1 INTERNET-DRAFT Brian Tung
2 draft-ietf-cat-kerberos-pk-init-17.txt Clifford Neuman
3 Updates: RFC 1510bis USC/ISI
4 expires May 31, 2004 Matthew Hur
13 Public Key Cryptography for Initial Authentication in Kerberos
15 0. Status Of This Memo
17 This document is an Internet-Draft and is in full conformance with
18 all provision of Section 10 of RFC 2026. Internet-Drafts are
19 working documents of the Internet Engineering Task Force (IETF), its
20 areas, and its working groups. Note that other groups may also
21 distribute working documents as Internet-Drafts.
23 Internet-Drafts are draft documents valid for a maximum of six
24 months and may be updated, replaced, or obsoleted by other documents
25 at any time. It is inappropriate to use Internet-Drafts as
26 reference material or to cite them other than as "work in progress."
28 The list of current Internet-Drafts can be accessed at
29 http://www.ietf.org/ietf/1id-abstracts.txt
31 The list of Internet-Draft Shadow Directories can be accessed at
32 http://www.ietf.org/shadow.html
34 The distribution of this memo is unlimited. It is filed as
35 draft-ietf-cat-kerberos-pk-init-17.txt and expires May 31, 2004.
36 Please send comments to the authors.
41 This draft describes protocol extensions (hereafter called PKINIT)
42 to the Kerberos protocol specification (RFC 1510bis [1]). These
43 extensions provide a method for integrating public key cryptography
44 into the initial authentication exchange, by passing cryptographic
45 certificates and associated authenticators in preauthentication data
51 A client typically authenticates itself to a service in Kerberos
52 using three distinct though related exchanges. First, the client
53 requests a ticket-granting ticket (TGT) from the Kerberos
54 authentication server (AS). Then, it uses the TGT to request a
55 service ticket from the Kerberos ticket-granting server (TGS).
56 Usually, the AS and TGS are integrated in a single device known as
57 a Kerberos Key Distribution Center, or KDC. (In this draft, we will
58 refer to both the AS and the TGS as the KDC.) Finally, the client
59 uses the service ticket to authenticate itself to the service.
61 The advantage afforded by the TGT is that the user need only
62 explicitly request a ticket and expose his credentials once. The
63 TGT and its associated session key can then be used for any
64 subsequent requests. One implication of this is that all further
65 authentication is independent of the method by which the initial
66 authentication was performed. Consequently, initial authentication
67 provides a convenient place to integrate public-key cryptography
68 into Kerberos authentication.
70 As defined, Kerberos authentication exchanges use symmetric-key
71 cryptography, in part for performance. (Symmetric-key cryptography
72 is typically 10-100 times faster than public-key cryptography,
73 depending on the public-key operations. [c]) One cost of using
74 symmetric-key cryptography is that the keys must be shared, so that
75 before a user can authentication himself, he must already be
76 registered with the KDC.
78 Conversely, public-key cryptography--in conjunction with an
79 established certification infrastructure--permits authentication
80 without prior registration. Adding it to Kerberos allows the
81 widespread use of Kerberized applications by users without requiring
82 them to register first--a requirement that has no inherent security
85 As noted above, a convenient and efficient place to introduce
86 public-key cryptography into Kerberos is in the initial
87 authentication exchange. This document describes the methods and
88 data formats for integrating public-key cryptography into Kerberos
89 initial authentication. Another document (PKCROSS) describes a
90 similar protocol for Kerberos cross-realm authentication.
95 This section describes extensions to RFC 1510bis for supporting the
96 use of public-key cryptography in the initial request for a ticket
97 granting ticket (TGT).
99 Briefly, the following changes to RFC 1510bis are proposed:
101 1. If public-key authentication is indicated, the client sends
102 the user's public-key data and an authenticator in a
103 preauthentication field accompanying the usual request.
104 This authenticator is signed by the user's private
107 2. The KDC verifies the client's request against its own
108 policy and certification authorities.
110 3. If the request passes the verification tests, the KDC
111 replies as usual, but the reply is encrypted using either:
113 a. a randomly generated key, signed using the KDC's
114 signature key and encrypted using the user's encryption
117 b. a key generated through a Diffie-Hellman exchange with
118 the client, signed using the KDC's signature key.
