtKeyUsageIPSECEndSystem},
        {ExtKeyUsageIPSECTunnel, oidExtKeyUsageIPSECTunnel},
        {ExtKeyUsageIPSECUser, oidExtKeyUsageIPSECUser},
        {ExtKeyUsageTimeStamping, oidExtKeyUsageTimeStamping},
        {ExtKeyUsageOCSPSigning, oidExtKeyUsageOCSPSigning},
        {ExtKeyUsageMicrosoftServerGatedCrypto, oidExtKeyUsageMicrosoftServerGatedCrypto},
        {ExtKeyUsageNetscapeServerGatedCrypto, oidExtKeyUsageNetscapeServerGatedCrypto},
}

func extKeyUsageFromOID(oid asn1.ObjectIdentifier) (eku ExtKeyUsage, ok bool) {
        for _, pair := range extKeyUsageOIDs {
                if oid.Equal(pair.oid) {
                        return pair.extKeyUsage, true
                }
        }
        return
}

func oidFromExtKeyUsage(eku ExtKeyUsage) (oid asn1.ObjectIdentifier, ok bool) {
        for _, pair := range extKeyUsageOIDs {
                if eku == pair.extKeyUsage {
                        return pair.oid, true
                }
        }
        return
}

// A Certificate represents an X.509 certificate.
type Certificate struct {
        Raw                     []byte // Complete ASN.1 DER content (certificate, signature algorithm and signature).
        RawTBSCertificate       []byte // Certificate part of raw ASN.1 DER content.
        RawSubjectPublicKeyInfo []byte // DER encoded SubjectPublicKeyInfo.
        RawSubject              []byte // DER encoded Subject
        RawIssuer               []byte // DER encoded Issuer

        Signature          []byte
        SignatureAlgorithm SignatureAlgorithm

        PublicKeyAlgorithm PublicKeyAlgorithm
        PublicKey          interface{}

        Version             int
        SerialNumber        *big.Int
        Issuer              pkix.Name
        Subject             pkix.Name
        NotBefore, NotAfter time.Time // Validity bounds.
        KeyUsage            KeyUsage

        // Extensions contains raw X.509 extensions. When parsing certificates,
        // this can be used to extract non-critical extensions that are not
        // parsed by this package. When marshaling certificates, the Extensions
        // field is ignored, see ExtraExtensions.
        Extensions []pkix.Extension

        // ExtraExtensions contains extensions to be copied, raw, into any
        // marshaled certificates. Values override any extensions that would
        // otherwise be produced based on the other fields. The ExtraExtensions
        // field is not populated when parsing certificates, see Extensions.
        ExtraExtensions []pkix.Extension

        // UnhandledCriticalExtensions contains a list of extension IDs that
        // were not (fully) processed when parsing. Verify will fail if this
        // slice is non-empty, unless verification is delegated to an OS
        // library which understands all the critical extensions.
        //
        // Users can access these extensions using Extensions and can remove
        // elements from this slice if they believe that they have been
        // handled.
        UnhandledCriticalExtensions []asn1.ObjectIdentifier

        ExtKeyUsage        []ExtKeyUsage           // Sequence of extended key usages.
        UnknownExtKeyUsage []asn1.ObjectIdentifier // Encountered extended key usages unknown to this package.

        // BasicConstraintsValid indicates whether IsCA, MaxPathLen,
        // and MaxPathLenZero are valid.
        BasicConstraintsValid bool
        IsCA                  bool

        // MaxPathLen and MaxPathLenZero indicate the presence and
        // value of the BasicConstraints' "pathLenConstraint".
        //
        // When parsing a certificate, a positive non-zero MaxPathLen
        // means that the field was specified, -1 means it was unset,
        // and MaxPathLenZero being true mean that the field was
        // explicitly set to zero. The case of MaxPathLen==0 with MaxPathLenZero==false
        // should be treated equivalent to -1 (unset).
        //
        // When generating a certificate, an unset pathLenConstraint
        // can be requested with either MaxPathLen == -1 or using the
        // zero value for both MaxPathLen and MaxPathLenZero.
        MaxPathLen int
        // MaxPathLenZero indicates that BasicConstraintsValid==true
        // and MaxPathLen==0 should be interpreted as an actual
        // maximum path length of zero. Otherwise, that combination is
        // interpreted as MaxPathLen not being set.
        MaxPathLenZero bool

        SubjectKeyId   []byte
        AuthorityKeyId []byte

        // RFC 5280, 4.2.2.1 (Authority Information Access)
        OCSPServer            []string
        IssuingCertificateURL []string

        // Subject Alternate Name values
        DNSNames       []string
        EmailAddresses []string
        IPAddresses    []net.IP

        // Name constraints
        PermittedDNSDomainsCritical bool // if true then the name constraints are marked critical.
        PermittedDNSDomains         []string
        ExcludedDNSDomains          []string

        // CRL Distribution Points
        CRLDistributionPoints []string

        PolicyIdentifiers []asn1.ObjectIdentifier
}

// ErrUnsupportedAlgorithm results from attempting to perform an operation that
// involves algorithms that are not currently implemented.
var ErrUnsupportedAlgorithm = errors.New("x509: cannot verify signature: algorithm unimplemented")

// An InsecureAlgorithmError
type InsecureAlgorithmError SignatureAlgorithm

func (e InsecureAlgorithmError) Error() string {
        return fmt.Sprintf("x509: cannot verify signature: insecure algorithm %v", SignatureAlgorithm(e))
}

// ConstraintViolationError results when a requested usage is not permitted by
// a certificate. For example: checking a signature when the public key isn't a
// certificate signing key.
type ConstraintViolationError struct{}

func (ConstraintViolationError) Error() string {
        return "x509: invalid signature: parent certificate cannot sign this kind of certificate"
}

func (c *Certificate) Equal(other *Certificate) bool {
        return bytes.Equal(c.Raw, other.Raw)
}

func (c *Certificate) hasSANExtension() bool {
        return oidInExtensions(oidExtensionSubjectAltName, c.Extensions)
}

