1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * CFB: Cipher FeedBack mode
  4 *
  5 * Copyright (c) 2018 James.Bottomley@HansenPartnership.com
  6 *
  7 * CFB is a stream cipher mode which is layered on to a block
  8 * encryption scheme.  It works very much like a one time pad where
  9 * the pad is generated initially from the encrypted IV and then
 10 * subsequently from the encrypted previous block of ciphertext.  The
 11 * pad is XOR'd into the plain text to get the final ciphertext.
 12 *
 13 * The scheme of CFB is best described by wikipedia:
 14 *
 15 * https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#CFB
 16 *
 17 * Note that since the pad for both encryption and decryption is
 18 * generated by an encryption operation, CFB never uses the block
 19 * decryption function.
 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/string.h>
 30
 31static unsigned int crypto_cfb_bsize(struct crypto_skcipher *tfm)
 32{
 33	return crypto_cipher_blocksize(skcipher_cipher_simple(tfm));
 34}
 35
 36static void crypto_cfb_encrypt_one(struct crypto_skcipher *tfm,
 37					  const u8 *src, u8 *dst)
 38{
 39	crypto_cipher_encrypt_one(skcipher_cipher_simple(tfm), dst, src);
 40}
 41
 42/* final encrypt and decrypt is the same */
 43static void crypto_cfb_final(struct skcipher_walk *walk,
 44			     struct crypto_skcipher *tfm)
 45{
 46	const unsigned long alignmask = crypto_skcipher_alignmask(tfm);
 47	u8 tmp[MAX_CIPHER_BLOCKSIZE + MAX_CIPHER_ALIGNMASK];
 48	u8 *stream = PTR_ALIGN(tmp + 0, alignmask + 1);
 49	u8 *src = walk->src.virt.addr;
 50	u8 *dst = walk->dst.virt.addr;
 51	u8 *iv = walk->iv;
 52	unsigned int nbytes = walk->nbytes;
 53
 54	crypto_cfb_encrypt_one(tfm, iv, stream);
 55	crypto_xor_cpy(dst, stream, src, nbytes);
 56}
 57
 58static int crypto_cfb_encrypt_segment(struct skcipher_walk *walk,
 59				      struct crypto_skcipher *tfm)
 60{
 61	const unsigned int bsize = crypto_cfb_bsize(tfm);
 62	unsigned int nbytes = walk->nbytes;
 63	u8 *src = walk->src.virt.addr;
 64	u8 *dst = walk->dst.virt.addr;
 65	u8 *iv = walk->iv;
 66
 67	do {
 68		crypto_cfb_encrypt_one(tfm, iv, dst);
 69		crypto_xor(dst, src, bsize);
 70		iv = dst;
 71
 72		src += bsize;
 73		dst += bsize;
 74	} while ((nbytes -= bsize) >= bsize);
 75
 76	memcpy(walk->iv, iv, bsize);
 77
 78	return nbytes;
 79}
 80
 81static int crypto_cfb_encrypt_inplace(struct skcipher_walk *walk,
 82				      struct crypto_skcipher *tfm)
 83{
 84	const unsigned int bsize = crypto_cfb_bsize(tfm);
 85	unsigned int nbytes = walk->nbytes;
 86	u8 *src = walk->src.virt.addr;
 87	u8 *iv = walk->iv;
 88	u8 tmp[MAX_CIPHER_BLOCKSIZE];
 89
 90	do {
 91		crypto_cfb_encrypt_one(tfm, iv, tmp);
 92		crypto_xor(src, tmp, bsize);
 93		iv = src;
 94
 95		src += bsize;
 96	} while ((nbytes -= bsize) >= bsize);
 97
 98	memcpy(walk->iv, iv, bsize);
 99
100	return nbytes;
101}
102
103static int crypto_cfb_encrypt(struct skcipher_request *req)
104{
105	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
106	struct skcipher_walk walk;
107	unsigned int bsize = crypto_cfb_bsize(tfm);
108	int err;
109
110	err = skcipher_walk_virt(&walk, req, false);
111
112	while (walk.nbytes >= bsize) {
113		if (walk.src.virt.addr == walk.dst.virt.addr)
114			err = crypto_cfb_encrypt_inplace(&walk, tfm);
115		else
116			err = crypto_cfb_encrypt_segment(&walk, tfm);
117		err = skcipher_walk_done(&walk, err);
118	}
119
120	if (walk.nbytes) {
