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/skcipher.h>
24#include <linux/err.h>
25#include <linux/init.h>
26#include <linux/kernel.h>
27#include <linux/module.h>
28#include <linux/string.h>
29
30static unsigned int crypto_cfb_bsize(struct crypto_skcipher *tfm)
31{
32 return crypto_cipher_blocksize(skcipher_cipher_simple(tfm));
33}
34
35static void crypto_cfb_encrypt_one(struct crypto_skcipher *tfm,
36 const u8 *src, u8 *dst)
37{
38 crypto_cipher_encrypt_one(skcipher_cipher_simple(tfm), dst, src);
39}
40
41/* final encrypt and decrypt is the same */
42static void crypto_cfb_final(struct skcipher_walk *walk,
43 struct crypto_skcipher *tfm)
44{
45 const unsigned long alignmask = crypto_skcipher_alignmask(tfm);
46 u8 tmp[MAX_CIPHER_BLOCKSIZE + MAX_CIPHER_ALIGNMASK];
47 u8 *stream = PTR_ALIGN(tmp + 0, alignmask + 1);
48 u8 *src = walk->src.virt.addr;
49 u8 *dst = walk->dst.virt.addr;
50 u8 *iv = walk->iv;
51 unsigned int nbytes = walk->nbytes;
52
53 crypto_cfb_encrypt_one(tfm, iv, stream);
54 crypto_xor_cpy(dst, stream, src, nbytes);
55}
56
57static int crypto_cfb_encrypt_segment(struct skcipher_walk *walk,
58 struct crypto_skcipher *tfm)
59{
60 const unsigned int bsize = crypto_cfb_bsize(tfm);
61 unsigned int nbytes = walk->nbytes;
62 u8 *src = walk->src.virt.addr;
63 u8 *dst = walk->dst.virt.addr;
64 u8 *iv = walk->iv;
65
66 do {
67 crypto_cfb_encrypt_one(tfm, iv, dst);
68 crypto_xor(dst, src, bsize);
69 iv = dst;
70
71 src += bsize;
72 dst += bsize;
73 } while ((nbytes -= bsize) >= bsize);
74
75 memcpy(walk->iv, iv, bsize);
76
77 return nbytes;
78}
79
80static int crypto_cfb_encrypt_inplace(struct skcipher_walk *walk,
81 struct crypto_skcipher *tfm)
82{
83 const unsigned int bsize = crypto_cfb_bsize(tfm);
84 unsigned int nbytes = walk->nbytes;
85 u8 *src = walk->src.virt.addr;
86 u8 *iv = walk->iv;
87 u8 tmp[MAX_CIPHER_BLOCKSIZE];
88
89 do {
90 crypto_cfb_encrypt_one(tfm, iv, tmp);
91 crypto_xor(src, tmp, bsize);
92 iv = src;
93
94 src += bsize;
95 } while ((nbytes -= bsize) >= bsize);
96
97 memcpy(walk->iv, iv, bsize);
98
99 return nbytes;
100}
101
102static int crypto_cfb_encrypt(struct skcipher_request *req)
103{
104 struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
105 struct skcipher_walk walk;
106 unsigned int bsize = crypto_cfb_bsize(tfm);
107 int err;
108
109 err = skcipher_walk_virt(&walk, req, false);
110
111 while (walk.nbytes >= bsize) {
112 if (walk.src.virt.addr == walk.dst.virt.addr)
113 err = crypto_cfb_encrypt_inplace(&walk, tfm);
114 else
115 err = crypto_cfb_encrypt_segment(&walk, tfm);
116 err = skcipher_walk_done(&walk, err);
117 }
118
119 if (walk.nbytes) {
120 crypto_cfb_final(&walk, tfm);
121 err = skcipher_walk_done(&walk, 0);
122 }
123
124 return err;
125}
126
127static int crypto_cfb_decrypt_segment(struct skcipher_walk *walk,
128 struct crypto_skcipher *tfm)
129{
130 const unsigned int bsize = crypto_cfb_bsize(tfm);
131 unsigned int nbytes = walk->nbytes;
132 u8 *src = walk->src.virt.addr;
