1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * HCTR2 length-preserving encryption mode
4 *
5 * Copyright 2021 Google LLC
6 */
7
8
9 /*
10 * HCTR2 is a length-preserving encryption mode that is efficient on
11 * processors with instructions to accelerate AES and carryless
12 * multiplication, e.g. x86 processors with AES-NI and CLMUL, and ARM
13 * processors with the ARMv8 crypto extensions.
14 *
15 * For more details, see the paper: "Length-preserving encryption with HCTR2"
16 * (https://eprint.iacr.org/2021/1441.pdf)
17 */
18
19 #include <crypto/internal/cipher.h>
20 #include <crypto/internal/hash.h>
21 #include <crypto/internal/skcipher.h>
22 #include <crypto/polyval.h>
23 #include <crypto/scatterwalk.h>
24 #include <linux/module.h>
25
26 #define BLOCKCIPHER_BLOCK_SIZE 16
27
28 /*
29 * The specification allows variable-length tweaks, but Linux's crypto API
30 * currently only allows algorithms to support a single length. The "natural"
31 * tweak length for HCTR2 is 16, since that fits into one POLYVAL block for
32 * the best performance. But longer tweaks are useful for fscrypt, to avoid
33 * needing to derive per-file keys. So instead we use two blocks, or 32 bytes.
34 */
35 #define TWEAK_SIZE 32
36
37 struct hctr2_instance_ctx {
38 struct crypto_cipher_spawn blockcipher_spawn;
39 struct crypto_skcipher_spawn xctr_spawn;
40 struct crypto_shash_spawn polyval_spawn;
41 };
42
43 struct hctr2_tfm_ctx {
44 struct crypto_cipher *blockcipher;
45 struct crypto_skcipher *xctr;
46 struct crypto_shash *polyval;
47 u8 L[BLOCKCIPHER_BLOCK_SIZE];
48 int hashed_tweak_offset;
49 /*
50 * This struct is allocated with extra space for two exported hash
51 * states. Since the hash state size is not known at compile-time, we
52 * can't add these to the struct directly.
53 *
54 * hashed_tweaklen_divisible;
55 * hashed_tweaklen_remainder;
56 */
57 };
58
59 struct hctr2_request_ctx {
60 u8 first_block[BLOCKCIPHER_BLOCK_SIZE];
61 u8 xctr_iv[BLOCKCIPHER_BLOCK_SIZE];
62 struct scatterlist *bulk_part_dst;
63 struct scatterlist *bulk_part_src;
64 struct scatterlist sg_src[2];
65 struct scatterlist sg_dst[2];
66 /*
67 * Sub-request sizes are unknown at compile-time, so they need to go
68 * after the members with known sizes.
69 */
70 union {
71 struct shash_desc hash_desc;
72 struct skcipher_request xctr_req;
73 } u;
74 /*
75 * This struct is allocated with extra space for one exported hash
76 * state. Since the hash state size is not known at compile-time, we
77 * can't add it to the struct directly.
78 *
79 * hashed_tweak;
80 */
81 };
82
83 static inline u8 *hctr2_hashed_tweaklen(const struct hctr2_tfm_ctx *tctx,
84 bool has_remainder)
85 {
86 u8 *p = (u8 *)tctx + sizeof(*tctx);
87
88 if (has_remainder) /* For messages not a multiple of block length */
89 p += crypto_shash_statesize(tctx->polyval);
90 return p;
91 }
92
93 static inline u8 *hctr2_hashed_tweak(const struct hctr2_tfm_ctx *tctx,
94 struct hctr2_request_ctx *rctx)
95 {
96 return (u8 *)rctx + tctx->hashed_tweak_offset;
97 }
98
99 /*
100 * The input data for each HCTR2 hash step begins with a 16-byte block that
101 * contains the tweak length and a flag that indicates whether the input is evenly
102 * divisible into blocks. Since this implementation only supports one tweak
103 * length, we precompute the two hash states resulting from hashing the two
104 * possible values of this initial block. This reduces by one block the amount of
105 * data that needs to be hashed for each encryption/decryption
106 *
107 * These precomputed hashes are stored in hctr2_tfm_ctx.
