crypto/hctr2.c - linux/kernel/git/bpf/bpf-next - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * HCTR2 length-preserving encryption mode
  *
  * Copyright 2021 Google LLC
  */


 /*
  * HCTR2 is a length-preserving encryption mode that is efficient on
  * processors with instructions to accelerate AES and carryless
  * multiplication, e.g. x86 processors with AES-NI and CLMUL, and ARM
  * processors with the ARMv8 crypto extensions.
  *
  * For more details, see the paper: "Length-preserving encryption with HCTR2"
  * (https://eprint.iacr.org/2021/1441.pdf)
  */

 #include <crypto/internal/cipher.h>
 #include <crypto/internal/skcipher.h>
 #include <crypto/polyval.h>
 #include <crypto/scatterwalk.h>
 #include <linux/module.h>

 #define BLOCKCIPHER_BLOCK_SIZE		16

 /*
  * The specification allows variable-length tweaks, but Linux's crypto API
  * currently only allows algorithms to support a single length.  The "natural"
  * tweak length for HCTR2 is 16, since that fits into one POLYVAL block for
  * the best performance.  But longer tweaks are useful for fscrypt, to avoid
  * needing to derive per-file keys.  So instead we use two blocks, or 32 bytes.
  */
 #define TWEAK_SIZE		32

 struct hctr2_instance_ctx {
 	struct crypto_cipher_spawn blockcipher_spawn;
 	struct crypto_skcipher_spawn xctr_spawn;
 };

 struct hctr2_tfm_ctx {
 	struct crypto_cipher *blockcipher;
 	struct crypto_skcipher *xctr;
 	struct polyval_key poly_key;
 	struct polyval_elem hashed_tweaklens[2];
 	u8 L[BLOCKCIPHER_BLOCK_SIZE];
 };

 struct hctr2_request_ctx {
 	u8 first_block[BLOCKCIPHER_BLOCK_SIZE];
 	u8 xctr_iv[BLOCKCIPHER_BLOCK_SIZE];
 	struct scatterlist *bulk_part_dst;
 	struct scatterlist *bulk_part_src;
 	struct scatterlist sg_src[2];
 	struct scatterlist sg_dst[2];
 	struct polyval_elem hashed_tweak;
 	/*
 	 * skcipher sub-request size is unknown at compile-time, so it needs to
 	 * go after the members with known sizes.
 	 */
 	union {
 		struct polyval_ctx poly_ctx;
 		struct skcipher_request xctr_req;
 	} u;
 };

 /*
  * The input data for each HCTR2 hash step begins with a 16-byte block that
  * contains the tweak length and a flag that indicates whether the input is evenly
  * divisible into blocks.  Since this implementation only supports one tweak
  * length, we precompute the two hash states resulting from hashing the two
  * possible values of this initial block.  This reduces by one block the amount of
  * data that needs to be hashed for each encryption/decryption
  *
  * These precomputed hashes are stored in hctr2_tfm_ctx.
  */
 static void hctr2_hash_tweaklens(struct hctr2_tfm_ctx *tctx)
 {
 	struct polyval_ctx ctx;

 	for (int has_remainder = 0; has_remainder < 2; has_remainder++) {
 		const __le64 tweak_length_block[2] = {
 			cpu_to_le64(TWEAK_SIZE * 8 * 2 + 2 + has_remainder),
 		};

 		polyval_init(&ctx, &tctx->poly_key);
 		polyval_update(&ctx, (const u8 *)&tweak_length_block,
 			       sizeof(tweak_length_block));
 		static_assert(sizeof(tweak_length_block) == POLYVAL_BLOCK_SIZE);
 		polyval_export_blkaligned(
 			&ctx, &tctx->hashed_tweaklens[has_remainder]);
 	}
 	memzero_explicit(&ctx, sizeof(ctx));
 }

 static int hctr2_setkey(struct crypto_skcipher *tfm, const u8 *key,
 			unsigned int keylen)
 {
 	struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
 	u8 hbar[BLOCKCIPHER_BLOCK_SIZE];
 	int err;

 	crypto_cipher_clear_flags(tctx->blockcipher, CRYPTO_TFM_REQ_MASK);
 	crypto_cipher_set_flags(tctx->blockcipher,
 				crypto_skcipher_get_flags(tfm) &
 				CRYPTO_TFM_REQ_MASK);
 	err = crypto_cipher_setkey(tctx->blockcipher, key, keylen);
 	if (err)
 		return err;

