conformance test disagrees with our timestamp counting

[jku]

I'm fixing our conformance test suite support (#1443) and found a test that fails:
test_verify_dsse_bundle_with_trust_root

• test uses custom trust root trusted_root.d.json
• the bundle d.txt.good.sigstore.json contains a rekor v1 entry with integrated time and a TSA timestamp
• test expects success but sigstore-python thinks there are not enough valid timestamps

This is partially related to #1421 but I think it's also something more: "signer certificate not found?"

Should test the timestamp content with plain rfc3161-client with the cert chain from the custom trusted root.

E           
E           [18:33:39] ERROR    Error while verifying certificates: Unable to create signers:        verifier.py:138
E                               ErrorStack([Error { code: 276824192, library: "PKCS7 routines",                     
E                               function: "PKCS7_get0_signers", reason: "signer certificate not                     
E                               found", file: "crypto/pkcs7/pk7_smime.c", line: 413 }])                             
E                               Traceback (most recent call last):                                                  
E                                 File                                                                              
E                               "/home/jkukkonen/.venvs/sigstore-python/lib/python3.11/site-packages                
E                               /rfc3161_client/verify.py", line 321, in _verify_tsr_with_chains                    
E                                   self._verify_signed_data(p7, verification_certificate)                          
E                                 File                                                                              
E                               "/home/jkukkonen/.venvs/sigstore-python/lib/python3.11/site-packages                
E                               /rfc3161_client/verify.py", line 339, in _verify_signed_data                        
E                                   return _rust_verify.pkcs7_verify(sig, list(certificates))                       
E                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                       
E                               ValueError: Unable to create signers: ErrorStack([Error { code:                     
E                               276824192, library: "PKCS7 routines", function:                                     
E                               "PKCS7_get0_signers", reason: "signer certificate not found", file:                 
E                               "crypto/pkcs7/pk7_smime.c", line: 413 }])                                           
E                                                                                                                   
E                               During handling of the above exception, another exception occurred:                 
E                                                                                                                   
E                               Traceback (most recent call last):                                                  
E                                 File                                                                              
E                               "/home/jkukkonen/src/sigstore-python/sigstore/verify/verifier.py",                  
E                               line 135, in _verify_signed_timestamp                                               
E                                   verifier.verify_message(timestamp_response, message)  # type:                   
E                               ignore[attr-defined]                                                                
E                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                            
E                                 File                                                                              
E                               "/home/jkukkonen/.venvs/sigstore-python/lib/python3.11/site-packages                
E                               /rfc3161_client/verify.py", line 181, in verify_message                             
E                                   return self.verify(timestamp_response, hashed_message)                          
E                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                          
E                                 File                                                                              
E                               "/home/jkukkonen/.venvs/sigstore-python/lib/python3.11/site-packages                
E                               /rfc3161_client/verify.py", line 199, in verify                                     
E                                   self._verify_tsr_with_chains(timestamp_response)                                
E                                 File                                                                              
E                               "/home/jkukkonen/.venvs/sigstore-python/lib/python3.11/site-packages                
E                               /rfc3161_client/verify.py", line 324, in _verify_tsr_with_chains                    
E                                   raise VerificationError(msg)                                                    
E                               rfc3161_client.errors.VerificationError: Error while verifying                      
E                               certificates: Unable to create signers: ErrorStack([Error { code:                   
E                               276824192, library: "PKCS7 routines", function:                                     
E                               "PKCS7_get0_signers", reason: "signer certificate not found", file:                 
E                               "crypto/pkcs7/pk7_smime.c", line: 413 }])                                           
E                      ERROR    FAIL:                                                                   _cli.py:1071
E                               SHA2_256:330a043220fa13e01d68a7db39c89e12b0c4c3b6a0346fe624b0903f1303b5             
E                               b2                                                                                  
E                      ERROR    not enough timestamps validated to meet the validation threshold (0/1) 

