Include counter-signature hash algorithm into SignedData.digestAlgorithms

[bsanchezb]

Hello,

RFC 5652 highly recommends to add used hash algorithms within the SignedData.digestAlgorithms field:

digestAlgorithms is a collection of message digest algorithm identifiers.
...
Implementations MAY fail to validate signatures that use a digest algorithm that is not included in this set.

and

The message digest algorithm SHOULD be among those listed in the digestAlgorithms field of the associated SignerData.

However, with BC I have a problem to fulfil this requirement when working with counter-signatures.

When I create a counter-signature with a hash algorithm different from the one used in the main signature, the produced CMSSignedData does not contain the new hash algorithm and I do not see a way to add it.

The code what I use to create a counter-signature is:

SignerInformationStore signerInformationStore = originalCMSSignedData.getSignerInfos();
List<SignerInformation> result = new LinkedList<>();
for (SignerInformation signerInformation : signerInformationStore) {
    CMSSignedDataGenerator cmsSignedDataGenerator = *build the generator*;
    SignerInformationStore counterSignatureSignerInfoStore = CMSUtils.generateCounterSigners(cmsSignedDataGenerator, signerInformation);
    result.add(SignerInformation.addCounterSigners(signerInformation, counterSignatureSignerInfoStore));
}
CMSSignedData updatedCMSSignedData = CMSSignedData.replaceSigners(originalCMSSignedData, new SignerInformationStore(result));

The resulting updatedCMSSignedData does not contain the hash algorithm used to create the counter-signature, which is stored within a SignerInformation object corresponding to the counter-signature.

The method CMSSignedData.replaceSigners(...) collects all digest algorithms from the SignerInformations, therefore it collects the algorithms in case of parallel signatures, however it does not take counter-signatures into account.
Would be nice to collect the algorithms from counter-signatures as well, especially that the code could be adapted easely:

Iterator it = signerInformationStore.getSigners().iterator();
while (it.hasNext()) {
    SignerInformation signer = (SignerInformation)it.next();            
    digestAlgs.add(CMSSignedHelper.INSTANCE.fixAlgID(signer.getDigestAlgorithmID()));
    SignerInformationStore counterSignaturesStore = signerInformation.getCounterSignatures();
    Iterator<SignerInformation> counterSignatureIt = counterSignatureStore.iterator();
    while (counterSignatureIt.hasNext()) {
        SignerInformation counterSigner = counterSignatureIt.next();
        digestAlgs.add(CMSSignedHelper.INSTANCE.fixAlgID(counterSigner.getDigestAlgorithmID()));
    }
    vec.add(signer.toASN1Structure());        
}

Or, as an alternative, a new method, similar to CMSSignedData.replaceCertificatesAndCRLs(...) could be added, allowed replacing the digestAlgorithms field value.

If another solution is possible, I would appreciate any help as well.

Thank you.

Best regards,
Aleksandr.

[dghgit]

So, I've done this, as it's probably an improvement and does no harm. A word of caution though, I'm not entirely sure the interpretation is correct though as the purpose of digestAlgorithms is to allow for 1 pass processing of encapsulated signed data. This isn't required for countersignatures, but it very helpful if you have to stream your way through a gig of BER encoded data to get to the signers. For this reason I would not assume that counter signatures would be represented in the digestAlgorithms field as for large objects it will involved rewriting the start of the structure because of a change made to the end of it.

Update should appear here shortly and just made the cut for 1.69.

[bsanchezb]

Thanks! Seems very good!

Regards,
Aleksandr.