CCM mode: decrypting an empty ciphertext fails

[emillon]

Hi,

It looks like CCM decryption does not correctly handle the case when the ciphertext is empty. This happens when there is AAD and when there is not.

In the following example, I would expect Some Cstruct.empty to be returned:

let () =
  let key =
    Cstruct.of_hex "00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f"
    |> Nocrypto.Cipher_block.AES.CCM.of_secret ~maclen:16
  in
  let nonce = Cstruct.of_hex "00 01 02 03 04 05 06 07" in
  let adata = Cstruct.of_string "hello" in
  let recovered =
    Cstruct.empty
    |> Nocrypto.Cipher_block.AES.CCM.encrypt ~key ~nonce ~adata
    |> Nocrypto.Cipher_block.AES.CCM.decrypt ~key ~nonce ~adata
  in
  match recovered with
  | Some cs -> Cstruct.hexdump cs
  | None -> print_endline "None"

Output:

None

Thanks!