 	crypto_skcipher_clear_flags(tctx->xctr, CRYPTO_TFM_REQ_MASK);
 	crypto_skcipher_set_flags(tctx->xctr,
 				  crypto_skcipher_get_flags(tfm) &
 				  CRYPTO_TFM_REQ_MASK);
 	err = crypto_skcipher_setkey(tctx->xctr, key, keylen);
 	if (err)
 		return err;

 	memset(hbar, 0, sizeof(hbar));
 	crypto_cipher_encrypt_one(tctx->blockcipher, hbar, hbar);

 	memset(tctx->L, 0, sizeof(tctx->L));
 	tctx->L[0] = 0x01;
 	crypto_cipher_encrypt_one(tctx->blockcipher, tctx->L, tctx->L);

 	static_assert(sizeof(hbar) == POLYVAL_BLOCK_SIZE);
 	polyval_preparekey(&tctx->poly_key, hbar);
 	memzero_explicit(hbar, sizeof(hbar));

 	hctr2_hash_tweaklens(tctx);
 	return 0;
 }

 static void hctr2_hash_tweak(struct skcipher_request *req)
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
 	const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
 	struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
 	struct polyval_ctx *poly_ctx = &rctx->u.poly_ctx;
 	bool has_remainder = req->cryptlen % POLYVAL_BLOCK_SIZE;

 	polyval_import_blkaligned(poly_ctx, &tctx->poly_key,
 				  &tctx->hashed_tweaklens[has_remainder]);
 	polyval_update(poly_ctx, req->iv, TWEAK_SIZE);

 	// Store the hashed tweak, since we need it when computing both
 	// H(T || N) and H(T || V).
 	static_assert(TWEAK_SIZE % POLYVAL_BLOCK_SIZE == 0);
 	polyval_export_blkaligned(poly_ctx, &rctx->hashed_tweak);
 }

 static void hctr2_hash_message(struct skcipher_request *req,
 			       struct scatterlist *sgl,
 			       u8 digest[POLYVAL_DIGEST_SIZE])
 {
 	static const u8 padding = 0x1;
 	struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
 	struct polyval_ctx *poly_ctx = &rctx->u.poly_ctx;
 	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
 	struct sg_mapping_iter miter;
 	int i;
 	int n = 0;

 	sg_miter_start(&miter, sgl, sg_nents(sgl),
 		       SG_MITER_FROM_SG | SG_MITER_ATOMIC);
 	for (i = 0; i < bulk_len; i += n) {
 		sg_miter_next(&miter);
 		n = min_t(unsigned int, miter.length, bulk_len - i);
 		polyval_update(poly_ctx, miter.addr, n);
 	}
 	sg_miter_stop(&miter);

 	if (req->cryptlen % BLOCKCIPHER_BLOCK_SIZE)
 		polyval_update(poly_ctx, &padding, 1);
 	polyval_final(poly_ctx, digest);
 }

 static int hctr2_finish(struct skcipher_request *req)
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
 	const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
 	struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
 	struct polyval_ctx *poly_ctx = &rctx->u.poly_ctx;
 	u8 digest[POLYVAL_DIGEST_SIZE];

 	// U = UU ^ H(T || V)
 	// or M = MM ^ H(T || N)
 	polyval_import_blkaligned(poly_ctx, &tctx->poly_key,
 				  &rctx->hashed_tweak);
 	hctr2_hash_message(req, rctx->bulk_part_dst, digest);
 	crypto_xor(rctx->first_block, digest, BLOCKCIPHER_BLOCK_SIZE);

 	// Copy U (or M) into dst scatterlist
 	scatterwalk_map_and_copy(rctx->first_block, req->dst,
 				 0, BLOCKCIPHER_BLOCK_SIZE, 1);
 	return 0;
 }

 static void hctr2_xctr_done(void *data, int err)
 {
 	struct skcipher_request *req = data;

 	if (!err)
 		err = hctr2_finish(req);

 	skcipher_request_complete(req, err);
 }

 static int hctr2_crypt(struct skcipher_request *req, bool enc)
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
 	const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
 	struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
 	u8 digest[POLYVAL_DIGEST_SIZE];
 	int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;

