PDF digital signature

[Inma_M.@adobeforums.com]

Hello all,

I have a doubt regarding PDF PKCS7 signature. After signing a document (using Adobe Acrobat Professional 6.0), I extract the signature from the signature dictionary (content of 'Contents') and when I see it using an ASN.1 viewer, I notice that the field 'digestAlgorithm' in SignedData has value 'sha1withRSAEncryption', which is a signature (digest+encryption) algorithm rather than a digest algorithm, as indicated in RFC 2315 (PKCS7: Cryptographic Message Syntax v 1.5 Specification).

Could anybody be so kind as to tell me why Adobe uses this algorithm instead of a digest algorithm (such as sha1, md5,..), please?

Thank you very much in advance.

[Leonard_Rosenthol@adobeforums.com]

I can't explain why the ASN.1 viewer is reporting that, but I can verify that there isn't any encryption being performed - just digesting.

Leonard

[Inma_M.@adobeforums.com]

Besides, iText (a JAVA-PDF library) implementation finds that algorithm and converts it to 'sha1' in order to verify the signature using java.security.Signature and BouncyCastle as a cryptographic provider. The PKCS7 PDF signature, as-is, can not be verified using other cryptographic libraries, such as BouncyCastle or IAIK, due to the fact that it is specified 'sha1withRSAEncryption' as digest algorithm (which is considered a signature algorithm by these libraries), instead of a digest algorith like sha1 or md5.

[Leonard_Rosenthol@adobeforums.com]

Well, you did say you were using Acrobat 6, but the current version is Acrobat 8.

It is certainly possible/probably that we fixed this in later versions.

Leonard

[Eugene_Mayevski@adobeforums.com]

Hello!
You wrote on Mon, 20 Nov 2006 03:37:00 -0800

IM> I have a doubt regarding PDF PKCS7 signature. After signing a document
IM> (using Adobe Acrobat Professional 6.0), I extract the signature from
IM> the signature dictionary (content of 'Contents') and when I see it
IM> using an ASN.1 viewer, I notice that the field 'digestAlgorithm' in
IM> SignedData has value 'sha1withRSAEncryption', which is a signature
IM> (digest+encryption) algorithm rather than a digest algorithm, as
IM> indicated in RFC 2315 (PKCS7: Cryptographic Message Syntax v 1.5
IM> Specification).

Signing is a process of encrypting the hash (digest) with a secret key of
the signer. So encryption (asymmetric) is involved.

With best regards,
Eugene Mayevski