[jku]

trailofbits/rfc3161-client#163

[jku]

Ok situation evolves: Now that the linked rfc3161-client issue has been resolved, the error is now:

E                               Traceback (most recent call last):                                                  
E                                 File                                                                              
E                               "/home/jkukkonen/src/sigstore-conformance/selftest-env/lib/python3.1                
E                               2/site-packages/sigstore/verify/verifier.py", line 134, in                          
E                               _verify_signed_timestamp                                                            
E                                   verifier.verify_message(timestamp_response, message)                            
E                                 File                                                                              
E                               "/home/jkukkonen/src/sigstore-conformance/selftest-env/lib/python3.1                
E                               2/site-packages/rfc3161_client/verify.py", line 192, in                             
E                               verify_message                                                                      
E                                   return self.verify(timestamp_response, hashed_message)                          
E                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                          
E                                 File                                                                              
E                               "/home/jkukkonen/src/sigstore-conformance/selftest-env/lib/python3.1                
E                               2/site-packages/rfc3161_client/verify.py", line 210, in verify                      
E                                   self._verify_tsr_with_chains(timestamp_response)                                
E                                 File                                                                              
E                               "/home/jkukkonen/src/sigstore-conformance/selftest-env/lib/python3.1                
E                               2/site-packages/rfc3161_client/verify.py", line 341, in                             
E                               _verify_tsr_with_chains                                                             
E                                   raise VerificationError(msg)                                                    
E                               rfc3161_client.errors.VerificationError: Error while verifying                      
E                               certificates: Unable to verify pkcs7 signature: ErrorStack([Error {                 
E                               code: 276824181, library: "PKCS7 routines", function:                               
E                               "PKCS7_verify", reason: "certificate verify error", file:                           
E                               "crypto/pkcs7/pk7_smime.c", line: 301, data: "Verify error:                         
E                               certificate has expired" }])                                                        
E                      ERROR    FAIL:                                                                   _cli.py:1071
E                               SHA2_256:330a043220fa13e01d68a7db39c89e12b0c4c3b6a0346fe624b0903f1303b5             
E                               b2                                                                                  
E                      ERROR    not enough timestamps validated to meet the validation threshold (0/1)  errors.py:42
E                               For detailed error information, run sigstore with the `--verbose` flag.

One of the certificates is expired and by default PKCS7_verify() considers that a problem.

I'm not sure if this is documented anywhere but it seems like maybe for our use case this should not be considered an issue -- as long as the cert was valid at the time of the timestamp, I think we're happy?

[woodruffw]

Yeah, hmm. This might require another behavioral tweak in rfc3161-client -- we should consider a certificate valid for the purposes of verifying signed timestamps within its liveness period, even if that liveness period is over.

[jku]

Yeah, hmm. This might require another behavioral tweak in rfc3161-client -- we should consider a certificate valid for the purposes of verifying signed timestamps within its liveness period, even if that liveness period is over.

That's my first impression as well: the current behaviour can still be default for rfc3161-client but for the sigstore use case it looks like we would like the ability to do what you suggest... Sigstore signatures should remain valid into future and that includes timestamps.

[jku]

Reading the RFC:

B) The validity of the digital signature may then be verified in the
      following way:

...

      4)    The date/time indicated by the TSA MUST be within the
            validity period of the signer's certificate.

...

There seems to be no requirement for current time to be within certificate validity period so maybe rfc3161-client could just change how it works? (There is a requirement to check revocation status but that's not something rc3161-client can really do and sigstore-python handles that with trustedroot)

[woodruffw]

There seems to be no requirement for current time to be within certificate validity period so maybe rfc3161-client could just change how it works?

Agreed 🙂

And yeah, I think in practice ~nobody bothers to do in-client revocation with signed timestamping (most RFC 3161 implementations can't even bother to get DER right). But either way, we're in a good shape there per your point about TUF and our trusted root.