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Initial support for host certificates.

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Now we offer the OpenSSH certificate key types in our KEXINIT host key
algorithm list, so that if the server has a certificate, they can send
it to us.

There's a new storage.h abstraction for representing a list of trusted
host CAs, and which ones are trusted to certify hosts for what
domains. This is stored outside the normal saved session data, because
the whole point of host certificates is to avoid per-host faffing.

Configuring this set of trusted CAs is done via a new GUI dialog box,
separate from the main PuTTY config box (because it modifies a single
set of settings across all saved sessions), which you can launch by
clicking a button in the 'Host keys' pane. The GUI is pretty crude for
the moment, and very much at a 'just about usable' stage right now. It
will want some polishing.

If we have no CA configured that matches the hostname, we don't offer
to receive certified host keys in the first place. So for existing
users who haven't set any of this up yet, nothing will immediately
change.

Currently, if we do offer to receive certified host keys and the
server presents one signed by a CA we don't trust, PuTTY will bomb out
unconditionally with an error, instead of offering a confirmation box.
That's an unfinished part which I plan to fix before this goes into a
release.
This commit is contained in:
Simon Tatham
2022-04-22 12:07:24 +01:00
parent df3a21d97b
commit 21d4754b6a
15 changed files with 1024 additions and 53 deletions
Download Patch File Download Diff File Expand all files Collapse all files

105
ssh/kex2-client.c
View File

@ -718,7 +718,8 @@ void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
}
}
s->keystr = (s->hkey ? ssh_key_cache_str(s->hkey) : NULL);
s->keystr = (s->hkey && !ssh_key_alg(s->hkey)->is_certificate ?
ssh_key_cache_str(s->hkey) : NULL);
#ifndef NO_GSSAPI
if (s->gss_kex_used) {
/*
@ -851,19 +852,83 @@ void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
}
}
/*
* Authenticate remote host: verify host key. (We've already
* checked the signature of the exchange hash.)
*/
{
ssh2_userkey uk = { .key = s->hkey, .comment = NULL };
char *keydisp = ssh2_pubkey_openssh_str(&uk);
char **fingerprints = ssh2_all_fingerprints(s->hkey);
ssh2_userkey uk = { .key = s->hkey, .comment = NULL };
char **fingerprints = ssh2_all_fingerprints(s->hkey);
FingerprintType fptype_default =
ssh2_pick_default_fingerprint(fingerprints);
ppl_logevent("Host key fingerprint is:");
ppl_logevent("%s", fingerprints[fptype_default]);
FingerprintType fptype_default =
ssh2_pick_default_fingerprint(fingerprints);
ppl_logevent("Host key fingerprint is:");
ppl_logevent("%s", fingerprints[fptype_default]);
/*
* Authenticate remote host: verify host key, either by
* certification or by the local host key cache.
*
* (We've already checked the signature of the exchange
* hash.)
*/
if (ssh_key_alg(s->hkey)->is_certificate) {
ssh2_free_all_fingerprints(fingerprints);
char *base_fp = ssh2_fingerprint(ssh_key_base_key(s->hkey),
fptype_default);
ppl_logevent("Host key is a certificate, whose base key has "
"fingerprint:");
ppl_logevent("%s", base_fp);
sfree(base_fp);
strbuf *id_string = strbuf_new();
StripCtrlChars *id_string_scc = stripctrl_new(
BinarySink_UPCAST(id_string), false, L'\0');
ssh_key_cert_id_string(
s->hkey, BinarySink_UPCAST(id_string_scc));
stripctrl_free(id_string_scc);
ppl_logevent("Certificate ID string is \"%s\"", id_string->s);
strbuf_free(id_string);
strbuf *ca_pub = strbuf_new();
ssh_key_ca_public_blob(s->hkey, BinarySink_UPCAST(ca_pub));
host_ca hca_search = { .ca_public_key = ca_pub };
host_ca *hca_found = find234(s->host_cas, &hca_search, NULL);
char *ca_fp = ssh2_fingerprint_blob(ptrlen_from_strbuf(ca_pub),
fptype_default);
ppl_logevent("Fingerprint of certification authority:");
ppl_logevent("%s", ca_fp);
sfree(ca_fp);
strbuf_free(ca_pub);
strbuf *error = strbuf_new();
bool cert_ok = false;
if (!hca_found) {
put_fmt(error, "Certification authority is not trusted");
} else {
ppl_logevent("Certification authority matches '%s'",
hca_found->name);
cert_ok = ssh_key_check_cert(
s->hkey,
true, /* host certificate */
ptrlen_from_asciz(s->savedhost),
time(NULL),
BinarySink_UPCAST(error));
}
if (cert_ok) {
strbuf_free(error);
ppl_logevent("Accepted certificate");
} else {
ppl_logevent("Rejected host key certificate: %s",
error->s);
ssh_sw_abort(s->ppl.ssh,
"Rejected host key certificate: %s",
error->s);
*aborted = true;
strbuf_free(error);
return;
}
} else {
char *keydisp = ssh2_pubkey_openssh_str(&uk);
s->spr = verify_ssh_host_key(
ppl_get_iseat(&s->ppl), s->conf, s->savedhost, s->savedport,
@ -872,15 +937,15 @@ void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
ssh2_free_all_fingerprints(fingerprints);
sfree(keydisp);
}
#ifdef FUZZING
s->spr = SPR_OK;
s->spr = SPR_OK;
#endif
crMaybeWaitUntilV(s->spr.kind != SPRK_INCOMPLETE);
if (spr_is_abort(s->spr)) {
*aborted = true;
ssh_spr_close(s->ppl.ssh, s->spr, "host key verification");
return;
crMaybeWaitUntilV(s->spr.kind != SPRK_INCOMPLETE);
if (spr_is_abort(s->spr)) {
*aborted = true;
ssh_spr_close(s->ppl.ssh, s->spr, "host key verification");
return;
}
}
/*