120 Any key data required by the client to obtain the encryption
121 key is returned in a preauthentication field accompanying
124 4. The client obtains the encryption key, decrypts the reply,
125 and then proceeds as usual.
127 Section 3.1 of this document defines the necessary message formats.
128 Section 3.2 describes their syntax and use in greater detail.
129 Implementation of all specified formats and uses in these sections
130 is REQUIRED for compliance with PKINIT.
136 3.1.1. Required Algorithms
138 [What is the current list of required algorithm? --brian]
141 3.1.2. Defined Message and Encryption Types
143 PKINIT makes use of the following new preauthentication types:
148 PKINIT introduces the following new error types:
150 KDC_ERR_CLIENT_NOT_TRUSTED 62
151 KDC_ERR_KDC_NOT_TRUSTED 63
152 KDC_ERR_INVALID_SIG 64
153 KDC_ERR_KEY_TOO_WEAK 65
154 KDC_ERR_CERTIFICATE_MISMATCH 66
155 KDC_ERR_CANT_VERIFY_CERTIFICATE 70
156 KDC_ERR_INVALID_CERTIFICATE 71
157 KDC_ERR_REVOKED_CERTIFICATE 72
158 KDC_ERR_REVOCATION_STATUS_UNKNOWN 73
159 KDC_ERR_CLIENT_NAME_MISMATCH 75
161 PKINIT uses the following typed data types for errors:
164 TD-TRUSTED-CERTIFIERS 104
165 TD-CERTIFICATE-INDEX 105
167 PKINIT defines the following encryption types, for use in the AS-REQ
168 message (to indicate acceptance of the corresponding encryption OIDs
172 md5WithRSAEncryption-CmsOID 10
173 sha1WithRSAEncryption-CmsOID 11
175 rsaEncryption-EnvOID (PKCS1 v1.5) 13
176 rsaES-OAEP-ENV-OID (PKCS1 v2.0) 14
177 des-ede3-cbc-Env-OID 15
179 The above encryption types are used (in PKINIT) only within CMS [8]
180 structures within the PKINIT preauthentication fields. Their use
181 within Kerberos EncryptedData structures is unspecified.
184 3.1.3. Algorithm Identifiers
186 PKINIT does not define, but does make use of, the following
187 algorithm identifiers.
189 PKINIT uses the following algorithm identifier for Diffie-Hellman
194 PKINIT uses the following signature algorithm identifiers [8, 12]:
196 sha-1WithRSAEncryption (RSA with SHA1)
197 md5WithRSAEncryption (RSA with MD5)
198 id-dsa-with-sha1 (DSA with SHA1)
200 PKINIT uses the following encryption algorithm identifiers [12] for
201 encrypting the temporary key with a public key:
203 rsaEncryption (PKCS1 v1.5)
204 id-RSAES-OAEP (PKCS1 v2.0)
206 These OIDs are not to be confused with the encryption types listed
209 PKINIT uses the following algorithm identifiers [8] for encrypting
210 the reply key with the temporary key:
212 des-ede3-cbc (three-key 3DES, CBC mode)
213 rc2-cbc (RC2, CBC mode)
215 Again, these OIDs are not to be confused with the encryption types
219 3.2. PKINIT Preauthentication Syntax and Use
221 In this section, we describe the syntax and use of the various
222 preauthentication fields employed to implement PKINIT.
225 3.2.1. Client Request
227 The initial authentication request (AS-REQ) is sent as per RFC
228 1510bis, except that a preauthentication field containing data
229 signed by the user's private signature key accompanies the request,
232 PA-PK-AS-REQ ::= SEQUENCE {
234 signedAuthPack [0] ContentInfo,
236 -- Type is SignedData.
237 -- Content is AuthPack
239 trustedCertifiers [1] SEQUENCE OF TrustedCAs OPTIONAL,
240 -- A list of CAs, trusted by
241 -- the client, used to certify
243 kdcCert [2] IssuerAndSerialNumber OPTIONAL,
245 -- Identifies a particular KDC
246 -- certificate, if the client
248 encryptionCert [3] IssuerAndSerialNumber OPTIONAL,
249 -- May identify the user's
250 -- Diffie-Hellman certificate,
251 -- or an RSA encryption key
256 TrustedCAs ::= CHOICE {
258 -- Fully qualified X.500 name
259 -- as defined in X.509 [11].
