/* Copyright (c) 2004-2007, Sara Golemon <sarag@libssh2.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms,
 * with or without modification, are permitted provided
 * that the following conditions are met:
 *
 *   Redistributions of source code must retain the above
 *   copyright notice, this list of conditions and the
 *   following disclaimer.
 *
 *   Redistributions in binary form must reproduce the above
 *   copyright notice, this list of conditions and the following
 *   disclaimer in the documentation and/or other materials
 *   provided with the distribution.
 *
 *   Neither the name of the copyright holder nor the names
 *   of any other contributors may be used to endorse or
 *   promote products derived from this software without
 *   specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
 * USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
 * OF SUCH DAMAGE.
 */

#include "libssh2_priv.h"

#include "transport.h"
#include "comp.h"
#include "mac.h"

/* TODO: Switch this to an inline and handle alloc() failures */
/* Helper macro called from kex_method_diffie_hellman_group1_sha1_key_exchange */
#define LIBSSH2_KEX_METHOD_DIFFIE_HELLMAN_SHA1_HASH(value, reqlen, version) \
{                                                                           \
    libssh2_sha1_ctx hash;                                                  \
    unsigned long len = 0;                                                  \
    if (!(value)) {                                                         \
        value = LIBSSH2_ALLOC(session, reqlen + SHA_DIGEST_LENGTH);         \
    }                                                                       \
    if (value)                                                              \
        while (len < (unsigned long)reqlen) {                               \
            libssh2_sha1_init(&hash);                                       \
            libssh2_sha1_update(hash, exchange_state->k_value,              \
                                exchange_state->k_value_len);               \
            libssh2_sha1_update(hash, exchange_state->h_sig_comp,           \
                                SHA_DIGEST_LENGTH);                         \
            if (len > 0) {                                                  \
                libssh2_sha1_update(hash, value, len);                      \
            }    else {                                                     \
                libssh2_sha1_update(hash, (version), 1);                    \
                libssh2_sha1_update(hash, session->session_id,              \
                                    session->session_id_len);               \
            }                                                               \
            libssh2_sha1_final(hash, (value) + len);                        \
            len += SHA_DIGEST_LENGTH;                                       \
        }                                                                   \
}

/*
 * diffie_hellman_sha1
 *
 * Diffie Hellman Key Exchange, Group Agnostic
 */
static int diffie_hellman_sha1(LIBSSH2_SESSION *session,
                               _libssh2_bn *g,
                               _libssh2_bn *p,
                               int group_order,
                               unsigned char packet_type_init,
                               unsigned char packet_type_reply,
                               unsigned char *midhash,
                               unsigned long midhash_len,
                               kmdhgGPsha1kex_state_t *exchange_state)
{
    int ret = 0;
    int rc;

    if (exchange_state->state == libssh2_NB_state_idle) {
        /* Setup initial values */
        exchange_state->e_packet = NULL;
        exchange_state->s_packet = NULL;
        exchange_state->k_value = NULL;
        exchange_state->ctx = _libssh2_bn_ctx_new();
        exchange_state->x = _libssh2_bn_init(); /* Random from client */
        exchange_state->e = _libssh2_bn_init(); /* g^x mod p */
        exchange_state->f = _libssh2_bn_init(); /* g^(Random from server) mod p */
        exchange_state->k = _libssh2_bn_init(); /* The shared secret: f^x mod p */

        /* Zero the whole thing out */
        memset(&exchange_state->req_state, 0, sizeof(packet_require_state_t));

        /* Generate x and e */
        _libssh2_bn_rand(exchange_state->x, group_order, 0, -1);
        _libssh2_bn_mod_exp(exchange_state->e, g, exchange_state->x, p,
                            exchange_state->ctx);

        /* Send KEX init */
        /* packet_type(1) + String Length(4) + leading 0(1) */
        exchange_state->e_packet_len =
            _libssh2_bn_bytes(exchange_state->e) + 6;
        if (_libssh2_bn_bits(exchange_state->e) % 8) {
            /* Leading 00 not needed */
            exchange_state->e_packet_len--;
        }

        exchange_state->e_packet =
            LIBSSH2_ALLOC(session, exchange_state->e_packet_len);
        if (!exchange_state->e_packet) {
            libssh2_error(session, LIBSSH2_ERROR_ALLOC, "Out of memory error",
                          0);
            ret = LIBSSH2_ERROR_ALLOC;
            goto clean_exit;
        }
        exchange_state->e_packet[0] = packet_type_init;
        _libssh2_htonu32(exchange_state->e_packet + 1,
                        exchange_state->e_packet_len - 5);
        if (_libssh2_bn_bits(exchange_state->e) % 8) {
            _libssh2_bn_to_bin(exchange_state->e,
                               exchange_state->e_packet + 5);
        } else {
            exchange_state->e_packet[5] = 0;
            _libssh2_bn_to_bin(exchange_state->e,
                               exchange_state->e_packet + 6);
        }

        _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sending KEX packet %d",
                       (int) packet_type_init);
        exchange_state->state = libssh2_NB_state_created;
    }

    if (exchange_state->state == libssh2_NB_state_created) {
        rc = _libssh2_transport_write(session, exchange_state->e_packet,
                                      exchange_state->e_packet_len);
        if (rc == PACKET_EAGAIN) {
            return rc;
        } else if (rc) {
            libssh2_error(session, rc,
                          "Unable to send KEX init message", 0);
            ret = rc;
            goto clean_exit;
        }
        exchange_state->state = libssh2_NB_state_sent;
    }

    if (exchange_state->state == libssh2_NB_state_sent) {
        if (session->burn_optimistic_kexinit) {
            /* The first KEX packet to come along will be the guess initially
             * sent by the server.  That guess turned out to be wrong so we
             * need to silently ignore it */
            int burn_type;

            _libssh2_debug(session, LIBSSH2_DBG_KEX,
                           "Waiting for badly guessed KEX packet (to be ignored)");
            burn_type =
                _libssh2_packet_burn(session, &exchange_state->burn_state);
            if (burn_type == PACKET_EAGAIN) {
                return burn_type;
            } else if (burn_type <= 0) {
                /* Failed to receive a packet */
                ret = burn_type;
                goto clean_exit;
            }
            session->burn_optimistic_kexinit = 0;

            _libssh2_debug(session, LIBSSH2_DBG_KEX,
                           "Burnt packet of type: %02x",
                           (unsigned int) burn_type);
        }

        exchange_state->state = libssh2_NB_state_sent1;
    }

    if (exchange_state->state == libssh2_NB_state_sent1) {
        /* Wait for KEX reply */
        rc = _libssh2_packet_require(session, packet_type_reply,
                                     &exchange_state->s_packet,
                                     &exchange_state->s_packet_len, 0, NULL,
                                     0, &exchange_state->req_state);
        if (rc == PACKET_EAGAIN) {
            return rc;
        }
        if (rc) {
            libssh2_error(session, LIBSSH2_ERROR_TIMEOUT,
                          "Timed out waiting for KEX reply", 0);
            ret = rc;
            goto clean_exit;
        }

        /* Parse KEXDH_REPLY */
        exchange_state->s = exchange_state->s_packet + 1;

        session->server_hostkey_len = _libssh2_ntohu32(exchange_state->s);
        exchange_state->s += 4;
        session->server_hostkey =
            LIBSSH2_ALLOC(session, session->server_hostkey_len);
        if (!session->server_hostkey) {
            libssh2_error(session, LIBSSH2_ERROR_ALLOC,
                          "Unable to allocate memory for a copy of the host key",
                          0);
            ret = LIBSSH2_ERROR_ALLOC;
            goto clean_exit;
        }
        memcpy(session->server_hostkey, exchange_state->s,
               session->server_hostkey_len);
        exchange_state->s += session->server_hostkey_len;

#if LIBSSH2_MD5
        {
            libssh2_md5_ctx fingerprint_ctx;

            libssh2_md5_init(&fingerprint_ctx);
            libssh2_md5_update(fingerprint_ctx, session->server_hostkey,
                               session->server_hostkey_len);
            libssh2_md5_final(fingerprint_ctx, session->server_hostkey_md5);
        }
#ifdef LIBSSH2DEBUG
        {
            char fingerprint[50], *fprint = fingerprint;
            int i;
            for(i = 0; i < 16; i++, fprint += 3) {
                snprintf(fprint, 4, "%02x:", session->server_hostkey_md5[i]);
            }
            *(--fprint) = '\0';
            _libssh2_debug(session, LIBSSH2_DBG_KEX,
                           "Server's MD5 Fingerprint: %s", fingerprint);
        }
#endif /* LIBSSH2DEBUG */
#endif /* ! LIBSSH2_MD5 */