// Entrust have a broken root certificate (CN=Entrust.net Certification
// Authority (2048)) which isn't marked as a CA certificate and is thus invalid
// according to PKIX.
// We recognise this certificate by its SubjectPublicKeyInfo and exempt it
// from the Basic Constraints requirement.
// See http://www.entrust.net/knowledge-base/technote.cfm?tn=7869
//
// TODO(agl): remove this hack once their reissued root is sufficiently
// widespread.
var entrustBrokenSPKI = []byte{
        0x30, 0x82, 0x01, 0x22, 0x30, 0x0d, 0x06, 0x09,
        0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x01, 0x01,
        0x01, 0x05, 0x00, 0x03, 0x82, 0x01, 0x0f, 0x00,
        0x30, 0x82, 0x01, 0x0a, 0x02, 0x82, 0x01, 0x01,
        0x00, 0x97, 0xa3, 0x2d, 0x3c, 0x9e, 0xde, 0x05,
        0xda, 0x13, 0xc2, 0x11, 0x8d, 0x9d, 0x8e, 0xe3,
        0x7f, 0xc7, 0x4b, 0x7e, 0x5a, 0x9f, 0xb3, 0xff,
        0x62, 0xab, 0x73, 0xc8, 0x28, 0x6b, 0xba, 0x10,
        0x64, 0x82, 0x87, 0x13, 0xcd, 0x57, 0x18, 0xff,
        0x28, 0xce, 0xc0, 0xe6, 0x0e, 0x06, 0x91, 0x50,
        0x29, 0x83, 0xd1, 0xf2, 0xc3, 0x2a, 0xdb, 0xd8,
        0xdb, 0x4e, 0x04, 0xcc, 0x00, 0xeb, 0x8b, 0xb6,
        0x96, 0xdc, 0xbc, 0xaa, 0xfa, 0x52, 0x77, 0x04,
        0xc1, 0xdb, 0x19, 0xe4, 0xae, 0x9c, 0xfd, 0x3c,
        0x8b, 0x03, 0xef, 0x4d, 0xbc, 0x1a, 0x03, 0x65,
        0xf9, 0xc1, 0xb1, 0x3f, 0x72, 0x86, 0xf2, 0x38,
        0xaa, 0x19, 0xae, 0x10, 0x88, 0x78, 0x28, 0xda,
        0x75, 0xc3, 0x3d, 0x02, 0x82, 0x02, 0x9c, 0xb9,
        0xc1, 0x65, 0x77, 0x76, 0x24, 0x4c, 0x98, 0xf7,
        0x6d, 0x31, 0x38, 0xfb, 0xdb, 0xfe, 0xdb, 0x37,
        0x02, 0x76, 0xa1, 0x18, 0x97, 0xa6, 0xcc, 0xde,
        0x20, 0x09, 0x49, 0x36, 0x24, 0x69, 0x42, 0xf6,
        0xe4, 0x37, 0x62, 0xf1, 0x59, 0x6d, 0xa9, 0x3c,
        0xed, 0x34, 0x9c, 0xa3, 0x8e, 0xdb, 0xdc, 0x3a,
        0xd7, 0xf7, 0x0a, 0x6f, 0xef, 0x2e, 0xd8, 0xd5,
        0x93, 0x5a, 0x7a, 0xed, 0x08, 0x49, 0x68, 0xe2,
        0x41, 0xe3, 0x5a, 0x90, 0xc1, 0x86, 0x55, 0xfc,
        0x51, 0x43, 0x9d, 0xe0, 0xb2, 0xc4, 0x67, 0xb4,
        0xcb, 0x32, 0x31, 0x25, 0xf0, 0x54, 0x9f, 0x4b,
        0xd1, 0x6f, 0xdb, 0xd4, 0xdd, 0xfc, 0xaf, 0x5e,
        0x6c, 0x78, 0x90, 0x95, 0xde, 0xca, 0x3a, 0x48,
        0xb9, 0x79, 0x3c, 0x9b, 0x19, 0xd6, 0x75, 0x05,
        0xa0, 0xf9, 0x88, 0xd7, 0xc1, 0xe8, 0xa5, 0x09,
        0xe4, 0x1a, 0x15, 0xdc, 0x87, 0x23, 0xaa, 0xb2,
        0x75, 0x8c, 0x63, 0x25, 0x87, 0xd8, 0xf8, 0x3d,
        0xa6, 0xc2, 0xcc, 0x66, 0xff, 0xa5, 0x66, 0x68,
        0x55, 0x02, 0x03, 0x01, 0x00, 0x01,
}

// CheckSignatureFrom verifies that the signature on c is a valid signature
// from parent.
func (c *Certificate) CheckSignatureFrom(parent *Certificate) error {
        // RFC 5280, 4.2.1.9:
        // "If the basic constraints extension is not present in a version 3
        // certificate, or the extension is present but the cA boolean is not
        // asserted, then the certified public key MUST NOT be used to verify
        // certificate signatures."
        // (except for Entrust, see comment above entrustBrokenSPKI)
        if (parent.Version == 3 && !parent.BasicConstraintsValid ||
                parent.BasicConstraintsValid && !parent.IsCA) &&
                !bytes.Equal(c.RawSubjectPublicKeyInfo, entrustBrokenSPKI) {
                return ConstraintViolationError{}
        }

        if parent.KeyUsage != 0 && parent.KeyUsage&KeyUsageCertSign == 0 {
                return ConstraintViolationError{}
        }

        if parent.PublicKeyAlgorithm == UnknownPublicKeyAlgorithm {
                return ErrUnsupportedAlgorithm
        }

        // TODO(agl): don't ignore the path length constraint.

        return parent.CheckSignature(c.SignatureAlgorithm, c.RawTBSCertificate, c.Signature)
}

// CheckSignature verifies that signature is a valid signature over signed from
// c's public key.
func (c *Certificate) CheckSignature(algo SignatureAlgorithm, signed, signature []byte) error {
        return checkSignature(algo, signed, signature, c.PublicKey)
}

// CheckSignature verifies that signature is a valid signature over signed from
// a crypto.PublicKey.
func checkSignature(algo SignatureAlgorithm, signed, signature []byte, publicKey crypto.PublicKey) (err error) {
        var hashType crypto.Hash

        switch algo {
        case SHA1WithRSA, DSAWithSHA1, ECDSAWithSHA1:
                hashType = crypto.SHA1
        case SHA256WithRSA, SHA256WithRSAPSS, DSAWithSHA256, ECDSAWithSHA256:
                hashType = crypto.SHA256
        case SHA384WithRSA, SHA384WithRSAPSS, ECDSAWithSHA384:
                hashType = crypto.SHA384
        case SHA512WithRSA, SHA512WithRSAPSS, ECDSAWithSHA512:
                hashType = crypto.SHA512
        case MD2WithRSA, MD5WithRSA:
                return InsecureAlgorithmError(algo)
        default:
                return ErrUnsupportedAlgorithm
        }

        if !hashType.Available() {
                return ErrUnsupportedAlgorithm
        }
        h := hashType.New()

        h.Write(signed)
        digest := h.Sum(nil)

        switch pub := publicKey.(type) {
        case *rsa.PublicKey:
                if algo.isRSAPSS() {
                        return rsa.VerifyPSS(pub, hashType, digest, signature, &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash})
                } else {
                        return rsa.VerifyPKCS1v15(pub, hashType, digest, signature)
                }
        case *dsa.PublicKey:
                dsaSig := new(dsaSignature)
                if rest, err := asn1.Unmarshal(signature, dsaSig); err != nil {
                        return err
                } else if len(rest) != 0 {
                        return errors.New("x509: trailing data after DSA signature")
                }
                if dsaSig.R.Sign() <= 0 || dsaSig.S.Sign() <= 0 {
                        return errors.New("x509: DSA signature contained zero or negative values")
                }
                if !dsa.Verify(pub, digest, dsaSig.R, dsaSig.S) {
                        return errors.New("x509: DSA verification failure")
                }
                return
        case *ecdsa.PublicKey:
                ecdsaSig := new(ecdsaSignature)
                if rest, err := asn1.Unmarshal(signature, ecdsaSig); err != nil {
                        return err
                } else if len(rest) != 0 {
                        return errors.New("x509: trailing data after ECDSA signature")
                }
                if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
                        return errors.New("x509: ECDSA signature contained zero or negative values")
                }
                if !ecdsa.Verify(pub, digest, ecdsaSig.R, ecdsaSig.S) {
                        return errors.New("x509: ECDSA verification failure")
                }
                return
        }
        return ErrUnsupportedAlgorithm
}

// CheckCRLSignature checks that the signature in crl is from c.
func (c *Certificate) CheckCRLSignature(crl *pkix.CertificateList) error {
        algo := getSignatureAlgorithmFromAI(crl.SignatureAlgorithm)
        return c.CheckSignature(algo, crl.TBSCertList.Raw, crl.SignatureValue.RightAlign())
}

type UnhandledCriticalExtension struct{}

func (h UnhandledCriticalExtension) Error() string {
        return "x509: unhandled critical extension"
}

type basicConstraints struct {
        IsCA       bool `asn1:"optional"`
        MaxPathLen int  `asn1:"optional,default:-1"`
}

// RFC 5280 4.2.1.4
type policyInformation struct {
        Policy asn1.ObjectIdentifier
        // policyQualifiers omitted
}

// RFC 5280, 4.2.1.10
type nameConstraints struct {
        Permitted []generalSubtree `asn1:"optional,tag:0"`
        Excluded  []generalSubtree `asn1:"optional,tag:1"`
}

type generalSubtree struct {
        Name string `asn1:"tag:2,optional,ia5"`
}

// RFC 5280, 4.2.2.1
type authorityInfoAccess struct {
        Method   asn1.ObjectIdentifier
        Location asn1.RawValue
}