133 u8 *dst = walk->dst.virt.addr;
134 u8 *iv = walk->iv;
135
136 do {
137 crypto_cfb_encrypt_one(tfm, iv, dst);
138 crypto_xor(dst, src, bsize);
139 iv = src;
140
141 src += bsize;
142 dst += bsize;
143 } while ((nbytes -= bsize) >= bsize);
144
145 memcpy(walk->iv, iv, bsize);
146
147 return nbytes;
148}
149
150static int crypto_cfb_decrypt_inplace(struct skcipher_walk *walk,
151 struct crypto_skcipher *tfm)
152{
153 const unsigned int bsize = crypto_cfb_bsize(tfm);
154 unsigned int nbytes = walk->nbytes;
155 u8 *src = walk->src.virt.addr;
156 u8 * const iv = walk->iv;
157 u8 tmp[MAX_CIPHER_BLOCKSIZE];
158
159 do {
160 crypto_cfb_encrypt_one(tfm, iv, tmp);
161 memcpy(iv, src, bsize);
162 crypto_xor(src, tmp, bsize);
163 src += bsize;
164 } while ((nbytes -= bsize) >= bsize);
165
166 return nbytes;
167}
168
169static int crypto_cfb_decrypt_blocks(struct skcipher_walk *walk,
170 struct crypto_skcipher *tfm)
171{
172 if (walk->src.virt.addr == walk->dst.virt.addr)
173 return crypto_cfb_decrypt_inplace(walk, tfm);
174 else
175 return crypto_cfb_decrypt_segment(walk, tfm);
176}
177
178static int crypto_cfb_decrypt(struct skcipher_request *req)
179{
180 struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
181 struct skcipher_walk walk;
182 const unsigned int bsize = crypto_cfb_bsize(tfm);
183 int err;
184
185 err = skcipher_walk_virt(&walk, req, false);
186
187 while (walk.nbytes >= bsize) {
188 err = crypto_cfb_decrypt_blocks(&walk, tfm);
189 err = skcipher_walk_done(&walk, err);
190 }
191
192 if (walk.nbytes) {
193 crypto_cfb_final(&walk, tfm);
194 err = skcipher_walk_done(&walk, 0);
195 }
196
197 return err;
198}
199
200static int crypto_cfb_create(struct crypto_template *tmpl, struct rtattr **tb)
201{
202 struct skcipher_instance *inst;
203 struct crypto_alg *alg;
204 int err;
205
206 inst = skcipher_alloc_instance_simple(tmpl, tb, &alg);
207 if (IS_ERR(inst))
208 return PTR_ERR(inst);
209
210 /* CFB mode is a stream cipher. */
211 inst->alg.base.cra_blocksize = 1;
212
213 /*
214 * To simplify the implementation, configure the skcipher walk to only
215 * give a partial block at the very end, never earlier.
216 */
217 inst->alg.chunksize = alg->cra_blocksize;
218
219 inst->alg.encrypt = crypto_cfb_encrypt;
220 inst->alg.decrypt = crypto_cfb_decrypt;
221
222 err = skcipher_register_instance(tmpl, inst);
223 if (err)
224 inst->free(inst);
225
226 crypto_mod_put(alg);
227 return err;
228}
229
230static struct crypto_template crypto_cfb_tmpl = {
231 .name = "cfb",
232 .create = crypto_cfb_create,
233 .module = THIS_MODULE,
234};
235
236static int __init crypto_cfb_module_init(void)
237{
238 return crypto_register_template(&crypto_cfb_tmpl);
239}
240
241static void __exit crypto_cfb_module_exit(void)
242{
243 crypto_unregister_template(&crypto_cfb_tmpl);
244}
245
246module_init(crypto_cfb_module_init);
247module_exit(crypto_cfb_module_exit);
248
249MODULE_LICENSE("GPL");
250MODULE_DESCRIPTION("CFB block cipher mode of operation");
251MODULE_ALIAS_CRYPTO("cfb");
252