code: func (csp *impl) Sign(k bccsp.Key, digest []byte, opts bccsp.SignerOpts) (signature []byte, err error) {
  	return nil, nil
  }
docs: // Sign signs digest using key k.
  // The opts argument should be appropriate for the algorithm used.
  //
  // Note that when a signature of a hash of a larger message is needed,
  // the caller is responsible for hashing the larger message and passing
  // the hash (as digest).