260 issuerAndSerial [1] IssuerAndSerialNumber,
261 -- Identifies a specific CA
262 -- certificate, if the client
267 [Should we even allow principalName as a choice? --brian]
269 AuthPack ::= SEQUENCE {
270 pkAuthenticator [0] PKAuthenticator,
271 clientPublicValue [1] SubjectPublicKeyInfo OPTIONAL
274 -- Present only if the client
275 -- is using ephemeral-ephemeral
279 PKAuthenticator ::= SEQUENCE {
281 ctime [1] KerberosTime,
282 -- cusec and ctime are used as
283 -- in RFC 1510bis, for replay
286 -- Binds reply to request,
287 -- except is zero when client
288 -- will accept cached
289 -- Diffie-Hellman parameters
290 -- from KDC and MUST NOT be
292 paChecksum [3] Checksum,
293 -- Defined in RFC 1510bis.
294 -- Performed over KDC-REQ-BODY,
299 SubjectPublicKeyInfo ::= SEQUENCE {
300 -- As defined in X.509.
301 algorithm AlgorithmIdentifier,
302 -- Equals dhpublicnumber (see
303 -- AlgorithmIdentifier, below)
305 subjectPublicKey BIT STRING
306 -- Equals public exponent
307 -- (INTEGER encoded as payload
308 -- of BIT STRING) for PKINIT.
311 AlgorithmIdentifier ::= SEQUENCE {
312 -- As defined in X.509.
313 algorithm OBJECT IDENTIFIER,
315 -- { iso (1) member-body (2)
316 -- US (840) ansi-x942 (10046)
317 -- number-type (2) 1 }
318 -- From RFC 2459 [11].
319 parameters ANY DEFINED BY algorithm OPTIONAL
320 -- Content is DomainParameters
321 -- (see below) for PKINIT.
324 DomainParameters ::= SEQUENCE {
325 -- As defined in RFC 2459.
327 -- p is the odd prime, equals
335 validationParms ValidationParms OPTIONAL
338 ValidationParms ::= SEQUENCE {
339 -- As defined in RFC 2459.
341 -- Seed for the system parameter
342 -- generation process.
344 -- Integer value output as part
345 -- of the system parameter
346 -- generation process.
349 The ContentInfo in the signedAuthPack is filled out as follows:
351 1. The eContent field contains data of type AuthPack. It MUST
352 contain the pkAuthenticator, and MAY also contain the
353 user's Diffie-Hellman public value (clientPublicValue).
355 2. The eContentType field MUST contain the OID value for
356 pkauthdata: { iso (1) org (3) dod (6) internet (1)
357 security (5) kerberosv5 (2) pkinit (3) pkauthdata (1)}
359 3. The signerInfos field MUST contain the signature of the
362 4. The certificates field MUST contain at least a signature
363 verification certificate chain that the KDC can use to
364 verify the signature on the AuthPack. Additionally, the
365 client may also insert an encryption certificate chain, if
366 (for example) the client is not using ephemeral-ephemeral
369 5. If a Diffie-Hellman key is being used, the parameters SHOULD
370 be chosen from the First or Second defined Oakley Groups.
373 6. The KDC may wish to use cached Diffie-Hellman parameters.
374 To indicate acceptance of caching, the client sends zero in
375 the nonce field of the pkAuthenticator. Zero is not a valid
376 value for this field under any other circumstances. Since
377 zero is used to indicate acceptance of cached parameters,
378 message binding in this case is performed instead using the
379 nonce in the main request.
382 3.2.2. Validation of Client Request
384 Upon receiving the client's request, the KDC validates it. This
385 section describes the steps that the KDC MUST (unless otherwise
386 noted) take in validating the request.
388 The KDC must look for a user certificate in the signedAuthPack.
389 If it cannot find one signed by a CA it trusts, it sends back an
390 error of type KDC_ERR_CANT_VERIFY_CERTIFICATE. The accompanying
391 e-data for this error is a SEQUENCE OF TypedData:
393 TypedData ::= SEQUENCE {
394 -- As defined in RFC 1510bis.
