class Net::IMAP::SASL::DigestMD5Authenticator

def response_value(response)
  a1 = compute_a1(response)
  a2 = compute_a2(response)
  Digest::MD5.hexdigest(
    [
      Digest::MD5.hexdigest(a1),
      response.values_at(:nonce, :nc, :cnonce, :qop),
      Digest::MD5.hexdigest(a2)
    ].join(":")
  )
end