title: Use Poco::Crypto::DigestEngine in Poco::HMACEngine
body: <p>Poco supports <code>HMACEngine</code> with different hash functions. For example to create a HMAC-SHA1 function I can use <code>HMACEngine&lt;SHA1Engine&gt; hmca_sha1("secret");</code></p>
  
  <p>This works well with <code>DigestEngine</code> like <code>SHA1Engine</code> and <code>MD5Engine</code> that have a constructor without argument.</p>
  
  <p>Is it possible to use <code>HMACEngine</code> with hash functions from OpenSSL as they are provided by <code>Poco::Crypto::DigestEngine</code>?</p>
  
  <p>The problem is that these functions take a string parameter in their constructor that specifies which hashing algorithm to use. This means <code>HMACEngine&lt;Poco::Crypto:DigestEngine&gt; hmca_xxx("secret");</code> wouldn't work.</p>
  
  <p>Any idea how to do this?</p>