        {
            libssh2_sha1_ctx fingerprint_ctx;

            libssh2_sha1_init(&fingerprint_ctx);
            libssh2_sha1_update(fingerprint_ctx, session->server_hostkey,
                                session->server_hostkey_len);
            libssh2_sha1_final(fingerprint_ctx, session->server_hostkey_sha1);
        }
#ifdef LIBSSH2DEBUG
        {
            char fingerprint[64], *fprint = fingerprint;
            int i;

            for(i = 0; i < 20; i++, fprint += 3) {
                snprintf(fprint, 4, "%02x:", session->server_hostkey_sha1[i]);
            }
            *(--fprint) = '\0';
            _libssh2_debug(session, LIBSSH2_DBG_KEX,
                           "Server's SHA1 Fingerprint: %s", fingerprint);
        }
#endif /* LIBSSH2DEBUG */

        if (session->hostkey->init(session, session->server_hostkey,
                                   session->server_hostkey_len,
                                   &session->server_hostkey_abstract)) {
            libssh2_error(session, LIBSSH2_ERROR_HOSTKEY_INIT,
                          "Unable to initialize hostkey importer", 0);
            ret = LIBSSH2_ERROR_HOSTKEY_INIT;
            goto clean_exit;
        }

        exchange_state->f_value_len = _libssh2_ntohu32(exchange_state->s);
        exchange_state->s += 4;
        exchange_state->f_value = exchange_state->s;
        exchange_state->s += exchange_state->f_value_len;
        _libssh2_bn_from_bin(exchange_state->f, exchange_state->f_value_len,
                             exchange_state->f_value);

        exchange_state->h_sig_len = _libssh2_ntohu32(exchange_state->s);
        exchange_state->s += 4;
        exchange_state->h_sig = exchange_state->s;

        /* Compute the shared secret */
        _libssh2_bn_mod_exp(exchange_state->k, exchange_state->f,
                            exchange_state->x, p, exchange_state->ctx);
        exchange_state->k_value_len = _libssh2_bn_bytes(exchange_state->k) + 5;
        if (_libssh2_bn_bits(exchange_state->k) % 8) {
            /* don't need leading 00 */
            exchange_state->k_value_len--;
        }
        exchange_state->k_value =
            LIBSSH2_ALLOC(session, exchange_state->k_value_len);
        if (!exchange_state->k_value) {
            libssh2_error(session, LIBSSH2_ERROR_ALLOC,
                          "Unable to allocate buffer for K", 0);
            ret = LIBSSH2_ERROR_ALLOC;
            goto clean_exit;
        }
        _libssh2_htonu32(exchange_state->k_value,
                        exchange_state->k_value_len - 4);
        if (_libssh2_bn_bits(exchange_state->k) % 8) {
            _libssh2_bn_to_bin(exchange_state->k, exchange_state->k_value + 4);
        } else {
            exchange_state->k_value[4] = 0;
            _libssh2_bn_to_bin(exchange_state->k, exchange_state->k_value + 5);
        }

        libssh2_sha1_init(&exchange_state->exchange_hash);
        if (session->local.banner) {
            _libssh2_htonu32(exchange_state->h_sig_comp,
                            strlen((char *) session->local.banner) - 2);
            libssh2_sha1_update(exchange_state->exchange_hash,
                                exchange_state->h_sig_comp, 4);
            libssh2_sha1_update(exchange_state->exchange_hash,
                                (char *) session->local.banner,
                                strlen((char *) session->local.banner) - 2);
        } else {
            _libssh2_htonu32(exchange_state->h_sig_comp,
                            sizeof(LIBSSH2_SSH_DEFAULT_BANNER) - 1);
            libssh2_sha1_update(exchange_state->exchange_hash,
                                exchange_state->h_sig_comp, 4);
            libssh2_sha1_update(exchange_state->exchange_hash,
                                LIBSSH2_SSH_DEFAULT_BANNER,
                                sizeof(LIBSSH2_SSH_DEFAULT_BANNER) - 1);
        }

        _libssh2_htonu32(exchange_state->h_sig_comp,
                        strlen((char *) session->remote.banner));
        libssh2_sha1_update(exchange_state->exchange_hash,
                            exchange_state->h_sig_comp, 4);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            session->remote.banner,
                            strlen((char *) session->remote.banner));

        _libssh2_htonu32(exchange_state->h_sig_comp,
                        session->local.kexinit_len);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            exchange_state->h_sig_comp, 4);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            session->local.kexinit,
                            session->local.kexinit_len);

        _libssh2_htonu32(exchange_state->h_sig_comp,
                        session->remote.kexinit_len);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            exchange_state->h_sig_comp, 4);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            session->remote.kexinit,
                            session->remote.kexinit_len);

        _libssh2_htonu32(exchange_state->h_sig_comp,
                        session->server_hostkey_len);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            exchange_state->h_sig_comp, 4);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            session->server_hostkey,
                            session->server_hostkey_len);

        if (packet_type_init == SSH_MSG_KEX_DH_GEX_INIT) {
            /* diffie-hellman-group-exchange hashes additional fields */
#ifdef LIBSSH2_DH_GEX_NEW
            _libssh2_htonu32(exchange_state->h_sig_comp,
                            LIBSSH2_DH_GEX_MINGROUP);
            _libssh2_htonu32(exchange_state->h_sig_comp + 4,
                            LIBSSH2_DH_GEX_OPTGROUP);
            _libssh2_htonu32(exchange_state->h_sig_comp + 8,
                            LIBSSH2_DH_GEX_MAXGROUP);
            libssh2_sha1_update(exchange_state->exchange_hash,
                                exchange_state->h_sig_comp, 12);
#else
            _libssh2_htonu32(exchange_state->h_sig_comp,
                            LIBSSH2_DH_GEX_OPTGROUP);
            libssh2_sha1_update(exchange_state->exchange_hash,
                                exchange_state->h_sig_comp, 4);
#endif
        }

        if (midhash) {
            libssh2_sha1_update(exchange_state->exchange_hash, midhash,
                                midhash_len);
        }

        libssh2_sha1_update(exchange_state->exchange_hash,
                            exchange_state->e_packet + 1,
                            exchange_state->e_packet_len - 1);

        _libssh2_htonu32(exchange_state->h_sig_comp,
                        exchange_state->f_value_len);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            exchange_state->h_sig_comp, 4);
        libssh2_sha1_update(exchange_state->exchange_hash,
                            exchange_state->f_value,
                            exchange_state->f_value_len);

        libssh2_sha1_update(exchange_state->exchange_hash,
                            exchange_state->k_value,
                            exchange_state->k_value_len);

        libssh2_sha1_final(exchange_state->exchange_hash,
                           exchange_state->h_sig_comp);

        if (session->hostkey->
            sig_verify(session, exchange_state->h_sig,
                       exchange_state->h_sig_len, exchange_state->h_sig_comp,
                       20, &session->server_hostkey_abstract)) {
            libssh2_error(session, LIBSSH2_ERROR_HOSTKEY_SIGN,
                          "Unable to verify hostkey signature", 0);
            ret = -1;
            goto clean_exit;
        }

        _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sending NEWKEYS message");
        exchange_state->c = SSH_MSG_NEWKEYS;

        exchange_state->state = libssh2_NB_state_sent2;
    }

    if (exchange_state->state == libssh2_NB_state_sent2) {
        rc = _libssh2_transport_write(session, &exchange_state->c, 1);
        if (rc == PACKET_EAGAIN) {
            return rc;
        } else if (rc) {
            libssh2_error(session, rc, "Unable to send NEWKEYS message", 0);
            ret = rc;
            goto clean_exit;
        }

        exchange_state->state = libssh2_NB_state_sent3;
    }

    if (exchange_state->state == libssh2_NB_state_sent3) {
        rc = _libssh2_packet_require(session, SSH_MSG_NEWKEYS,
                                     &exchange_state->tmp,
                                     &exchange_state->tmp_len, 0, NULL, 0,
                                     &exchange_state->req_state);
        if (rc == PACKET_EAGAIN) {
            return rc;
        } else if (rc) {
            libssh2_error(session, rc, "Timed out waiting for NEWKEYS", 0);
            ret = rc;
            goto clean_exit;
        }
        /* The first key exchange has been performed,
           switch to active crypt/comp/mac mode */
        session->state |= LIBSSH2_STATE_NEWKEYS;
        _libssh2_debug(session, LIBSSH2_DBG_KEX, "Received NEWKEYS message");

