AES and EVP_CIPHER question

Philip Prindeville philipp_subx at
Fri May 13 15:49:07 UTC 2022


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) {
                if ((res = EVP_CipherUpdate(ctx, out, &outlen, in, inlen)) <= 0) {
                /* for ECB, this is a no-op */
                if ((res = EVP_CipherFinal(ctx, final, &finallen)) <= 0) {

                res = outlen;
        } while (0);


        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.



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