Subject Key Identifier hash method

[David von Oheimb]

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 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
> > 
> 
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