 	// Requests must be at least one block
 	if (req->cryptlen < BLOCKCIPHER_BLOCK_SIZE)
 		return -EINVAL;

 	// Copy M (or U) into a temporary buffer
 	scatterwalk_map_and_copy(rctx->first_block, req->src,
 				 0, BLOCKCIPHER_BLOCK_SIZE, 0);

 	// Create scatterlists for N and V
 	rctx->bulk_part_src = scatterwalk_ffwd(rctx->sg_src, req->src,
 					       BLOCKCIPHER_BLOCK_SIZE);
 	rctx->bulk_part_dst = scatterwalk_ffwd(rctx->sg_dst, req->dst,
 					       BLOCKCIPHER_BLOCK_SIZE);

 	// MM = M ^ H(T || N)
 	// or UU = U ^ H(T || V)
 	hctr2_hash_tweak(req);
 	hctr2_hash_message(req, rctx->bulk_part_src, digest);
 	crypto_xor(digest, rctx->first_block, BLOCKCIPHER_BLOCK_SIZE);

 	// UU = E(MM)
 	// or MM = D(UU)
 	if (enc)
 		crypto_cipher_encrypt_one(tctx->blockcipher, rctx->first_block,
 					  digest);
 	else
 		crypto_cipher_decrypt_one(tctx->blockcipher, rctx->first_block,
 					  digest);

 	// S = MM ^ UU ^ L
 	crypto_xor(digest, rctx->first_block, BLOCKCIPHER_BLOCK_SIZE);
 	crypto_xor_cpy(rctx->xctr_iv, digest, tctx->L, BLOCKCIPHER_BLOCK_SIZE);

 	// V = XCTR(S, N)
 	// or N = XCTR(S, V)
 	skcipher_request_set_tfm(&rctx->u.xctr_req, tctx->xctr);
 	skcipher_request_set_crypt(&rctx->u.xctr_req, rctx->bulk_part_src,
 				   rctx->bulk_part_dst, bulk_len,
 				   rctx->xctr_iv);
 	skcipher_request_set_callback(&rctx->u.xctr_req,
 				      req->base.flags,
 				      hctr2_xctr_done, req);
 	return crypto_skcipher_encrypt(&rctx->u.xctr_req) ?:
 		hctr2_finish(req);
 }

 static int hctr2_encrypt(struct skcipher_request *req)
 {
 	return hctr2_crypt(req, true);
 }

 static int hctr2_decrypt(struct skcipher_request *req)
 {
 	return hctr2_crypt(req, false);
 }

 static int hctr2_init_tfm(struct crypto_skcipher *tfm)
 {
 	struct skcipher_instance *inst = skcipher_alg_instance(tfm);
 	struct hctr2_instance_ctx *ictx = skcipher_instance_ctx(inst);
 	struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
 	struct crypto_skcipher *xctr;
 	struct crypto_cipher *blockcipher;
 	int err;

 	xctr = crypto_spawn_skcipher(&ictx->xctr_spawn);
 	if (IS_ERR(xctr))
 		return PTR_ERR(xctr);

 	blockcipher = crypto_spawn_cipher(&ictx->blockcipher_spawn);
 	if (IS_ERR(blockcipher)) {
 		err = PTR_ERR(blockcipher);
 		goto err_free_xctr;
 	}

 	tctx->xctr = xctr;
 	tctx->blockcipher = blockcipher;

 	BUILD_BUG_ON(offsetofend(struct hctr2_request_ctx, u) !=
 				 sizeof(struct hctr2_request_ctx));
 	crypto_skcipher_set_reqsize(
 		tfm, max(sizeof(struct hctr2_request_ctx),
 			 offsetofend(struct hctr2_request_ctx, u.xctr_req) +
 				 crypto_skcipher_reqsize(xctr)));
 	return 0;

 err_free_xctr:
 	crypto_free_skcipher(xctr);
 	return err;
 }

 static void hctr2_exit_tfm(struct crypto_skcipher *tfm)
 {
 	struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);

 	crypto_free_cipher(tctx->blockcipher);
 	crypto_free_skcipher(tctx->xctr);
 }

 static void hctr2_free_instance(struct skcipher_instance *inst)
 {
 	struct hctr2_instance_ctx *ictx = skcipher_instance_ctx(inst);

 	crypto_drop_cipher(&ictx->blockcipher_spawn);
 	crypto_drop_skcipher(&ictx->xctr_spawn);
 	kfree(inst);
 }

