Hi, I recently had to do some PKCS#7 signature validation and was disappointed to find that it didn't just work. After digging through RFCs to figure out the myriad of things I'd done wrong I was also left with a lack of 3 features in mbedtls:

1. The full certificate chain was not being loaded and explicitly not supported. I believe this is in error since the certificate was never actually used anywhere, meaning it basically errored out for no reason. Since this certificate chain also contains the key used to validate the signature in my case that presented a pretty fundamental problem.

2. signedAttributes were not supported at all.

3. All the fields in mbedtls_pkcs7 and its associated structures are marked as MBEDTLS_PRIVATE. I need to inspect both the certificates and the signedAttributes and would rather not have to parse the entire DER myself.

I have attached patches implementing the first two. I believe I've done so without altering the behavior of the calls although I'm a bit unsure as to why mbedtls_x509_crt_verify doesn't take a const mbedtls_x509_crt. The third issue is a matter of policy and I don't know what your opinion on it is. For the moment I can at least get away with poking at the internal fields.

I have tested it on several signatures, one of which I have included here and I apologize in advance for the absolute spew but I thought it better than attaching even more files.

The data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

It's SHA2-256 hash is: C76DE7AF191C16E01405CE8FE89136EB60AC035B3DDE34ACF65220AC7C3CA619

If you decode the signature DER you'll see a matching id-messageDigest in the signedAttributes towards the very bottom.

The signature in DER format: 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 886F70D010701301C06092A864886F70D010905310F170D3233303230393231333032345A302806092A864886F70D010934311B3019300B0609608648016503040201A10A06082A8648CE3D040302302F06092A864886F70D01090431220420C76DE7AF191C16E01405CE8FE89136EB60AC035B3DDE34ACF65220AC7C3CA619300A06082A8648CE3D04030204473045022100EE9B221CD9B5EEB1C6AF4160128D099C9414440A96C855A789846599E3BDE442022005AE51DC947E0B2AA6593E8FDB9F888782C71E5BB1CB6BF3C32B79C448C6A75D

Joakim