1 //SPDX-License-Identifier: GPL-2.0
3 * CFB: Cipher FeedBack mode
5 * Copyright (c) 2018 James.Bottomley@HansenPartnership.com
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.
13 * The scheme of CFB is best described by wikipedia:
15 * https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#CFB
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.
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>
31 static unsigned int crypto_cfb_bsize(struct crypto_skcipher
*tfm
)
33 return crypto_cipher_blocksize(skcipher_cipher_simple(tfm
));
36 static void crypto_cfb_encrypt_one(struct crypto_skcipher
*tfm
,
37 const u8
*src
, u8
*dst
)
39 crypto_cipher_encrypt_one(skcipher_cipher_simple(tfm
), dst
, src
);
42 /* final encrypt and decrypt is the same */
43 static void crypto_cfb_final(struct skcipher_walk
*walk
,
44 struct crypto_skcipher
*tfm
)
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
;
52 unsigned int nbytes
= walk
->nbytes
;
54 crypto_cfb_encrypt_one(tfm
, iv
, stream
);
55 crypto_xor_cpy(dst
, stream
, src
, nbytes
);
58 static int crypto_cfb_encrypt_segment(struct skcipher_walk
*walk
,
59 struct crypto_skcipher
*tfm
)
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
;
68 crypto_cfb_encrypt_one(tfm
, iv
, dst
);
69 crypto_xor(dst
, src
, bsize
);
74 } while ((nbytes
-= bsize
) >= bsize
);
76 memcpy(walk
->iv
, iv
, bsize
);
81 static int crypto_cfb_encrypt_inplace(struct skcipher_walk
*walk
,
82 struct crypto_skcipher
*tfm
)
84 const unsigned int bsize
= crypto_cfb_bsize(tfm
);
85 unsigned int nbytes
= walk
->nbytes
;
86 u8
*src
= walk
->src
.virt
.addr
;
88 u8 tmp
[MAX_CIPHER_BLOCKSIZE
];
91 crypto_cfb_encrypt_one(tfm
, iv
, tmp
);
92 crypto_xor(src
, tmp
, bsize
);
96 } while ((nbytes
-= bsize
) >= bsize
);
98 memcpy(walk
->iv
, iv
, bsize
);
103 static int crypto_cfb_encrypt(struct skcipher_request
*req
)
105 struct crypto_skcipher
*tfm
= crypto_skcipher_reqtfm(req
);
106 struct skcipher_walk walk
;
107 unsigned int bsize
= crypto_cfb_bsize(tfm
);
110 err
= skcipher_walk_virt(&walk
, req
, false);
112 while (walk
.nbytes
>= bsize
) {
113 if (walk
.src
.virt
.addr
== walk
.dst
.virt
.addr
)
114 err
= crypto_cfb_encrypt_inplace(&walk
, tfm
);
116 err
= crypto_cfb_encrypt_segment(&walk
, tfm
);
117 err
= skcipher_walk_done(&walk
, err
);
121 crypto_cfb_final(&walk
, tfm
);
122 err
= skcipher_walk_done(&walk
, 0);
128 static int crypto_cfb_decrypt_segment(struct skcipher_walk
*walk
,
129 struct crypto_skcipher
*tfm
)
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
;
138 crypto_cfb_encrypt_one(tfm
, iv
, dst
);
139 crypto_xor(dst
, src
, bsize
);
144 } while ((nbytes
-= bsize
) >= bsize
);
146 memcpy(walk
->iv
, iv
, bsize
);
151 static int crypto_cfb_decrypt_inplace(struct skcipher_walk
*walk
,
152 struct crypto_skcipher
*tfm
)
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
];
161 crypto_cfb_encrypt_one(tfm
, iv
, tmp
);
162 memcpy(iv
, src
, bsize
);
163 crypto_xor(src
, tmp
, bsize
);
165 } while ((nbytes
-= bsize
) >= bsize
);
170 static int crypto_cfb_decrypt_blocks(struct skcipher_walk
*walk
,
171 struct crypto_skcipher
*tfm
)
173 if (walk
->src
.virt
.addr
== walk
->dst
.virt
.addr
)
174 return crypto_cfb_decrypt_inplace(walk
, tfm
);
176 return crypto_cfb_decrypt_segment(walk
, tfm
);
179 static int crypto_cfb_decrypt(struct skcipher_request
*req
)
181 struct crypto_skcipher
*tfm
= crypto_skcipher_reqtfm(req
);
182 struct skcipher_walk walk
;
183 const unsigned int bsize
= crypto_cfb_bsize(tfm
);
186 err
= skcipher_walk_virt(&walk
, req
, false);
188 while (walk
.nbytes
>= bsize
) {
189 err
= crypto_cfb_decrypt_blocks(&walk
, tfm
);
190 err
= skcipher_walk_done(&walk
, err
);
194 crypto_cfb_final(&walk
, tfm
);
195 err
= skcipher_walk_done(&walk
, 0);
201 static int crypto_cfb_create(struct crypto_template
*tmpl
, struct rtattr
**tb
)
203 struct skcipher_instance
*inst
;
204 struct crypto_alg
*alg
;
207 inst
= skcipher_alloc_instance_simple(tmpl
, tb
);
209 return PTR_ERR(inst
);
211 alg
= skcipher_ialg_simple(inst
);
213 /* CFB mode is a stream cipher. */
214 inst
->alg
.base
.cra_blocksize
= 1;
217 * To simplify the implementation, configure the skcipher walk to only
218 * give a partial block at the very end, never earlier.
220 inst
->alg
.chunksize
= alg
->cra_blocksize
;
222 inst
->alg
.encrypt
= crypto_cfb_encrypt
;
223 inst
->alg
.decrypt
= crypto_cfb_decrypt
;
225 err
= skcipher_register_instance(tmpl
, inst
);
232 static struct crypto_template crypto_cfb_tmpl
= {
234 .create
= crypto_cfb_create
,
235 .module
= THIS_MODULE
,
238 static int __init
crypto_cfb_module_init(void)
240 return crypto_register_template(&crypto_cfb_tmpl
);
243 static void __exit
crypto_cfb_module_exit(void)
245 crypto_unregister_template(&crypto_cfb_tmpl
);
248 subsys_initcall(crypto_cfb_module_init
);
249 module_exit(crypto_cfb_module_exit
);
251 MODULE_LICENSE("GPL");
252 MODULE_DESCRIPTION("CFB block cipher mode of operation");
253 MODULE_ALIAS_CRYPTO("cfb");
254 MODULE_IMPORT_NS(CRYPTO_INTERNAL
);