Subject Key Identifier hash method

[Corey Bonnell]

The hash method isn't explicitly encoded in the certificate, but it can be 
derived if you have the SubjectPublicKey(Info). If you have the public key, 
then you can calculate the IDs using the various methods and seeing which one 
matches the ID encoded in the certificate. The first method defined in RFC 
5280, section https://datatracker.ietf.org/doc/html/rfc5280#section-4.2.1.2 
(SHA-1 of the subjectPublicKey field (not the SPKI as a whole)) is by far the 
most common method. The two methods in RFC 5280 require only the 
subjectPublicKey, whereas some of the methods defined in RFC 7093 use the 
SubjectPublicKeyInfo as a whole.

Thanks,
Corey

-----Original Message-----
From: openssl-users <openssl-users-bounces at openssl.org> On Behalf Of Robert 
Moskowitz
Sent: Wednesday, June 7, 2023 8:57 AM
To: openssl-users at openssl.org
Subject: Subject Key Identifier hash method

I am trying to figure out if the Subject Key Identifier hash method is carried 
in the certificate.  An asn1dump of a "regular" cert shows:

   276:d=4  hl=2 l=  29 cons:     SEQUENCE
   278:d=5  hl=2 l=   3 prim:      OBJECT            :X509v3 Subject Key 
Identifier
   283:d=5  hl=2 l=  22 prim:      OCTET STRING      [HEX 
DUMP]:04144F0C1A75F4AF13DC67EC18465C020FC22A82616B
   307:d=4  hl=2 l=  31 cons:     SEQUENCE
   309:d=5  hl=2 l=   3 prim:      OBJECT            :X509v3 Authority Key 
Identifier
   314:d=5  hl=2 l=  24 prim:      OCTET STRING      [HEX
DUMP]:30168014A8885F91878E4ED6AA2056C535E2212413F96BA2


I cannot easily see if the hashing method is contained here.  I am assuming it 
is a sha2 hash of the EdDSA public keys, but how do I tell?

Of course I am asking as I want to use the rfc9374 DETs here.

thanks

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 4990 bytes
Desc: not available
URL: <https://mta.openssl.org/pipermail/openssl-users/attachments/20230607/25a988ed/attachment-0001.p7s>