How can I see what attributes are signed inside a PKCS 7 file?

Question

I have a PKCS 7 file, which contains signed data. It successfully verifies: ``` openssl smime -verify -in data.p7s -CAfile root-certificate.pem ``` Output: ``` Verification successful Signed data ``` But when I extract the signed part, I do not see that it is actually the same as what was signed. I mean the following steps: ``` openssl asn1parse -in data.p7s ``` Output: ``` ... 35:d=4 hl=2 l= 9 prim: OBJECT :pkcs7-data 46:d=4 hl=2 l=inf cons: cont [ 0 ] 48:d=5 hl=2 l=inf cons: OCTET STRING 50:d=6 hl=2 l= 5 prim: OCTET STRING :(my data is here in plaintext) ... (then the signed block starts:) 2861:d=6 hl=2 l= 9 prim: OBJECT :rsaEncryption 2872:d=6 hl=2 l= 0 prim: NULL 2874:d=5 hl=4 l= 256 prim: OCTET STRING [HEX DUMP]:<signed data is here> ``` I cut the `[HEX DUMP]` with the signed data: ``` dd if=data.p7s of=signed-part.bin bs=1 skip=2878 count=256 ``` Decrypt it with the corresponding public key: ``` openssl rsautl -verify -in signed-part.bin -pubin -inkey root-public-key.pem -out ve…

Accepted Answer

Using a different sample:

OpenSSL `asn1parse` does not help much identifying the authenticated attributes. You can use OpenSSL `cms`:

```
openssl cms -in data.p7s -noout -cmsout -print
```

Look for `signedAttrs` ("signed attributes" is how the "authenticated attributes" are now called, in CMS terminology)

It will look like this:

```
    ...
    signerInfos:
        ...
        signedAttrs:
            object: contentType (1.2.840.113549.1.9.3)
            value.set:
              OBJECT:pkcs7-data (1.2.840.113549.1.7.1)
            object: signingTime (1.2.840.113549.1.9.5)
            ...
```

Now go back to `asn1parse` output, and find the corresponding part, which may look like:

```
 ...
 1343:d=5  hl=3 l= 216 cons:      cont [ 0 ]
 1346:d=6  hl=2 l=  24 cons:       SEQUENCE
 1348:d=7  hl=2 l=   9 prim:        OBJECT            :contentType
 1359:d=7  hl=2 l=  11 cons:        SET
 1361:d=8  hl=2 l=   9 prim:         OBJECT            :pkcs7-data
 1372:d=6  hl=2 l=  28 cons:       SEQUENCE
 1374:d=7  hl=2 l=   9 prim:        OBJECT            :signingTime
 ...
```

(for this nice indentation, add the `-i` option)

Now, extract (`dd ...`) the data, including the DER context tag header, i.e., offset 1343, length 219, in this case. Then replace the `0xa0` byte at the beginning by `0x31`. Why you have to do it, is described in DER encoding - How to convert implicit tag to explicit tag, or RFC5652, section 5.4

The SHA-1 hash of this data should now match.

How can I see what attributes are signed inside a PKCS 7 file?

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