summaryrefslogtreecommitdiff
path: root/crypto/xctr.c
blob: 5c00147e8ec40b08d8b77c59ae8d9d190bf9839d (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * XCTR: XOR Counter mode - Adapted from ctr.c
 *
 * (C) Copyright IBM Corp. 2007 - Joy Latten <latten@us.ibm.com>
 * Copyright 2021 Google LLC
 */

/*
 * XCTR mode is a blockcipher mode of operation used to implement HCTR2. XCTR is
 * closely related to the CTR mode of operation; the main difference is that CTR
 * generates the keystream using E(CTR + IV) whereas XCTR generates the
 * keystream using E(CTR ^ IV). This allows implementations to avoid dealing
 * with multi-limb integers (as is required in CTR mode). XCTR is also specified
 * using little-endian arithmetic which makes it slightly faster on LE machines.
 *
 * See the HCTR2 paper for more details:
 *	Length-preserving encryption with HCTR2
 *      (https://eprint.iacr.org/2021/1441.pdf)
 */

#include <crypto/algapi.h>
#include <crypto/internal/cipher.h>
#include <crypto/internal/skcipher.h>
#include <linux/err.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/slab.h>

/* For now this implementation is limited to 16-byte blocks for simplicity */
#define XCTR_BLOCKSIZE 16

static void crypto_xctr_crypt_final(struct skcipher_walk *walk,
				   struct crypto_cipher *tfm, u32 byte_ctr)
{
	u8 keystream[XCTR_BLOCKSIZE];
	const u8 *src = walk->src.virt.addr;
	u8 *dst = walk->dst.virt.addr;
	unsigned int nbytes = walk->nbytes;
	__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);

	crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
	crypto_cipher_encrypt_one(tfm, keystream, walk->iv);
	crypto_xor_cpy(dst, keystream, src, nbytes);
	crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
}

static int crypto_xctr_crypt_segment(struct skcipher_walk *walk,
				    struct crypto_cipher *tfm, u32 byte_ctr)
{
	void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =
		   crypto_cipher_alg(tfm)->cia_encrypt;
	const u8 *src = walk->src.virt.addr;
	u8 *dst = walk->dst.virt.addr;
	unsigned int nbytes = walk->nbytes;
	__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);

	do {
		crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
		fn(crypto_cipher_tfm(tfm), dst, walk->iv);
		crypto_xor(dst, src, XCTR_BLOCKSIZE);
		crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));

		le32_add_cpu(&ctr32, 1);

		src += XCTR_BLOCKSIZE;
		dst += XCTR_BLOCKSIZE;
	} while ((nbytes -= XCTR_BLOCKSIZE) >= XCTR_BLOCKSIZE);

	return nbytes;
}

static int crypto_xctr_crypt_inplace(struct skcipher_walk *walk,
				    struct crypto_cipher *tfm, u32 byte_ctr)
{
	void (*fn)(struct crypto_tfm *, u8 *, const u8 *) =
		   crypto_cipher_alg(tfm)->cia_encrypt;
	unsigned long alignmask = crypto_cipher_alignmask(tfm);
	unsigned int nbytes = walk->nbytes;
	u8 *data = walk->src.virt.addr;
	u8 tmp[XCTR_BLOCKSIZE + MAX_CIPHER_ALIGNMASK];
	u8 *keystream = PTR_ALIGN(tmp + 0, alignmask + 1);
	__le32 ctr32 = cpu_to_le32(byte_ctr / XCTR_BLOCKSIZE + 1);

	do {
		crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));
		fn(crypto_cipher_tfm(tfm), keystream, walk->iv);
		crypto_xor(data, keystream, XCTR_BLOCKSIZE);
		crypto_xor(walk->iv, (u8 *)&ctr32, sizeof(ctr32));

		le32_add_cpu(&ctr32, 1);

		data += XCTR_BLOCKSIZE;
	} while ((nbytes -= XCTR_BLOCKSIZE) >= XCTR_BLOCKSIZE);

	return nbytes;
}

static int crypto_xctr_crypt(struct skcipher_request *req)
{
	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
	struct crypto_cipher *cipher = skcipher_cipher_simple(tfm);
	struct skcipher_walk walk;
	unsigned int nbytes;
	int err;
	u32 byte_ctr = 0;

	err = skcipher_walk_virt(&walk, req, false);

	while (walk.nbytes >= XCTR_BLOCKSIZE) {
		if (walk.src.virt.addr == walk.dst.virt.addr)
			nbytes = crypto_xctr_crypt_inplace(&walk, cipher,
							   byte_ctr);
		else
			nbytes = crypto_xctr_crypt_segment(&walk, cipher,
							   byte_ctr);

		byte_ctr += walk.nbytes - nbytes;
		err = skcipher_walk_done(&walk, nbytes);
	}

	if (walk.nbytes) {
		crypto_xctr_crypt_final(&walk, cipher, byte_ctr);
		err = skcipher_walk_done(&walk, 0);
	}

	return err;
}

static int crypto_xctr_create(struct crypto_template *tmpl, struct rtattr **tb)
{
	struct skcipher_instance *inst;
	struct crypto_alg *alg;
	int err;

	inst = skcipher_alloc_instance_simple(tmpl, tb);
	if (IS_ERR(inst))
		return PTR_ERR(inst);

	alg = skcipher_ialg_simple(inst);

	/* Block size must be 16 bytes. */
	err = -EINVAL;
	if (alg->cra_blocksize != XCTR_BLOCKSIZE)
		goto out_free_inst;

	/* XCTR mode is a stream cipher. */
	inst->alg.base.cra_blocksize = 1;

	/*
	 * To simplify the implementation, configure the skcipher walk to only
	 * give a partial block at the very end, never earlier.
	 */
	inst->alg.chunksize = alg->cra_blocksize;

	inst->alg.encrypt = crypto_xctr_crypt;
	inst->alg.decrypt = crypto_xctr_crypt;

	err = skcipher_register_instance(tmpl, inst);
	if (err) {
out_free_inst:
		inst->free(inst);
	}

	return err;
}

static struct crypto_template crypto_xctr_tmpl = {
	.name = "xctr",
	.create = crypto_xctr_create,
	.module = THIS_MODULE,
};

static int __init crypto_xctr_module_init(void)
{
	return crypto_register_template(&crypto_xctr_tmpl);
}

static void __exit crypto_xctr_module_exit(void)
{
	crypto_unregister_template(&crypto_xctr_tmpl);
}

subsys_initcall(crypto_xctr_module_init);
module_exit(crypto_xctr_module_exit);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("XCTR block cipher mode of operation");
MODULE_ALIAS_CRYPTO("xctr");
MODULE_IMPORT_NS(CRYPTO_INTERNAL);