[openssl-users] PKCS7 and RSA_verify

[Dr. Stephen Henson]

On Thu, Sep 28, 2017, ch wrote:

> Hello!
> 
> I am working on a tool for verifying SMIME-messages.
> Because cms and smime is only able to verify base64 pkcs7-signatures
> I try to do it "manually" and I now have a problem with the
> signing-timestamp.
> 

I'm not sure what you mean by "only able to verify base64 pkcs7-signatures"
it can handle PEM and DER forms too.

> Lets do an example:
> 
> openssl smime -sign -md sha1  -in plain.txt  -inkey mykey -signer
> mycert  -noattr  -outform der | openssl asn1parse -inform der
> 
> If I put plain.txt and the 128 byte signature (from asn1parse out of
> the pkcs7) into RSA_verify it works perfectly.
> Every call would produce the same signature-hexdump.
> 
> But if I remove the -noattr the signature-value will be different
> every second and then RSA_verify it not working anymore.
> 
> How can I handle this?
> 

When you don't use attributes the signature is over performed over the
content. If you use attributes then the signature is over the encoding of a
bunch of attributes including a signing time and the digest of the content.
Because the signing time changes the data being signed in the attributes
changes too.

Steve.
--
Dr Stephen N. Henson. OpenSSL project core developer.
Commercial tech support now available see: http://www.openssl.org