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server: Correctly handle extensions

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If the server had an RSA host key, it provided unconditionally SHA2
signatures without consulting the client proposed list of supported host
keys.

This commit implements more fine-grained detection of the extension
to provide the client with valid signatures according to RFC 8332
Section 3.1.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 27fe60954c)
This commit is contained in:
Jakub Jelen
2019-01-07 18:49:58 +01:00
committed by Andreas Schneider
parent d028b2495d
commit 0acfd81f85
3 changed files with 25 additions and 8 deletions
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24
src/kex.c
View File

@@ -526,13 +526,29 @@ SSH_PACKET_CALLBACK(ssh_packet_kexinit){
ok = ssh_match_group(session->next_crypto->client_kex.methods[SSH_KEX],
KEX_EXTENSION_CLIENT);
if (ok) {
const char *hostkeys = NULL;
/* The client supports extension negotiation */
session->extensions |= SSH_EXT_NEGOTIATION;
/*
* Enable all the supported extensions and when the time comes
* (after NEWKEYS) send them to the client.
* RFC 8332 Section 3.1: Use for Server Authentication
* Check what algorithms were provided in the SSH_HOSTKEYS list
* by the client and enable the respective extensions to provide
* correct signature in the next packet if RSA is negotiated
*/
hostkeys = session->next_crypto->client_kex.methods[SSH_HOSTKEYS];
ok = ssh_match_group(hostkeys, "rsa-sha2-512");
if (ok) {
session->extensions |= SSH_EXT_SIG_RSA_SHA512;
}
ok = ssh_match_group(hostkeys, "rsa-sha2-256");
if (ok) {
session->extensions |= SSH_EXT_SIG_RSA_SHA256;
}
SSH_LOG(SSH_LOG_DEBUG, "The client supports extension "
"negotiation: enabling all extensions");
session->extensions = SSH_EXT_ALL;
"negotiation. Enabled signature algorithms: %s%s",
session->extensions & SSH_EXT_SIG_RSA_SHA256 ? "SHA256" : "",
session->extensions & SSH_EXT_SIG_RSA_SHA512 ? " SHA512" : "");
}
/*

2
src/server.c
View File

@@ -523,7 +523,7 @@ static void ssh_server_connection_callback(ssh_session session){
* our supported extensions now. This is the first message after
* sending NEWKEYS message and after turning on crypto.
*/
if (session->extensions &&
if (session->extensions & SSH_EXT_NEGOTIATION &&
session->session_state != SSH_SESSION_STATE_AUTHENTICATED) {
ssh_server_send_extensions(session);
}