remove unused variable

[dropbear.git] / common-kex.c

/*
 * Dropbear SSH
 * 
 * Copyright (c) 2002-2004 Matt Johnston
 * Portions Copyright (c) 2004 by Mihnea Stoenescu
 * All rights reserved.
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE. */

#include "includes.h"
#include "dbutil.h"
#include "algo.h"
#include "buffer.h"
#include "session.h"
#include "kex.h"
#include "ssh.h"
#include "packet.h"
#include "bignum.h"
#include "random.h"
#include "runopts.h"

/* diffie-hellman-group1-sha1 value for p */
static const unsigned char dh_p_val[] = {
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xC9, 0x0F, 0xDA, 0xA2,
    0x21, 0x68, 0xC2, 0x34, 0xC4, 0xC6, 0x62, 0x8B, 0x80, 0xDC, 0x1C, 0xD1,
        0x29, 0x02, 0x4E, 0x08, 0x8A, 0x67, 0xCC, 0x74, 0x02, 0x0B, 0xBE, 0xA6,
        0x3B, 0x13, 0x9B, 0x22, 0x51, 0x4A, 0x08, 0x79, 0x8E, 0x34, 0x04, 0xDD,
        0xEF, 0x95, 0x19, 0xB3, 0xCD, 0x3A, 0x43, 0x1B, 0x30, 0x2B, 0x0A, 0x6D,
        0xF2, 0x5F, 0x14, 0x37, 0x4F, 0xE1, 0x35, 0x6D, 0x6D, 0x51, 0xC2, 0x45,
        0xE4, 0x85, 0xB5, 0x76, 0x62, 0x5E, 0x7E, 0xC6, 0xF4, 0x4C, 0x42, 0xE9,
        0xA6, 0x37, 0xED, 0x6B, 0x0B, 0xFF, 0x5C, 0xB6, 0xF4, 0x06, 0xB7, 0xED,
        0xEE, 0x38, 0x6B, 0xFB, 0x5A, 0x89, 0x9F, 0xA5, 0xAE, 0x9F, 0x24, 0x11,
        0x7C, 0x4B, 0x1F, 0xE6, 0x49, 0x28, 0x66, 0x51, 0xEC, 0xE6, 0x53, 0x81,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF};
#define DH_P_LEN sizeof(dh_p_val)

static const int DH_G_VAL = 2;

static void kexinitialise();
void gen_new_keys();
#ifndef DISABLE_ZLIB
static void gen_new_zstreams();
#endif
static void read_kex_algos();
/* helper function for gen_new_keys */
static void hashkeys(unsigned char *out, int outlen, 
                const hash_state * hs, unsigned const char X);


/* Send our list of algorithms we can use */
void send_msg_kexinit() {

        CHECKCLEARTOWRITE();
        buf_putbyte(ses.writepayload, SSH_MSG_KEXINIT);

        /* cookie */
        genrandom(buf_getwriteptr(ses.writepayload, 16), 16);
        buf_incrwritepos(ses.writepayload, 16);

        /* kex algos */
        buf_put_algolist(ses.writepayload, sshkex);

        /* server_host_key_algorithms */
        buf_put_algolist(ses.writepayload, sshhostkey);

        /* encryption_algorithms_client_to_server */
        buf_put_algolist(ses.writepayload, sshciphers);

        /* encryption_algorithms_server_to_client */
        buf_put_algolist(ses.writepayload, sshciphers);

        /* mac_algorithms_client_to_server */
        buf_put_algolist(ses.writepayload, sshhashes);

        /* mac_algorithms_server_to_client */
        buf_put_algolist(ses.writepayload, sshhashes);

        /* compression_algorithms_client_to_server */
        buf_put_algolist(ses.writepayload, ses.compress_algos);

        /* compression_algorithms_server_to_client */
        buf_put_algolist(ses.writepayload, ses.compress_algos);

        /* languages_client_to_server */
        buf_putstring(ses.writepayload, "", 0);

        /* languages_server_to_client */
        buf_putstring(ses.writepayload, "", 0);

        /* first_kex_packet_follows - unimplemented for now */
        buf_putbyte(ses.writepayload, 0x00);

        /* reserved unit32 */
        buf_putint(ses.writepayload, 0);

        /* set up transmitted kex packet buffer for hashing. 
         * This is freed after the end of the kex */
        ses.transkexinit = buf_newcopy(ses.writepayload);

        encrypt_packet();
        ses.dataallowed = 0; /* don't send other packets during kex */