108 */
109 static int hctr2_hash_tweaklen(struct hctr2_tfm_ctx *tctx, bool has_remainder)
110 {
111 SHASH_DESC_ON_STACK(shash, tfm->polyval);
112 __le64 tweak_length_block[2];
113 int err;
114
115 shash->tfm = tctx->polyval;
116 memset(tweak_length_block, 0, sizeof(tweak_length_block));
117
118 tweak_length_block[0] = cpu_to_le64(TWEAK_SIZE * 8 * 2 + 2 + has_remainder);
119 err = crypto_shash_init(shash);
120 if (err)
121 return err;
122 err = crypto_shash_update(shash, (u8 *)tweak_length_block,
123 POLYVAL_BLOCK_SIZE);
124 if (err)
125 return err;
126 return crypto_shash_export(shash, hctr2_hashed_tweaklen(tctx, has_remainder));
127 }
128
129 static int hctr2_setkey(struct crypto_skcipher *tfm, const u8 *key,
130 unsigned int keylen)
131 {
132 struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
133 u8 hbar[BLOCKCIPHER_BLOCK_SIZE];
134 int err;
135
136 crypto_cipher_clear_flags(tctx->blockcipher, CRYPTO_TFM_REQ_MASK);
137 crypto_cipher_set_flags(tctx->blockcipher,
138 crypto_skcipher_get_flags(tfm) &
139 CRYPTO_TFM_REQ_MASK);
140 err = crypto_cipher_setkey(tctx->blockcipher, key, keylen);
141 if (err)
142 return err;
143
144 crypto_skcipher_clear_flags(tctx->xctr, CRYPTO_TFM_REQ_MASK);
145 crypto_skcipher_set_flags(tctx->xctr,
146 crypto_skcipher_get_flags(tfm) &
147 CRYPTO_TFM_REQ_MASK);
148 err = crypto_skcipher_setkey(tctx->xctr, key, keylen);
149 if (err)
150 return err;
151
152 memset(hbar, 0, sizeof(hbar));
153 crypto_cipher_encrypt_one(tctx->blockcipher, hbar, hbar);
154
155 memset(tctx->L, 0, sizeof(tctx->L));
156 tctx->L[0] = 0x01;
157 crypto_cipher_encrypt_one(tctx->blockcipher, tctx->L, tctx->L);
158
159 crypto_shash_clear_flags(tctx->polyval, CRYPTO_TFM_REQ_MASK);
160 crypto_shash_set_flags(tctx->polyval, crypto_skcipher_get_flags(tfm) &
161 CRYPTO_TFM_REQ_MASK);
162 err = crypto_shash_setkey(tctx->polyval, hbar, BLOCKCIPHER_BLOCK_SIZE);
163 if (err)
164 return err;
165 memzero_explicit(hbar, sizeof(hbar));
166
167 return hctr2_hash_tweaklen(tctx, true) ?: hctr2_hash_tweaklen(tctx, false);
168 }
169
170 static int hctr2_hash_tweak(struct skcipher_request *req)
171 {
172 struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
173 const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
174 struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
175 struct shash_desc *hash_desc = &rctx->u.hash_desc;
176 int err;
177 bool has_remainder = req->cryptlen % POLYVAL_BLOCK_SIZE;
178
179 hash_desc->tfm = tctx->polyval;
180 err = crypto_shash_import(hash_desc, hctr2_hashed_tweaklen(tctx, has_remainder));
181 if (err)
182 return err;
183 err = crypto_shash_update(hash_desc, req->iv, TWEAK_SIZE);
184 if (err)
185 return err;
186
187 // Store the hashed tweak, since we need it when computing both
188 // H(T || N) and H(T || V).
