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Now that we've got at least some SDCTR modes working (and aes256-ctr is our

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default preferred cipher), add code to inject SSH_MSG_IGNOREs to randomise
the IV when using CBC-mode ciphers.  Each cipher has a flag to indicate
whether it needs this workaround, and the SSH packet output maze has gained
some extra complexity to implement it.

[originally from svn r5659]
This commit is contained in:
Ben Harris
2005-04-23 16:22:51 +00:00
parent b28330fc35
commit f2b0335c48
6 changed files with 48 additions and 19 deletions
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35
ssh.c
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@ -1797,8 +1797,19 @@ static int ssh2_pkt_construct(Ssh ssh, struct Packet *pkt)
* ssh2_pkt_send() or ssh2_pkt_defer() either go straight to one of
* these or get queued, and then when the queue is later emptied
* the packets are all passed to defer_noqueue().
*
* When using a CBC-mode cipher, it's necessary to ensure that an
* attacker can't provide data to be encrypted using an IV that they
* know. We ensure this by prefixing each packet that might contain
* user data with an SSH_MSG_IGNORE. This is done using the deferral
* mechanism, so in this case send_noqueue() ends up redirecting to
* defer_noqueue(). If you don't like this inefficiency, don't use
* CBC.
*/
static void ssh2_pkt_defer_noqueue(Ssh, struct Packet *, int);
static void ssh_pkt_defersend(Ssh);
/*
* Send an SSH-2 packet immediately, without queuing or deferring.
*/
@ -1806,6 +1817,12 @@ static void ssh2_pkt_send_noqueue(Ssh ssh, struct Packet *pkt)
{
int len;
int backlog;
if (ssh->cscipher != NULL && (ssh->cscipher->flags & SSH_CIPHER_IS_CBC)) {
/* We need to send two packets, so use the deferral mechanism. */
ssh2_pkt_defer_noqueue(ssh, pkt, FALSE);
ssh_pkt_defersend(ssh);
return;
}
len = ssh2_pkt_construct(ssh, pkt);
backlog = sk_write(ssh->s, (char *)pkt->data, len);
if (backlog > SSH_MAX_BACKLOG)
@ -1823,9 +1840,19 @@ static void ssh2_pkt_send_noqueue(Ssh ssh, struct Packet *pkt)
/*
* Defer an SSH-2 packet.
*/
static void ssh2_pkt_defer_noqueue(Ssh ssh, struct Packet *pkt)
static void ssh2_pkt_defer_noqueue(Ssh ssh, struct Packet *pkt, int noignore)
{
int len = ssh2_pkt_construct(ssh, pkt);
int len;
if (ssh->cscipher != NULL && (ssh->cscipher->flags & SSH_CIPHER_IS_CBC) &&
ssh->deferred_len == 0 && !noignore) {
/*
* Interpose an SSH_MSG_IGNORE to ensure that user data don't
* get encrypted with a known IV.
*/
struct Packet *ipkt = ssh2_pkt_init(SSH2_MSG_IGNORE);
ssh2_pkt_defer_noqueue(ssh, ipkt, TRUE);
}
len = ssh2_pkt_construct(ssh, pkt);
if (ssh->deferred_len + len > ssh->deferred_size) {
ssh->deferred_size = ssh->deferred_len + len + 128;
ssh->deferred_send_data = sresize(ssh->deferred_send_data,
@ -1875,7 +1902,7 @@ static void ssh2_pkt_defer(Ssh ssh, struct Packet *pkt)
if (ssh->queueing)
ssh2_pkt_queue(ssh, pkt);
else
ssh2_pkt_defer_noqueue(ssh, pkt);
ssh2_pkt_defer_noqueue(ssh, pkt, FALSE);
}
#endif
@ -1923,7 +1950,7 @@ static void ssh2_pkt_queuesend(Ssh ssh)
assert(!ssh->queueing);
for (i = 0; i < ssh->queuelen; i++)
ssh2_pkt_defer_noqueue(ssh, ssh->queue[i]);
ssh2_pkt_defer_noqueue(ssh, ssh->queue[i], FALSE);
ssh->queuelen = 0;
ssh_pkt_defersend(ssh);