395 data-type [0] INTEGER,
396 data-value [1] OCTET STRING
399 For this error, the data-type is TD-TRUSTED-CERTIFIERS, and the
400 data-value is an OCTET STRING containing the DER encoding of
402 TrustedCertifiers ::= SEQUENCE OF Name
404 If, while verifying the certificate chain, the KDC determines that
405 the signature on one of the certificates in the signedAuthPack is
406 invalid, it returns an error of type KDC_ERR_INVALID_CERTIFICATE.
407 The accompanying e-data for this error is a SEQUENCE OF TypedData,
408 whose data-type is TD-CERTIFICATE-INDEX, and whose data-value is an
409 OCTET STRING containing the DER encoding of the index into the
410 CertificateSet field, ordered as sent by the client:
412 CertificateIndex ::= INTEGER
413 -- 0 = first certificate (in
414 -- order of encoding),
415 -- 1 = second certificate, etc.
417 If more than one signature is invalid, the KDC sends one TypedData
418 per invalid signature.
420 The KDC MAY also check whether any of the certificates in the user's
421 chain have been revoked. If any of them have been revoked, the KDC
422 returns an error of type KDC_ERR_REVOKED_CERTIFICATE; if the KDC
423 attempts to determine the revocation status but is unable to do so,
424 it returns an error of type KDC_ERR_REVOCATION_STATUS_UNKNOWN. In
425 either case, the certificate or certificates affected are identified
426 exactly as for an error of type KDC_ERR_INVALID_CERTIFICATE (see
429 If the certificate chain is successfully validated, but the name in
430 the user's certificate does not match the name given in the request,
431 the KDC returns an error of type KDC_ERR_CLIENT_NAME_MISMATCH. There
432 is no accompanying e-data for this error.
434 Even if the chain is validated, and the names in the certificate and
435 the request match, the KDC may decide not to trust the client. For
436 example, the certificate may include (or not include) an Enhanced
437 Key Usage (EKU) OID in the extensions field. As a matter of local
438 policy, the KDC may decide to reject requests on the basis of the
439 absence or presence of specific EKU OIDs. In this case, the KDC
440 returns an error of type KDC_ERR_CLIENT_NOT_TRUSTED. For the
441 benefit of implementors, we define a PKINIT EKU OID as follows:
442 { iso (1) org (3) dod (6) internet (1) security (5) kerberosv5 (2)
443 pkinit (3) pkekuoid (2) }.
445 If the certificate chain and usage check out, but the client's
446 signature on the signedAuthPack fails to verify, the KDC returns an
447 error of type KDC_ERR_INVALID_SIG. There is no accompanying e-data
450 [What about the case when all this checks out but one or more
451 certificates is rejected for other reasons? For example, perhaps
452 the key is too short for local policy. --DRE]
454 The KDC must check the timestamp to ensure that the request is not
455 a replay, and that the time skew falls within acceptable limits. If
456 the check fails, the KDC returns an error of type KRB_AP_ERR_REPEAT
457 or KRB_AP_ERR_SKEW, respectively.
459 Finally, if the clientPublicValue is filled in, indicating that the
460 client wishes to use ephemeral-ephemeral Diffie-Hellman, the KDC
461 checks to see if the parameters satisfy its policy. If they do not,
462 it returns an error of type KDC_ERR_KEY_TOO_WEAK. The accompanying
463 e-data is a SEQUENCE OF TypedData, whose data-type is
464 TD-DH-PARAMETERS, and whose data-value is an OCTET STRING containing
465 the DER encoding of a DomainParameters (see above), including
466 appropriate Diffie-Hellman parameters with which to retry the
469 [This makes no sense. For example, maybe the key is too strong for
472 In order to establish authenticity of the reply, the KDC will sign
473 some key data (either the random key used to encrypt the reply in
474 the case of a KDCDHKeyInfo, or the Diffie-Hellman parameters used to
475 generate the reply-encrypting key in the case of a ReplyKeyPack).
476 The signature certificate to be used is to be selected as follows:
478 1. If the client included a kdcCert field in the PA-PK-AS-REQ,
479 use the referred-to certificate, if the KDC has it. If it
480 does not, the KDC returns an error of type
481 KDC_ERR_CERTIFICATE_MISMATCH.
483 2. Otherwise, if the client did not include a kdcCert field,
484 but did include a trustedCertifiers field, and the KDC
485 possesses a certificate issued by one of the listed
486 certifiers, use that certificate. if it does not possess
487 one, it returns an error of type KDC_ERR_KDC_NOT_TRUSTED.
