Verifying S/MIME signed message with OpenSSL without checking the certificate’s purpose

The answer is here: https://stackoverflow.com/questions/2385320/verifying-a-file-signature-with-openssl-dgst

The structure of this IIS7 renewal request is actually quite elegant. It seems to start from the premise that because this is a request to renew a current certificate, it needs to prove that the request is coming from the correct host -- i.e. the host that is actually using the current certificate & ∴ owns the associated private key. In the Internet world, you prove that you are allowed to request renewals for a certificate by authenticating to your CA as the original user, rather than creating a signed CSR.

To prove the right to issue a renewal request, IIS7 creates a normal CSR (PKCS#10 object), and then signs it, and provides the cert of the key that signed it.

• IIS7 renewal CSR
  • PKCS#7 Data
    • PKCS#10 Data (the ordinary CSR)
  • Normal server certificate
  • Issuing CA data
  • RSA signature (I assume)

Use openssl asn1parse -in iis7rcsr -i to see the structure of the file, and compare this to normal CSRs. You should see an OCTET STRING near the beginning, in an object labelled ":pkcs7-data", which is what you need to extract to get the CSR.

$ openssl asn1parse -in iis7rcsr -i
0:d=0  hl=4 l=4273 cons: SEQUENCE          
4:d=1  hl=2 l=   9 prim:  OBJECT            :pkcs7-signedData
15:d=1  hl=4 l=4258 cons:  cont [ 0 ]        
19:d=2  hl=4 l=4254 cons:   SEQUENCE          
23:d=3  hl=2 l=   1 prim:    INTEGER           :01
26:d=3  hl=2 l=  11 cons:    SET               
28:d=4  hl=2 l=   9 cons:     SEQUENCE          
30:d=5  hl=2 l=   5 prim:      OBJECT            :sha1
37:d=5  hl=2 l=   0 prim:      NULL              
39:d=3  hl=4 l=2426 cons:    SEQUENCE          
43:d=4  hl=2 l=   9 prim:     OBJECT            :pkcs7-data
54:d=4  hl=4 l=2411 cons:     cont [ 0 ]        
58:d=5  hl=4 l=2407 prim:      OCTET STRING      [HEX DUMP]:3082096330820...

In order to get the actual PKCS#10 CSR out of here, we need that offset number, "58" in this example. Then we can use that offset to extract the binary version of that object :-

$ openssl asn1parse -in iis7rcsr -strparse 58 -out thecsr -noout

Next we can read that output file 'thecsr' with openssl req, remembering to specify the input format DER.

$ openssl req -in thecsr -inform DER -text -noout
Certificate Request:
Data:
    Version: 0 (0x0)
    Subject: (normal CSR Subject: line, censored)
    Subject Public Key Info:
        Public Key Algorithm: rsaEncryption
...

I can wrap all this up into one command-line with no temporary files (but sadly 2 reads of the original cert), as long as I can use Linux's /proc/self/fd/ to fool openssl (it will do native tricks with file descriptors for password handling, but not normal output).

$ openssl asn1parse -in iis7rcsr -strparse $(openssl asn1parse -in iis7rcsr | grep -A2 ':pkcs7-data'|tail -1|cut -d: -f1) -out /dev/stdout -noout | openssl req -inform DER -noout -text

Certificate Request:
Data:
    Version: 0 (0x0)
    Subject: (Subject: line censored again)
    Subject Public Key Info:
        Public Key Algorithm: rsaEncryption
        RSA Public Key: (1024 bit)
            Modulus (1024 bit):
...

This long command line is directly equivalent to the simple openssl req -in non-iis7rcsr -noout -text that I normally use :-)