gnutls-symmetric-encrypt/decrypt in GCM mode requires plaintext/ciphertext size to be multiple of 16

gnutls-symmetric-encrypt/decrypt in GCM mode requires plaintext/ciphertext size to be multiple of 16

[Jürgen Hötzel]

Hi,

GCM doesn't require any padding of the plaintext before it used, so this is
IMO an invalid assumption. Evaluating

(gnutls-symmetric-encrypt 'AES-128-GCM "0123456789ABCDEF" "0123456789AB"
"hello")

results in:

(error "GnuTLS AEAD cipher AES-128-GCM/encrypt input block length 5 is
not 0 greater than a multiple of the required 16")

whereas the corresponding C code using GnuTLS works as expected:

#include <gnutls/crypto.h>
#include <gnutls/gnutls.h>
#include <stdio.h>

int main(int argc, char *argv[]) {
  gnutls_datum_t key = {.data = (unsigned char *)"0123456789ABCDEF", .size = 16};
  char iv[12] = "0123456789AB";
  char plaintext[5] = "hello";
  int tlserr;
  gnutls_session_t session;
  gnutls_aead_cipher_hd_t hd;
  if ((tlserr = gnutls_init(&session, 0) != GNUTLS_E_SUCCESS)) {
  };

  if (gnutls_aead_cipher_init(&hd, GNUTLS_CIPHER_AES_128_GCM, &key) < 0) {
    fprintf(stderr, "gnutls_cipher_init failed: %s", gnutls_strerror(tlserr));
    goto cleanup;
  }
  char ctext[5 + 16]; /* plaintext + tagsize */
  size_t ctext_len = 5 + 16;
  if ((tlserr = gnutls_aead_cipher_encrypt(hd, iv, sizeof(iv), NULL, 0, 16, plaintext, 5, ctext, &ctext_len)) < 0) {
    fprintf(stderr, "gnutls_aead_cipher_decrypt failed: %s\n", gnutls_strerror(tlserr));
    goto cleanup;
  }
  fwrite(ctext, 1, ctext_len, stdout);
cleanup:
  gnutls_deinit(session);
  return tlserr;
}

Best regards,

Jürgen

Re: gnutls-symmetric-encrypt/decrypt in GCM mode requires plaintext/ciphertext size to be multiple of 16

[Eli Zaretskii]

> From: Jürgen Hötzel <juergen@hoetzel.info>
> Date: Sun, 08 Jan 2023 17:03:48 +0100
> 
> GCM doesn't require any padding of the plaintext before it used, so this is
> IMO an invalid assumption. Evaluating
> 
> (gnutls-symmetric-encrypt 'AES-128-GCM "0123456789ABCDEF" "0123456789AB"
> "hello")
> 
> results in:
> 
> (error "GnuTLS AEAD cipher AES-128-GCM/encrypt input block length 5 is
> not 0 greater than a multiple of the required 16")
> 
> whereas the corresponding C code using GnuTLS works as expected:

Please report this using "M-x report-emacs-bug RET", and please
include with the report the version of the GnuTLS library you are
using.

Thanks.