        TRACE(("DATAALLOWED=0"))
        TRACE(("-> KEXINIT"))
        ses.kexstate.sentkexinit = 1;
}

/* *** NOTE regarding (send|recv)_msg_newkeys *** 
 * Changed by mihnea from the original kex.c to set dataallowed after a 
 * completed key exchange, no matter the order in which it was performed.
 * This enables client mode without affecting server functionality.
 */

/* Bring new keys into use after a key exchange, and let the client know*/
void send_msg_newkeys() {

        TRACE(("enter send_msg_newkeys"))

        /* generate the kexinit request */
        CHECKCLEARTOWRITE();
        buf_putbyte(ses.writepayload, SSH_MSG_NEWKEYS);
        encrypt_packet();
        

        /* set up our state */
        if (ses.kexstate.recvnewkeys) {
                TRACE(("while RECVNEWKEYS=1"))
                gen_new_keys();
                kexinitialise(); /* we've finished with this kex */
                TRACE((" -> DATAALLOWED=1"))
                ses.dataallowed = 1; /* we can send other packets again now */
                ses.kexstate.donefirstkex = 1;
        } else {
                ses.kexstate.sentnewkeys = 1;
                TRACE(("SENTNEWKEYS=1"))
        }

        TRACE(("-> MSG_NEWKEYS"))
        TRACE(("leave send_msg_newkeys"))
}

/* Bring the new keys into use after a key exchange */
void recv_msg_newkeys() {

        TRACE(("<- MSG_NEWKEYS"))
        TRACE(("enter recv_msg_newkeys"))

        /* simply check if we've sent SSH_MSG_NEWKEYS, and if so,
         * switch to the new keys */
        if (ses.kexstate.sentnewkeys) {
                TRACE(("while SENTNEWKEYS=1"))
                gen_new_keys();
                kexinitialise(); /* we've finished with this kex */
            TRACE((" -> DATAALLOWED=1"))
            ses.dataallowed = 1; /* we can send other packets again now */
                ses.kexstate.donefirstkex = 1;
        } else {
                TRACE(("RECVNEWKEYS=1"))
                ses.kexstate.recvnewkeys = 1;
        }
        
        TRACE(("leave recv_msg_newkeys"))
}


/* Set up the kex for the first time */
void kexfirstinitialise() {
        ses.kexstate.donefirstkex = 0;

#ifndef DISABLE_ZLIB
        if (opts.enable_compress) {
                ses.compress_algos = ssh_compress;
        } else
#endif
        {
                ses.compress_algos = ssh_nocompress;
        }
        kexinitialise();
}

/* Reset the kex state, ready for a new negotiation */
static void kexinitialise() {

        TRACE(("kexinitialise()"))

        /* sent/recv'd MSG_KEXINIT */
        ses.kexstate.sentkexinit = 0;
        ses.kexstate.recvkexinit = 0;

        /* sent/recv'd MSG_NEWKEYS */
        ses.kexstate.recvnewkeys = 0;
        ses.kexstate.sentnewkeys = 0;

        /* first_packet_follows */
        ses.kexstate.firstfollows = 0;

        ses.kexstate.datatrans = 0;
        ses.kexstate.datarecv = 0;

        ses.kexstate.lastkextime = time(NULL);

}

/* Helper function for gen_new_keys, creates a hash. It makes a copy of the
 * already initialised hash_state hs, which should already have processed
 * the dh_K and hash, since these are common. X is the letter 'A', 'B' etc.
 * out must have at least min(SHA1_HASH_SIZE, outlen) bytes allocated.
 * The output will only be expanded once, as we are assured that
 * outlen <= 2*SHA1_HASH_SIZE for all known hashes.
 *
 * See Section 7.2 of rfc4253 (ssh transport) for details */
static void hashkeys(unsigned char *out, int outlen, 
                const hash_state * hs, const unsigned char X) {

        hash_state hs2;
        unsigned char k2[SHA1_HASH_SIZE]; /* used to extending */

        memcpy(&hs2, hs, sizeof(hash_state));
        sha1_process(&hs2, &X, 1);
        sha1_process(&hs2, ses.session_id, SHA1_HASH_SIZE);
        sha1_done(&hs2, out);
        if (SHA1_HASH_SIZE < outlen) {
                /* need to extend */
                memcpy(&hs2, hs, sizeof(hash_state));
                sha1_process(&hs2, out, SHA1_HASH_SIZE);
                sha1_done(&hs2, k2);
                memcpy(&out[SHA1_HASH_SIZE], k2, outlen - SHA1_HASH_SIZE);
        }
}