489 3. Otherwise, if the client included neither a kdcCert field
490 nor a trustedCertifiers field, and the KDC has only one
491 signature certificate, use that certificate. If it has
492 more than one certificate, it returns an error of type
493 KDC_ERR_CERTIFICATE_MISMATCH.
498 Assuming that the client's request has been properly validated, the
499 KDC proceeds as per RFC 1510bis, except as follows.
501 The user's name as represented in the AS-REP must be derived from
502 the certificate provided in the client's request. If the KDC has
503 its own mapping from the name in the certificate to a Kerberos name,
504 it uses that Kerberos name.
506 Otherwise, if the certificate contains a subjectAltName extension
507 with PrincipalName, it uses that name. In this case, the realm in
508 the ticket is that of the local realm (or some other realm name
509 chosen by that realm). (OID and syntax for this extension to be
510 specified here.) Otherwise, the KDC returns an error of type
511 KDC_ERR_CLIENT_NAME_MISMATCH.
513 In addition, the certifiers in the certification path of the user's
514 certificate MUST be added to an authdata (to be specified at a later
517 The AS-REP is otherwise unchanged from RFC 1510bis. The KDC then
518 encrypts the reply as usual, but not with the user's long-term key.
519 Instead, it encrypts it with either a random encryption key, or a
520 key derived through a Diffie-Hellman exchange. Which is the case is
521 indicated by the contents of the PA-PK-AS-REP (note tags):
523 PA-PK-AS-REP ::= CHOICE {
525 dhSignedData [0] ContentInfo,
526 -- Type is SignedData.
527 -- Content is KDCDHKeyInfo
529 encKeyPack [1] ContentInfo,
530 -- Type is EnvelopedData.
531 -- Content is ReplyKeyPack
536 Note that PA-PK-AS-REP is a CHOICE: either a dhSignedData, or an
537 encKeyPack, but not both. The former contains data of type
538 KDCDHKeyInfo, and is used only when the reply is encrypted using a
539 Diffie-Hellman derived key:
541 KDCDHKeyInfo ::= SEQUENCE {
542 subjectPublicKey [0] BIT STRING,
543 -- Equals public exponent
545 -- INTEGER encoded as payload
548 -- Binds reply to request.
549 -- Exception: A value of zero
550 -- indicates that the KDC is
551 -- using cached values.
552 dhKeyExpiration [2] KerberosTime OPTIONAL,
553 -- Expiration time for KDC's
558 The fields of the ContentInfo for dhSignedData are to be filled in
561 1. The eContent field contains data of type KDCDHKeyInfo.
563 2. The eContentType field contains the OID value for
564 pkdhkeydata: { iso (1) org (3) dod (6) internet (1)
565 security (5) kerberosv5 (2) pkinit (3) pkdhkeydata (2) }
567 3. The signerInfos field contains a single signerInfo, which is
568 the signature of the KDCDHKeyInfo.
570 4. The certificates field contains a signature verification
571 certificate chain that the client may use to verify the
572 KDC's signature over the KDCDHKeyInfo.) It may only be left
573 empty if the client did not include a trustedCertifiers
574 field in the PA-PK-AS-REQ, indicating that it has the KDC's
577 5. If the client and KDC agree to use cached parameters, the
578 KDC SHOULD return a zero in the nonce field and include the
579 expiration time of the cached values in the dhKeyExpiration
580 field. If this time is exceeded, the client SHOULD NOT use
581 the reply. If the time is absent, the client SHOULD NOT use
582 the reply and MAY resubmit a request with a non-zero nonce,
583 thus indicating non-acceptance of the cached parameters.
585 The key is derived as follows: Both the KDC and the client calculate
586 the value g^(ab) mod p, where a and b are the client and KDC's
587 private exponents, respectively. They both take the first N bits of
588 this secret value and convert it into a reply key, where N depends
591 1. For example, if the key type is DES, N = 64 bits, where some
592 of the bits are replaced with parity bits, according to FIPS
595 2. If the key type is (three-key) 3DES, N = 192 bits, where
596 some of the bits are replaced with parity bits, again
597 according to FIPS PUB 74.
