[openssl-users] BIO_seek() on bio_f_cipher with EVP_aes_256_ctr

[William King]

Without any of the error checking code in place, this is what I'm testing:

/* START CODE BLOCK */
const EVP_CIPHER *cipher = EVP_aes_256_ctr();
const EVP_MD *digest = EVP_sha256();
BIO *enc = BIO_new(BIO_f_cipher());
BIO *in = BIO_new(BIO_s_file());
EVP_CIPHER_CTX *ctx = NULL;
char *path = "/path/to/file.wav", *key = "fake secret key", *iv = "fake iv";

BIO_get_cipher_ctx(enc, &context->ctx);

if (BIO_read_filename(in, path) <= 0 ) {
	assert(0);
}

BIO_push(enc, in);

BIO_set_cipher(enc, cipher, key, iv, 0);

BIO_seek(enc, 2056);

BIO_read(enc, buffer, 128);

/* END CODE BLOCK */

What I'm finding is that using fread() of the unencrypted file and
comparing that to the BIO_seek() then BIO_read(), the data is not
properly decrypted. Comparing fread() of the unencrypted file, to just
doing BIO_read()'s does decrypt the file correctly.

William King
Senior Engineer
Quentus Technologies, INC
1037 NE 65th St Suite 273
Seattle, WA 98115
Main:   (877) 211-9337
Office: (206) 388-4772
Cell:   (253) 686-5518
william.king at quentustech.com

On 8/9/16 2:10 PM, William King wrote:
> What is needed to be able to BIO_seek() on a bio_f_cipher() with a
> cipher of EVP_aes_256_ctr() without the counter, or IV or another
> internal state getting corrupted?
> 
> It seems that doing a seek any direction results in corrupted output.
>