1// SPDX-License-Identifier: GPL-2.0-or-later
  2/*
  3 * XCTR: XOR Counter mode - Adapted from ctr.c
  4 *
  5 * (C) Copyright IBM Corp. 2007 - Joy Latten <latten@us.ibm.com>
  6 * Copyright 2021 Google LLC
  7 */
  8
  9/*
 10 * XCTR mode is a blockcipher mode of operation used to implement HCTR2. XCTR is
 11 * closely related to the CTR mode of operation; the main difference is that CTR
 12 * generates the keystream using E(CTR + IV) whereas XCTR generates the
 13 * keystream using E(CTR ^ IV). This allows implementations to avoid dealing
 14 * with multi-limb integers (as is required in CTR mode). XCTR is also specified
 15 * using little-endian arithmetic which makes it slightly faster on LE machines.
 16 *
 17 * See the HCTR2 paper for more details:
 18 *	Length-preserving encryption with HCTR2
 19 *      (https://eprint.iacr.org/2021/1441.pdf)
 20 */
 21
 22#include <crypto/algapi.h>
 23#include <crypto/internal/cipher.h>
 24#include <crypto/internal/skcipher.h>
 25#include <linux/err.h>
 26#include <linux/init.h>
 27#include <linux/kernel.h>
 28#include <linux/module.h>
 29#include <linux/slab.h>
 30
 31/* For now this implementation is limited to 16-byte blocks for simplicity */
 32#define XCTR_BLOCKSIZE 16
 33
 34static void crypto_xctr_crypt_final(struct skcipher_walk *walk,
 35				   struct crypto_cipher *tfm, u32 byte_ctr)
 36{
 37	u8 keystream[XCTR_BLOCKSIZE];
 38	const u8 *src = walk->src.virt.addr;
 39	u8 *dst = walk->dst.virt.addr;
 40	unsigned int nbytes = walk->nbytes;
 41	__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);
 42
 43	crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
 44	crypto_cipher_encrypt_one(tfm, keystream, walk->iv);
 45	crypto_xor_cpy(dst, keystream, src, nbytes);
 46	crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
 47}
 48
 49static int crypto_xctr_crypt_segment(struct skcipher_walk *walk,
 50				    struct crypto_cipher *tfm, u32 byte_ctr)
 51{
 52	void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =
 53		   crypto_cipher_alg(tfm)->cia_encrypt;
 54	const u8 *src = walk->src.virt.addr;
 55	u8 *dst = walk->dst.virt.addr;
 56	unsigned int nbytes = walk->nbytes;
 57	__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);
 58
 59	do {
 60		crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
 61		fn(crypto_cipher_tfm(tfm), dst, walk->iv);
 62		crypto_xor(dst, src, XCTR_BLOCKSIZE);
 63		crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
 64
 65		le32_add_cpu(&ctr32, 1);
 66
 67		src += XCTR_BLOCKSIZE;
 68		dst += XCTR_BLOCKSIZE;
 69	} while ((nbytes -= XCTR_BLOCKSIZE) >= XCTR_BLOCKSIZE);
 70
 71	return nbytes;
 72}
 73
 74static int crypto_xctr_crypt_inplace(struct skcipher_walk *walk,
 75				    struct crypto_cipher *tfm, u32 byte_ctr)
 76{
 77	void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =
 78		   crypto_cipher_alg(tfm)->cia_encrypt;
 79	unsigned long alignmask = crypto_cipher_alignmask(tfm);
 80	unsigned int nbytes = walk->nbytes;
 81	u8 *data = walk->src.virt.addr;
 82	u8 tmp[XCTR_BLOCKSIZE + MAX_CIPHER_ALIGNMASK];
 83	u8 *keystream = PTR_ALIGN(tmp + 0, alignmask + 1);
 84	__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);
 85
 86	do {
 87		crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
 88		fn(crypto_cipher_tfm(tfm), keystream, walk->iv);
 89		crypto_xor(data, keystream, XCTR_BLOCKSIZE);
 90		crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
 91
 92		le32_add_cpu(&ctr32, 1);
 93
 94		data += XCTR_BLOCKSIZE;
 95	} while ((nbytes -= XCTR_BLOCKSIZE) >= XCTR_BLOCKSIZE);
 96
 97	return nbytes;
 98}
 99
100static int crypto_xctr_crypt(struct skcipher_request *req)
101{
102	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
103	struct crypto_cipher *cipher = skcipher_cipher_simple(tfm);
104	struct skcipher_walk walk;
105	unsigned int nbytes;
106	int err;
107	u32 byte_ctr = 0;
108
109	err = skcipher_walk_virt(&walk, req, false);
110
111	while (walk.nbytes >= XCTR_BLOCKSIZE) {
112		if (walk.src.virt.addr == walk.dst.virt.addr)
113			nbytes = crypto_xctr_crypt_inplace(&walk, cipher,
114							   byte_ctr);
115		else
116			nbytes = crypto_xctr_crypt_segment(&walk, cipher,
117							   byte_ctr);
118
119		byte_ctr += walk.nbytes - nbytes;
120		err = skcipher_walk_done(&walk, nbytes);
121	}
122
123	if (walk.nbytes) {
124		crypto_xctr_crypt_final(&walk, cipher, byte_ctr);
125		err = skcipher_walk_done(&walk, 0);
126	}
127
128	return err;
129}
130
131static int crypto_xctr_create(struct crypto_template *tmpl, struct rtattr **tb)
132{
133	struct skcipher_instance *inst;
134	struct crypto_alg *alg;
135	int err;
136
137	inst = skcipher_alloc_instance_simple(tmpl, tb);
138	if (IS_ERR(inst))
139		return PTR_ERR(inst);
140
141	alg = skcipher_ialg_simple(inst);
142
143	/* Block size must be 16 bytes. */
144	err = -EINVAL;
145	if (alg->cra_blocksize != XCTR_BLOCKSIZE)
146		goto out_free_inst;
147
148	/* XCTR mode is a stream cipher. */
149	inst->alg.base.cra_blocksize = 1;
150
151	/*
152	 * To simplify the implementation, configure the skcipher walk to only
153	 * give a partial block at the very end, never earlier.
154	 */
155	inst->alg.chunksize = alg->cra_blocksize;
156
157	inst->alg.encrypt = crypto_xctr_crypt;
158	inst->alg.decrypt = crypto_xctr_crypt;
159
160	err = skcipher_register_instance(tmpl, inst);
161	if (err) {
162out_free_inst:
163		inst->free(inst);
164	}
165
166	return err;
167}
168
169static struct crypto_template crypto_xctr_tmpl = {
170	.name = "xctr",
171	.create = crypto_xctr_create,
172	.module = THIS_MODULE,
173};
174
175static int __init crypto_xctr_module_init(void)
176{
177	return crypto_register_template(&crypto_xctr_tmpl);
178}
179
180static void __exit crypto_xctr_module_exit(void)
181{
182	crypto_unregister_template(&crypto_xctr_tmpl);
183}
184
185subsys_initcall(crypto_xctr_module_init);
186module_exit(crypto_xctr_module_exit);
187
188MODULE_LICENSE("GPL");
189MODULE_DESCRIPTION("XCTR block cipher mode of operation");
190MODULE_ALIAS_CRYPTO("xctr");
191MODULE_IMPORT_NS(CRYPTO_INTERNAL);