 static int hctr2_create_common(struct crypto_template *tmpl, struct rtattr **tb,
 			       const char *xctr_name)
 {
 	struct skcipher_alg_common *xctr_alg;
 	u32 mask;
 	struct skcipher_instance *inst;
 	struct hctr2_instance_ctx *ictx;
 	struct crypto_alg *blockcipher_alg;
 	char blockcipher_name[CRYPTO_MAX_ALG_NAME];
 	int len;
 	int err;

 	err = crypto_check_attr_type(tb, CRYPTO_ALG_TYPE_SKCIPHER, &mask);
 	if (err)
 		return err;

 	inst = kzalloc(sizeof(*inst) + sizeof(*ictx), GFP_KERNEL);
 	if (!inst)
 		return -ENOMEM;
 	ictx = skcipher_instance_ctx(inst);

 	/* Stream cipher, xctr(block_cipher) */
 	err = crypto_grab_skcipher(&ictx->xctr_spawn,
 				   skcipher_crypto_instance(inst),
 				   xctr_name, 0, mask);
 	if (err)
 		goto err_free_inst;
 	xctr_alg = crypto_spawn_skcipher_alg_common(&ictx->xctr_spawn);

 	err = -EINVAL;
 	if (strncmp(xctr_alg->base.cra_name, "xctr(", 5))
 		goto err_free_inst;
 	len = strscpy(blockcipher_name, xctr_alg->base.cra_name + 5,
 		      sizeof(blockcipher_name));
 	if (len < 1)
 		goto err_free_inst;
 	if (blockcipher_name[len - 1] != ')')
 		goto err_free_inst;
 	blockcipher_name[len - 1] = 0;

 	/* Block cipher, e.g. "aes" */
 	err = crypto_grab_cipher(&ictx->blockcipher_spawn,
 				 skcipher_crypto_instance(inst),
 				 blockcipher_name, 0, mask);
 	if (err)
 		goto err_free_inst;
 	blockcipher_alg = crypto_spawn_cipher_alg(&ictx->blockcipher_spawn);

 	/* Require blocksize of 16 bytes */
 	err = -EINVAL;
 	if (blockcipher_alg->cra_blocksize != BLOCKCIPHER_BLOCK_SIZE)
 		goto err_free_inst;

 	/* Instance fields */

 	err = -ENAMETOOLONG;
 	if (snprintf(inst->alg.base.cra_name, CRYPTO_MAX_ALG_NAME, "hctr2(%s)",
 		     blockcipher_alg->cra_name) >= CRYPTO_MAX_ALG_NAME)
 		goto err_free_inst;
 	if (snprintf(inst->alg.base.cra_driver_name, CRYPTO_MAX_ALG_NAME,
 		     "hctr2_base(%s,polyval-lib)",
 		     xctr_alg->base.cra_driver_name) >= CRYPTO_MAX_ALG_NAME)
 		goto err_free_inst;

 	inst->alg.base.cra_blocksize = BLOCKCIPHER_BLOCK_SIZE;
 	inst->alg.base.cra_ctxsize = sizeof(struct hctr2_tfm_ctx);
 	inst->alg.base.cra_alignmask = xctr_alg->base.cra_alignmask;
 	inst->alg.base.cra_priority = (2 * xctr_alg->base.cra_priority +
 				       blockcipher_alg->cra_priority) /
 				      3;

 	inst->alg.setkey = hctr2_setkey;
 	inst->alg.encrypt = hctr2_encrypt;
 	inst->alg.decrypt = hctr2_decrypt;
 	inst->alg.init = hctr2_init_tfm;
 	inst->alg.exit = hctr2_exit_tfm;
 	inst->alg.min_keysize = xctr_alg->min_keysize;
 	inst->alg.max_keysize = xctr_alg->max_keysize;
 	inst->alg.ivsize = TWEAK_SIZE;

 	inst->free = hctr2_free_instance;

 	err = skcipher_register_instance(tmpl, inst);
 	if (err) {
 err_free_inst:
 		hctr2_free_instance(inst);
 	}
 	return err;
 }

 static int hctr2_create_base(struct crypto_template *tmpl, struct rtattr **tb)
 {
 	const char *xctr_name;
 	const char *polyval_name;