// RFC 5280, 4.2.1.14
type distributionPoint struct {
        DistributionPoint distributionPointName `asn1:"optional,tag:0"`
        Reason            asn1.BitString        `asn1:"optional,tag:1"`
        CRLIssuer         asn1.RawValue         `asn1:"optional,tag:2"`
}

type distributionPointName struct {
        FullName     asn1.RawValue    `asn1:"optional,tag:0"`
        RelativeName pkix.RDNSequence `asn1:"optional,tag:1"`
}

func parsePublicKey(algo PublicKeyAlgorithm, keyData *publicKeyInfo) (interface{}, error) {
        asn1Data := keyData.PublicKey.RightAlign()
        switch algo {
        case RSA:
                // RSA public keys must have a NULL in the parameters
                // (https://tools.ietf.org/html/rfc3279#section-2.3.1).
                if !bytes.Equal(keyData.Algorithm.Parameters.FullBytes, asn1.NullBytes) {
                        return nil, errors.New("x509: RSA key missing NULL parameters")
                }

                p := new(pkcs1PublicKey)
                rest, err := asn1.Unmarshal(asn1Data, p)
                if err != nil {
                        return nil, err
                }
                if len(rest) != 0 {
                        return nil, errors.New("x509: trailing data after RSA public key")
                }

                if p.N.Sign() <= 0 {
                        return nil, errors.New("x509: RSA modulus is not a positive number")
                }
                if p.E <= 0 {
                        return nil, errors.New("x509: RSA public exponent is not a positive number")
                }

                pub := &rsa.PublicKey{
                        E: p.E,
                        N: p.N,
                }
                return pub, nil
        case DSA:
                var p *big.Int
                rest, err := asn1.Unmarshal(asn1Data, &p)
                if err != nil {
                        return nil, err
                }
                if len(rest) != 0 {
                        return nil, errors.New("x509: trailing data after DSA public key")
                }
                paramsData := keyData.Algorithm.Parameters.FullBytes
                params := new(dsaAlgorithmParameters)
                rest, err = asn1.Unmarshal(paramsData, params)
                if err != nil {
                        return nil, err
                }
                if len(rest) != 0 {
                        return nil, errors.New("x509: trailing data after DSA parameters")
                }
                if p.Sign() <= 0 || params.P.Sign() <= 0 || params.Q.Sign() <= 0 || params.G.Sign() <= 0 {
                        return nil, errors.New("x509: zero or negative DSA parameter")
                }
                pub := &dsa.PublicKey{
                        Parameters: dsa.Parameters{
                                P: params.P,
                                Q: params.Q,
                                G: params.G,
                        },
                        Y: p,
                }
                return pub, nil
        case ECDSA:
                paramsData := keyData.Algorithm.Parameters.FullBytes
                namedCurveOID := new(asn1.ObjectIdentifier)
                rest, err := asn1.Unmarshal(paramsData, namedCurveOID)
                if err != nil {
                        return nil, err
                }
                if len(rest) != 0 {
                        return nil, errors.New("x509: trailing data after ECDSA parameters")
                }
                namedCurve := namedCurveFromOID(*namedCurveOID)
                if namedCurve == nil {
                        return nil, errors.New("x509: unsupported elliptic curve")
                }
                x, y := elliptic.Unmarshal(namedCurve, asn1Data)
                if x == nil {
                        return nil, errors.New("x509: failed to unmarshal elliptic curve point")
                }
                pub := &ecdsa.PublicKey{
                        Curve: namedCurve,
                        X:     x,
                        Y:     y,
                }
                return pub, nil
        default:
                return nil, nil
        }
}

func parseSANExtension(value []byte) (dnsNames, emailAddresses []string, ipAddresses []net.IP, err error) {
        // RFC 5280, 4.2.1.6

        // SubjectAltName ::= GeneralNames
        //
        // GeneralNames ::= SEQUENCE SIZE (1..MAX) OF GeneralName
        //
        // GeneralName ::= CHOICE {
        //      otherName                       [0]     OtherName,
        //      rfc822Name                      [1]     IA5String,
        //      dNSName                         [2]     IA5String,
        //      x400Address                     [3]     ORAddress,
        //      directoryName                   [4]     Name,
        //      ediPartyName                    [5]     EDIPartyName,
        //      uniformResourceIdentifier       [6]     IA5String,
        //      iPAddress                       [7]     OCTET STRING,
        //      registeredID                    [8]     OBJECT IDENTIFIER }
        var seq asn1.RawValue
        var rest []byte
        if rest, err = asn1.Unmarshal(value, &seq); err != nil {
                return
        } else if len(rest) != 0 {
                err = errors.New("x509: trailing data after X.509 extension")
                return
        }
        if !seq.IsCompound || seq.Tag != 16 || seq.Class != 0 {
                err = asn1.StructuralError{Msg: "bad SAN sequence"}
                return
        }

        rest = seq.Bytes
        for len(rest) > 0 {
                var v asn1.RawValue
                rest, err = asn1.Unmarshal(rest, &v)
                if err != nil {
                        return
                }
                switch v.Tag {
                case 1:
                        emailAddresses = append(emailAddresses, string(v.Bytes))
                case 2:
                        dnsNames = append(dnsNames, string(v.Bytes))
                case 7:
                        switch len(v.Bytes) {
                        case net.IPv4len, net.IPv6len:
                                ipAddresses = append(ipAddresses, v.Bytes)
                        default:
                                err = errors.New("x509: certificate contained IP address of length " + strconv.Itoa(len(v.Bytes)))
                                return
                        }
                }
        }

        return
}

func parseCertificate(in *certificate) (*Certificate, error) {
        out := new(Certificate)
        out.Raw = in.Raw
        out.RawTBSCertificate = in.TBSCertificate.Raw
        out.RawSubjectPublicKeyInfo = in.TBSCertificate.PublicKey.Raw
        out.RawSubject = in.TBSCertificate.Subject.FullBytes
        out.RawIssuer = in.TBSCertificate.Issuer.FullBytes

        out.Signature = in.SignatureValue.RightAlign()
        out.SignatureAlgorithm =
                getSignatureAlgorithmFromAI(in.TBSCertificate.SignatureAlgorithm)

        out.PublicKeyAlgorithm =
                getPublicKeyAlgorithmFromOID(in.TBSCertificate.PublicKey.Algorithm.Algorithm)
        var err error
        out.PublicKey, err = parsePublicKey(out.PublicKeyAlgorithm, &in.TBSCertificate.PublicKey)
        if err != nil {
                return nil, err
        }

        out.Version = in.TBSCertificate.Version + 1
        out.SerialNumber = in.TBSCertificate.SerialNumber

        var issuer, subject pkix.RDNSequence
        if rest, err := asn1.Unmarshal(in.TBSCertificate.Subject.FullBytes, &subject); err != nil {
                return nil, err
        } else if len(rest) != 0 {
                return nil, errors.New("x509: trailing data after X.509 subject")
        }
        if rest, err := asn1.Unmarshal(in.TBSCertificate.Issuer.FullBytes, &issuer); err != nil {
                return nil, err
        } else if len(rest) != 0 {
                return nil, errors.New("x509: trailing data after X.509 subject")
        }

        out.Issuer.FillFromRDNSequence(&issuer)
        out.Subject.FillFromRDNSequence(&subject)

        out.NotBefore = in.TBSCertificate.Validity.NotBefore
        out.NotAfter = in.TBSCertificate.Validity.NotAfter

        for _, e := range in.TBSCertificate.Extensions {
                out.Extensions = append(out.Extensions, e)
                unhandled := false

                if len(e.Id) == 4 && e.Id[0] == 2 && e.Id[1] == 5 && e.Id[2] == 29 {
                        switch e.Id[3] {
                        case 15:
                                // RFC 5280, 4.2.1.3
                                var usageBits asn1.BitString
                                if rest, err := asn1.Unmarshal(e.Value, &usageBits); err != nil {
                                        return nil, err
                                } else if len(rest) != 0 {
                                        return nil, errors.New("x509: trailing data after X.509 KeyUsage")
                                }

                                var usage int
                                for i := 0; i < 9; i++ {
                                        if usageBits.At(i) != 0 {
                                                usage |= 1 << uint(i)
                                        }
                                }
                                out.KeyUsage = KeyUsage(usage)

