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Reject multilayer certificates in check_cert.

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Rejecting them in the CA config box reminded me that the main checking
code also ought to do the same thing.
This commit is contained in:
Simon Tatham
2022-05-07 12:25:54 +01:00
parent 4b0e54c22a
commit b753cf6e3b
2 changed files with 67 additions and 1 deletions
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31
crypto/openssh-certs.c
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@ -726,11 +726,40 @@ static bool opensshcert_check_cert(
ptrlen signature = ptrlen_from_strbuf(ck->signature);
ca_key = opensshcert_ca_pub_key(ck, signature, NULL);
/*
* The OpenSSH certificate spec is one-layer only: it explicitly
* forbids using a certified key in turn as the CA.
*
* If it did not, then we'd also have to recursively verify
* everything up the CA chain until we reached the ultimate root,
* and then make sure _that_ was something we trusted. (Not to
* mention that there'd probably be an additional SSH_CERT_TYPE_CA
* or some such, and certificate options saying what kinds of
* certificate a CA was trusted to sign for, and ...)
*/
ca_key = opensshcert_ca_pub_key(ck, make_ptrlen(NULL, 0), NULL);
if (!ca_key) {
put_fmt(error, "Certificate's signing key is invalid");
goto out;
}
if (ssh_key_alg(ca_key)->is_certificate) {
put_fmt(error, "Certificate is signed with a certified key "
"(forbidden by OpenSSH certificate specification)");
goto out;
}
/*
* Now re-instantiate the key in a way that matches the signature
* (i.e. so that if the key is an RSA one we get the right subtype
* of RSA).
*/
ssh_key_free(ca_key);
ca_key = opensshcert_ca_pub_key(ck, signature, NULL);
if (!ca_key) {
put_fmt(error, "Certificate's signing key does not match "
"signature type");
goto out;
}
/* Check which signature algorithm is actually in use, because
* that might be a reason to reject the certificate (e.g. ssh-rsa