Re: Subject Key Identifier hash method

[David von Oheimb <it@xxxxxxxxxxxxx>]

On Wed, 2023-06-07 at 10:46 -0400, Robert Moskowitz wrote:

thanks all.  It is as I thought.  You have to pretty much know what the

CA did.  You can guess, but go read the CP!

I doubt that you'll find such inessential info on SKIDs in a CP.
As Tomas wrote, it's just any identifier for the public key that is unique per CA.
Since RFC 5280 suggests using the SHA1 hash value of the key and this is a reasonably
unique and convenient way to achieve that, this is what most implementations do.
There is no need to use anything more involved such as any of the SHA-2 algos.

David

On 6/7/23 10:37, Corey Bonnell wrote:

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@xxxxxxxxxxx> On Behalf Of Robert

Moskowitz

Sent: Wednesday, June 7, 2023 8:57 AM

To: openssl-users@xxxxxxxxxxx

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