fix: [#630] Use the correct algorithm for at_hash and c_hash

[vivshankar]

Related Issue or Design Document

The defect is described in #630 along with the discussion.

1. at_hash: https://openid.net/specs/openid-connect-core-1_0.html#CodeIDToken
2. c_hash: https://openid.net/specs/openid-connect-core-1_0.html#HybridIDToken

Checklist

• I have read the contributing guidelines
  and signed the CLA.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added necessary documentation within the code base (if
  appropriate).

Further comments

There are two key changes:

1. Standardize computation of the hash across the different flows
2. Introduce ComputeHash that extracts the algorithm, if available, from the openid.Session header and computes the hash. It falls back to 256 key length to retain backward compatibility.

[aeneasr]

Awesome, thank you for your contribution! This looks pretty good and I have some ideas how to improve it further :)

[aeneasr]

I think it would make sense to set hash directly, without the intermediate hashSize variable :)

[vivshankar]

Hmm, could you elaborate on this please? The alg may or may not be present in the header, so the first step here is to compute the hash size.

[aeneasr]

I guess something along this pseudo-code:

hash := sha256.New()

if alg, ok := sess.IDTokenHeaders().Get("alg").(string); ok {
 		hashSize, _ := strconv.Atoi(alg[2:]) // 0 per default
 		switch hashSize {
 			case 384
				hash = sha512.New()
 			// ...
 		}
 	}

[vivshankar]

@aeneasr Made the change.

[vivshankar]

@aeneasr Pushed in the change based on your review comments. Hope this works.

[aeneasr]

Awesome, thank you for your contribution!