AES and EVP_CIPHER question

[Philip Prindeville]

> On May 13, 2022, at 10:34 AM, Matt Caswell <matt at openssl.org> 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