        /* This will actually end up being just packet_type(1)
           for this packet type anyway */
        LIBSSH2_FREE(session, exchange_state->tmp);

        if (!session->session_id) {
            session->session_id = LIBSSH2_ALLOC(session, SHA_DIGEST_LENGTH);
            if (!session->session_id) {
                libssh2_error(session, LIBSSH2_ERROR_ALLOC,
                              "Unable to allocate buffer for SHA digest", 0);
                ret = LIBSSH2_ERROR_ALLOC;
                goto clean_exit;
            }
            memcpy(session->session_id, exchange_state->h_sig_comp,
                   SHA_DIGEST_LENGTH);
            session->session_id_len = SHA_DIGEST_LENGTH;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "session_id calculated");
        }

        /* Cleanup any existing cipher */
        if (session->local.crypt->dtor) {
            session->local.crypt->dtor(session,
                                       &session->local.crypt_abstract);
        }

        /* Calculate IV/Secret/Key for each direction */
        if (session->local.crypt->init) {
            unsigned char *iv = NULL, *secret = NULL;
            int free_iv = 0, free_secret = 0;

            LIBSSH2_KEX_METHOD_DIFFIE_HELLMAN_SHA1_HASH(iv,
                                                        session->local.crypt->
                                                        iv_len, "A");
            if (!iv) {
                ret = -1;
                goto clean_exit;
            }
            LIBSSH2_KEX_METHOD_DIFFIE_HELLMAN_SHA1_HASH(secret,
                                                        session->local.crypt->
                                                        secret_len, "C");
            if (!secret) {
                LIBSSH2_FREE(session, iv);
                ret = LIBSSH2_ERROR_KEX_FAILURE;
                goto clean_exit;
            }
            if (session->local.crypt->
                init(session, session->local.crypt, iv, &free_iv, secret,
                     &free_secret, 1, &session->local.crypt_abstract)) {
                LIBSSH2_FREE(session, iv);
                LIBSSH2_FREE(session, secret);
                ret = LIBSSH2_ERROR_KEX_FAILURE;
                goto clean_exit;
            }

            if (free_iv) {
                memset(iv, 0, session->local.crypt->iv_len);
                LIBSSH2_FREE(session, iv);
            }

            if (free_secret) {
                memset(secret, 0, session->local.crypt->secret_len);
                LIBSSH2_FREE(session, secret);
            }
        }
        _libssh2_debug(session, LIBSSH2_DBG_KEX,
                       "Client to Server IV and Key calculated");

        if (session->remote.crypt->dtor) {
            /* Cleanup any existing cipher */
            session->remote.crypt->dtor(session,
                                        &session->remote.crypt_abstract);
        }

        if (session->remote.crypt->init) {
            unsigned char *iv = NULL, *secret = NULL;
            int free_iv = 0, free_secret = 0;

            LIBSSH2_KEX_METHOD_DIFFIE_HELLMAN_SHA1_HASH(iv,
                                                        session->remote.crypt->
                                                        iv_len, "B");
            if (!iv) {
                ret = LIBSSH2_ERROR_KEX_FAILURE;
                goto clean_exit;
            }
            LIBSSH2_KEX_METHOD_DIFFIE_HELLMAN_SHA1_HASH(secret,
                                                        session->remote.crypt->
                                                        secret_len, "D");
            if (!secret) {
                LIBSSH2_FREE(session, iv);
                ret = LIBSSH2_ERROR_KEX_FAILURE;
                goto clean_exit;
            }
            if (session->remote.crypt->
                init(session, session->remote.crypt, iv, &free_iv, secret,
                     &free_secret, 0, &session->remote.crypt_abstract)) {
                LIBSSH2_FREE(session, iv);
                LIBSSH2_FREE(session, secret);
                ret = LIBSSH2_ERROR_KEX_FAILURE;
                goto clean_exit;
            }

            if (free_iv) {
                memset(iv, 0, session->remote.crypt->iv_len);
                LIBSSH2_FREE(session, iv);
            }

            if (free_secret) {
                memset(secret, 0, session->remote.crypt->secret_len);
                LIBSSH2_FREE(session, secret);
            }
        }
        _libssh2_debug(session, LIBSSH2_DBG_KEX,
                       "Server to Client IV and Key calculated");

        if (session->local.mac->dtor) {
            session->local.mac->dtor(session, &session->local.mac_abstract);
        }

        if (session->local.mac->init) {
            unsigned char *key = NULL;
            int free_key = 0;

            LIBSSH2_KEX_METHOD_DIFFIE_HELLMAN_SHA1_HASH(key,
                                                        session->local.mac->
                                                        key_len, "E");
            if (!key) {
                ret = LIBSSH2_ERROR_KEX_FAILURE;
                goto clean_exit;
            }
            session->local.mac->init(session, key, &free_key,
                                     &session->local.mac_abstract);

            if (free_key) {
                memset(key, 0, session->local.mac->key_len);
                LIBSSH2_FREE(session, key);
            }
        }
        _libssh2_debug(session, LIBSSH2_DBG_KEX,
                       "Client to Server HMAC Key calculated");

        if (session->remote.mac->dtor) {
            session->remote.mac->dtor(session, &session->remote.mac_abstract);
        }

        if (session->remote.mac->init) {
            unsigned char *key = NULL;
            int free_key = 0;

            LIBSSH2_KEX_METHOD_DIFFIE_HELLMAN_SHA1_HASH(key,
                                                        session->remote.mac->
                                                        key_len, "F");
            if (!key) {
                ret = LIBSSH2_ERROR_KEX_FAILURE;
                goto clean_exit;
            }
            session->remote.mac->init(session, key, &free_key,
                                      &session->remote.mac_abstract);

            if (free_key) {
                memset(key, 0, session->remote.mac->key_len);
                LIBSSH2_FREE(session, key);
            }
        }
        _libssh2_debug(session, LIBSSH2_DBG_KEX,
                       "Server to Client HMAC Key calculated");
    }

  clean_exit:
    _libssh2_bn_free(exchange_state->x);
    exchange_state->x = NULL;
    _libssh2_bn_free(exchange_state->e);
    exchange_state->e = NULL;
    _libssh2_bn_free(exchange_state->f);
    exchange_state->f = NULL;
    _libssh2_bn_free(exchange_state->k);
    exchange_state->k = NULL;
    _libssh2_bn_ctx_free(exchange_state->ctx);
    exchange_state->ctx = NULL;

    if (exchange_state->e_packet) {
        LIBSSH2_FREE(session, exchange_state->e_packet);
        exchange_state->e_packet = NULL;
    }

    if (exchange_state->s_packet) {
        LIBSSH2_FREE(session, exchange_state->s_packet);
        exchange_state->s_packet = NULL;
    }

    if (exchange_state->k_value) {
        LIBSSH2_FREE(session, exchange_state->k_value);
        exchange_state->k_value = NULL;
    }

    exchange_state->state = libssh2_NB_state_idle;

    return ret;
}


/* kex_method_diffie_hellman_group1_sha1_key_exchange
 * Diffie-Hellman Group1 (Actually Group2) Key Exchange using SHA1
 */
static int
kex_method_diffie_hellman_group1_sha1_key_exchange(LIBSSH2_SESSION *session,
                                                   key_exchange_state_low_t
                                                   * key_state)
{
    static const unsigned char p_value[128] = {
        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
    };

    int ret;

    if (key_state->state == libssh2_NB_state_idle) {
        /* g == 2 */
        key_state->p = _libssh2_bn_init();      /* SSH2 defined value (p_value) */
        key_state->g = _libssh2_bn_init();      /* SSH2 defined value (2) */

        /* Initialize P and G */
        _libssh2_bn_set_word(key_state->g, 2);
        _libssh2_bn_from_bin(key_state->p, 128, p_value);

