code: function( signature, input, key ) {
  
      var check = this.sign( input, key )
  
      return signature.toString( 'hex' ) ===
        check.toString( 'hex' )
  
    }
docs: Verify an HMAC signature
  @internal used by `.verify()`
  @param  {Buffer} signature
  @param  {Buffer} input
  @param  {Buffer} key
  @return {Boolean}