/* Generate the actual encryption/integrity keys, using the results of the
 * key exchange, as specified in section 5.2 of the IETF secsh-transport
 * draft. This occurs after the DH key-exchange.
 *
 * ses.newkeys is the new set of keys which are generated, these are only
 * taken into use after both sides have sent a newkeys message */

/* Originally from kex.c, generalized for cli/svr mode --mihnea */
void gen_new_keys() {

        unsigned char C2S_IV[MAX_IV_LEN];
        unsigned char C2S_key[MAX_KEY_LEN];
        unsigned char S2C_IV[MAX_IV_LEN];
        unsigned char S2C_key[MAX_KEY_LEN];
        /* unsigned char key[MAX_KEY_LEN]; */
        unsigned char *trans_IV, *trans_key, *recv_IV, *recv_key;

        hash_state hs;
        unsigned int C2S_keysize, S2C_keysize;
        char mactransletter, macrecvletter; /* Client or server specific */
        int recv_cipher = 0, trans_cipher = 0;

        TRACE(("enter gen_new_keys"))
        /* the dh_K and hash are the start of all hashes, we make use of that */

        sha1_init(&hs);
        sha1_process_mp(&hs, ses.dh_K);
        mp_clear(ses.dh_K);
        m_free(ses.dh_K);
        sha1_process(&hs, ses.hash, SHA1_HASH_SIZE);
        m_burn(ses.hash, SHA1_HASH_SIZE);

        if (IS_DROPBEAR_CLIENT) {
            trans_IV    = C2S_IV;
            recv_IV             = S2C_IV;
            trans_key   = C2S_key;
            recv_key    = S2C_key;
            C2S_keysize = ses.newkeys->trans.algo_crypt->keysize;
            S2C_keysize = ses.newkeys->recv.algo_crypt->keysize;
                mactransletter = 'E';
                macrecvletter = 'F';
        } else {
            trans_IV    = S2C_IV;
            recv_IV             = C2S_IV;
            trans_key   = S2C_key;
            recv_key    = C2S_key;
            C2S_keysize = ses.newkeys->recv.algo_crypt->keysize;
            S2C_keysize = ses.newkeys->trans.algo_crypt->keysize;
                mactransletter = 'F';
                macrecvletter = 'E';
        }

        hashkeys(C2S_IV, SHA1_HASH_SIZE, &hs, 'A');
        hashkeys(S2C_IV, SHA1_HASH_SIZE, &hs, 'B');
        hashkeys(C2S_key, C2S_keysize, &hs, 'C');
        hashkeys(S2C_key, S2C_keysize, &hs, 'D');

        recv_cipher = find_cipher(ses.newkeys->recv.algo_crypt->cipherdesc->name);
        if (recv_cipher < 0)
            dropbear_exit("crypto error");
        if (ses.newkeys->recv.crypt_mode->start(recv_cipher, 
                        recv_IV, recv_key, 
                        ses.newkeys->recv.algo_crypt->keysize, 0, 
                        &ses.newkeys->recv.cipher_state) != CRYPT_OK) {
                dropbear_exit("crypto error");
        }

        trans_cipher = find_cipher(ses.newkeys->trans.algo_crypt->cipherdesc->name);
        if (trans_cipher < 0)
            dropbear_exit("crypto error");
        if (ses.newkeys->trans.crypt_mode->start(trans_cipher, 
                        trans_IV, trans_key, 
                        ses.newkeys->trans.algo_crypt->keysize, 0, 
                        &ses.newkeys->trans.cipher_state) != CRYPT_OK) {
                dropbear_exit("crypto error");
        }
        
        /* MAC keys */
        hashkeys(ses.newkeys->trans.mackey, 
                        ses.newkeys->trans.algo_mac->keysize, &hs, mactransletter);
        hashkeys(ses.newkeys->recv.mackey, 
                        ses.newkeys->recv.algo_mac->keysize, &hs, macrecvletter);
        ses.newkeys->trans.hash_index = find_hash(ses.newkeys->trans.algo_mac->hashdesc->name),
        ses.newkeys->recv.hash_index = find_hash(ses.newkeys->recv.algo_mac->hashdesc->name),