                        case 19:
                                // RFC 5280, 4.2.1.9
                                var constraints basicConstraints
                                if rest, err := asn1.Unmarshal(e.Value, &constraints); err != nil {
                                        return nil, err
                                } else if len(rest) != 0 {
                                        return nil, errors.New("x509: trailing data after X.509 BasicConstraints")
                                }

                                out.BasicConstraintsValid = true
                                out.IsCA = constraints.IsCA
                                out.MaxPathLen = constraints.MaxPathLen
                                out.MaxPathLenZero = out.MaxPathLen == 0
                                // TODO: map out.MaxPathLen to 0 if it has the -1 default value? (Issue 19285)
                        case 17:
                                out.DNSNames, out.EmailAddresses, out.IPAddresses, err = parseSANExtension(e.Value)
                                if err != nil {
                                        return nil, err
                                }

                                if len(out.DNSNames) == 0 && len(out.EmailAddresses) == 0 && len(out.IPAddresses) == 0 {
                                        // If we didn't parse anything then we do the critical check, below.
                                        unhandled = true
                                }

                        case 30:
                                // RFC 5280, 4.2.1.10

                                // NameConstraints ::= SEQUENCE {
                                //      permittedSubtrees       [0]     GeneralSubtrees OPTIONAL,
                                //      excludedSubtrees        [1]     GeneralSubtrees OPTIONAL }
                                //
                                // GeneralSubtrees ::= SEQUENCE SIZE (1..MAX) OF GeneralSubtree
                                //
                                // GeneralSubtree ::= SEQUENCE {
                                //      base                    GeneralName,
                                //      minimum         [0]     BaseDistance DEFAULT 0,
                                //      maximum         [1]     BaseDistance OPTIONAL }
                                //
                                // BaseDistance ::= INTEGER (0..MAX)

                                var constraints nameConstraints
                                if rest, err := asn1.Unmarshal(e.Value, &constraints); err != nil {
                                        return nil, err
                                } else if len(rest) != 0 {
                                        return nil, errors.New("x509: trailing data after X.509 NameConstraints")
                                }

                                getDNSNames := func(subtrees []generalSubtree, isCritical bool) (dnsNames []string, err error) {
                                        for _, subtree := range subtrees {
                                                if len(subtree.Name) == 0 {
                                                        if isCritical {
                                                                return nil, UnhandledCriticalExtension{}
                                                        }
                                                        continue
                                                }
                                                dnsNames = append(dnsNames, subtree.Name)
                                        }

                                        return dnsNames, nil
                                }

                                if out.PermittedDNSDomains, err = getDNSNames(constraints.Permitted, e.Critical); err != nil {
                                        return out, err
                                }
                                if out.ExcludedDNSDomains, err = getDNSNames(constraints.Excluded, e.Critical); err != nil {
                                        return out, err
                                }
                                out.PermittedDNSDomainsCritical = e.Critical

                        case 31:
                                // RFC 5280, 4.2.1.13

                                // CRLDistributionPoints ::= SEQUENCE SIZE (1..MAX) OF DistributionPoint
                                //
                                // DistributionPoint ::= SEQUENCE {
                                //     distributionPoint       [0]     DistributionPointName OPTIONAL,
                                //     reasons                 [1]     ReasonFlags OPTIONAL,
                                //     cRLIssuer               [2]     GeneralNames OPTIONAL }
                                //
                                // DistributionPointName ::= CHOICE {
                                //     fullName                [0]     GeneralNames,
                                //     nameRelativeToCRLIssuer [1]     RelativeDistinguishedName }

                                var cdp []distributionPoint
                                if rest, err := asn1.Unmarshal(e.Value, &cdp); err != nil {
                                        return nil, err
                                } else if len(rest) != 0 {
                                        return nil, errors.New("x509: trailing data after X.509 CRL distribution point")
                                }

                                for _, dp := range cdp {
                                        // Per RFC 5280, 4.2.1.13, one of distributionPoint or cRLIssuer may be empty.
                                        if len(dp.DistributionPoint.FullName.Bytes) == 0 {
                                                continue
                                        }

                                        var n asn1.RawValue
                                        if _, err := asn1.Unmarshal(dp.DistributionPoint.FullName.Bytes, &n); err != nil {
                                                return nil, err
                                        }
                                        // Trailing data after the fullName is
                                        // allowed because other elements of
                                        // the SEQUENCE can appear.

                                        if n.Tag == 6 {
                                                out.CRLDistributionPoints = append(out.CRLDistributionPoints, string(n.Bytes))
                                        }
                                }

                        case 35:
                                // RFC 5280, 4.2.1.1
                                var a authKeyId
                                if rest, err := asn1.Unmarshal(e.Value, &a); err != nil {
                                        return nil, err
                                } else if len(rest) != 0 {
                                        return nil, errors.New("x509: trailing data after X.509 authority key-id")
                                }
                                out.AuthorityKeyId = a.Id

                        case 37:
                                // RFC 5280, 4.2.1.12.  Extended Key Usage

                                // id-ce-extKeyUsage OBJECT IDENTIFIER ::= { id-ce 37 }
                                //
                                // ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId
                                //
                                // KeyPurposeId ::= OBJECT IDENTIFIER

                                var keyUsage []asn1.ObjectIdentifier
                                if rest, err := asn1.Unmarshal(e.Value, &keyUsage); err != nil {
                                        return nil, err
                                } else if len(rest) != 0 {
                                        return nil, errors.New("x509: trailing data after X.509 ExtendedKeyUsage")
                                }

                                for _, u := range keyUsage {
                                        if extKeyUsage, ok := extKeyUsageFromOID(u); ok {
                                                out.ExtKeyUsage = append(out.ExtKeyUsage, extKeyUsage)
                                        } else {
                                                out.UnknownExtKeyUsage = append(out.UnknownExtKeyUsage, u)
                                        }
                                }

                        case 14:
                                // RFC 5280, 4.2.1.2
                                var keyid []byte
                                if rest, err := asn1.Unmarshal(e.Value, &keyid); err != nil {
                                        return nil, err
                                } else if len(rest) != 0 {
                                        return nil, errors.New("x509: trailing data after X.509 key-id")
                                }
                                out.SubjectKeyId = keyid

                        case 32:
                                // RFC 5280 4.2.1.4: Certificate Policies
                                var policies []policyInformation
                                if rest, err := asn1.Unmarshal(e.Value, &policies); err != nil {
                                        return nil, err
                                } else if len(rest) != 0 {
                                        return nil, errors.New("x509: trailing data after X.509 certificate policies")
                                }
                                out.PolicyIdentifiers = make([]asn1.ObjectIdentifier, len(policies))
                                for i, policy := range policies {
                                        out.PolicyIdentifiers[i] = policy.Policy
                                }

                        default:
                                // Unknown extensions are recorded if critical.
                                unhandled = true
                        }
                } else if e.Id.Equal(oidExtensionAuthorityInfoAccess) {
                        // RFC 5280 4.2.2.1: Authority Information Access
                        var aia []authorityInfoAccess
                        if rest, err := asn1.Unmarshal(e.Value, &aia); err != nil {
                                return nil, err
                        } else if len(rest) != 0 {
                                return nil, errors.New("x509: trailing data after X.509 authority information")
                        }