 	xctr_name = crypto_attr_alg_name(tb[1]);
 	if (IS_ERR(xctr_name))
 		return PTR_ERR(xctr_name);

 	polyval_name = crypto_attr_alg_name(tb[2]);
 	if (IS_ERR(polyval_name))
 		return PTR_ERR(polyval_name);
 	if (strcmp(polyval_name, "polyval") != 0 &&
 	    strcmp(polyval_name, "polyval-lib") != 0)
 		return -ENOENT;

 	return hctr2_create_common(tmpl, tb, xctr_name);
 }

 static int hctr2_create(struct crypto_template *tmpl, struct rtattr **tb)
 {
 	const char *blockcipher_name;
 	char xctr_name[CRYPTO_MAX_ALG_NAME];

 	blockcipher_name = crypto_attr_alg_name(tb[1]);
 	if (IS_ERR(blockcipher_name))
 		return PTR_ERR(blockcipher_name);

 	if (snprintf(xctr_name, CRYPTO_MAX_ALG_NAME, "xctr(%s)",
 		    blockcipher_name) >= CRYPTO_MAX_ALG_NAME)
 		return -ENAMETOOLONG;

 	return hctr2_create_common(tmpl, tb, xctr_name);
 }

 static struct crypto_template hctr2_tmpls[] = {
 	{
 		/* hctr2_base(xctr_name, polyval_name) */
 		.name = "hctr2_base",
 		.create = hctr2_create_base,
 		.module = THIS_MODULE,
 	}, {
 		/* hctr2(blockcipher_name) */
 		.name = "hctr2",
 		.create = hctr2_create,
 		.module = THIS_MODULE,
 	}
 };

 static int __init hctr2_module_init(void)
 {
 	return crypto_register_templates(hctr2_tmpls, ARRAY_SIZE(hctr2_tmpls));
 }

 static void __exit hctr2_module_exit(void)
 {
 	return crypto_unregister_templates(hctr2_tmpls,
 					   ARRAY_SIZE(hctr2_tmpls));
 }

 module_init(hctr2_module_init);
 module_exit(hctr2_module_exit);

 MODULE_DESCRIPTION("HCTR2 length-preserving encryption mode");
 MODULE_LICENSE("GPL v2");
 MODULE_ALIAS_CRYPTO("hctr2");
 MODULE_IMPORT_NS("CRYPTO_INTERNAL");
	// SPDX-License-Identifier: GPL-2.0
	/*
	* HCTR2 length-preserving encryption mode
	*
	* Copyright 2021 Google LLC
	*/


	/*
	* HCTR2 is a length-preserving encryption mode that is efficient on
	* processors with instructions to accelerate AES and carryless
	* multiplication, e.g. x86 processors with AES-NI and CLMUL, and ARM
	* processors with the ARMv8 crypto extensions.
	*
	* For more details, see the paper: "Length-preserving encryption with HCTR2"
	* (https://eprint.iacr.org/2021/1441.pdf)
	*/

	#include <crypto/internal/cipher.h>
	#include <crypto/internal/skcipher.h>
	#include <crypto/polyval.h>
	#include <crypto/scatterwalk.h>
	#include <linux/module.h>

	#define BLOCKCIPHER_BLOCK_SIZE 16

	/*
	* The specification allows variable-length tweaks, but Linux's crypto API
	* currently only allows algorithms to support a single length. The "natural"
	* tweak length for HCTR2 is 16, since that fits into one POLYVAL block for
	* the best performance. But longer tweaks are useful for fscrypt, to avoid
	* needing to derive per-file keys. So instead we use two blocks, or 32 bytes.
	*/
	#define TWEAK_SIZE 32

	struct hctr2_instance_ctx {
	struct crypto_cipher_spawn blockcipher_spawn;
	struct crypto_skcipher_spawn xctr_spawn;
	};

	struct hctr2_tfm_ctx {
	struct crypto_cipher *blockcipher;
	struct crypto_skcipher *xctr;
	struct polyval_key poly_key;
	struct polyval_elem hashed_tweaklens[2];
	u8 L[BLOCKCIPHER_BLOCK_SIZE];
	};

	struct hctr2_request_ctx {
	u8 first_block[BLOCKCIPHER_BLOCK_SIZE];
	u8 xctr_iv[BLOCKCIPHER_BLOCK_SIZE];
	struct scatterlist *bulk_part_dst;
	struct scatterlist *bulk_part_src;
	struct scatterlist sg_src[2];
	struct scatterlist sg_dst[2];
	struct polyval_elem hashed_tweak;
	/*
	* skcipher sub-request size is unknown at compile-time, so it needs to
	* go after the members with known sizes.
	*/
	union {
	struct polyval_ctx poly_ctx;
	struct skcipher_request xctr_req;
	} u;
	};