#ifndef DISABLE_ZLIB
        gen_new_zstreams();
#endif
        
        /* Switch over to the new keys */
        m_burn(ses.keys, sizeof(struct key_context));
        m_free(ses.keys);
        ses.keys = ses.newkeys;
        ses.newkeys = NULL;

        TRACE(("leave gen_new_keys"))
}

#ifndef DISABLE_ZLIB

int is_compress_trans() {
        return ses.keys->trans.algo_comp == DROPBEAR_COMP_ZLIB
                || (ses.authstate.authdone
                        && ses.keys->trans.algo_comp == DROPBEAR_COMP_ZLIB_DELAY);
}

int is_compress_recv() {
        return ses.keys->recv.algo_comp == DROPBEAR_COMP_ZLIB
                || (ses.authstate.authdone
                        && ses.keys->recv.algo_comp == DROPBEAR_COMP_ZLIB_DELAY);
}

/* Set up new zlib compression streams, close the old ones. Only
 * called from gen_new_keys() */
static void gen_new_zstreams() {

        /* create new zstreams */
        if (ses.newkeys->recv.algo_comp == DROPBEAR_COMP_ZLIB
                        || ses.newkeys->recv.algo_comp == DROPBEAR_COMP_ZLIB_DELAY) {
                ses.newkeys->recv.zstream = (z_streamp)m_malloc(sizeof(z_stream));
                ses.newkeys->recv.zstream->zalloc = Z_NULL;
                ses.newkeys->recv.zstream->zfree = Z_NULL;
                
                if (inflateInit(ses.newkeys->recv.zstream) != Z_OK) {
                        dropbear_exit("zlib error");
                }
        } else {
                ses.newkeys->recv.zstream = NULL;
        }

        if (ses.newkeys->trans.algo_comp == DROPBEAR_COMP_ZLIB
                        || ses.newkeys->trans.algo_comp == DROPBEAR_COMP_ZLIB_DELAY) {
                ses.newkeys->trans.zstream = (z_streamp)m_malloc(sizeof(z_stream));
                ses.newkeys->trans.zstream->zalloc = Z_NULL;
                ses.newkeys->trans.zstream->zfree = Z_NULL;
        
                if (deflateInit2(ses.newkeys->trans.zstream, Z_DEFAULT_COMPRESSION,
                                        Z_DEFLATED, DROPBEAR_ZLIB_WINDOW_BITS, 
                                        DROPBEAR_ZLIB_MEM_LEVEL, Z_DEFAULT_STRATEGY)
                                != Z_OK) {
                        dropbear_exit("zlib error");
                }
        } else {
                ses.newkeys->trans.zstream = NULL;
        }

        /* clean up old keys */
        if (ses.keys->recv.zstream != NULL) {
                if (inflateEnd(ses.keys->recv.zstream) == Z_STREAM_ERROR) {
                        /* Z_DATA_ERROR is ok, just means that stream isn't ended */
                        dropbear_exit("crypto error");
                }
                m_free(ses.keys->recv.zstream);
        }
        if (ses.keys->trans.zstream != NULL) {
                if (deflateEnd(ses.keys->trans.zstream) == Z_STREAM_ERROR) {
                        /* Z_DATA_ERROR is ok, just means that stream isn't ended */
                        dropbear_exit("crypto error");
                }
                m_free(ses.keys->trans.zstream);
        }
}
#endif /* DISABLE_ZLIB */


/* Executed upon receiving a kexinit message from the client to initiate
 * key exchange. If we haven't already done so, we send the list of our
 * preferred algorithms. The client's requested algorithms are processed,
 * and we calculate the first portion of the key-exchange-hash for used
 * later in the key exchange. No response is sent, as the client should
 * initiate the diffie-hellman key exchange */

/* Originally from kex.c, generalized for cli/svr mode --mihnea  */
/* Belongs in common_kex.c where it should be moved after review */
void recv_msg_kexinit() {
        
        unsigned int kexhashbuf_len = 0;
        unsigned int remote_ident_len = 0;
        unsigned int local_ident_len = 0;

        TRACE(("<- KEXINIT"))
        TRACE(("enter recv_msg_kexinit"))
        
        if (!ses.kexstate.sentkexinit) {
                /* we need to send a kex packet */
                send_msg_kexinit();
                TRACE(("continue recv_msg_kexinit: sent kexinit"))
        }