                        for _, v := range aia {
                                // GeneralName: uniformResourceIdentifier [6] IA5String
                                if v.Location.Tag != 6 {
                                        continue
                                }
                                if v.Method.Equal(oidAuthorityInfoAccessOcsp) {
                                        out.OCSPServer = append(out.OCSPServer, string(v.Location.Bytes))
                                } else if v.Method.Equal(oidAuthorityInfoAccessIssuers) {
                                        out.IssuingCertificateURL = append(out.IssuingCertificateURL, string(v.Location.Bytes))
                                }
                        }
                } else {
                        // Unknown extensions are recorded if critical.
                        unhandled = true
                }

                if e.Critical && unhandled {
                        out.UnhandledCriticalExtensions = append(out.UnhandledCriticalExtensions, e.Id)
                }
        }

        return out, nil
}

// ParseCertificate parses a single certificate from the given ASN.1 DER data.
func ParseCertificate(asn1Data []byte) (*Certificate, error) {
        var cert certificate
        rest, err := asn1.Unmarshal(asn1Data, &cert)
        if err != nil {
                return nil, err
        }
        if len(rest) > 0 {
                return nil, asn1.SyntaxError{Msg: "trailing data"}
        }

        return parseCertificate(&cert)
}

// ParseCertificates parses one or more certificates from the given ASN.1 DER
// data. The certificates must be concatenated with no intermediate padding.
func ParseCertificates(asn1Data []byte) ([]*Certificate, error) {
        var v []*certificate

        for len(asn1Data) > 0 {
                cert := new(certificate)
                var err error
                asn1Data, err = asn1.Unmarshal(asn1Data, cert)
                if err != nil {
                        return nil, err
                }
                v = append(v, cert)
        }

        ret := make([]*Certificate, len(v))
        for i, ci := range v {
                cert, err := parseCertificate(ci)
                if err != nil {
                        return nil, err
                }
                ret[i] = cert
        }

        return ret, nil
}

func reverseBitsInAByte(in byte) byte {
        b1 := in>>4 | in<<4
        b2 := b1>>2&0x33 | b1<<2&0xcc
        b3 := b2>>1&0x55 | b2<<1&0xaa
        return b3
}

// asn1BitLength returns the bit-length of bitString by considering the
// most-significant bit in a byte to be the "first" bit. This convention
// matches ASN.1, but differs from almost everything else.
func asn1BitLength(bitString []byte) int {
        bitLen := len(bitString) * 8

        for i := range bitString {
                b := bitString[len(bitString)-i-1]

                for bit := uint(0); bit < 8; bit++ {
                        if (b>>bit)&1 == 1 {
                                return bitLen
                        }
                        bitLen--
                }
        }

        return 0
}

var (
        oidExtensionSubjectKeyId          = []int{2, 5, 29, 14}
        oidExtensionKeyUsage              = []int{2, 5, 29, 15}
        oidExtensionExtendedKeyUsage      = []int{2, 5, 29, 37}
        oidExtensionAuthorityKeyId        = []int{2, 5, 29, 35}
        oidExtensionBasicConstraints      = []int{2, 5, 29, 19}
        oidExtensionSubjectAltName        = []int{2, 5, 29, 17}
        oidExtensionCertificatePolicies   = []int{2, 5, 29, 32}
        oidExtensionNameConstraints       = []int{2, 5, 29, 30}
        oidExtensionCRLDistributionPoints = []int{2, 5, 29, 31}
        oidExtensionAuthorityInfoAccess   = []int{1, 3, 6, 1, 5, 5, 7, 1, 1}
)

var (
        oidAuthorityInfoAccessOcsp    = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 48, 1}
        oidAuthorityInfoAccessIssuers = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 48, 2}
)

// oidNotInExtensions returns whether an extension with the given oid exists in
// extensions.
func oidInExtensions(oid asn1.ObjectIdentifier, extensions []pkix.Extension) bool {
        for _, e := range extensions {
                if e.Id.Equal(oid) {
                        return true
                }
        }
        return false
}

// marshalSANs marshals a list of addresses into a the contents of an X.509
// SubjectAlternativeName extension.
func marshalSANs(dnsNames, emailAddresses []string, ipAddresses []net.IP) (derBytes []byte, err error) {
        var rawValues []asn1.RawValue
        for _, name := range dnsNames {
                rawValues = append(rawValues, asn1.RawValue{Tag: 2, Class: 2, Bytes: []byte(name)})
        }
        for _, email := range emailAddresses {
                rawValues = append(rawValues, asn1.RawValue{Tag: 1, Class: 2, Bytes: []byte(email)})
        }
        for _, rawIP := range ipAddresses {
                // If possible, we always want to encode IPv4 addresses in 4 bytes.
                ip := rawIP.To4()
                if ip == nil {
                        ip = rawIP
                }
                rawValues = append(rawValues, asn1.RawValue{Tag: 7, Class: 2, Bytes: ip})
        }
        return asn1.Marshal(rawValues)
}

func buildExtensions(template *Certificate, authorityKeyId []byte) (ret []pkix.Extension, err error) {
        ret = make([]pkix.Extension, 10 /* maximum number of elements. */)
        n := 0

        if template.KeyUsage != 0 &&
                !oidInExtensions(oidExtensionKeyUsage, template.ExtraExtensions) {
                ret[n].Id = oidExtensionKeyUsage
                ret[n].Critical = true

                var a [2]byte
                a[0] = reverseBitsInAByte(byte(template.KeyUsage))
                a[1] = reverseBitsInAByte(byte(template.KeyUsage >> 8))

                l := 1
                if a[1] != 0 {
                        l = 2
                }

                bitString := a[:l]
                ret[n].Value, err = asn1.Marshal(asn1.BitString{Bytes: bitString, BitLength: asn1BitLength(bitString)})
                if err != nil {
                        return
                }
                n++
        }

        if (len(template.ExtKeyUsage) > 0 || len(template.UnknownExtKeyUsage) > 0) &&
                !oidInExtensions(oidExtensionExtendedKeyUsage, template.ExtraExtensions) {
                ret[n].Id = oidExtensionExtendedKeyUsage

                var oids []asn1.ObjectIdentifier
                for _, u := range template.ExtKeyUsage {
                        if oid, ok := oidFromExtKeyUsage(u); ok {
                                oids = append(oids, oid)
                        } else {
                                panic("internal error")
                        }
                }

                oids = append(oids, template.UnknownExtKeyUsage...)

                ret[n].Value, err = asn1.Marshal(oids)
                if err != nil {
                        return
                }
                n++
        }

        if template.BasicConstraintsValid && !oidInExtensions(oidExtensionBasicConstraints, template.ExtraExtensions) {
                // Leaving MaxPathLen as zero indicates that no maximum path
                // length is desired, unless MaxPathLenZero is set. A value of
                // -1 causes encoding/asn1 to omit the value as desired.
                maxPathLen := template.MaxPathLen
                if maxPathLen == 0 && !template.MaxPathLenZero {
                        maxPathLen = -1
                }
                ret[n].Id = oidExtensionBasicConstraints
                ret[n].Value, err = asn1.Marshal(basicConstraints{template.IsCA, maxPathLen})
                ret[n].Critical = true
                if err != nil {
                        return
                }
                n++
        }

        if len(template.SubjectKeyId) > 0 && !oidInExtensions(oidExtensionSubjectKeyId, template.ExtraExtensions) {
                ret[n].Id = oidExtensionSubjectKeyId
                ret[n].Value, err = asn1.Marshal(template.SubjectKeyId)
                if err != nil {
                        return
                }
                n++
        }

        if len(authorityKeyId) > 0 && !oidInExtensions(oidExtensionAuthorityKeyId, template.ExtraExtensions) {
                ret[n].Id = oidExtensionAuthorityKeyId
                ret[n].Value, err = asn1.Marshal(authKeyId{authorityKeyId})
                if err != nil {
                        return
                }
                n++
        }

        if (len(template.OCSPServer) > 0 || len(template.IssuingCertificateURL) > 0) &&
                !oidInExtensions(oidExtensionAuthorityInfoAccess, template.ExtraExtensions) {
                ret[n].Id = oidExtensionAuthorityInfoAccess
                var aiaValues []authorityInfoAccess
                for _, name := range template.OCSPServer {
                        aiaValues = append(aiaValues, authorityInfoAccess{
                                Method:   oidAuthorityInfoAccessOcsp,
                                Location: asn1.RawValue{Tag: 6, Class: 2, Bytes: []byte(name)},
                        })
                }
                for _, name := range template.IssuingCertificateURL {
                        aiaValues = append(aiaValues, authorityInfoAccess{
                                Method:   oidAuthorityInfoAccessIssuers,
                                Location: asn1.RawValue{Tag: 6, Class: 2, Bytes: []byte(name)},
                        })
                }
                ret[n].Value, err = asn1.Marshal(aiaValues)
                if err != nil {
                        return
                }
                n++
        }