189 return crypto_shash_export(hash_desc, hctr2_hashed_tweak(tctx, rctx));
190 }
191
192 static int hctr2_hash_message(struct skcipher_request *req,
193 struct scatterlist *sgl,
194 u8 digest[POLYVAL_DIGEST_SIZE])
195 {
196 static const u8 padding[BLOCKCIPHER_BLOCK_SIZE] = { 0x1 };
197 struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
198 struct shash_desc *hash_desc = &rctx->u.hash_desc;
199 const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
200 struct sg_mapping_iter miter;
201 unsigned int remainder = bulk_len % BLOCKCIPHER_BLOCK_SIZE;
202 int i;
203 int err = 0;
204 int n = 0;
205
206 sg_miter_start(&miter, sgl, sg_nents(sgl),
207 SG_MITER_FROM_SG | SG_MITER_ATOMIC);
208 for (i = 0; i < bulk_len; i += n) {
209 sg_miter_next(&miter);
210 n = min_t(unsigned int, miter.length, bulk_len - i);
211 err = crypto_shash_update(hash_desc, miter.addr, n);
212 if (err)
213 break;
214 }
215 sg_miter_stop(&miter);
216
217 if (err)
218 return err;
219
220 if (remainder) {
221 err = crypto_shash_update(hash_desc, padding,
222 BLOCKCIPHER_BLOCK_SIZE - remainder);
223 if (err)
224 return err;
225 }
226 return crypto_shash_final(hash_desc, digest);
227 }
228
229 static int hctr2_finish(struct skcipher_request *req)
230 {
231 struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
232 const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
233 struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
234 u8 digest[POLYVAL_DIGEST_SIZE];
235 struct shash_desc *hash_desc = &rctx->u.hash_desc;
236 int err;
237
238 // U = UU ^ H(T || V)
239 // or M = MM ^ H(T || N)
240 hash_desc->tfm = tctx->polyval;
241 err = crypto_shash_import(hash_desc, hctr2_hashed_tweak(tctx, rctx));
242 if (err)
243 return err;
244 err = hctr2_hash_message(req, rctx->bulk_part_dst, digest);
245 if (err)
246 return err;
247 crypto_xor(rctx->first_block, digest, BLOCKCIPHER_BLOCK_SIZE);
248
249 // Copy U (or M) into dst scatterlist
250 scatterwalk_map_and_copy(rctx->first_block, req->dst,
251 0, BLOCKCIPHER_BLOCK_SIZE, 1);
252 return 0;
253 }
254
255 static void hctr2_xctr_done(void *data, int err)
256 {
257 struct skcipher_request *req = data;
258
259 if (!err)
260 err = hctr2_finish(req);
261
262 skcipher_request_complete(req, err);
263 }
264
265 static int hctr2_crypt(struct skcipher_request *req, bool enc)
266 {
267 struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
268 const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
269 struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
270 u8 digest[POLYVAL_DIGEST_SIZE];
271 int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
272 int err;
273
274 // Requests must be at least one block
275 if (req->cryptlen < BLOCKCIPHER_BLOCK_SIZE)
276 return -EINVAL;
277
278 // Copy M (or U) into a temporary buffer
279 scatterwalk_map_and_copy(rctx->first_block, req->src,
280 0, BLOCKCIPHER_BLOCK_SIZE, 0);
281
282 // Create scatterlists for N and V
283 rctx->bulk_part_src = scatterwalk_ffwd(rctx->sg_src, req->src,
284 BLOCKCIPHER_BLOCK_SIZE);
285 rctx->bulk_part_dst = scatterwalk_ffwd(rctx->sg_dst, req->dst,
286 BLOCKCIPHER_BLOCK_SIZE);
287
288 // MM = M ^ H(T || N)
