Use Poco::Crypto::DigestEngine in Poco::HMACEngine

Question

Poco supports HMACEngine with different hash functions. For example to create a HMAC-SHA1 function I can use HMACEngine<SHA1Engine> hmca_sha1("secret");

This works well with DigestEngine like SHA1Engine and MD5Engine that have a constructor without argument.

Is it possible to use HMACEngine with hash functions from OpenSSL as they are provided by Poco::Crypto::DigestEngine?

The problem is that these functions take a string parameter in their constructor that specifies which hashing algorithm to use. This means HMACEngine<Poco::Crypto:DigestEngine> hmca_xxx("secret"); wouldn't work.

Any idea how to do this?

Answer 1

I think it will require some coding on Poco side. HMAC is accepting any class as template argument, and hoping it is a Digest class ans using methods like digest() that may not even exist. The HMACEngine that instantiates the class as template, so you can´t passa anything to its constructor as it is. In fact I don´t think that is a good use of templates, while I´m not very familiar with using this C++ resource yet, I think one should not expect anything to be available from templated class.

Probably a new HMACEngine for the Crypto module (instead of Foundation module) would be good. But looks like an approach more like the Poco::Crypto::RSADigestEngine would be better than the current template based one. I´ve recently modified Poco::Crypto::RSADigestEngine to be based on Poco::Crypto::DigestEngine instead off old Foundation Poco::DigestEngine, and therefore for next release it will also support all hashes that OpenSSL support. Basically RSADigestEngine creates an instance of a Poco::Crypto::DigestEngine as base and uses it.

I could help on a patch proposal if you create one at github. Poco::Crypto does need some updates :)