How to sign the signed_attrs? How was the test file cms-signed.der created?

[mholschbach]

Hi,

I'm struggling with creating a valid CMS signature when I try to use signed attributes. I tried to simplify my problem to rebuilding the cms-signed.der from the tests/fixtures directory.

What works:
I'm able to create the ASN.1 structure of ContentInfo, SignedData and SignerInfo. I'm using cryptography.hazmat... for signing my data = b'This is the message to encapsulate in PKCS#7/CMS\r\n' like this:
signer_info['signature'] = key.sign(data, padding.PKCS1v15(), hashes.SHA256())
For validation I use "openssl cms -verify -in mytestdata.der -inform DER -CAfile myCA.crt" and get a "CMS Verification successful"

Instead of signing the data directly, I create and sign the signed_attrs like this:

signer_info['signed_attrs'] = cms.CMSAttributes([
        cms.CMSAttribute({
            'type':   cms.CMSAttributeType('content_type'),
            'values': ('data',)
        }),
        cms.CMSAttribute({
            'type':   cms.CMSAttributeType('signing_time'),
            'values': (cms.Time({
                'utc_time': core.UTCTime(datetime.now(timezone.utc))
            }), )
        }),
        cms.CMSAttribute({
            'type':   cms.CMSAttributeType('message_digest'),
            'values': ( md_value, ) # sha256 of the data parameter
        }),
])

signer_info['signature'] = key.sign(
        signer_info['signed_attrs'].dump(),
        padding.PKCS1v15(),
        hashes.SHA256()      
)

The ASN.1 structure looks just like I expect it to and the md_value equals to the sha256 value of the data parameter. But I can't verify with openssl (Version 3.0.10):
CMS Verification failure
402789FF7C7F0000:error:02000068:rsa routines:ossl_rsa_verify:bad signature:../crypto/rsa/rsa_sign.c:430:
402789FF7C7F0000:error:1C880004:Provider routines:rsa_verify:RSA lib:../providers/implementations/signature/rsa_sig.c:774:
402789FF7C7F0000:error:1700009E:CMS routines:CMS_SignerInfo_verify:verification failure:../crypto/cms/cms_sd.c:899:

Many thanks for your help and asn1crypto
Michael

[MatthiasValvekens]

Try replacing signer_info['signed_attrs'].dump() with signer_info['signed_attrs'].untag().dump() and see if that works. :)

You can also put the signed attrs in a variable, use that to compute the signature, and then assign it to signer_info.

(This often comes up because the signed attributes field has context-dependent tagging in SignerInfo, whereas the CMS spec requires the signature to be computed using universal tags)

[mholschbach]

Thank you very much, the untag() did it.