Emil Assarsson wrote:
I want to create a "Subject Key ID" extension and place in a
certificate. My plan is to follow RFC 3280 (Page 27) and sha-1-hash the
public key.
Are you a CA?
Subject Key ID and Authority Key ID extensions are generally created
by a CA, not by the party requesting the cert.
Can I use CERTCertificateRequest.subjectPublicKeyInfo.subjectPublicKey?
Or must I encode the key to DER first?
As the CA, you can use any algorithm that you see fit, that is likely
to produce unique results for each key that you certify. You can
hash the key, raw or encoded; you could even keep a table of all the
keys you receive, and use the index into that table (a monotonically
increasing index) as the key id, if that's what you choose as the CA.
But you should be consistent about it. And no two CAs need to choose
the same algorithm for mapping keys into key IDs.
As a CA, if you receive a cert request that contains a particular
subject key ID value, you are not obligated to use that value, and
probably should not use it, unless it just happens to match the value
that you would choose anyway. The discussion in RFC 3280 on page 27
about a "previously established identifier" doesn't mean "identifier
found in the request". Rather it means that if the CA has previously
issued a cert with this public key in it, and that cert has a subject
key ID, then the CA should use that same subject key ID in any
subsequently issued certs with that same public key in them.
I can't find any example code for this.
This is what I have for the moment. It works but the SKI does not match
the original certificate when I'm doing cross-certifications (on a bunch
of certs).
Doesn't match the original certificate?
Let me explain: No code except the CA's own code that issues the cert,
should ever be trying to create the key ID from the key. USERS of
issued certs should be trying to match the "authority key ID" in one
cert against the "subject key ID" in the issuer's (CA's) cert, and they
should do so without any knowledge of the issuer's key id generation
algorithm. They should NOT be trying to compute a key ID of their own
from some cert and then match that up with key IDs computed by someone
else.
BTW, This is why RFC 3280 says that only CA certs need subject key IDs,
and root CA certs do not need authority Key IDs.
---
ski = PK11_MakeIDFromPubKey(
&(request->subjectPublicKeyInfo.subjectPublicKey));
This function doesn't create a Subject Key ID extension.
It creates a PKCS11 ID attribute (CKA_ID) value.
(And it's just a duplicate of SHA1_HashBuf.)
As a CA, you *could* choose to use this function to generate your
SKID. But it sounds to me like you're not a CA, and you're trying
to duplicate the key->SKID generation algorithm used by some CA,
for some reason. That approach is doomed, I'm afraid. Remember,
not all CAs use the same algorithm to map keys to key IDs.
if(NULL != ski) {
encodedSki = SEC_ASN1EncodeItem (NULL, NULL, ski,
CERTSubjectKeyIDTemplate);
CERT_AddExtension(extHandle, SEC_OID_X509_SUBJECT_KEY_ID,
encodedSki, PR_FALSE, PR_TRUE);
} else {
puts("failed to create SKI");
};
---
Emil Assarsson
--
Nelson B
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