optional ID Token signature validation for code flow

[Filip Skokan]

This issue calls for clarification on inheriting optional response_type=code OIDC behaviour which allows the client to omit verifying the ID Token signature.

If the ID Token is received via direct communication between the Client and the Token Endpoint (which it is in this flow), the TLS server validation MAY be used to validate the issuer in place of checking the token signature.

Both FAPI 2.0 and FAPI 1.0 (only when using response_type=code and JARM) are in question here.

Having checked FAPI 2.0, 1.0 Baseline and 1.0 Advanced I have not found a statement which would further profile client behaviour to require verifying these signatures.

[Joseph Heenan]

My notes for FAPI2 RP certification tests says “Remove various tests that return invalid frontchannel id_tokens, as there isn't one in FAPI2 (under discussion with FAPI WG; we may want to have these checks used for backchannel id token instead)”.

Unfortunately I don’t appear to have made any note of where/how it was under discussion, and I can’t find anything relevant in the issue tracker. I know I made that note on 10th April this year.

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[Joseph Heenan]

Ah, it was very briefly discussed in https://bitbucket.org/openid/fapi/issues/490/request-for-suggestions-for-tests-for without reaching a conclusion/consensus.

[Filip Skokan]

On last week’s call we’ve explored this and in order to decide we need the information (@Daniel Fett offered to collect it) about whether the signature validation step is part of the security analysis mathematical model and whether it in any way shape or form affects the outcome.

[Daniel Fett]

Thanks for the reminder! The signature validation will be considered optional in the analysis.

[Joseph Heenan]

Thanks all.

So given the various confirmations, I’ve raised https://gitlab.com/openid/conformance-suite/-/issues/1114 so we make sure the conformance tests are correct in this regard.

I think we can close this ticket.

[Nat Sakimura]

We will wait for the analysis result to come out. If it is found that verification is needed, then we will add text.

[Nat Sakimura]

Security analysis concluded that ID Token signature verification is not required if it is received within a correct TLS session.