code: function iter (block) {
      var hash = crypto.createHash(opts.digest || 'md5')
      hash.update(block)
      hash.update(data)
      hash.update(salt)
      block = hash.digest()
  
      for (var i = 1; i < (opts.count || 1); i++) {
        hash = crypto.createHash(opts.digest || 'md5')
        hash.update(block)
        block = hash.digest()
      }
  
      return block
    }
docs: eslint-disable-line A single EVP iteration, returns `D_i`, where block equlas to `D_(i-1)`