        if (len(template.DNSNames) > 0 || len(template.EmailAddresses) > 0 || len(template.IPAddresses) > 0) &&
                !oidInExtensions(oidExtensionSubjectAltName, template.ExtraExtensions) {
                ret[n].Id = oidExtensionSubjectAltName
                ret[n].Value, err = marshalSANs(template.DNSNames, template.EmailAddresses, template.IPAddresses)
                if err != nil {
                        return
                }
                n++
        }

        if len(template.PolicyIdentifiers) > 0 &&
                !oidInExtensions(oidExtensionCertificatePolicies, template.ExtraExtensions) {
                ret[n].Id = oidExtensionCertificatePolicies
                policies := make([]policyInformation, len(template.PolicyIdentifiers))
                for i, policy := range template.PolicyIdentifiers {
                        policies[i].Policy = policy
                }
                ret[n].Value, err = asn1.Marshal(policies)
                if err != nil {
                        return
                }
                n++
        }

        if (len(template.PermittedDNSDomains) > 0 || len(template.ExcludedDNSDomains) > 0) &&
                !oidInExtensions(oidExtensionNameConstraints, template.ExtraExtensions) {
                ret[n].Id = oidExtensionNameConstraints
                ret[n].Critical = template.PermittedDNSDomainsCritical

                var out nameConstraints

                out.Permitted = make([]generalSubtree, len(template.PermittedDNSDomains))
                for i, permitted := range template.PermittedDNSDomains {
                        out.Permitted[i] = generalSubtree{Name: permitted}
                }
                out.Excluded = make([]generalSubtree, len(template.ExcludedDNSDomains))
                for i, excluded := range template.ExcludedDNSDomains {
                        out.Excluded[i] = generalSubtree{Name: excluded}
                }

                ret[n].Value, err = asn1.Marshal(out)
                if err != nil {
                        return
                }
                n++
        }

        if len(template.CRLDistributionPoints) > 0 &&
                !oidInExtensions(oidExtensionCRLDistributionPoints, template.ExtraExtensions) {
                ret[n].Id = oidExtensionCRLDistributionPoints

                var crlDp []distributionPoint
                for _, name := range template.CRLDistributionPoints {
                        rawFullName, _ := asn1.Marshal(asn1.RawValue{Tag: 6, Class: 2, Bytes: []byte(name)})

                        dp := distributionPoint{
                                DistributionPoint: distributionPointName{
                                        FullName: asn1.RawValue{Tag: 0, Class: 2, IsCompound: true, Bytes: rawFullName},
                                },
                        }
                        crlDp = append(crlDp, dp)
                }

                ret[n].Value, err = asn1.Marshal(crlDp)
                if err != nil {
                        return
                }
                n++
        }

        // Adding another extension here? Remember to update the maximum number
        // of elements in the make() at the top of the function.

        return append(ret[:n], template.ExtraExtensions...), nil
}

func subjectBytes(cert *Certificate) ([]byte, error) {
        if len(cert.RawSubject) > 0 {
                return cert.RawSubject, nil
        }

        return asn1.Marshal(cert.Subject.ToRDNSequence())
}

// signingParamsForPublicKey returns the parameters to use for signing with
// priv. If requestedSigAlgo is not zero then it overrides the default
// signature algorithm.
func signingParamsForPublicKey(pub interface{}, requestedSigAlgo SignatureAlgorithm) (hashFunc crypto.Hash, sigAlgo pkix.AlgorithmIdentifier, err error) {
        var pubType PublicKeyAlgorithm

        switch pub := pub.(type) {
        case *rsa.PublicKey:
                pubType = RSA
                hashFunc = crypto.SHA256
                sigAlgo.Algorithm = oidSignatureSHA256WithRSA
                sigAlgo.Parameters = asn1.NullRawValue

        case *ecdsa.PublicKey:
                pubType = ECDSA

                switch pub.Curve {
                case elliptic.P224(), elliptic.P256():
                        hashFunc = crypto.SHA256
                        sigAlgo.Algorithm = oidSignatureECDSAWithSHA256
                case elliptic.P384():
                        hashFunc = crypto.SHA384
                        sigAlgo.Algorithm = oidSignatureECDSAWithSHA384
                case elliptic.P521():
                        hashFunc = crypto.SHA512
                        sigAlgo.Algorithm = oidSignatureECDSAWithSHA512
                default:
                        err = errors.New("x509: unknown elliptic curve")
                }

        default:
                err = errors.New("x509: only RSA and ECDSA keys supported")
        }

        if err != nil {
                return
        }

        if requestedSigAlgo == 0 {
                return
        }

        found := false
        for _, details := range signatureAlgorithmDetails {
                if details.algo == requestedSigAlgo {
                        if details.pubKeyAlgo != pubType {
                                err = errors.New("x509: requested SignatureAlgorithm does not match private key type")
                                return
                        }
                        sigAlgo.Algorithm, hashFunc = details.oid, details.hash
                        if hashFunc == 0 {
                                err = errors.New("x509: cannot sign with hash function requested")
                                return
                        }
                        if requestedSigAlgo.isRSAPSS() {
                                sigAlgo.Parameters = rsaPSSParameters(hashFunc)
                        }
                        found = true
                        break
                }
        }

        if !found {
                err = errors.New("x509: unknown SignatureAlgorithm")
        }

        return
}

// CreateCertificate creates a new certificate based on a template.
// The following members of template are used: AuthorityKeyId,
// BasicConstraintsValid, DNSNames, ExcludedDNSDomains, ExtKeyUsage,
// IsCA, KeyUsage, MaxPathLen, MaxPathLenZero, NotAfter, NotBefore,
// PermittedDNSDomains, PermittedDNSDomainsCritical, SerialNumber,
// SignatureAlgorithm, Subject, SubjectKeyId, and UnknownExtKeyUsage.
//
// The certificate is signed by parent. If parent is equal to template then the
// certificate is self-signed. The parameter pub is the public key of the
// signee and priv is the private key of the signer.
//
// The returned slice is the certificate in DER encoding.
//
// All keys types that are implemented via crypto.Signer are supported (This
// includes *rsa.PublicKey and *ecdsa.PublicKey.)
//
// The AuthorityKeyId will be taken from the SubjectKeyId of parent, if any,
// unless the resulting certificate is self-signed. Otherwise the value from
// template will be used.
func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv interface{}) (cert []byte, err error) {
        key, ok := priv.(crypto.Signer)
        if !ok {
                return nil, errors.New("x509: certificate private key does not implement crypto.Signer")
        }

        if template.SerialNumber == nil {
                return nil, errors.New("x509: no SerialNumber given")
        }

        hashFunc, signatureAlgorithm, err := signingParamsForPublicKey(key.Public(), template.SignatureAlgorithm)
        if err != nil {
                return nil, err
        }

        publicKeyBytes, publicKeyAlgorithm, err := marshalPublicKey(pub)
        if err != nil {
                return nil, err
        }

        asn1Issuer, err := subjectBytes(parent)
        if err != nil {
                return
        }

        asn1Subject, err := subjectBytes(template)
        if err != nil {
                return
        }

        authorityKeyId := template.AuthorityKeyId
        if !bytes.Equal(asn1Issuer, asn1Subject) && len(parent.SubjectKeyId) > 0 {
                authorityKeyId = parent.SubjectKeyId
        }

        extensions, err := buildExtensions(template, authorityKeyId)
        if err != nil {
                return
        }

        encodedPublicKey := asn1.BitString{BitLength: len(publicKeyBytes) * 8, Bytes: publicKeyBytes}
        c := tbsCertificate{
                Version:            2,
                SerialNumber:       template.SerialNumber,
                SignatureAlgorithm: signatureAlgorithm,
                Issuer:             asn1.RawValue{FullBytes: asn1Issuer},
                Validity:           validity{template.NotBefore.UTC(), template.NotAfter.UTC()},
                Subject:            asn1.RawValue{FullBytes: asn1Subject},
                PublicKey:          publicKeyInfo{nil, publicKeyAlgorithm, encodedPublicKey},
                Extensions:         extensions,
        }

        tbsCertContents, err := asn1.Marshal(c)
        if err != nil {
                return
        }

        c.Raw = tbsCertContents

        h := hashFunc.New()
        h.Write(tbsCertContents)
        digest := h.Sum(nil)

        var signerOpts crypto.SignerOpts
        signerOpts = hashFunc
        if template.SignatureAlgorithm != 0 && template.SignatureAlgorithm.isRSAPSS() {
                signerOpts = &rsa.PSSOptions{
                        SaltLength: rsa.PSSSaltLengthEqualsHash,
                        Hash:       hashFunc,
                }
        }

        var signature []byte
        signature, err = key.Sign(rand, digest, signerOpts)
        if err != nil {
                return
        }

        return asn1.Marshal(certificate{
                nil,
                c,
                signatureAlgorithm,
                asn1.BitString{Bytes: signature, BitLength: len(signature) * 8},
        })
}

