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putty-source/ssh2kex-server.c
Simon Tatham 34a0460f05 New functions to shrink a strbuf. 
These are better than my previous approach of just assigning to
sb->len, because firstly they check by assertion that the new length
is within range, and secondly they preserve the invariant that the
byte stored in the buffer just after the length runs out is \0.

Switched to using the new functions everywhere a grep could turn up
opportunities.

(cherry picked from commit 5891142aee)
2020-02-09 08:51:37 +00:00

320 lines
11 KiB
C
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/*
* Server side of key exchange for the SSH-2 transport protocol (RFC 4253).
*/
#include <assert.h>
#include "putty.h"
#include "ssh.h"
#include "sshbpp.h"
#include "sshppl.h"
#include "sshcr.h"
#include "sshserver.h"
#include "storage.h"
#include "ssh2transport.h"
#include "mpint.h"
void ssh2_transport_provide_hostkeys(PacketProtocolLayer *ppl,
ssh_key *const *hostkeys, int nhostkeys)
{
struct ssh2_transport_state *s =
container_of(ppl, struct ssh2_transport_state, ppl);
s->hostkeys = hostkeys;
s->nhostkeys = nhostkeys;
}
static strbuf *finalise_and_sign_exhash(struct ssh2_transport_state *s)
{
strbuf *sb;
ssh2transport_finalise_exhash(s);
sb = strbuf_new();
ssh_key_sign(
s->hkey, make_ptrlen(s->exchange_hash, s->kex_alg->hash->hlen),
0, BinarySink_UPCAST(sb));
return sb;
}
static void no_progress(void *param, int action, int phase, int iprogress)
{
}
void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
{
PacketProtocolLayer *ppl = &s->ppl; /* for ppl_logevent */
PktIn *pktin;
PktOut *pktout;
crBegin(s->crStateKex);
{
int i;
for (i = 0; i < s->nhostkeys; i++)
if (ssh_key_alg(s->hostkeys[i]) == s->hostkey_alg) {
s->hkey = s->hostkeys[i];
break;
}
assert(s->hkey);
}
strbuf_clear(s->hostkeyblob);
ssh_key_public_blob(s->hkey, BinarySink_UPCAST(s->hostkeyblob));
s->hostkeydata = ptrlen_from_strbuf(s->hostkeyblob);
put_stringpl(s->exhash, s->hostkeydata);
if (s->kex_alg->main_type == KEXTYPE_DH) {
/*
* If we're doing Diffie-Hellman group exchange, start by
* waiting for the group request.
*/
if (dh_is_gex(s->kex_alg)) {
ppl_logevent("Doing Diffie-Hellman group exchange");
s->ppl.bpp->pls->kctx = SSH2_PKTCTX_DHGEX;
crMaybeWaitUntilV((pktin = ssh2_transport_pop(s)) != NULL);
if (pktin->type != SSH2_MSG_KEX_DH_GEX_REQUEST &&
pktin->type != SSH2_MSG_KEX_DH_GEX_REQUEST_OLD) {
ssh_proto_error(s->ppl.ssh, "Received unexpected packet when "
"expecting Diffie-Hellman group exchange "
"request, type %d (%s)", pktin->type,
ssh2_pkt_type(s->ppl.bpp->pls->kctx,
s->ppl.bpp->pls->actx,
pktin->type));
*aborted = true;
return;
}
if (pktin->type != SSH2_MSG_KEX_DH_GEX_REQUEST_OLD) {
s->dh_got_size_bounds = true;
s->dh_min_size = get_uint32(pktin);
s->pbits = get_uint32(pktin);
s->dh_max_size = get_uint32(pktin);
} else {
s->dh_got_size_bounds = false;
s->pbits = get_uint32(pktin);
}
/*
* This is a hopeless strategy for making a secure DH
* group! It's good enough for testing a client against,
* but not for serious use.