        _libssh2_debug(session, LIBSSH2_DBG_KEX,
                       "Initiating Diffie-Hellman Group1 Key Exchange");

        key_state->state = libssh2_NB_state_created;
    }
    ret = diffie_hellman_sha1(session, key_state->g, key_state->p, 128,
                              SSH_MSG_KEXDH_INIT, SSH_MSG_KEXDH_REPLY,
                              NULL, 0, &key_state->exchange_state);
    if (ret == PACKET_EAGAIN) {
        return ret;
    }

    _libssh2_bn_free(key_state->p);
    key_state->p = NULL;
    _libssh2_bn_free(key_state->g);
    key_state->g = NULL;
    key_state->state = libssh2_NB_state_idle;

    return ret;
}


/* kex_method_diffie_hellman_group14_sha1_key_exchange
 * Diffie-Hellman Group14 Key Exchange using SHA1
 */
static int
kex_method_diffie_hellman_group14_sha1_key_exchange(LIBSSH2_SESSION *session,
                                                    key_exchange_state_low_t
                                                    * key_state)
{
    static const unsigned char p_value[256] = {
        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, 0xE4, 0x5B, 0x3D,
        0xC2, 0x00, 0x7C, 0xB8, 0xA1, 0x63, 0xBF, 0x05,
        0x98, 0xDA, 0x48, 0x36, 0x1C, 0x55, 0xD3, 0x9A,
        0x69, 0x16, 0x3F, 0xA8, 0xFD, 0x24, 0xCF, 0x5F,
        0x83, 0x65, 0x5D, 0x23, 0xDC, 0xA3, 0xAD, 0x96,
        0x1C, 0x62, 0xF3, 0x56, 0x20, 0x85, 0x52, 0xBB,
        0x9E, 0xD5, 0x29, 0x07, 0x70, 0x96, 0x96, 0x6D,
        0x67, 0x0C, 0x35, 0x4E, 0x4A, 0xBC, 0x98, 0x04,
        0xF1, 0x74, 0x6C, 0x08, 0xCA, 0x18, 0x21, 0x7C,
        0x32, 0x90, 0x5E, 0x46, 0x2E, 0x36, 0xCE, 0x3B,
        0xE3, 0x9E, 0x77, 0x2C, 0x18, 0x0E, 0x86, 0x03,
        0x9B, 0x27, 0x83, 0xA2, 0xEC, 0x07, 0xA2, 0x8F,
        0xB5, 0xC5, 0x5D, 0xF0, 0x6F, 0x4C, 0x52, 0xC9,
        0xDE, 0x2B, 0xCB, 0xF6, 0x95, 0x58, 0x17, 0x18,
        0x39, 0x95, 0x49, 0x7C, 0xEA, 0x95, 0x6A, 0xE5,
        0x15, 0xD2, 0x26, 0x18, 0x98, 0xFA, 0x05, 0x10,
        0x15, 0x72, 0x8E, 0x5A, 0x8A, 0xAC, 0xAA, 0x68,
        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
    };
    int ret;

    if (key_state->state == libssh2_NB_state_idle) {
        key_state->p = _libssh2_bn_init();      /* SSH2 defined value (p_value) */
        key_state->g = _libssh2_bn_init();      /* SSH2 defined value (2) */

        /* g == 2 */
        /* Initialize P and G */
        _libssh2_bn_set_word(key_state->g, 2);
        _libssh2_bn_from_bin(key_state->p, 256, p_value);

        _libssh2_debug(session, LIBSSH2_DBG_KEX,
                       "Initiating Diffie-Hellman Group14 Key Exchange");

        key_state->state = libssh2_NB_state_created;
    }
    ret = diffie_hellman_sha1(session, key_state->g, key_state->p,
                              256, SSH_MSG_KEXDH_INIT, SSH_MSG_KEXDH_REPLY,
                              NULL, 0, &key_state->exchange_state);
    if (ret == PACKET_EAGAIN) {
        return ret;
    }

    key_state->state = libssh2_NB_state_idle;
    _libssh2_bn_free(key_state->p);
    key_state->p = NULL;
    _libssh2_bn_free(key_state->g);
    key_state->g = NULL;

    return ret;
}


/* kex_method_diffie_hellman_group_exchange_sha1_key_exchange
 * Diffie-Hellman Group Exchange Key Exchange using SHA1
 * Negotiates random(ish) group for secret derivation
 */
static int
kex_method_diffie_hellman_group_exchange_sha1_key_exchange
(LIBSSH2_SESSION * session, key_exchange_state_low_t * key_state)
{
    unsigned char *s;
    unsigned long p_len, g_len;
    int ret = 0;
    int rc;

    if (key_state->state == libssh2_NB_state_idle) {
        key_state->p = _libssh2_bn_init();
        key_state->g = _libssh2_bn_init();
        /* Ask for a P and G pair */
#ifdef LIBSSH2_DH_GEX_NEW
        key_state->request[0] = SSH_MSG_KEX_DH_GEX_REQUEST;
        _libssh2_htonu32(key_state->request + 1, LIBSSH2_DH_GEX_MINGROUP);
        _libssh2_htonu32(key_state->request + 5, LIBSSH2_DH_GEX_OPTGROUP);
        _libssh2_htonu32(key_state->request + 9, LIBSSH2_DH_GEX_MAXGROUP);
        key_state->request_len = 13;
        _libssh2_debug(session, LIBSSH2_DBG_KEX,
                       "Initiating Diffie-Hellman Group-Exchange (New Method)");
#else
        key_state->request[0] = SSH_MSG_KEX_DH_GEX_REQUEST_OLD;
        _libssh2_htonu32(key_state->request + 1, LIBSSH2_DH_GEX_OPTGROUP);
        key_state->request_len = 5;
        _libssh2_debug(session, LIBSSH2_DBG_KEX,
                       "Initiating Diffie-Hellman Group-Exchange (Old Method)");
#endif

        key_state->state = libssh2_NB_state_created;
    }

    if (key_state->state == libssh2_NB_state_created) {
        rc = _libssh2_transport_write(session, key_state->request,
                                   key_state->request_len);
        if (rc == PACKET_EAGAIN) {
            return rc;
        } else if (rc) {
            libssh2_error(session, rc,
                          "Unable to send Group Exchange Request", 0);
            ret = rc;
            goto dh_gex_clean_exit;
        }

        key_state->state = libssh2_NB_state_sent;
    }

    if (key_state->state == libssh2_NB_state_sent) {
        rc = _libssh2_packet_require(session, SSH_MSG_KEX_DH_GEX_GROUP,
                                     &key_state->data, &key_state->data_len,
                                     0, NULL, 0, &key_state->req_state);
        if (rc == PACKET_EAGAIN) {
            return rc;
        } else if (rc) {
            libssh2_error(session, rc,
                          "Timeout waiting for GEX_GROUP reply", 0);
            ret = rc;
            goto dh_gex_clean_exit;
        }

        key_state->state = libssh2_NB_state_sent1;
    }

    if (key_state->state == libssh2_NB_state_sent1) {
        s = key_state->data + 1;
        p_len = _libssh2_ntohu32(s);
        s += 4;
        _libssh2_bn_from_bin(key_state->p, p_len, s);
        s += p_len;

        g_len = _libssh2_ntohu32(s);
        s += 4;
        _libssh2_bn_from_bin(key_state->g, g_len, s);
        s += g_len;

        ret = diffie_hellman_sha1(session, key_state->g, key_state->p, p_len,
                                  SSH_MSG_KEX_DH_GEX_INIT,
                                  SSH_MSG_KEX_DH_GEX_REPLY,
                                  key_state->data + 1,
                                  key_state->data_len - 1,
                                  &key_state->exchange_state);
        if (ret == PACKET_EAGAIN) {
            return ret;
        }