        /* start the kex hash */
        local_ident_len = strlen(LOCAL_IDENT);
        remote_ident_len = strlen((char*)ses.remoteident);

        kexhashbuf_len = local_ident_len + remote_ident_len
                + ses.transkexinit->len + ses.payload->len
                + KEXHASHBUF_MAX_INTS;

        ses.kexhashbuf = buf_new(kexhashbuf_len);

        if (IS_DROPBEAR_CLIENT) {

                /* read the peer's choice of algos */
                read_kex_algos();

                /* V_C, the client's version string (CR and NL excluded) */
            buf_putstring(ses.kexhashbuf,
                        (unsigned char*)LOCAL_IDENT, local_ident_len);
                /* V_S, the server's version string (CR and NL excluded) */
            buf_putstring(ses.kexhashbuf, ses.remoteident, remote_ident_len);

                /* I_C, the payload of the client's SSH_MSG_KEXINIT */
            buf_putstring(ses.kexhashbuf,
                        ses.transkexinit->data, ses.transkexinit->len);
                /* I_S, the payload of the server's SSH_MSG_KEXINIT */
            buf_setpos(ses.payload, 0);
            buf_putstring(ses.kexhashbuf, ses.payload->data, ses.payload->len);

        } else {
                /* SERVER */

                /* read the peer's choice of algos */
                read_kex_algos();
                /* V_C, the client's version string (CR and NL excluded) */
            buf_putstring(ses.kexhashbuf, ses.remoteident, remote_ident_len);
                /* V_S, the server's version string (CR and NL excluded) */
            buf_putstring(ses.kexhashbuf, 
                                (unsigned char*)LOCAL_IDENT, local_ident_len);

                /* I_C, the payload of the client's SSH_MSG_KEXINIT */
            buf_setpos(ses.payload, 0);
            buf_putstring(ses.kexhashbuf, ses.payload->data, ses.payload->len);

                /* I_S, the payload of the server's SSH_MSG_KEXINIT */
            buf_putstring(ses.kexhashbuf,
                        ses.transkexinit->data, ses.transkexinit->len);

                ses.requirenext = SSH_MSG_KEXDH_INIT;
        }

        buf_free(ses.transkexinit);
        ses.transkexinit = NULL;
        /* the rest of ses.kexhashbuf will be done after DH exchange */

        ses.kexstate.recvkexinit = 1;

        TRACE(("leave recv_msg_kexinit"))
}

/* Initialises and generate one side of the diffie-hellman key exchange values.
 * See the ietf-secsh-transport draft, section 6, for details */
/* dh_pub and dh_priv MUST be already initialised */
void gen_kexdh_vals(mp_int *dh_pub, mp_int *dh_priv) {

        DEF_MP_INT(dh_p);
        DEF_MP_INT(dh_q);
        DEF_MP_INT(dh_g);

        TRACE(("enter send_msg_kexdh_reply"))
        
        m_mp_init_multi(&dh_g, &dh_p, &dh_q, NULL);

        /* read the prime and generator*/
        bytes_to_mp(&dh_p, (unsigned char*)dh_p_val, DH_P_LEN);
        
        if (mp_set_int(&dh_g, DH_G_VAL) != MP_OKAY) {
                dropbear_exit("Diffie-Hellman error");
        }

        /* calculate q = (p-1)/2 */
        /* dh_priv is just a temp var here */
        if (mp_sub_d(&dh_p, 1, dh_priv) != MP_OKAY) { 
                dropbear_exit("Diffie-Hellman error");
        }
        if (mp_div_2(dh_priv, &dh_q) != MP_OKAY) {
                dropbear_exit("Diffie-Hellman error");
        }

        /* Generate a private portion 0 < dh_priv < dh_q */
        gen_random_mpint(&dh_q, dh_priv);

        /* f = g^y mod p */
        if (mp_exptmod(&dh_g, dh_priv, &dh_p, dh_pub) != MP_OKAY) {
                dropbear_exit("Diffie-Hellman error");
        }
        mp_clear_multi(&dh_g, &dh_p, &dh_q, NULL);
}

/* This function is fairly common between client/server, with some substitution
 * of dh_e/dh_f etc. Hence these arguments:
 * dh_pub_us is 'e' for the client, 'f' for the server. dh_pub_them is 
 * vice-versa. dh_priv is the x/y value corresponding to dh_pub_us */
void kexdh_comb_key(mp_int *dh_pub_us, mp_int *dh_priv, mp_int *dh_pub_them,
                sign_key *hostkey) {

        mp_int dh_p;
        mp_int *dh_e = NULL, *dh_f = NULL;
        hash_state hs;

