> On May 13, 2022, at 10:34 AM, Matt Caswell <matt@xxxxxxxxxxx> wrote:
>
>
>
> 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.
The length is 16, which matches the key size (and the AES-128-ECB 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