        LIBSSH2_FREE(session, key_state->data);
    }

  dh_gex_clean_exit:
    key_state->state = libssh2_NB_state_idle;
    _libssh2_bn_free(key_state->g);
    key_state->g = NULL;
    _libssh2_bn_free(key_state->p);
    key_state->p = NULL;

    return ret;
}


#define LIBSSH2_KEX_METHOD_FLAG_REQ_ENC_HOSTKEY     0x0001
#define LIBSSH2_KEX_METHOD_FLAG_REQ_SIGN_HOSTKEY    0x0002

static const LIBSSH2_KEX_METHOD kex_method_diffie_helman_group1_sha1 = {
    "diffie-hellman-group1-sha1",
    kex_method_diffie_hellman_group1_sha1_key_exchange,
    LIBSSH2_KEX_METHOD_FLAG_REQ_SIGN_HOSTKEY,
};

static const LIBSSH2_KEX_METHOD kex_method_diffie_helman_group14_sha1 = {
    "diffie-hellman-group14-sha1",
    kex_method_diffie_hellman_group14_sha1_key_exchange,
    LIBSSH2_KEX_METHOD_FLAG_REQ_SIGN_HOSTKEY,
};

static const LIBSSH2_KEX_METHOD
    kex_method_diffie_helman_group_exchange_sha1 = {
    "diffie-hellman-group-exchange-sha1",
    kex_method_diffie_hellman_group_exchange_sha1_key_exchange,
    LIBSSH2_KEX_METHOD_FLAG_REQ_SIGN_HOSTKEY,
};

static const LIBSSH2_KEX_METHOD *libssh2_kex_methods[] = {
    &kex_method_diffie_helman_group14_sha1,
    &kex_method_diffie_helman_group_exchange_sha1,
    &kex_method_diffie_helman_group1_sha1,
    NULL
};

typedef struct _LIBSSH2_COMMON_METHOD
{
    const char *name;
} LIBSSH2_COMMON_METHOD;

/* kex_method_strlen
 * Calculate the length of a particular method list's resulting string
 * Includes SUM(strlen() of each individual method plus 1 (for coma)) - 1 (because the last coma isn't used)
 * Another sign of bad coding practices gone mad.  Pretend you don't see this.
 */
static size_t
kex_method_strlen(LIBSSH2_COMMON_METHOD ** method)
{
    size_t len = 0;

    if (!method || !*method) {
        return 0;
    }

    while (*method && (*method)->name) {
        len += strlen((*method)->name) + 1;
        method++;
    }

    return len - 1;
}


/* kex_method_list
 * Generate formatted preference list in buf
 */
static size_t
kex_method_list(unsigned char *buf, size_t list_strlen,
                LIBSSH2_COMMON_METHOD ** method)
{
    _libssh2_htonu32(buf, list_strlen);
    buf += 4;

    if (!method || !*method) {
        return 4;
    }

    while (*method && (*method)->name) {
        int mlen = strlen((*method)->name);
        memcpy(buf, (*method)->name, mlen);
        buf += mlen;
        *(buf++) = ',';
        method++;
    }

    return list_strlen + 4;
}


#define LIBSSH2_METHOD_PREFS_LEN(prefvar, defaultvar) \
    ((prefvar) ? strlen(prefvar) : \
     kex_method_strlen((LIBSSH2_COMMON_METHOD**)(defaultvar)))

#define LIBSSH2_METHOD_PREFS_STR(buf, prefvarlen, prefvar, defaultvar)  \
    if (prefvar) {                                                      \
        _libssh2_htonu32((buf), (prefvarlen));                           \
        buf += 4;                                                       \
        memcpy((buf), (prefvar), (prefvarlen));                         \
        buf += (prefvarlen);                                            \
    } else {                                                            \
        buf += kex_method_list((buf), (prefvarlen),                     \
                               (LIBSSH2_COMMON_METHOD**)(defaultvar));  \
    }

/* kexinit
 * Send SSH_MSG_KEXINIT packet
 */
static int kexinit(LIBSSH2_SESSION * session)
{
    /* 62 = packet_type(1) + cookie(16) + first_packet_follows(1) +
       reserved(4) + length longs(40) */
    size_t data_len = 62;
    size_t kex_len, hostkey_len = 0;
    size_t crypt_cs_len, crypt_sc_len;
    size_t comp_cs_len, comp_sc_len;
    size_t mac_cs_len, mac_sc_len;
    size_t lang_cs_len, lang_sc_len;
    unsigned char *data, *s;
    int rc;

    if (session->kexinit_state == libssh2_NB_state_idle) {
        kex_len =
            LIBSSH2_METHOD_PREFS_LEN(session->kex_prefs, libssh2_kex_methods);
        hostkey_len =
            LIBSSH2_METHOD_PREFS_LEN(session->hostkey_prefs,
                                     libssh2_hostkey_methods());
        crypt_cs_len =
            LIBSSH2_METHOD_PREFS_LEN(session->local.crypt_prefs,
                                     libssh2_crypt_methods());
        crypt_sc_len =
            LIBSSH2_METHOD_PREFS_LEN(session->remote.crypt_prefs,
                                     libssh2_crypt_methods());
        mac_cs_len =
            LIBSSH2_METHOD_PREFS_LEN(session->local.mac_prefs,
                                     _libssh2_mac_methods());
        mac_sc_len =
            LIBSSH2_METHOD_PREFS_LEN(session->remote.mac_prefs,
                                     _libssh2_mac_methods());
        comp_cs_len =
            LIBSSH2_METHOD_PREFS_LEN(session->local.comp_prefs,
                                     _libssh2_comp_methods());
        comp_sc_len =
            LIBSSH2_METHOD_PREFS_LEN(session->remote.comp_prefs,
                                     _libssh2_comp_methods());
        lang_cs_len =
            LIBSSH2_METHOD_PREFS_LEN(session->local.lang_prefs, NULL);
        lang_sc_len =
            LIBSSH2_METHOD_PREFS_LEN(session->remote.lang_prefs, NULL);

        data_len += kex_len + hostkey_len + crypt_cs_len + crypt_sc_len +
            comp_cs_len + comp_sc_len + mac_cs_len + mac_sc_len +
            lang_cs_len + lang_sc_len;

        s = data = LIBSSH2_ALLOC(session, data_len);
        if (!data) {
            libssh2_error(session, LIBSSH2_ERROR_ALLOC,
                          "Unable to allocate memory", 0);
            return LIBSSH2_ERROR_ALLOC;
        }

        *(s++) = SSH_MSG_KEXINIT;

        libssh2_random(s, 16);
        s += 16;

        /* Ennumerating through these lists twice is probably (certainly?)
           inefficient from a CPU standpoint, but it saves multiple
           malloc/realloc calls */
        LIBSSH2_METHOD_PREFS_STR(s, kex_len, session->kex_prefs,
                                 libssh2_kex_methods);
        LIBSSH2_METHOD_PREFS_STR(s, hostkey_len, session->hostkey_prefs,
                                 libssh2_hostkey_methods());
        LIBSSH2_METHOD_PREFS_STR(s, crypt_cs_len, session->local.crypt_prefs,
                                 libssh2_crypt_methods());
        LIBSSH2_METHOD_PREFS_STR(s, crypt_sc_len, session->remote.crypt_prefs,
                                 libssh2_crypt_methods());
        LIBSSH2_METHOD_PREFS_STR(s, mac_cs_len, session->local.mac_prefs,
                                 _libssh2_mac_methods());
        LIBSSH2_METHOD_PREFS_STR(s, mac_sc_len, session->remote.mac_prefs,
                                 _libssh2_mac_methods());
        LIBSSH2_METHOD_PREFS_STR(s, comp_cs_len, session->local.comp_prefs,
                                 _libssh2_comp_methods());
        LIBSSH2_METHOD_PREFS_STR(s, comp_sc_len, session->remote.comp_prefs,
                                 _libssh2_comp_methods());
        LIBSSH2_METHOD_PREFS_STR(s, lang_cs_len, session->local.lang_prefs,
                                 NULL);
        LIBSSH2_METHOD_PREFS_STR(s, lang_sc_len, session->remote.lang_prefs,
                                 NULL);

        /* No optimistic KEX packet follows */
        /* Deal with optimistic packets
         * session->flags |= KEXINIT_OPTIMISTIC
         * session->flags |= KEXINIT_METHODSMATCH
         */
        *(s++) = 0;

        /* Reserved == 0 */
        *(s++) = 0;
        *(s++) = 0;
        *(s++) = 0;
        *(s++) = 0;

#ifdef LIBSSH2DEBUG
        {
            /* Funnily enough, they'll all "appear" to be '\0' terminated */
            unsigned char *p = data + 21;       /* type(1) + cookie(16) + len(4) */

