Verifying an IIS7 SSL renewal request using OpenSSL

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Recently I received an SSL “renewal request” generated by an IIS7 server. These SSL certificate requests are interesting because they are very different from the usual CSRs generated.

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:
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:
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 🙂

This entry was posted in Authentication, Encryption and tagged , by Jim Cheetham. Bookmark the permalink.

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