289 // or UU = U ^ H(T || V)
290 err = hctr2_hash_tweak(req);
291 if (err)
292 return err;
293 err = hctr2_hash_message(req, rctx->bulk_part_src, digest);
294 if (err)
295 return err;
296 crypto_xor(digest, rctx->first_block, BLOCKCIPHER_BLOCK_SIZE);
297
298 // UU = E(MM)
299 // or MM = D(UU)
300 if (enc)
301 crypto_cipher_encrypt_one(tctx->blockcipher, rctx->first_block,
302 digest);
303 else
304 crypto_cipher_decrypt_one(tctx->blockcipher, rctx->first_block,
305 digest);
306
307 // S = MM ^ UU ^ L
308 crypto_xor(digest, rctx->first_block, BLOCKCIPHER_BLOCK_SIZE);
309 crypto_xor_cpy(rctx->xctr_iv, digest, tctx->L, BLOCKCIPHER_BLOCK_SIZE);
310
311 // V = XCTR(S, N)
312 // or N = XCTR(S, V)
313 skcipher_request_set_tfm(&rctx->u.xctr_req, tctx->xctr);
314 skcipher_request_set_crypt(&rctx->u.xctr_req, rctx->bulk_part_src,
315 rctx->bulk_part_dst, bulk_len,
316 rctx->xctr_iv);
317 skcipher_request_set_callback(&rctx->u.xctr_req,
318 req->base.flags,
319 hctr2_xctr_done, req);
320 return crypto_skcipher_encrypt(&rctx->u.xctr_req) ?:
321 hctr2_finish(req);
322 }
323
324 static int hctr2_encrypt(struct skcipher_request *req)
325 {
326 return hctr2_crypt(req, true);
327 }
328
329 static int hctr2_decrypt(struct skcipher_request *req)
330 {
331 return hctr2_crypt(req, false);
332 }
333
334 static int hctr2_init_tfm(struct crypto_skcipher *tfm)
335 {
336 struct skcipher_instance *inst = skcipher_alg_instance(tfm);
337 struct hctr2_instance_ctx *ictx = skcipher_instance_ctx(inst);
338 struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
339 struct crypto_skcipher *xctr;
340 struct crypto_cipher *blockcipher;
341 struct crypto_shash *polyval;
342 unsigned int subreq_size;
343 int err;
344
345 xctr = crypto_spawn_skcipher(&ictx->xctr_spawn);
346 if (IS_ERR(xctr))
347 return PTR_ERR(xctr);
348
349 blockcipher = crypto_spawn_cipher(&ictx->blockcipher_spawn);
350 if (IS_ERR(blockcipher)) {
351 err = PTR_ERR(blockcipher);
352 goto err_free_xctr;
353 }
354
355 polyval = crypto_spawn_shash(&ictx->polyval_spawn);
356 if (IS_ERR(polyval)) {
357 err = PTR_ERR(polyval);
358 goto err_free_blockcipher;
359 }
360
361 tctx->xctr = xctr;
362 tctx->blockcipher = blockcipher;
363 tctx->polyval = polyval;
364
365 BUILD_BUG_ON(offsetofend(struct hctr2_request_ctx, u) !=
366 sizeof(struct hctr2_request_ctx));
367 subreq_size = max(sizeof_field(struct hctr2_request_ctx, u.hash_desc) +
368 crypto_shash_descsize(polyval),
369 sizeof_field(struct hctr2_request_ctx, u.xctr_req) +
370 crypto_skcipher_reqsize(xctr));
371
372 tctx->hashed_tweak_offset = offsetof(struct hctr2_request_ctx, u) +
373 subreq_size;
374 crypto_skcipher_set_reqsize(tfm, tctx->hashed_tweak_offset +
375 crypto_shash_statesize(polyval));
376 return 0;
377
378 err_free_blockcipher:
379 crypto_free_cipher(blockcipher);
380 err_free_xctr:
381 crypto_free_skcipher(xctr);
382 return err;
383 }
384
385 static void hctr2_exit_tfm(struct crypto_skcipher *tfm)
386 {
387 struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
388
389 crypto_free_cipher(tctx->blockcipher);
390 crypto_free_skcipher(tctx->xctr);
391 crypto_free_shash(tctx->polyval);
392 }
393
