include/crypto/aes-ctr.h

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * AES-CTR and AES-XCTR stream ciphers
 *
 * Copyright 2026 Google LLC
 */
#ifndef _CRYPTO_AES_CTR_H
#define _CRYPTO_AES_CTR_H

#include <crypto/aes.h>

/**
 * aes_ctr() - AES-CTR en/decryption
 * @dst: The destination buffer.  Can be in-place or out-of-place.  For other
 *	 overlaps the behavior is unspecified.
 * @src: The source data
 * @len: Number of bytes to en/decrypt
 * @ctr: The counter.  It will be incremented by ceil(@len / AES_BLOCK_SIZE).
 * @key: The key, already prepared using aes_preparekey() or aes_prepareenckey()
 *
 * This implements AES in counter mode with a 128-bit big endian counter.
 *
 * This exists only for use by the implementation of modes built on top of CTR
 * (e.g., GCM and CCM) and some legacy protocols that use CTR mode directly.
 * Callers are expected to know how to use CTR mode appropriately, including
 * choosing (key, counter) pairs appropriately to avoid keystream reuse.
 *
 * This supports incremental en/decryption.  The length of each non-final chunk
 * must be a multiple of AES_BLOCK_SIZE, and the updated @ctr must be passed in
 * each time.
 *
 * Context: Any context.
 */
void aes_ctr(u8 *dst, const u8 *src, size_t len,
	     u8 ctr[at_least AES_BLOCK_SIZE], aes_encrypt_arg key);

/**
 * aes_xctr() - AES-XCTR en/decryption
 * @dst: The destination buffer.  Can be in-place or out-of-place.  For other
 *	 overlaps the behavior is unspecified.
 * @src: The source data
 * @len: Number of bytes to en/decrypt
 * @ctr: The block counter (in host endianness).  For the first call, set it to
 *	 1.  It will be incremented by ceil(@len / AES_BLOCK_SIZE).
 * @iv: The initialization vector
 * @key: The key, already prepared using aes_preparekey() or aes_prepareenckey()
 *
 * This implements AES in XOR Counter mode, as specified in the paper
 * "Length-preserving encryption with HCTR2"
 * (https://eprint.iacr.org/2021/1441.pdf).
 *
 * This exists only for use by the implementation of modes built on top of XCTR.
 * Callers are expected to know how to use XCTR mode appropriately, including
 * choosing (key, IV) pairs appropriately to avoid keystream reuse.
 *
 * This supports incremental en/decryption.  The length of each non-final chunk
 * must be a multiple of AES_BLOCK_SIZE, and the updated @ctr must be passed in
 * each time.
 *
 * Context: Any context.
 */
void aes_xctr(u8 *dst, const u8 *src, size_t len, u64 *ctr,
	      const u8 iv[at_least AES_BLOCK_SIZE], aes_encrypt_arg key);

#endif /* _CRYPTO_AES_CTR_H */