AES and EVP_CIPHER question

[Philip Prindeville]

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
> 
>