AES and EVP_CIPHER question

[Matt Caswell]

On 13/05/2022 16:49, Philip Prindeville wrote:
> Hi,
> 
> I'm trying to rewrite some legacy AES_* code to use EVP_CIPHER_* so it's forward compatible into 3.x.
> 
> My code, in a nutshell, looks like:
> 
> static int evp_cipher_aes_decrypt(const unsigned char *in, unsigned char *out, unsigned inlen, const ast_aes_decrypt_key *key)
> {
>          EVP_CIPHER_CTX *ctx;
>          int res, outlen, finallen;
>          unsigned char final[AST_CRYPTO_AES_BLOCKSIZE / 8];
> 
>          if ((ctx = EVP_CIPHER_CTX_new()) == NULL) {
>                  return -1;
>          }
> 
>          EVP_CIPHER_CTX_set_padding(ctx, 0);
> 
>          do {
>                  if ((res = EVP_CipherInit(ctx, EVP_aes_128_ecb(), key->raw, NULL, 0)) <= 0) {
>                          break;
>                  }
>                  if ((res = EVP_CipherUpdate(ctx, out, &outlen, in, inlen)) <= 0) {
>                          break;
>                  }
>                  /* for ECB, this is a no-op */
>                  if ((res = EVP_CipherFinal(ctx, final, &finallen)) <= 0) {
>                          break;
>                  }
> 
>                  res = outlen;
>          } while (0);
> 
>          EVP_CIPHER_CTX_free(ctx);
> 
>          return res;
> }
> 
> It's ECB, so there's no IV.  Or padding.  The block size and key size are both 128 bits.
> 
> One thing I noticed right away is that EVP_CipherUpdate() returns 1, and sees "outlen" to zero.

What value does inlen have? If you're not doing padding then it must be 
a multiple of the block size.

Matt


> 
> And then EVP_CipherFinal() returns 0, and sets "finallen" to zero.
> 
> What's wrong with this code?
> 
> I'm trying to write "naive" code that counts on the primitives to indicate how much resultant output is generated for the input I've given (yes, I know that it's 1:1 in the case of ECB, but I shouldn't have to hard-code that in case I want to use the same code with multiple block modes).
> 
> The function is supposed to return <= 0 on error, otherwise the number of bytes decrypted into "out" on success.
> 
> Thanks,
> 
> -Philip
>