The EVP_CIPHER_CTX_set_padding(ctx, 0) must be called after the
EVP_CipherInit() to have an effect.
Also what is the AST_CRYPTO_AES_BLOCKSIZE value? Is it in bits (i.e,
128)?
Also res should be initialized to -1 so you do not return uninitialized
value on error.
Tomas Mraz
On Fri, 2022-05-13 at 09:49 -0600, 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.
>
> 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
>
--
Tomáš Mráz, OpenSSL
