Implements HTTP request and response signing and verification. Supports the major MAC and asymmetric key signature algorithms. It has several safety restrictions: One, none of the widely known non-cryptographically safe algorithms are permitted; Two, the RSA SHA256 algorithms must be available in the binary (and it should, barring export restrictions); Finally, the library assumes either the 'Authorizationn' or 'Signature' headers are to be set (but not both).