	/*
	* The input data for each HCTR2 hash step begins with a 16-byte block that
	* contains the tweak length and a flag that indicates whether the input is evenly
	* divisible into blocks. Since this implementation only supports one tweak
	* length, we precompute the two hash states resulting from hashing the two
	* possible values of this initial block. This reduces by one block the amount of
	* data that needs to be hashed for each encryption/decryption
	*
	* These precomputed hashes are stored in hctr2_tfm_ctx.
	*/
	static void hctr2_hash_tweaklens(struct hctr2_tfm_ctx *tctx)
	{
	struct polyval_ctx ctx;

	for (int has_remainder = 0; has_remainder < 2; has_remainder++) {
	const __le64 tweak_length_block[2] = {
	cpu_to_le64(TWEAK_SIZE * 8 * 2 + 2 + has_remainder),
	};

	polyval_init(&ctx, &tctx->poly_key);
	polyval_update(&ctx, (const u8 *)&tweak_length_block,
	sizeof(tweak_length_block));
	static_assert(sizeof(tweak_length_block) == POLYVAL_BLOCK_SIZE);
	polyval_export_blkaligned(
	&ctx, &tctx->hashed_tweaklens[has_remainder]);
	}
	memzero_explicit(&ctx, sizeof(ctx));
	}

	static int hctr2_setkey(struct crypto_skcipher tfm, const u8 key,
	unsigned int keylen)
	{
	struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
	u8 hbar[BLOCKCIPHER_BLOCK_SIZE];
	int err;

	crypto_cipher_clear_flags(tctx->blockcipher, CRYPTO_TFM_REQ_MASK);
	crypto_cipher_set_flags(tctx->blockcipher,
	crypto_skcipher_get_flags(tfm) &
	CRYPTO_TFM_REQ_MASK);
	err = crypto_cipher_setkey(tctx->blockcipher, key, keylen);
	if (err)
	return err;

	crypto_skcipher_clear_flags(tctx->xctr, CRYPTO_TFM_REQ_MASK);
	crypto_skcipher_set_flags(tctx->xctr,
	crypto_skcipher_get_flags(tfm) &
	CRYPTO_TFM_REQ_MASK);
	err = crypto_skcipher_setkey(tctx->xctr, key, keylen);
	if (err)
	return err;

	memset(hbar, 0, sizeof(hbar));
	crypto_cipher_encrypt_one(tctx->blockcipher, hbar, hbar);

	memset(tctx->L, 0, sizeof(tctx->L));
	tctx->L[0] = 0x01;
	crypto_cipher_encrypt_one(tctx->blockcipher, tctx->L, tctx->L);

	static_assert(sizeof(hbar) == POLYVAL_BLOCK_SIZE);
	polyval_preparekey(&tctx->poly_key, hbar);
	memzero_explicit(hbar, sizeof(hbar));

	hctr2_hash_tweaklens(tctx);
	return 0;
	}

	static void hctr2_hash_tweak(struct skcipher_request *req)
	{
	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
	const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
	struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
	struct polyval_ctx *poly_ctx = &rctx->u.poly_ctx;
	bool has_remainder = req->cryptlen % POLYVAL_BLOCK_SIZE;

	polyval_import_blkaligned(poly_ctx, &tctx->poly_key,
	&tctx->hashed_tweaklens[has_remainder]);
	polyval_update(poly_ctx, req->iv, TWEAK_SIZE);