            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent KEX: %s", p);
            p += kex_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent HOSTKEY: %s", p);
            p += hostkey_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent CRYPT_CS: %s", p);
            p += crypt_cs_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent CRYPT_SC: %s", p);
            p += crypt_sc_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent MAC_CS: %s", p);
            p += mac_cs_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent MAC_SC: %s", p);
            p += mac_sc_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent COMP_CS: %s", p);
            p += comp_cs_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent COMP_SC: %s", p);
            p += comp_sc_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent LANG_CS: %s", p);
            p += lang_cs_len + 4;
            _libssh2_debug(session, LIBSSH2_DBG_KEX, "Sent LANG_SC: %s", p);
            p += lang_sc_len + 4;
        }
#endif /* LIBSSH2DEBUG */

        session->kexinit_state = libssh2_NB_state_created;
    } else {
        data = session->kexinit_data;
        data_len = session->kexinit_data_len;
    }

    rc = _libssh2_transport_write(session, data, data_len);
    if (rc == PACKET_EAGAIN) {
        session->kexinit_data = data;
        session->kexinit_data_len = data_len;
        return rc;
    }
    else if (rc) {
        LIBSSH2_FREE(session, data);
        libssh2_error(session, rc,
                      "Unable to send KEXINIT packet to remote host", 0);
        session->kexinit_state = libssh2_NB_state_idle;
        return rc;
    }

    if (session->local.kexinit) {
        LIBSSH2_FREE(session, session->local.kexinit);
    }

    session->local.kexinit = data;
    session->local.kexinit_len = data_len;

    session->kexinit_state = libssh2_NB_state_idle;

    return 0;
}

/* kex_agree_instr
 * Kex specific variant of strstr()
 * Needle must be preceed by BOL or ',', and followed by ',' or EOL
 */
static unsigned char *
kex_agree_instr(unsigned char *haystack, unsigned long haystack_len,
                const unsigned char *needle, unsigned long needle_len)
{
    unsigned char *s;

    /* Haystack too short to bother trying */
    if (haystack_len < needle_len) {
        return NULL;
    }

    /* Needle at start of haystack */
    if ((strncmp((char *) haystack, (char *) needle, needle_len) == 0) &&
        (needle_len == haystack_len || haystack[needle_len] == ',')) {
        return haystack;
    }

    s = haystack;
    /* Search until we run out of comas or we run out of haystack,
       whichever comes first */
    while ((s = (unsigned char *) strchr((char *) s, ','))
           && ((haystack_len - (s - haystack)) > needle_len)) {
        s++;
        /* Needle at X position */
        if ((strncmp((char *) s, (char *) needle, needle_len) == 0) &&
            (((s - haystack) + needle_len) == haystack_len
             || s[needle_len] == ',')) {
            return s;
        }
    }

    return NULL;
}


/* kex_get_method_by_name
 */
static const LIBSSH2_COMMON_METHOD *
kex_get_method_by_name(const char *name, size_t name_len,
                           const LIBSSH2_COMMON_METHOD ** methodlist)
{
    while (*methodlist) {
        if ((strlen((*methodlist)->name) == name_len) &&
            (strncmp((*methodlist)->name, name, name_len) == 0)) {
            return *methodlist;
        }
        methodlist++;
    }
    return NULL;
}


/* kex_agree_hostkey
 * Agree on a Hostkey which works with this kex
 */
static int kex_agree_hostkey(LIBSSH2_SESSION * session,
                             unsigned long kex_flags,
                             unsigned char *hostkey, unsigned long hostkey_len)
{
    const LIBSSH2_HOSTKEY_METHOD **hostkeyp = libssh2_hostkey_methods();
    unsigned char *s;

    if (session->hostkey_prefs) {
        s = (unsigned char *) session->hostkey_prefs;

        while (s && *s) {
            unsigned char *p = (unsigned char *) strchr((char *) s, ',');
            size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
            if (kex_agree_instr(hostkey, hostkey_len, s, method_len)) {
                const LIBSSH2_HOSTKEY_METHOD *method =
                    (const LIBSSH2_HOSTKEY_METHOD *)
                    kex_get_method_by_name((char *) s, method_len,
                                           (const LIBSSH2_COMMON_METHOD **)
                                           hostkeyp);

                if (!method) {
                    /* Invalid method -- Should never be reached */
                    return -1;
                }

                /* So far so good, but does it suit our purposes? (Encrypting
                   vs Signing) */
                if (((kex_flags & LIBSSH2_KEX_METHOD_FLAG_REQ_ENC_HOSTKEY) ==
                     0) || (method->encrypt)) {
                    /* Either this hostkey can do encryption or this kex just
                       doesn't require it */
                    if (((kex_flags & LIBSSH2_KEX_METHOD_FLAG_REQ_SIGN_HOSTKEY)
                         == 0) || (method->sig_verify)) {
                        /* Either this hostkey can do signing or this kex just
                           doesn't require it */
                        session->hostkey = method;
                        return 0;
                    }
                }
            }

            s = p ? p + 1 : NULL;
        }
        return -1;
    }

    while (hostkeyp && (*hostkeyp)->name) {
        s = kex_agree_instr(hostkey, hostkey_len,
                            (unsigned char *) (*hostkeyp)->name,
                            strlen((*hostkeyp)->name));
        if (s) {
            /* So far so good, but does it suit our purposes? (Encrypting vs
               Signing) */
            if (((kex_flags & LIBSSH2_KEX_METHOD_FLAG_REQ_ENC_HOSTKEY) == 0) ||
                ((*hostkeyp)->encrypt)) {
                /* Either this hostkey can do encryption or this kex just
                   doesn't require it */
                if (((kex_flags & LIBSSH2_KEX_METHOD_FLAG_REQ_SIGN_HOSTKEY) ==
                     0) || ((*hostkeyp)->sig_verify)) {
                    /* Either this hostkey can do signing or this kex just
                       doesn't require it */
                    session->hostkey = *hostkeyp;
                    return 0;
                }
            }
        }
        hostkeyp++;
    }

    return -1;
}


/* kex_agree_kex_hostkey
 * Agree on a Key Exchange method and a hostkey encoding type
 */
static int kex_agree_kex_hostkey(LIBSSH2_SESSION * session, unsigned char *kex,
                                 unsigned long kex_len, unsigned char *hostkey,
                                 unsigned long hostkey_len)
{
    const LIBSSH2_KEX_METHOD **kexp = libssh2_kex_methods;
    unsigned char *s;

    if (session->kex_prefs) {
        s = (unsigned char *) session->kex_prefs;

        while (s && *s) {
            unsigned char *q, *p = (unsigned char *) strchr((char *) s, ',');
            size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));
            if ((q = kex_agree_instr(kex, kex_len, s, method_len))) {
                const LIBSSH2_KEX_METHOD *method = (const LIBSSH2_KEX_METHOD *)
                    kex_get_method_by_name((char *) s, method_len,
                                           (const LIBSSH2_COMMON_METHOD **)
                                           kexp);

                if (!method) {
                    /* Invalid method -- Should never be reached */
                    return -1;
                }

                /* We've agreed on a key exchange method,
                 * Can we agree on a hostkey that works with this kex?
                 */
                if (kex_agree_hostkey(session, method->flags, hostkey,
                                      hostkey_len) == 0) {
                    session->kex = method;
                    if (session->burn_optimistic_kexinit && (kex == q)) {
                        /* Server sent an optimistic packet,
                         * and client agrees with preference
                         * cancel burning the first KEX_INIT packet that comes in */
                        session->burn_optimistic_kexinit = 0;
                    }
                    return 0;
                }
            }

            s = p ? p + 1 : NULL;
        }
        return -1;
    }

    while (*kexp && (*kexp)->name) {
        s = kex_agree_instr(kex, kex_len,
                            (unsigned char *) (*kexp)->name,
                            strlen((*kexp)->name));
        if (s) {
            /* We've agreed on a key exchange method,
             * Can we agree on a hostkey that works with this kex?
             */
            if (kex_agree_hostkey(session, (*kexp)->flags, hostkey,
                                  hostkey_len) == 0) {
                session->kex = *kexp;
                if (session->burn_optimistic_kexinit && (kex == s)) {
                    /* Server sent an optimistic packet,
                     * and client agrees with preference
                     * cancel burning the first KEX_INIT packet that comes in */
                    session->burn_optimistic_kexinit = 0;
                }
                return 0;
            }
        }
        kexp++;
    }
    return -1;
}