// pemCRLPrefix is the magic string that indicates that we have a PEM encoded
// CRL.
var pemCRLPrefix = []byte("-----BEGIN X509 CRL")

// pemType is the type of a PEM encoded CRL.
var pemType = "X509 CRL"

// ParseCRL parses a CRL from the given bytes. It's often the case that PEM
// encoded CRLs will appear where they should be DER encoded, so this function
// will transparently handle PEM encoding as long as there isn't any leading
// garbage.
func ParseCRL(crlBytes []byte) (*pkix.CertificateList, error) {
        if bytes.HasPrefix(crlBytes, pemCRLPrefix) {
                block, _ := pem.Decode(crlBytes)
                if block != nil && block.Type == pemType {
                        crlBytes = block.Bytes
                }
        }
        return ParseDERCRL(crlBytes)
}

// ParseDERCRL parses a DER encoded CRL from the given bytes.
func ParseDERCRL(derBytes []byte) (*pkix.CertificateList, error) {
        certList := new(pkix.CertificateList)
        if rest, err := asn1.Unmarshal(derBytes, certList); err != nil {
                return nil, err
        } else if len(rest) != 0 {
                return nil, errors.New("x509: trailing data after CRL")
        }
        return certList, nil
}

// CreateCRL returns a DER encoded CRL, signed by this Certificate, that
// contains the given list of revoked certificates.
func (c *Certificate) CreateCRL(rand io.Reader, priv interface{}, revokedCerts []pkix.RevokedCertificate, now, expiry time.Time) (crlBytes []byte, err error) {
        key, ok := priv.(crypto.Signer)
        if !ok {
                return nil, errors.New("x509: certificate private key does not implement crypto.Signer")
        }

        hashFunc, signatureAlgorithm, err := signingParamsForPublicKey(key.Public(), 0)
        if err != nil {
                return nil, err
        }

        // Force revocation times to UTC per RFC 5280.
        revokedCertsUTC := make([]pkix.RevokedCertificate, len(revokedCerts))
        for i, rc := range revokedCerts {
                rc.RevocationTime = rc.RevocationTime.UTC()
                revokedCertsUTC[i] = rc
        }

        tbsCertList := pkix.TBSCertificateList{
                Version:             1,
                Signature:           signatureAlgorithm,
                Issuer:              c.Subject.ToRDNSequence(),
                ThisUpdate:          now.UTC(),
                NextUpdate:          expiry.UTC(),
                RevokedCertificates: revokedCertsUTC,
        }

        // Authority Key Id
        if len(c.SubjectKeyId) > 0 {
                var aki pkix.Extension
                aki.Id = oidExtensionAuthorityKeyId
                aki.Value, err = asn1.Marshal(authKeyId{Id: c.SubjectKeyId})
                if err != nil {
                        return
                }
                tbsCertList.Extensions = append(tbsCertList.Extensions, aki)
        }

        tbsCertListContents, err := asn1.Marshal(tbsCertList)
        if err != nil {
                return
        }

        h := hashFunc.New()
        h.Write(tbsCertListContents)
        digest := h.Sum(nil)

        var signature []byte
        signature, err = key.Sign(rand, digest, hashFunc)
        if err != nil {
                return
        }

        return asn1.Marshal(pkix.CertificateList{
                TBSCertList:        tbsCertList,
                SignatureAlgorithm: signatureAlgorithm,
                SignatureValue:     asn1.BitString{Bytes: signature, BitLength: len(signature) * 8},
        })
}

// CertificateRequest represents a PKCS #10, certificate signature request.
type CertificateRequest struct {
        Raw                      []byte // Complete ASN.1 DER content (CSR, signature algorithm and signature).
        RawTBSCertificateRequest []byte // Certificate request info part of raw ASN.1 DER content.
        RawSubjectPublicKeyInfo  []byte // DER encoded SubjectPublicKeyInfo.
        RawSubject               []byte // DER encoded Subject.

        Version            int
        Signature          []byte
        SignatureAlgorithm SignatureAlgorithm

        PublicKeyAlgorithm PublicKeyAlgorithm
        PublicKey          interface{}

        Subject pkix.Name

        // Attributes is the dried husk of a bug and shouldn't be used.
        Attributes []pkix.AttributeTypeAndValueSET

        // Extensions contains raw X.509 extensions. When parsing CSRs, this
        // can be used to extract extensions that are not parsed by this
        // package.
        Extensions []pkix.Extension

        // ExtraExtensions contains extensions to be copied, raw, into any
        // marshaled CSR. Values override any extensions that would otherwise
        // be produced based on the other fields but are overridden by any
        // extensions specified in Attributes.
        //
        // The ExtraExtensions field is not populated when parsing CSRs, see
        // Extensions.
        ExtraExtensions []pkix.Extension

        // Subject Alternate Name values.
        DNSNames       []string
        EmailAddresses []string
        IPAddresses    []net.IP
}

// These structures reflect the ASN.1 structure of X.509 certificate
// signature requests (see RFC 2986):

type tbsCertificateRequest struct {
        Raw           asn1.RawContent
        Version       int
        Subject       asn1.RawValue
        PublicKey     publicKeyInfo
        RawAttributes []asn1.RawValue `asn1:"tag:0"`
}

type certificateRequest struct {
        Raw                asn1.RawContent
        TBSCSR             tbsCertificateRequest
        SignatureAlgorithm pkix.AlgorithmIdentifier
        SignatureValue     asn1.BitString
}

// oidExtensionRequest is a PKCS#9 OBJECT IDENTIFIER that indicates requested
// extensions in a CSR.
var oidExtensionRequest = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 14}

// newRawAttributes converts AttributeTypeAndValueSETs from a template
// CertificateRequest's Attributes into tbsCertificateRequest RawAttributes.
func newRawAttributes(attributes []pkix.AttributeTypeAndValueSET) ([]asn1.RawValue, error) {
        var rawAttributes []asn1.RawValue
        b, err := asn1.Marshal(attributes)
        if err != nil {
                return nil, err
        }
        rest, err := asn1.Unmarshal(b, &rawAttributes)
        if err != nil {
                return nil, err
        }
        if len(rest) != 0 {
                return nil, errors.New("x509: failed to unmarshal raw CSR Attributes")
        }
        return rawAttributes, nil
}

// parseRawAttributes Unmarshals RawAttributes intos AttributeTypeAndValueSETs.
func parseRawAttributes(rawAttributes []asn1.RawValue) []pkix.AttributeTypeAndValueSET {
        var attributes []pkix.AttributeTypeAndValueSET
        for _, rawAttr := range rawAttributes {
                var attr pkix.AttributeTypeAndValueSET
                rest, err := asn1.Unmarshal(rawAttr.FullBytes, &attr)
                // Ignore attributes that don't parse into pkix.AttributeTypeAndValueSET
                // (i.e.: challengePassword or unstructuredName).
                if err == nil && len(rest) == 0 {
                        attributes = append(attributes, attr)
                }
        }
        return attributes
}

