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
34 static 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
49 static 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
74 static 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
100 static 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
131 static 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) {
162 out_free_inst:
163 inst->free(inst);
164 }
165
166 return err;
167 }
168
169 static struct crypto_template crypto_xctr_tmpl = {
170 .name = "xctr",
171 .create = crypto_xctr_create,
172 .module = THIS_MODULE,
173 };
174
175 static int __init crypto_xctr_module_init(void)
176 {
177 return crypto_register_template(&crypto_xctr_tmpl);
178 }
179
180 static void __exit crypto_xctr_module_exit(void)
181 {
182 crypto_unregister_template(&crypto_xctr_tmpl);
183 }
184
185 subsys_initcall(crypto_xctr_module_init);
186 module_exit(crypto_xctr_module_exit);
187
188 MODULE_LICENSE("GPL");
189 MODULE_DESCRIPTION("XCTR block cipher mode of operation");
190 MODULE_ALIAS_CRYPTO("xctr");
191 MODULE_IMPORT_NS(CRYPTO_INTERNAL);
192
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.