        /* read the prime and generator*/
        m_mp_init(&dh_p);
        bytes_to_mp(&dh_p, dh_p_val, DH_P_LEN);

        /* Check that dh_pub_them (dh_e or dh_f) is in the range [1, p-1] */
        if (mp_cmp(dh_pub_them, &dh_p) != MP_LT 
                        || mp_cmp_d(dh_pub_them, 0) != MP_GT) {
                dropbear_exit("Diffie-Hellman error");
        }
        
        /* K = e^y mod p = f^x mod p */
        ses.dh_K = (mp_int*)m_malloc(sizeof(mp_int));
        m_mp_init(ses.dh_K);
        if (mp_exptmod(dh_pub_them, dh_priv, &dh_p, ses.dh_K) != MP_OKAY) {
                dropbear_exit("Diffie-Hellman error");
        }

        /* clear no longer needed vars */
        mp_clear_multi(&dh_p, NULL);

        /* From here on, the code needs to work with the _same_ vars on each side,
         * not vice-versaing for client/server */
        if (IS_DROPBEAR_CLIENT) {
                dh_e = dh_pub_us;
                dh_f = dh_pub_them;
        } else {
                dh_e = dh_pub_them;
                dh_f = dh_pub_us;
        } 

        /* Create the remainder of the hash buffer, to generate the exchange hash */
        /* K_S, the host key */
        buf_put_pub_key(ses.kexhashbuf, hostkey, ses.newkeys->algo_hostkey);
        /* e, exchange value sent by the client */
        buf_putmpint(ses.kexhashbuf, dh_e);
        /* f, exchange value sent by the server */
        buf_putmpint(ses.kexhashbuf, dh_f);
        /* K, the shared secret */
        buf_putmpint(ses.kexhashbuf, ses.dh_K);

        /* calculate the hash H to sign */
        sha1_init(&hs);
        buf_setpos(ses.kexhashbuf, 0);
        sha1_process(&hs, buf_getptr(ses.kexhashbuf, ses.kexhashbuf->len),
                        ses.kexhashbuf->len);
        sha1_done(&hs, ses.hash);

        buf_burn(ses.kexhashbuf);
        buf_free(ses.kexhashbuf);
        ses.kexhashbuf = NULL;
        
        /* first time around, we set the session_id to H */
        if (ses.session_id == NULL) {
                /* create the session_id, this never needs freeing */
                ses.session_id = (unsigned char*)m_malloc(SHA1_HASH_SIZE);
                memcpy(ses.session_id, ses.hash, SHA1_HASH_SIZE);
        }
}

/* read the other side's algo list. buf_match_algo is a callback to match
 * algos for the client or server. */
static void read_kex_algos() {

        /* for asymmetry */
        algo_type * c2s_hash_algo = NULL;
        algo_type * s2c_hash_algo = NULL;
        algo_type * c2s_cipher_algo = NULL;
        algo_type * s2c_cipher_algo = NULL;
        algo_type * c2s_comp_algo = NULL;
        algo_type * s2c_comp_algo = NULL;
        /* the generic one */
        algo_type * algo = NULL;

        /* which algo couldn't match */
        char * erralgo = NULL;

        int goodguess = 0;
        int allgood = 1; /* we AND this with each goodguess and see if its still
                                                true after */

        buf_incrpos(ses.payload, 16); /* start after the cookie */

        ses.newkeys = (struct key_context*)m_malloc(sizeof(struct key_context));

        /* kex_algorithms */
        algo = ses.buf_match_algo(ses.payload, sshkex, &goodguess);
        allgood &= goodguess;
        if (algo == NULL) {
                erralgo = "kex";
                goto error;
        }
        TRACE(("kex algo %s", algo->name))
        ses.newkeys->algo_kex = algo->val;

        /* server_host_key_algorithms */
        algo = ses.buf_match_algo(ses.payload, sshhostkey, &goodguess);
        allgood &= goodguess;
        if (algo == NULL) {
                erralgo = "hostkey";
                goto error;
        }
        TRACE(("hostkey algo %s", algo->name))
        ses.newkeys->algo_hostkey = algo->val;