394 static void hctr2_free_instance(struct skcipher_instance *inst)
395 {
396 struct hctr2_instance_ctx *ictx = skcipher_instance_ctx(inst);
397
398 crypto_drop_cipher(&ictx->blockcipher_spawn);
399 crypto_drop_skcipher(&ictx->xctr_spawn);
400 crypto_drop_shash(&ictx->polyval_spawn);
401 kfree(inst);
402 }
403
404 static int hctr2_create_common(struct crypto_template *tmpl,
405 struct rtattr **tb,
406 const char *xctr_name,
407 const char *polyval_name)
408 {
409 struct skcipher_alg_common *xctr_alg;
410 u32 mask;
411 struct skcipher_instance *inst;
412 struct hctr2_instance_ctx *ictx;
413 struct crypto_alg *blockcipher_alg;
414 struct shash_alg *polyval_alg;
415 char blockcipher_name[CRYPTO_MAX_ALG_NAME];
416 int len;
417 int err;
418
419 err = crypto_check_attr_type(tb, CRYPTO_ALG_TYPE_SKCIPHER, &mask);
420 if (err)
421 return err;
422
423 inst = kzalloc(sizeof(*inst) + sizeof(*ictx), GFP_KERNEL);
424 if (!inst)
425 return -ENOMEM;
426 ictx = skcipher_instance_ctx(inst);
427
428 /* Stream cipher, xctr(block_cipher) */
429 err = crypto_grab_skcipher(&ictx->xctr_spawn,
430 skcipher_crypto_instance(inst),
431 xctr_name, 0, mask);
432 if (err)
433 goto err_free_inst;
434 xctr_alg = crypto_spawn_skcipher_alg_common(&ictx->xctr_spawn);
435
436 err = -EINVAL;
437 if (strncmp(xctr_alg->base.cra_name, "xctr(", 5))
438 goto err_free_inst;
439 len = strscpy(blockcipher_name, xctr_alg->base.cra_name + 5,
440 sizeof(blockcipher_name));
441 if (len < 1)
442 goto err_free_inst;
443 if (blockcipher_name[len - 1] != ')')
444 goto err_free_inst;
445 blockcipher_name[len - 1] = 0;
446
447 /* Block cipher, e.g. "aes" */
448 err = crypto_grab_cipher(&ictx->blockcipher_spawn,
449 skcipher_crypto_instance(inst),
450 blockcipher_name, 0, mask);
451 if (err)
452 goto err_free_inst;
453 blockcipher_alg = crypto_spawn_cipher_alg(&ictx->blockcipher_spawn);
454
455 /* Require blocksize of 16 bytes */
456 err = -EINVAL;
457 if (blockcipher_alg->cra_blocksize != BLOCKCIPHER_BLOCK_SIZE)
458 goto err_free_inst;
459
460 /* Polyval ε-∆U hash function */
461 err = crypto_grab_shash(&ictx->polyval_spawn,
462 skcipher_crypto_instance(inst),
463 polyval_name, 0, mask);
464 if (err)
465 goto err_free_inst;
466 polyval_alg = crypto_spawn_shash_alg(&ictx->polyval_spawn);
467
468 /* Ensure Polyval is being used */
469 err = -EINVAL;
470 if (strcmp(polyval_alg->base.cra_name, "polyval") != 0)
471 goto err_free_inst;
472
473 /* Instance fields */
474
475 err = -ENAMETOOLONG;
476 if (snprintf(inst->alg.base.cra_name, CRYPTO_MAX_ALG_NAME, "hctr2(%s)",
477 blockcipher_alg->cra_name) >= CRYPTO_MAX_ALG_NAME)
478 goto err_free_inst;
479 if (snprintf(inst->alg.base.cra_driver_name, CRYPTO_MAX_ALG_NAME,
480 "hctr2_base(%s,%s)",
481 xctr_alg->base.cra_driver_name,
482 polyval_alg->base.cra_driver_name) >= CRYPTO_MAX_ALG_NAME)
483 goto err_free_inst;
484
485 inst->alg.base.cra_blocksize = BLOCKCIPHER_BLOCK_SIZE;
486 inst->alg.base.cra_ctxsize = sizeof(struct hctr2_tfm_ctx) +
487 polyval_alg->statesize * 2;
488 inst->alg.base.cra_alignmask = xctr_alg->base.cra_alignmask;
489 /*
490 * The hash function is called twice, so it is weighted higher than the
491 * xctr and blockcipher.