599 If the KDC and client are not using Diffie-Hellman, the KDC encrypts
600 the reply with an encryption key, packed in the encKeyPack, which
601 contains data of type ReplyKeyPack:
603 ReplyKeyPack ::= SEQUENCE {
604 replyKey [0] EncryptionKey,
605 -- Defined in RFC 1510bis.
606 -- Used to encrypt main reply.
607 -- MUST be at least as strong as
608 -- enctype of session key.
610 -- Binds reply to request.
614 [What exactly does "at least as strong" mean? --DRE]
616 The fields of the ContentInfo for encKeyPack MUST be filled in as
619 1. The innermost data is of type SignedData. The eContent for
620 this data is of type ReplyKeyPack.
622 2. The eContentType for this data contains the OID value for
623 pkrkeydata: { iso (1) org (3) dod (6) internet (1)
624 security (5) kerberosv5 (2) pkinit (3) pkrkeydata (3) }
626 3. The signerInfos field contains a single signerInfo, which is
627 the signature of the ReplyKeyPack.
629 4. The certificates field contains a signature verification
630 certificate chain, which the client may use to verify the
631 KDC's signature over the ReplyKeyPack.) It may only be left
632 empty if the client did not include a trustedCertifiers
633 field in the PA-PK-AS-REQ, indicating that it has the KDC's
636 5. The outer data is of type EnvelopedData. The
637 encryptedContent for this data is the SignedData described
638 in items 1 through 4, above.
640 6. The encryptedContentType for this data contains the OID
641 value for id-signedData: { iso (1) member-body (2) us (840)
642 rsadsi (113549) pkcs (1) pkcs7 (7) signedData (2) }
644 7. The recipientInfos field is a SET which MUST contain exactly
645 one member of type KeyTransRecipientInfo. The encryptedKey
646 for this member contains the temporary key which is
647 encrypted using the client's public key.
649 8. Neither the unprotectedAttrs field nor the originatorInfo
650 field is required for PKINIT.
653 3.2.4. Validation of KDC Reply
655 Upon receipt of the KDC's reply, the client proceeds as follows. If
656 the PA-PK-AS-REP contains a dhSignedData, the client obtains and
657 verifies the Diffie-Hellman parameters, and obtains the shared key
658 as described above. Otherwise, the message contains an encKeyPack,
659 and the client decrypts and verifies the temporary encryption key.
660 In either case, the client then decrypts the main reply with the
661 resulting key, and then proceeds as described in RFC 1510bis.
664 4. Security Considerations
666 PKINIT raises certain security considerations beyond those that can
667 be regulated strictly in protocol definitions. We will address them
670 PKINIT extends the cross-realm model to the public-key
671 infrastructure. Anyone using PKINIT must be aware of how the
672 certification infrastructure they are linking to works.
674 Also, as in standard Kerberos, PKINIT presents the possibility of
675 interactions between cryptosystems of varying strengths, and this
676 now includes public-key cryptosystems. Many systems, for example,
677 allow the use of 512-bit public keys. Using such keys to wrap data
678 encrypted under strong conventional cryptosystems, such as 3DES, may
681 PKINIT calls for randomly generated keys for conventional
682 cryptosystems. Many such systems contain systematically "weak"
683 keys. For recommendations regarding these weak keys, see RFC
686 Care should be taken in how certificates are chosen for the purposes
687 of authentication using PKINIT. Some local policies may require
688 that key escrow be applied for certain certificate types. People
689 deploying PKINIT should be aware of the implications of using
690 certificates that have escrowed keys for the purposes of
693 PKINIT does not provide for a "return routability" test to prevent
694 attackers from mounting a denial-of-service attack on the KDC by
695 causing it to perform unnecessary and expensive public-key
696 operations. Strictly speaking, this is also true of standard
697 Kerberos, although the potential cost is not as great, because
698 standard Kerberos does not make use of public-key cryptography.
703 Some of the ideas on which this proposal is based arose during
704 discussions over several years between members of the SAAG, the IETF
705 CAT working group, and the PSRG, regarding integration of Kerberos
706 and SPX. Some ideas have also been drawn from the DASS system.
707 These changes are by no means endorsed by these groups. This is an
708 attempt to revive some of the goals of those groups, and this
709 proposal approaches those goals primarily from the Kerberos
710 perspective. Lastly, comments from groups working on similar ideas
711 in DCE have been invaluable.
716 This draft expires May 31, 2004.
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