crypto: ctr - add helper for performing a CTR encryption walk

#ifndef _CRYPTO_CTR_H

#define _CRYPTO_CTR_H

#include <crypto/algapi.h>

#include <crypto/internal/skcipher.h>

#include <linux/string.h>

#include <linux/types.h>

#define CTR_RFC3686_NONCE_SIZE 4

#define CTR_RFC3686_IV_SIZE 8

#define CTR_RFC3686_BLOCK_SIZE 16

static inline int crypto_ctr_encrypt_walk(struct skcipher_request *req,

					  void (*fn)(struct crypto_skcipher *,

						     const u8 *, u8 *))

{

	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);

	int blocksize = crypto_skcipher_chunksize(tfm);

	u8 buf[MAX_CIPHER_BLOCKSIZE];

	struct skcipher_walk walk;

	int err;

	/* avoid integer division due to variable blocksize parameter */

	if (WARN_ON_ONCE(!is_power_of_2(blocksize)))

		return -EINVAL;

	err = skcipher_walk_virt(&walk, req, false);

	while (walk.nbytes > 0) {

		u8 *dst = walk.dst.virt.addr;

		u8 *src = walk.src.virt.addr;

		int nbytes = walk.nbytes;

		int tail = 0;

		if (nbytes < walk.total) {

			tail = walk.nbytes & (blocksize - 1);

			nbytes -= tail;

		}

		do {

			int bsize = min(nbytes, blocksize);

			fn(tfm, walk.iv, buf);

			crypto_xor_cpy(dst, src, buf, bsize);

			crypto_inc(walk.iv, blocksize);

			dst += bsize;

			src += bsize;

			nbytes -= bsize;

		} while (nbytes > 0);

		err = skcipher_walk_done(&walk, tail);

	}

	return err;

}

#endif  /* _CRYPTO_CTR_H */