Hi Joakim,

The current PKCS #7 implementation indeed does not currently support certificate chains, and does not use a certificate from the PKCS #7 file to validate the signature, and does not support authenticatedAttributes.

We’ve tried to document these limitations clearly in include/mbedtls/pkcs7.h – if you think it’s not sufficiently clear, please raise an issue or PR with points for further improvement. We are currently tidying up the existing PKCS #7 functionality so want to get this right before the next release.

Regarding use of MBEDTLS_PRIVATE – if there are particular fields that it’s useful to access, the preferred approach would probably be to add functions to the PKCS #7 API to access the fields in question, rather than remove MBEDTLS_PRIVATE.

Thank you for providing these patches. Would you be able to submit them as a PR for review in the normal way via GitHub (see https://github.com/Mbed-TLS/mbedtls/blob/development/CONTRIBUTING.md for details)? They would also need some tests adding. If you don’t have time to work on these, I can create a PR but would need you to confirm that these submissions are made under the terms of our DCO.

Thanks

Dave Rodgman

From: Joakim Sindholt via mbed-tls <mbed-tls@lists.trustedfirmware.org>
Date: Tuesday, 21 February 2023 at 11:30
To: mbed-tls@lists.trustedfirmware.org <mbed-tls@lists.trustedfirmware.org>
Subject: [mbed-tls] PKCS#7 signedAttributes and embedded cert chain

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:
3059301306072A8648CE3D020106082A8648CE3D030107034200041B58AEE5B7C868AE0EED554133774B7C16802062EC2EB5A1A053318A9ED7DB943CE1F877671DADCFEB4D3171ACC2714ECBC0C0BCADE0569DF7CD0031623AF1BBDE72D8FFF60EC6D04B0D6C93658FC40F4AF2E22E1F68BF1A500FE00A94B804B7CA1DD7AF825C000FCF77A0ABB6918D2DF55C7A3CBC90BE14C59D8EE62A02B988DF7F32117770012AE362C03FB3AA3918AB9D10BE78442EFA7FCBA3CB02D6E196CE4137C6C3F280D963A5CD050EAC8871B3EB2C4500916FB3E39840D8EB198D58F49A434B2736385075BF9397405360B8694D0AD1E5DA8291F9EA47743F803F4A409EA72C2DF94F43DB60A0EAB8A8BD409C4D684200778B23D2A8126827995C3A80CAACA4AB7077266CED79BF91725330C9FCE3B8F1F5316C08C17B9994CF0ECB26D87A5C1F8FEB1FA737F78794895F6C5153B575EF8E95A8D433FD9486AA6C2B46ED6633D2BE0938202B99BEC21895018FB6FC59BAD662815A2C65200B8B1BAA4B69A8C89204B1EA70094026E9F710E599E14941476C8CD2E2879CF58E850DC732A506885F646BB9BE96B7A65355A6A9B907956E6DF65336504300CF062BA9DE50

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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
 082B0601
 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
 FE71E1E7
 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
 6170706C
 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
 6092A864
 886F70D010701301C06092A864886F70D010905310F170D3233303230393231333032345A302806092A864886F70D010934311B3019300B0609608648016503040201A10A06082A8648CE3D040302302F06092A864886F70D01090431220420C76DE7AF191C16E01405CE8FE89136EB60AC035B3DDE34ACF65220AC7C3CA619300A06082A8648CE3D04030204473045022100EE9B221CD9B5EEB1C6AF4160128D099C9414440A96C855A789846599E3BDE442022005AE51DC947E0B2AA6593E8FDB9F888782C71E5BB1CB6BF3C32B79C448C6A75D

Joakim