	// Store the hashed tweak, since we need it when computing both
	// H(T \|\| N) and H(T \|\| V).
	static_assert(TWEAK_SIZE % POLYVAL_BLOCK_SIZE == 0);
	polyval_export_blkaligned(poly_ctx, &rctx->hashed_tweak);
	}

	static void hctr2_hash_message(struct skcipher_request *req,
	struct scatterlist *sgl,
	u8 digest[POLYVAL_DIGEST_SIZE])
	{
	static const u8 padding = 0x1;
	struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
	struct polyval_ctx *poly_ctx = &rctx->u.poly_ctx;
	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
	struct sg_mapping_iter miter;
	int i;
	int n = 0;

	sg_miter_start(&miter, sgl, sg_nents(sgl),
	SG_MITER_FROM_SG \| SG_MITER_ATOMIC);
	for (i = 0; i < bulk_len; i += n) {
	sg_miter_next(&miter);
	n = min_t(unsigned int, miter.length, bulk_len - i);
	polyval_update(poly_ctx, miter.addr, n);
	}
	sg_miter_stop(&miter);

	if (req->cryptlen % BLOCKCIPHER_BLOCK_SIZE)
	polyval_update(poly_ctx, &padding, 1);
	polyval_final(poly_ctx, digest);
	}

	static int hctr2_finish(struct skcipher_request *req)
	{
	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
	const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
	struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
	struct polyval_ctx *poly_ctx = &rctx->u.poly_ctx;
	u8 digest[POLYVAL_DIGEST_SIZE];

	// U = UU ^ H(T \|\| V)
	// or M = MM ^ H(T \|\| N)
	polyval_import_blkaligned(poly_ctx, &tctx->poly_key,
	&rctx->hashed_tweak);
	hctr2_hash_message(req, rctx->bulk_part_dst, digest);
	crypto_xor(rctx->first_block, digest, BLOCKCIPHER_BLOCK_SIZE);

	// Copy U (or M) into dst scatterlist
	scatterwalk_map_and_copy(rctx->first_block, req->dst,
	0, BLOCKCIPHER_BLOCK_SIZE, 1);
	return 0;
	}

	static void hctr2_xctr_done(void *data, int err)
	{
	struct skcipher_request *req = data;

	if (!err)
	err = hctr2_finish(req);

	skcipher_request_complete(req, err);
	}

	static int hctr2_crypt(struct skcipher_request *req, bool enc)
	{
	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
	const struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
	struct hctr2_request_ctx *rctx = skcipher_request_ctx(req);
	u8 digest[POLYVAL_DIGEST_SIZE];
	int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;

	// Requests must be at least one block
	if (req->cryptlen < BLOCKCIPHER_BLOCK_SIZE)
	return -EINVAL;

	// Copy M (or U) into a temporary buffer
	scatterwalk_map_and_copy(rctx->first_block, req->src,
	0, BLOCKCIPHER_BLOCK_SIZE, 0);

	// Create scatterlists for N and V
	rctx->bulk_part_src = scatterwalk_ffwd(rctx->sg_src, req->src,
	BLOCKCIPHER_BLOCK_SIZE);
	rctx->bulk_part_dst = scatterwalk_ffwd(rctx->sg_dst, req->dst,
	BLOCKCIPHER_BLOCK_SIZE);

	// MM = M ^ H(T \|\| N)
	// or UU = U ^ H(T \|\| V)
	hctr2_hash_tweak(req);
	hctr2_hash_message(req, rctx->bulk_part_src, digest);
	crypto_xor(digest, rctx->first_block, BLOCKCIPHER_BLOCK_SIZE);

	// UU = E(MM)
	// or MM = D(UU)
	if (enc)
	crypto_cipher_encrypt_one(tctx->blockcipher, rctx->first_block,
	digest);
	else
	crypto_cipher_decrypt_one(tctx->blockcipher, rctx->first_block,
	digest);

	// S = MM ^ UU ^ L
	crypto_xor(digest, rctx->first_block, BLOCKCIPHER_BLOCK_SIZE);
	crypto_xor_cpy(rctx->xctr_iv, digest, tctx->L, BLOCKCIPHER_BLOCK_SIZE);

	// V = XCTR(S, N)
	// or N = XCTR(S, V)
	skcipher_request_set_tfm(&rctx->u.xctr_req, tctx->xctr);
	skcipher_request_set_crypt(&rctx->u.xctr_req, rctx->bulk_part_src,
	rctx->bulk_part_dst, bulk_len,
	rctx->xctr_iv);
	skcipher_request_set_callback(&rctx->u.xctr_req,
	req->base.flags,
	hctr2_xctr_done, req);
	return crypto_skcipher_encrypt(&rctx->u.xctr_req) ?:
	hctr2_finish(req);
	}

	static int hctr2_encrypt(struct skcipher_request *req)
	{
	return hctr2_crypt(req, true);
	}