/* kex_agree_crypt
 * Agree on a cipher algo
 */
static int kex_agree_crypt(LIBSSH2_SESSION * session,
                           libssh2_endpoint_data *endpoint,
                           unsigned char *crypt,
                           unsigned long crypt_len)
{
    const LIBSSH2_CRYPT_METHOD **cryptp = libssh2_crypt_methods();
    unsigned char *s;

    (void) session;

    if (endpoint->crypt_prefs) {
        s = (unsigned char *) endpoint->crypt_prefs;

        while (s && *s) {
            unsigned char *p = (unsigned char *) strchr((char *) s, ',');
            size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));

            if (kex_agree_instr(crypt, crypt_len, s, method_len)) {
                const LIBSSH2_CRYPT_METHOD *method =
                    (const LIBSSH2_CRYPT_METHOD *)
                    kex_get_method_by_name((char *) s, method_len,
                                           (const LIBSSH2_COMMON_METHOD **)
                                           cryptp);

                if (!method) {
                    /* Invalid method -- Should never be reached */
                    return -1;
                }

                endpoint->crypt = method;
                return 0;
            }

            s = p ? p + 1 : NULL;
        }
        return -1;
    }

    while (*cryptp && (*cryptp)->name) {
        s = kex_agree_instr(crypt, crypt_len,
                            (unsigned char *) (*cryptp)->name,
                            strlen((*cryptp)->name));
        if (s) {
            endpoint->crypt = *cryptp;
            return 0;
        }
        cryptp++;
    }

    return -1;
}


/* kex_agree_mac
 * Agree on a message authentication hash
 */
static int kex_agree_mac(LIBSSH2_SESSION * session,
                         libssh2_endpoint_data * endpoint, unsigned char *mac,
                         unsigned long mac_len)
{
    const LIBSSH2_MAC_METHOD **macp = _libssh2_mac_methods();
    unsigned char *s;
    (void) session;

    if (endpoint->mac_prefs) {
        s = (unsigned char *) endpoint->mac_prefs;

        while (s && *s) {
            unsigned char *p = (unsigned char *) strchr((char *) s, ',');
            size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));

            if (kex_agree_instr(mac, mac_len, s, method_len)) {
                const LIBSSH2_MAC_METHOD *method = (const LIBSSH2_MAC_METHOD *)
                    kex_get_method_by_name((char *) s, method_len,
                                           (const LIBSSH2_COMMON_METHOD **)
                                           macp);

                if (!method) {
                    /* Invalid method -- Should never be reached */
                    return -1;
                }

                endpoint->mac = method;
                return 0;
            }

            s = p ? p + 1 : NULL;
        }
        return -1;
    }

    while (*macp && (*macp)->name) {
        s = kex_agree_instr(mac, mac_len, (unsigned char *) (*macp)->name,
                            strlen((*macp)->name));
        if (s) {
            endpoint->mac = *macp;
            return 0;
        }
        macp++;
    }

    return -1;
}


/* kex_agree_comp
 * Agree on a compression scheme
 */
static int kex_agree_comp(LIBSSH2_SESSION * session,
                          libssh2_endpoint_data * endpoint, unsigned char *comp,
                          unsigned long comp_len)
{
    const LIBSSH2_COMP_METHOD **compp = _libssh2_comp_methods();
    unsigned char *s;
    (void) session;

    if (endpoint->comp_prefs) {
        s = (unsigned char *) endpoint->comp_prefs;

        while (s && *s) {
            unsigned char *p = (unsigned char *) strchr((char *) s, ',');
            size_t method_len = (p ? (size_t)(p - s) : strlen((char *) s));

            if (kex_agree_instr(comp, comp_len, s, method_len)) {
                const LIBSSH2_COMP_METHOD *method =
                    (const LIBSSH2_COMP_METHOD *)
                    kex_get_method_by_name((char *) s, method_len,
                                           (const LIBSSH2_COMMON_METHOD **)
                                           compp);

                if (!method) {
                    /* Invalid method -- Should never be reached */
                    return -1;
                }

                endpoint->comp = method;
                return 0;
            }

            s = p ? p + 1 : NULL;
        }
        return -1;
    }

    while (*compp && (*compp)->name) {
        s = kex_agree_instr(comp, comp_len, (unsigned char *) (*compp)->name,
                            strlen((*compp)->name));
        if (s) {
            endpoint->comp = *compp;
            return 0;
        }
        compp++;
    }

    return -1;
}


/* TODO: When in server mode we need to turn this logic on its head
 * The Client gets to make the final call on "agreed methods"
 */

/* kex_agree_methods
 * Decide which specific method to use of the methods offered by each party
 */
static int kex_agree_methods(LIBSSH2_SESSION * session, unsigned char *data,
                             unsigned data_len)
{
    unsigned char *kex, *hostkey, *crypt_cs, *crypt_sc, *comp_cs, *comp_sc,
        *mac_cs, *mac_sc, *lang_cs, *lang_sc;
    size_t kex_len, hostkey_len, crypt_cs_len, crypt_sc_len, comp_cs_len;
    size_t comp_sc_len, mac_cs_len, mac_sc_len, lang_cs_len, lang_sc_len;
    unsigned char *s = data;

    /* Skip packet_type, we know it already */
    s++;

    /* Skip cookie, don't worry, it's preserved in the kexinit field */
    s += 16;

    /* Locate each string */
    kex_len = _libssh2_ntohu32(s);
    kex = s + 4;
    s += 4 + kex_len;
    hostkey_len = _libssh2_ntohu32(s);
    hostkey = s + 4;
    s += 4 + hostkey_len;
    crypt_cs_len = _libssh2_ntohu32(s);
    crypt_cs = s + 4;
    s += 4 + crypt_cs_len;
    crypt_sc_len = _libssh2_ntohu32(s);
    crypt_sc = s + 4;
    s += 4 + crypt_sc_len;
    mac_cs_len = _libssh2_ntohu32(s);
    mac_cs = s + 4;
    s += 4 + mac_cs_len;
    mac_sc_len = _libssh2_ntohu32(s);
    mac_sc = s + 4;
    s += 4 + mac_sc_len;
    comp_cs_len = _libssh2_ntohu32(s);
    comp_cs = s + 4;
    s += 4 + comp_cs_len;
    comp_sc_len = _libssh2_ntohu32(s);
    comp_sc = s + 4;
    s += 4 + comp_sc_len;
    lang_cs_len = _libssh2_ntohu32(s);
    lang_cs = s + 4;
    s += 4 + lang_cs_len;
    lang_sc_len = _libssh2_ntohu32(s);
    lang_sc = s + 4;
    s += 4 + lang_sc_len;

    /* If the server sent an optimistic packet, assume that it guessed wrong.
     * If the guess is determined to be right (by kex_agree_kex_hostkey)
     * This flag will be reset to zero so that it's not ignored */
    session->burn_optimistic_kexinit = *(s++);
    /* Next uint32 in packet is all zeros (reserved) */

    if (data_len < (unsigned) (s - data))
        return -1;              /* short packet */

    if (kex_agree_kex_hostkey(session, kex, kex_len, hostkey, hostkey_len)) {
        return -1;
    }

    if (kex_agree_crypt(session, &session->local, crypt_cs, crypt_cs_len)
        || kex_agree_crypt(session, &session->remote, crypt_sc, crypt_sc_len)) {
        return -1;
    }

    if (kex_agree_mac(session, &session->local, mac_cs, mac_cs_len) ||
        kex_agree_mac(session, &session->remote, mac_sc, mac_sc_len)) {
        return -1;
    }

    if (kex_agree_comp(session, &session->local, comp_cs, comp_cs_len) ||
        kex_agree_comp(session, &session->remote, comp_sc, comp_sc_len)) {
        return -1;
    }

    if (libssh2_kex_agree_lang(session, &session->local, lang_cs, lang_cs_len)
        || libssh2_kex_agree_lang(session, &session->remote, lang_sc,
                                  lang_sc_len)) {
        return -1;
    }

    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on KEX method: %s",
                   session->kex->name);
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on HOSTKEY method: %s",
                   session->hostkey->name);
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on CRYPT_CS method: %s",
                   session->local.crypt->name);
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on CRYPT_SC method: %s",
                   session->remote.crypt->name);
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on MAC_CS method: %s",
                   session->local.mac->name);
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on MAC_SC method: %s",
                   session->remote.mac->name);
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on COMP_CS method: %s",
                   session->local.comp->name);
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on COMP_SC method: %s",
                   session->remote.comp->name);
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on LANG_CS method:");      /* None yet */
    _libssh2_debug(session, LIBSSH2_DBG_KEX, "Agreed on LANG_SC method:");      /* None yet */

