AES and EVP_CIPHER question

[Philip Prindeville]

Sorry, I shouldn't have phrased that inartfully.

There is no EVP_CIPHER_CTX_get_padding(), so how does one achieve something analogous?


> On May 16, 2022, at 1:00 PM, Philip Prindeville <philipp_subx at redfish-solutions.com> wrote:
> 
> Thanks.  That fixed the return value of EVP_CipherFinal().
> 
> Is there a reciprocal EVP_CIPHER_CTX_get_padding() method to find out what the default padding method is for ECB?
> 
> 
> 
>> On May 16, 2022, at 12:41 AM, Tomas Mraz <tomas at openssl.org> wrote:
>> 
>> 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
>> 
>> 
>