// parseCSRExtensions parses the attributes from a CSR and extracts any
// requested extensions.
func parseCSRExtensions(rawAttributes []asn1.RawValue) ([]pkix.Extension, error) {
        // pkcs10Attribute reflects the Attribute structure from section 4.1 of
        // https://tools.ietf.org/html/rfc2986.
        type pkcs10Attribute struct {
                Id     asn1.ObjectIdentifier
                Values []asn1.RawValue `asn1:"set"`
        }

        var ret []pkix.Extension
        for _, rawAttr := range rawAttributes {
                var attr pkcs10Attribute
                if rest, err := asn1.Unmarshal(rawAttr.FullBytes, &attr); err != nil || len(rest) != 0 || len(attr.Values) == 0 {
                        // Ignore attributes that don't parse.
                        continue
                }

                if !attr.Id.Equal(oidExtensionRequest) {
                        continue
                }

                var extensions []pkix.Extension
                if _, err := asn1.Unmarshal(attr.Values[0].FullBytes, &extensions); err != nil {
                        return nil, err
                }
                ret = append(ret, extensions...)
        }

        return ret, nil
}

// CreateCertificateRequest creates a new certificate request based on a
// template. The following members of template are used: Attributes, DNSNames,
// EmailAddresses, ExtraExtensions, IPAddresses, SignatureAlgorithm, and
// Subject. The private key is the private key of the signer.
//
// The returned slice is the certificate request in DER encoding.
//
// All keys types that are implemented via crypto.Signer are supported (This
// includes *rsa.PublicKey and *ecdsa.PublicKey.)
func CreateCertificateRequest(rand io.Reader, template *CertificateRequest, priv interface{}) (csr []byte, err error) {
        key, ok := priv.(crypto.Signer)
        if !ok {
                return nil, errors.New("x509: certificate private key does not implement crypto.Signer")
        }

        var hashFunc crypto.Hash
        var sigAlgo pkix.AlgorithmIdentifier
        hashFunc, sigAlgo, err = signingParamsForPublicKey(key.Public(), template.SignatureAlgorithm)
        if err != nil {
                return nil, err
        }

        var publicKeyBytes []byte
        var publicKeyAlgorithm pkix.AlgorithmIdentifier
        publicKeyBytes, publicKeyAlgorithm, err = marshalPublicKey(key.Public())
        if err != nil {
                return nil, err
        }

        var extensions []pkix.Extension

        if (len(template.DNSNames) > 0 || len(template.EmailAddresses) > 0 || len(template.IPAddresses) > 0) &&
                !oidInExtensions(oidExtensionSubjectAltName, template.ExtraExtensions) {
                sanBytes, err := marshalSANs(template.DNSNames, template.EmailAddresses, template.IPAddresses)
                if err != nil {
                        return nil, err
                }

                extensions = append(extensions, pkix.Extension{
                        Id:    oidExtensionSubjectAltName,
                        Value: sanBytes,
                })
        }

        extensions = append(extensions, template.ExtraExtensions...)

        var attributes []pkix.AttributeTypeAndValueSET
        attributes = append(attributes, template.Attributes...)

        if len(extensions) > 0 {
                // specifiedExtensions contains all the extensions that we
                // found specified via template.Attributes.
                specifiedExtensions := make(map[string]bool)

                for _, atvSet := range template.Attributes {
                        if !atvSet.Type.Equal(oidExtensionRequest) {
                                continue
                        }

                        for _, atvs := range atvSet.Value {
                                for _, atv := range atvs {
                                        specifiedExtensions[atv.Type.String()] = true
                                }
                        }
                }

                atvs := make([]pkix.AttributeTypeAndValue, 0, len(extensions))
                for _, e := range extensions {
                        if specifiedExtensions[e.Id.String()] {
                                // Attributes already contained a value for
                                // this extension and it takes priority.
                                continue
                        }

                        atvs = append(atvs, pkix.AttributeTypeAndValue{
                                // There is no place for the critical flag in a CSR.
                                Type:  e.Id,
                                Value: e.Value,
                        })
                }

                // Append the extensions to an existing attribute if possible.
                appended := false
                for _, atvSet := range attributes {
                        if !atvSet.Type.Equal(oidExtensionRequest) || len(atvSet.Value) == 0 {
                                continue
                        }

                        atvSet.Value[0] = append(atvSet.Value[0], atvs...)
                        appended = true
                        break
                }

                // Otherwise, add a new attribute for the extensions.
                if !appended {
                        attributes = append(attributes, pkix.AttributeTypeAndValueSET{
                                Type: oidExtensionRequest,
                                Value: [][]pkix.AttributeTypeAndValue{
                                        atvs,
                                },
                        })
                }
        }

        asn1Subject := template.RawSubject
        if len(asn1Subject) == 0 {
                asn1Subject, err = asn1.Marshal(template.Subject.ToRDNSequence())
                if err != nil {
                        return
                }
        }

        rawAttributes, err := newRawAttributes(attributes)
        if err != nil {
                return
        }

        tbsCSR := tbsCertificateRequest{
                Version: 0, // PKCS #10, RFC 2986
                Subject: asn1.RawValue{FullBytes: asn1Subject},
                PublicKey: publicKeyInfo{
                        Algorithm: publicKeyAlgorithm,
                        PublicKey: asn1.BitString{
                                Bytes:     publicKeyBytes,
                                BitLength: len(publicKeyBytes) * 8,
                        },
                },
                RawAttributes: rawAttributes,
        }

        tbsCSRContents, err := asn1.Marshal(tbsCSR)
        if err != nil {
                return
        }
        tbsCSR.Raw = tbsCSRContents

        h := hashFunc.New()
        h.Write(tbsCSRContents)
        digest := h.Sum(nil)

        var signature []byte
        signature, err = key.Sign(rand, digest, hashFunc)
        if err != nil {
                return
        }

        return asn1.Marshal(certificateRequest{
                TBSCSR:             tbsCSR,
                SignatureAlgorithm: sigAlgo,
                SignatureValue: asn1.BitString{
                        Bytes:     signature,
                        BitLength: len(signature) * 8,
                },
        })
}

// ParseCertificateRequest parses a single certificate request from the
// given ASN.1 DER data.
func ParseCertificateRequest(asn1Data []byte) (*CertificateRequest, error) {
        var csr certificateRequest

        rest, err := asn1.Unmarshal(asn1Data, &csr)
        if err != nil {
                return nil, err
        } else if len(rest) != 0 {
                return nil, asn1.SyntaxError{Msg: "trailing data"}
        }

        return parseCertificateRequest(&csr)
}

func parseCertificateRequest(in *certificateRequest) (*CertificateRequest, error) {
        out := &CertificateRequest{
                Raw: in.Raw,
                RawTBSCertificateRequest: in.TBSCSR.Raw,
                RawSubjectPublicKeyInfo:  in.TBSCSR.PublicKey.Raw,
                RawSubject:               in.TBSCSR.Subject.FullBytes,

                Signature:          in.SignatureValue.RightAlign(),
                SignatureAlgorithm: getSignatureAlgorithmFromAI(in.SignatureAlgorithm),

                PublicKeyAlgorithm: getPublicKeyAlgorithmFromOID(in.TBSCSR.PublicKey.Algorithm.Algorithm),

                Version:    in.TBSCSR.Version,
                Attributes: parseRawAttributes(in.TBSCSR.RawAttributes),
        }

        var err error
        out.PublicKey, err = parsePublicKey(out.PublicKeyAlgorithm, &in.TBSCSR.PublicKey)
        if err != nil {
                return nil, err
        }

        var subject pkix.RDNSequence
        if rest, err := asn1.Unmarshal(in.TBSCSR.Subject.FullBytes, &subject); err != nil {
                return nil, err
        } else if len(rest) != 0 {
                return nil, errors.New("x509: trailing data after X.509 Subject")
        }

        out.Subject.FillFromRDNSequence(&subject)

        if out.Extensions, err = parseCSRExtensions(in.TBSCSR.RawAttributes); err != nil {
                return nil, err
        }

        for _, extension := range out.Extensions {
                if extension.Id.Equal(oidExtensionSubjectAltName) {
                        out.DNSNames, out.EmailAddresses, out.IPAddresses, err = parseSANExtension(extension.Value)
                        if err != nil {
                                return nil, err
                        }
                }
        }

        return out, nil
}

// CheckSignature reports whether the signature on c is valid.
func (c *CertificateRequest) CheckSignature() error {
        return checkSignature(c.SignatureAlgorithm, c.RawTBSCertificateRequest, c.Signature, c.PublicKey)
}