*/
s->p = primegen(s->pbits, 2, 2, NULL, 1, no_progress, NULL, 1);
s->g = mp_from_integer(2);
s->dh_ctx = dh_setup_gex(s->p, s->g);
s->kex_init_value = SSH2_MSG_KEX_DH_GEX_INIT;
s->kex_reply_value = SSH2_MSG_KEX_DH_GEX_REPLY;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH2_MSG_KEX_DH_GEX_GROUP);
put_mp_ssh2(pktout, s->p);
put_mp_ssh2(pktout, s->g);
pq_push(s->ppl.out_pq, pktout);
} else {
s->ppl.bpp->pls->kctx = SSH2_PKTCTX_DHGROUP;
s->dh_ctx = dh_setup_group(s->kex_alg);
s->kex_init_value = SSH2_MSG_KEXDH_INIT;
s->kex_reply_value = SSH2_MSG_KEXDH_REPLY;
ppl_logevent("Using Diffie-Hellman with standard group \"%s\"",
s->kex_alg->groupname);
}
ppl_logevent("Doing Diffie-Hellman key exchange with hash %s",
ssh_hash_alg(s->exhash)->text_name);
/*
* Generate e for Diffie-Hellman.
*/
s->e = dh_create_e(s->dh_ctx, s->nbits * 2);
/*
* Wait to receive f.
*/
crMaybeWaitUntilV((pktin = ssh2_transport_pop(s)) != NULL);
if (pktin->type != s->kex_init_value) {
ssh_proto_error(s->ppl.ssh, "Received unexpected packet when "
"expecting Diffie-Hellman initial packet, "
"type %d (%s)", pktin->type,
ssh2_pkt_type(s->ppl.bpp->pls->kctx,
s->ppl.bpp->pls->actx,
pktin->type));
*aborted = true;
return;
}
s->f = get_mp_ssh2(pktin);
if (get_err(pktin)) {
ssh_proto_error(s->ppl.ssh,
"Unable to parse Diffie-Hellman initial packet");
*aborted = true;
return;
}
{
const char *err = dh_validate_f(s->dh_ctx, s->f);
if (err) {
ssh_proto_error(s->ppl.ssh, "Diffie-Hellman initial packet "
"failed validation: %s", err);
*aborted = true;
return;
}
}
s->K = dh_find_K(s->dh_ctx, s->f);
if (dh_is_gex(s->kex_alg)) {
if (s->dh_got_size_bounds)
put_uint32(s->exhash, s->dh_min_size);
put_uint32(s->exhash, s->pbits);
if (s->dh_got_size_bounds)
put_uint32(s->exhash, s->dh_max_size);
put_mp_ssh2(s->exhash, s->p);
put_mp_ssh2(s->exhash, s->g);
}
put_mp_ssh2(s->exhash, s->f);
put_mp_ssh2(s->exhash, s->e);
pktout = ssh_bpp_new_pktout(s->ppl.bpp, s->kex_reply_value);
put_stringpl(pktout, s->hostkeydata);
put_mp_ssh2(pktout, s->e);
put_stringsb(pktout, finalise_and_sign_exhash(s));
pq_push(s->ppl.out_pq, pktout);
dh_cleanup(s->dh_ctx);
s->dh_ctx = NULL;
mp_free(s->f); s->f = NULL;
if (dh_is_gex(s->kex_alg)) {
mp_free(s->g); s->g = NULL;
mp_free(s->p); s->p = NULL;
}
} else if (s->kex_alg->main_type == KEXTYPE_ECDH) {
ppl_logevent("Doing ECDH key exchange with curve %s and hash %s",
ssh_ecdhkex_curve_textname(s->kex_alg),
ssh_hash_alg(s->exhash)->text_name);
s->ppl.bpp->pls->kctx = SSH2_PKTCTX_ECDHKEX;
s->ecdh_key = ssh_ecdhkex_newkey(s->kex_alg);
if (!s->ecdh_key) {
ssh_sw_abort(s->ppl.ssh, "Unable to generate key for ECDH");
*aborted = true;
return;
}
crMaybeWaitUntilV((pktin = ssh2_transport_pop(s)) != NULL);
if (pktin->type != SSH2_MSG_KEX_ECDH_INIT) {
ssh_proto_error(s->ppl.ssh, "Received unexpected packet when "
"expecting ECDH initial packet, type %d (%s)",
pktin->type,
ssh2_pkt_type(s->ppl.bpp->pls->kctx,
s->ppl.bpp->pls->actx,
pktin->type));
*aborted = true;
return;
}