121		crypto_cfb_final(&walk, tfm);
122		err = skcipher_walk_done(&walk, 0);
123	}
124
125	return err;
126}
127
128static int crypto_cfb_decrypt_segment(struct skcipher_walk *walk,
129				      struct crypto_skcipher *tfm)
130{
131	const unsigned int bsize = crypto_cfb_bsize(tfm);
132	unsigned int nbytes = walk->nbytes;
133	u8 *src = walk->src.virt.addr;
134	u8 *dst = walk->dst.virt.addr;
135	u8 *iv = walk->iv;
136
137	do {
138		crypto_cfb_encrypt_one(tfm, iv, dst);
139		crypto_xor(dst, src, bsize);
140		iv = src;
141
142		src += bsize;
143		dst += bsize;
144	} while ((nbytes -= bsize) >= bsize);
145
146	memcpy(walk->iv, iv, bsize);
147
148	return nbytes;
149}
150
151static int crypto_cfb_decrypt_inplace(struct skcipher_walk *walk,
152				      struct crypto_skcipher *tfm)
153{
154	const unsigned int bsize = crypto_cfb_bsize(tfm);
155	unsigned int nbytes = walk->nbytes;
156	u8 *src = walk->src.virt.addr;
157	u8 * const iv = walk->iv;
158	u8 tmp[MAX_CIPHER_BLOCKSIZE];
159
160	do {
161		crypto_cfb_encrypt_one(tfm, iv, tmp);
162		memcpy(iv, src, bsize);
163		crypto_xor(src, tmp, bsize);
164		src += bsize;
165	} while ((nbytes -= bsize) >= bsize);
166
167	return nbytes;
168}
169
170static int crypto_cfb_decrypt_blocks(struct skcipher_walk *walk,
171				     struct crypto_skcipher *tfm)
172{
173	if (walk->src.virt.addr == walk->dst.virt.addr)
174		return crypto_cfb_decrypt_inplace(walk, tfm);
175	else
176		return crypto_cfb_decrypt_segment(walk, tfm);
177}
178
179static int crypto_cfb_decrypt(struct skcipher_request *req)
180{
181	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
182	struct skcipher_walk walk;
183	const unsigned int bsize = crypto_cfb_bsize(tfm);
184	int err;
185
186	err = skcipher_walk_virt(&walk, req, false);
187
188	while (walk.nbytes >= bsize) {
189		err = crypto_cfb_decrypt_blocks(&walk, tfm);
190		err = skcipher_walk_done(&walk, err);
191	}
192
193	if (walk.nbytes) {
194		crypto_cfb_final(&walk, tfm);
195		err = skcipher_walk_done(&walk, 0);
196	}
197
198	return err;
199}
200
201static int crypto_cfb_create(struct crypto_template *tmpl, struct rtattr **tb)
202{
203	struct skcipher_instance *inst;
204	struct crypto_alg *alg;
205	int err;
206
207	inst = skcipher_alloc_instance_simple(tmpl, tb);
208	if (IS_ERR(inst))
209		return PTR_ERR(inst);
210
211	alg = skcipher_ialg_simple(inst);
212
213	/* CFB mode is a stream cipher. */
214	inst->alg.base.cra_blocksize = 1;
215
216	/*
217	 * To simplify the implementation, configure the skcipher walk to only
218	 * give a partial block at the very end, never earlier.
219	 */
220	inst->alg.chunksize = alg->cra_blocksize;
221
222	inst->alg.encrypt = crypto_cfb_encrypt;
223	inst->alg.decrypt = crypto_cfb_decrypt;
224
225	err = skcipher_register_instance(tmpl, inst);
226	if (err)
227		inst->free(inst);
228
229	return err;
230}
231
232static struct crypto_template crypto_cfb_tmpl = {
233	.name = "cfb",
234	.create = crypto_cfb_create,
235	.module = THIS_MODULE,
236};
237
238static int __init crypto_cfb_module_init(void)
239{
240	return crypto_register_template(&crypto_cfb_tmpl);
241}
242
243static void __exit crypto_cfb_module_exit(void)
244{
245	crypto_unregister_template(&crypto_cfb_tmpl);
246}
247
248subsys_initcall(crypto_cfb_module_init);
249module_exit(crypto_cfb_module_exit);
250
251MODULE_LICENSE("GPL");
252MODULE_DESCRIPTION("CFB block cipher mode of operation");
253MODULE_ALIAS_CRYPTO("cfb");
254MODULE_IMPORT_NS(CRYPTO_INTERNAL);