        /* encryption_algorithms_client_to_server */
        c2s_cipher_algo = ses.buf_match_algo(ses.payload, sshciphers, &goodguess);
        if (c2s_cipher_algo == NULL) {
                erralgo = "enc c->s";
                goto error;
        }
        TRACE(("enc c2s is  %s", c2s_cipher_algo->name))

        /* encryption_algorithms_server_to_client */
        s2c_cipher_algo = ses.buf_match_algo(ses.payload, sshciphers, &goodguess);
        if (s2c_cipher_algo == NULL) {
                erralgo = "enc s->c";
                goto error;
        }
        TRACE(("enc s2c is  %s", s2c_cipher_algo->name))

        /* mac_algorithms_client_to_server */
        c2s_hash_algo = ses.buf_match_algo(ses.payload, sshhashes, &goodguess);
        if (c2s_hash_algo == NULL) {
                erralgo = "mac c->s";
                goto error;
        }
        TRACE(("hash c2s is  %s", c2s_hash_algo->name))

        /* mac_algorithms_server_to_client */
        s2c_hash_algo = ses.buf_match_algo(ses.payload, sshhashes, &goodguess);
        if (s2c_hash_algo == NULL) {
                erralgo = "mac s->c";
                goto error;
        }
        TRACE(("hash s2c is  %s", s2c_hash_algo->name))

        /* compression_algorithms_client_to_server */
        c2s_comp_algo = ses.buf_match_algo(ses.payload, ses.compress_algos, &goodguess);
        if (c2s_comp_algo == NULL) {
                erralgo = "comp c->s";
                goto error;
        }
        TRACE(("hash c2s is  %s", c2s_comp_algo->name))

        /* compression_algorithms_server_to_client */
        s2c_comp_algo = ses.buf_match_algo(ses.payload, ses.compress_algos, &goodguess);
        if (s2c_comp_algo == NULL) {
                erralgo = "comp s->c";
                goto error;
        }
        TRACE(("hash s2c is  %s", s2c_comp_algo->name))

        /* languages_client_to_server */
        buf_eatstring(ses.payload);

        /* languages_server_to_client */
        buf_eatstring(ses.payload);

        /* first_kex_packet_follows */
        if (buf_getbool(ses.payload)) {
                ses.kexstate.firstfollows = 1;
                /* if the guess wasn't good, we ignore the packet sent */
                if (!allgood) {
                        ses.ignorenext = 1;
                }
        }

        /* Handle the asymmetry */
        if (IS_DROPBEAR_CLIENT) {
                ses.newkeys->recv.algo_crypt = 
                        (struct dropbear_cipher*)s2c_cipher_algo->data;
                ses.newkeys->trans.algo_crypt = 
                        (struct dropbear_cipher*)c2s_cipher_algo->data;
                ses.newkeys->recv.crypt_mode = 
                        (struct dropbear_cipher_mode*)s2c_cipher_algo->mode;
                ses.newkeys->trans.crypt_mode =
                        (struct dropbear_cipher_mode*)c2s_cipher_algo->mode;
                ses.newkeys->recv.algo_mac = 
                        (struct dropbear_hash*)s2c_hash_algo->data;
                ses.newkeys->trans.algo_mac = 
                        (struct dropbear_hash*)c2s_hash_algo->data;
                ses.newkeys->recv.algo_comp = s2c_comp_algo->val;
                ses.newkeys->trans.algo_comp = c2s_comp_algo->val;
        } else {
                /* SERVER */
                ses.newkeys->recv.algo_crypt = 
                        (struct dropbear_cipher*)c2s_cipher_algo->data;
                ses.newkeys->trans.algo_crypt = 
                        (struct dropbear_cipher*)s2c_cipher_algo->data;
                ses.newkeys->recv.crypt_mode =
                        (struct dropbear_cipher_mode*)c2s_cipher_algo->mode;
                ses.newkeys->trans.crypt_mode =
                        (struct dropbear_cipher_mode*)s2c_cipher_algo->mode;
                ses.newkeys->recv.algo_mac = 
                        (struct dropbear_hash*)c2s_hash_algo->data;
                ses.newkeys->trans.algo_mac = 
                        (struct dropbear_hash*)s2c_hash_algo->data;
                ses.newkeys->recv.algo_comp = c2s_comp_algo->val;
                ses.newkeys->trans.algo_comp = s2c_comp_algo->val;
        }

        /* reserved for future extensions */
        buf_getint(ses.payload);
        return;

error:
        dropbear_exit("no matching algo %s", erralgo);
}