{
ptrlen keydata = get_string(pktin);
put_stringpl(s->exhash, keydata);
s->K = ssh_ecdhkex_getkey(s->ecdh_key, keydata);
if (!get_err(pktin) && !s->K) {
ssh_proto_error(s->ppl.ssh, "Received invalid elliptic curve "
"point in ECDH initial packet");
*aborted = true;
return;
}
}
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH2_MSG_KEX_ECDH_REPLY);
put_stringpl(pktout, s->hostkeydata);
{
strbuf *pubpoint = strbuf_new();
ssh_ecdhkex_getpublic(s->ecdh_key, BinarySink_UPCAST(pubpoint));
put_string(s->exhash, pubpoint->u, pubpoint->len);
put_stringsb(pktout, pubpoint);
}
put_stringsb(pktout, finalise_and_sign_exhash(s));
pq_push(s->ppl.out_pq, pktout);
ssh_ecdhkex_freekey(s->ecdh_key);
s->ecdh_key = NULL;
} else if (s->kex_alg->main_type == KEXTYPE_GSS) {
ssh_sw_abort(s->ppl.ssh, "GSS key exchange not supported in server");
} else {
assert(s->kex_alg->main_type == KEXTYPE_RSA);
ppl_logevent("Doing RSA key exchange with hash %s",
ssh_hash_alg(s->exhash)->text_name);
s->ppl.bpp->pls->kctx = SSH2_PKTCTX_RSAKEX;
const struct ssh_rsa_kex_extra *extra =
(const struct ssh_rsa_kex_extra *)s->kex_alg->extra;
if (s->ssc && s->ssc->rsa_kex_key) {
int klen = ssh_rsakex_klen(s->ssc->rsa_kex_key);
if (klen >= extra->minklen) {
ppl_logevent("Using configured %d-bit RSA key", klen);
s->rsa_kex_key = s->ssc->rsa_kex_key;
} else {
ppl_logevent("Configured %d-bit RSA key is too short (min %d)",
klen, extra->minklen);
}
}
if (!s->rsa_kex_key) {
ppl_logevent("Generating a %d-bit RSA key", extra->minklen);
s->rsa_kex_key = snew(RSAKey);
rsa_generate(s->rsa_kex_key, extra->minklen, no_progress, NULL);
s->rsa_kex_key->comment = NULL;
s->rsa_kex_key_needs_freeing = true;
}
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH2_MSG_KEXRSA_PUBKEY);
put_stringpl(pktout, s->hostkeydata);
{
strbuf *pubblob = strbuf_new();
ssh_key_public_blob(&s->rsa_kex_key->sshk,
BinarySink_UPCAST(pubblob));
put_string(s->exhash, pubblob->u, pubblob->len);
put_stringsb(pktout, pubblob);
}
pq_push(s->ppl.out_pq, pktout);
crMaybeWaitUntilV((pktin = ssh2_transport_pop(s)) != NULL);
if (pktin->type != SSH2_MSG_KEXRSA_SECRET) {
ssh_proto_error(s->ppl.ssh, "Received unexpected packet when "
"expecting RSA kex secret, type %d (%s)",
pktin->type,
ssh2_pkt_type(s->ppl.bpp->pls->kctx,
s->ppl.bpp->pls->actx,
pktin->type));
*aborted = true;
return;
}
{
ptrlen encrypted_secret = get_string(pktin);
put_stringpl(s->exhash, encrypted_secret);
s->K = ssh_rsakex_decrypt(
s->rsa_kex_key, s->kex_alg->hash, encrypted_secret);
}
if (!s->K) {
ssh_proto_error(s->ppl.ssh, "Unable to decrypt RSA kex secret");
*aborted = true;
return;
}
if (s->rsa_kex_key_needs_freeing) {
ssh_rsakex_freekey(s->rsa_kex_key);
sfree(s->rsa_kex_key);
}
s->rsa_kex_key = NULL;
s->rsa_kex_key_needs_freeing = false;
pktout = ssh_bpp_new_pktout(s->ppl.bpp, SSH2_MSG_KEXRSA_DONE);
put_stringsb(pktout, finalise_and_sign_exhash(s));
pq_push(s->ppl.out_pq, pktout);
}
crFinishV;
}
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