492 */
493 inst->alg.base.cra_priority = (2 * xctr_alg->base.cra_priority +
494 4 * polyval_alg->base.cra_priority +
495 blockcipher_alg->cra_priority) / 7;
496
497 inst->alg.setkey = hctr2_setkey;
498 inst->alg.encrypt = hctr2_encrypt;
499 inst->alg.decrypt = hctr2_decrypt;
500 inst->alg.init = hctr2_init_tfm;
501 inst->alg.exit = hctr2_exit_tfm;
502 inst->alg.min_keysize = xctr_alg->min_keysize;
503 inst->alg.max_keysize = xctr_alg->max_keysize;
504 inst->alg.ivsize = TWEAK_SIZE;
505
506 inst->free = hctr2_free_instance;
507
508 err = skcipher_register_instance(tmpl, inst);
509 if (err) {
510 err_free_inst:
511 hctr2_free_instance(inst);
512 }
513 return err;
514 }
515
516 static int hctr2_create_base(struct crypto_template *tmpl, struct rtattr **tb)
517 {
518 const char *xctr_name;
519 const char *polyval_name;
520
521 xctr_name = crypto_attr_alg_name(tb[1]);
522 if (IS_ERR(xctr_name))
523 return PTR_ERR(xctr_name);
524
525 polyval_name = crypto_attr_alg_name(tb[2]);
526 if (IS_ERR(polyval_name))
527 return PTR_ERR(polyval_name);
528
529 return hctr2_create_common(tmpl, tb, xctr_name, polyval_name);
530 }
531
532 static int hctr2_create(struct crypto_template *tmpl, struct rtattr **tb)
533 {
534 const char *blockcipher_name;
535 char xctr_name[CRYPTO_MAX_ALG_NAME];
536
537 blockcipher_name = crypto_attr_alg_name(tb[1]);
538 if (IS_ERR(blockcipher_name))
539 return PTR_ERR(blockcipher_name);
540
541 if (snprintf(xctr_name, CRYPTO_MAX_ALG_NAME, "xctr(%s)",
542 blockcipher_name) >= CRYPTO_MAX_ALG_NAME)
543 return -ENAMETOOLONG;
544
545 return hctr2_create_common(tmpl, tb, xctr_name, "polyval");
546 }
547
548 static struct crypto_template hctr2_tmpls[] = {
549 {
550 /* hctr2_base(xctr_name, polyval_name) */
551 .name = "hctr2_base",
552 .create = hctr2_create_base,
553 .module = THIS_MODULE,
554 }, {
555 /* hctr2(blockcipher_name) */
556 .name = "hctr2",
557 .create = hctr2_create,
558 .module = THIS_MODULE,
559 }
560 };
561
562 static int __init hctr2_module_init(void)
563 {
564 return crypto_register_templates(hctr2_tmpls, ARRAY_SIZE(hctr2_tmpls));
565 }
566
567 static void __exit hctr2_module_exit(void)
568 {
569 return crypto_unregister_templates(hctr2_tmpls,
570 ARRAY_SIZE(hctr2_tmpls));
571 }
572
573 subsys_initcall(hctr2_module_init);
574 module_exit(hctr2_module_exit);
575
576 MODULE_DESCRIPTION("HCTR2 length-preserving encryption mode");
577 MODULE_LICENSE("GPL v2");
578 MODULE_ALIAS_CRYPTO("hctr2");
579 MODULE_IMPORT_NS(CRYPTO_INTERNAL);
580
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