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
crypto_cfb_bsize(struct crypto_skcipher * tfm)30 static unsigned int crypto_cfb_bsize(struct crypto_skcipher *tfm)
31 {
32 return crypto_cipher_blocksize(skcipher_cipher_simple(tfm));
33 }
34
crypto_cfb_encrypt_one(struct crypto_skcipher * tfm,const u8 * src,u8 * dst)35 static 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 */
crypto_cfb_final(struct skcipher_walk * walk,struct crypto_skcipher * tfm)42 static 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
crypto_cfb_encrypt_segment(struct skcipher_walk * walk,struct crypto_skcipher * tfm)57 static 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
crypto_cfb_encrypt_inplace(struct skcipher_walk * walk,struct crypto_skcipher * tfm)80 static 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
crypto_cfb_encrypt(struct skcipher_request * req)102 static 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
crypto_cfb_decrypt_segment(struct skcipher_walk * walk,struct crypto_skcipher * tfm)127 static 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
crypto_cfb_decrypt_inplace(struct skcipher_walk * walk,struct crypto_skcipher * tfm)150 static 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
crypto_cfb_decrypt_blocks(struct skcipher_walk * walk,struct crypto_skcipher * tfm)169 static 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
crypto_cfb_decrypt(struct skcipher_request * req)178 static 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
crypto_cfb_create(struct crypto_template * tmpl,struct rtattr ** tb)200 static 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
230 static struct crypto_template crypto_cfb_tmpl = {
231 .name = "cfb",
232 .create = crypto_cfb_create,
233 .module = THIS_MODULE,
234 };
235
crypto_cfb_module_init(void)236 static int __init crypto_cfb_module_init(void)
237 {
238 return crypto_register_template(&crypto_cfb_tmpl);
239 }
240
crypto_cfb_module_exit(void)241 static void __exit crypto_cfb_module_exit(void)
242 {
243 crypto_unregister_template(&crypto_cfb_tmpl);
244 }
245
246 subsys_initcall(crypto_cfb_module_init);
247 module_exit(crypto_cfb_module_exit);
248
249 MODULE_LICENSE("GPL");
250 MODULE_DESCRIPTION("CFB block cipher mode of operation");
251 MODULE_ALIAS_CRYPTO("cfb");
252