    /* Initialize compression layer */
    if (session->local.comp && session->local.comp->init &&
        session->local.comp->init(session, 1, &session->local.comp_abstract)) {
        return -1;
    }

    if (session->remote.comp && session->remote.comp->init &&
        session->remote.comp->init(session, 0,
                                   &session->remote.comp_abstract)) {
        return -1;
    }

    return 0;
}


/* libssh2_kex_exchange
 * Exchange keys
 * Returns 0 on success, non-zero on failure
 *
 * Returns some errors without libssh2_error()
 */
int
libssh2_kex_exchange(LIBSSH2_SESSION * session, int reexchange,
                     key_exchange_state_t * key_state)
{
    int rc = 0;
    int retcode;

    session->state |= LIBSSH2_STATE_KEX_ACTIVE;

    if (key_state->state == libssh2_NB_state_idle) {
        /* Prevent loop in packet_add() */
        session->state |= LIBSSH2_STATE_EXCHANGING_KEYS;

        if (reexchange) {
            session->kex = NULL;

            if (session->hostkey && session->hostkey->dtor) {
                session->hostkey->dtor(session,
                                       &session->server_hostkey_abstract);
            }
            session->hostkey = NULL;
        }

        key_state->state = libssh2_NB_state_created;
    }

    if (!session->kex || !session->hostkey) {
        if (key_state->state == libssh2_NB_state_created) {
            /* Preserve in case of failure */
            key_state->oldlocal = session->local.kexinit;
            key_state->oldlocal_len = session->local.kexinit_len;

            session->local.kexinit = NULL;

            key_state->state = libssh2_NB_state_sent;
        }

        if (key_state->state == libssh2_NB_state_sent) {
            retcode = kexinit(session);
            if (retcode == PACKET_EAGAIN) {
                session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
                return retcode;
            } else if (retcode) {
                session->local.kexinit = key_state->oldlocal;
                session->local.kexinit_len = key_state->oldlocal_len;
                key_state->state = libssh2_NB_state_idle;
                session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
                session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
                return -1;
            }

            key_state->state = libssh2_NB_state_sent1;
        }

        if (key_state->state == libssh2_NB_state_sent1) {
            retcode =
                _libssh2_packet_require(session, SSH_MSG_KEXINIT,
                                        &key_state->data,
                                        &key_state->data_len, 0, NULL, 0,
                                        &key_state->req_state);
            if (retcode == PACKET_EAGAIN) {
                session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
                return retcode;
            }
            else if (retcode) {
                if (session->local.kexinit) {
                    LIBSSH2_FREE(session, session->local.kexinit);
                }
                session->local.kexinit = key_state->oldlocal;
                session->local.kexinit_len = key_state->oldlocal_len;
                key_state->state = libssh2_NB_state_idle;
                session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
                session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;
                return -1;
            }

            if (session->remote.kexinit) {
                LIBSSH2_FREE(session, session->remote.kexinit);
            }
            session->remote.kexinit = key_state->data;
            session->remote.kexinit_len = key_state->data_len;

            if (kex_agree_methods(session, key_state->data,
                                  key_state->data_len))
                rc = LIBSSH2_ERROR_KEX_FAILURE;

            key_state->state = libssh2_NB_state_sent2;
        }
    } else {
        key_state->state = libssh2_NB_state_sent2;
    }

    if (rc == 0) {
        if (key_state->state == libssh2_NB_state_sent2) {
            retcode = session->kex->exchange_keys(session,
                                                  &key_state->key_state_low);
            if (retcode == PACKET_EAGAIN) {
                session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
                return retcode;
            } else if (retcode) {
                libssh2_error(session, LIBSSH2_ERROR_KEY_EXCHANGE_FAILURE,
                              "Unrecoverable error exchanging keys", 0);
                rc = retcode;
            }
        }
    }

    /* Done with kexinit buffers */
    if (session->local.kexinit) {
        LIBSSH2_FREE(session, session->local.kexinit);
        session->local.kexinit = NULL;
    }
    if (session->remote.kexinit) {
        LIBSSH2_FREE(session, session->remote.kexinit);
        session->remote.kexinit = NULL;
    }

    session->state &= ~LIBSSH2_STATE_KEX_ACTIVE;
    session->state &= ~LIBSSH2_STATE_EXCHANGING_KEYS;

    key_state->state = libssh2_NB_state_idle;

    return rc;
}


/* libssh2_session_method_pref
 * Set preferred method
 */
LIBSSH2_API int
libssh2_session_method_pref(LIBSSH2_SESSION * session, int method_type,
                            const char *prefs)
{
    char **prefvar, *s, *newprefs;
    int prefs_len = strlen(prefs);
    const LIBSSH2_COMMON_METHOD **mlist;

    switch (method_type) {
    case LIBSSH2_METHOD_KEX:
        prefvar = &session->kex_prefs;
        mlist = (const LIBSSH2_COMMON_METHOD **) libssh2_kex_methods;
        break;

    case LIBSSH2_METHOD_HOSTKEY:
        prefvar = &session->hostkey_prefs;
        mlist = (const LIBSSH2_COMMON_METHOD **) libssh2_hostkey_methods();
        break;

    case LIBSSH2_METHOD_CRYPT_CS:
        prefvar = &session->local.crypt_prefs;
        mlist = (const LIBSSH2_COMMON_METHOD **) libssh2_crypt_methods();
        break;

    case LIBSSH2_METHOD_CRYPT_SC:
        prefvar = &session->remote.crypt_prefs;
        mlist = (const LIBSSH2_COMMON_METHOD **) libssh2_crypt_methods();
        break;

    case LIBSSH2_METHOD_MAC_CS:
        prefvar = &session->local.mac_prefs;
        mlist = (const LIBSSH2_COMMON_METHOD **) _libssh2_mac_methods();
        break;

    case LIBSSH2_METHOD_MAC_SC:
        prefvar = &session->remote.mac_prefs;
        mlist = (const LIBSSH2_COMMON_METHOD **) _libssh2_mac_methods();
        break;

    case LIBSSH2_METHOD_COMP_CS:
        prefvar = &session->local.comp_prefs;
        mlist = (const LIBSSH2_COMMON_METHOD **) _libssh2_comp_methods();
        break;

    case LIBSSH2_METHOD_COMP_SC:
        prefvar = &session->remote.comp_prefs;
        mlist = (const LIBSSH2_COMMON_METHOD **) _libssh2_comp_methods();
        break;

    case LIBSSH2_METHOD_LANG_CS:
        prefvar = &session->local.lang_prefs;
        mlist = NULL;
        break;

    case LIBSSH2_METHOD_LANG_SC:
        prefvar = &session->remote.lang_prefs;
        mlist = NULL;
        break;

    default:
        libssh2_error(session, LIBSSH2_ERROR_INVAL,
                      "Invalid parameter specified for method_type", 0);
        return -1;
    }

    s = newprefs = LIBSSH2_ALLOC(session, prefs_len + 1);
    if (!newprefs) {
        libssh2_error(session, LIBSSH2_ERROR_ALLOC,
                      "Error allocated space for method preferences", 0);
        return -1;
    }
    memcpy(s, prefs, prefs_len + 1);

    while (s && *s) {
        char *p = strchr(s, ',');
        int method_len = p ? (p - s) : (int) strlen(s);

        if (!kex_get_method_by_name(s, method_len, mlist)) {
            /* Strip out unsupported method */
            if (p) {
                memcpy(s, p + 1, strlen(s) - method_len);
            } else {
                if (s > newprefs) {
                    *(--s) = '\0';
                } else {
                    *s = '\0';
                }
            }
        }

        s = p ? (p + 1) : NULL;
    }

    if (strlen(newprefs) == 0) {
        libssh2_error(session, LIBSSH2_ERROR_METHOD_NOT_SUPPORTED,
                      "The requested method(s) are not currently supported",
                      0);
        LIBSSH2_FREE(session, newprefs);
        return -1;
    }

    if (*prefvar) {
        LIBSSH2_FREE(session, *prefvar);
    }
    *prefvar = newprefs;

    return 0;
}