	static int hctr2_decrypt(struct skcipher_request *req)
	{
	return hctr2_crypt(req, false);
	}

	static int hctr2_init_tfm(struct crypto_skcipher *tfm)
	{
	struct skcipher_instance *inst = skcipher_alg_instance(tfm);
	struct hctr2_instance_ctx *ictx = skcipher_instance_ctx(inst);
	struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
	struct crypto_skcipher *xctr;
	struct crypto_cipher *blockcipher;
	int err;

	xctr = crypto_spawn_skcipher(&ictx->xctr_spawn);
	if (IS_ERR(xctr))
	return PTR_ERR(xctr);

	blockcipher = crypto_spawn_cipher(&ictx->blockcipher_spawn);
	if (IS_ERR(blockcipher)) {
	err = PTR_ERR(blockcipher);
	goto err_free_xctr;
	}

	tctx->xctr = xctr;
	tctx->blockcipher = blockcipher;

	BUILD_BUG_ON(offsetofend(struct hctr2_request_ctx, u) !=
	sizeof(struct hctr2_request_ctx));
	crypto_skcipher_set_reqsize(
	tfm, max(sizeof(struct hctr2_request_ctx),
	offsetofend(struct hctr2_request_ctx, u.xctr_req) +
	crypto_skcipher_reqsize(xctr)));
	return 0;

	err_free_xctr:
	crypto_free_skcipher(xctr);
	return err;
	}

	static void hctr2_exit_tfm(struct crypto_skcipher *tfm)
	{
	struct hctr2_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);

	crypto_free_cipher(tctx->blockcipher);
	crypto_free_skcipher(tctx->xctr);
	}

	static void hctr2_free_instance(struct skcipher_instance *inst)
	{
	struct hctr2_instance_ctx *ictx = skcipher_instance_ctx(inst);

	crypto_drop_cipher(&ictx->blockcipher_spawn);
	crypto_drop_skcipher(&ictx->xctr_spawn);
	kfree(inst);
	}

	static int hctr2_create_common(struct crypto_template tmpl, struct rtattr *tb,
	const char *xctr_name)
	{
	struct skcipher_alg_common *xctr_alg;
	u32 mask;
	struct skcipher_instance *inst;
	struct hctr2_instance_ctx *ictx;
	struct crypto_alg *blockcipher_alg;
	char blockcipher_name[CRYPTO_MAX_ALG_NAME];
	int len;
	int err;

	err = crypto_check_attr_type(tb, CRYPTO_ALG_TYPE_SKCIPHER, &mask);
	if (err)
	return err;

	inst = kzalloc(sizeof(inst) + sizeof(ictx), GFP_KERNEL);
	if (!inst)
	return -ENOMEM;
	ictx = skcipher_instance_ctx(inst);

	/* Stream cipher, xctr(block_cipher) */
	err = crypto_grab_skcipher(&ictx->xctr_spawn,
	skcipher_crypto_instance(inst),
	xctr_name, 0, mask);
	if (err)
	goto err_free_inst;
	xctr_alg = crypto_spawn_skcipher_alg_common(&ictx->xctr_spawn);

	err = -EINVAL;
	if (strncmp(xctr_alg->base.cra_name, "xctr(", 5))
	goto err_free_inst;
	len = strscpy(blockcipher_name, xctr_alg->base.cra_name + 5,
	sizeof(blockcipher_name));
	if (len < 1)
	goto err_free_inst;
	if (blockcipher_name[len - 1] != ')')
	goto err_free_inst;
	blockcipher_name[len - 1] = 0;

	/* Block cipher, e.g. "aes" */
	err = crypto_grab_cipher(&ictx->blockcipher_spawn,
	skcipher_crypto_instance(inst),
	blockcipher_name, 0, mask);
	if (err)
	goto err_free_inst;
	blockcipher_alg = crypto_spawn_cipher_alg(&ictx->blockcipher_spawn);

	/* Require blocksize of 16 bytes */
	err = -EINVAL;
	if (blockcipher_alg->cra_blocksize != BLOCKCIPHER_BLOCK_SIZE)
	goto err_free_inst;

	/* Instance fields */

	err = -ENAMETOOLONG;
	if (snprintf(inst->alg.base.cra_name, CRYPTO_MAX_ALG_NAME, "hctr2(%s)",
	blockcipher_alg->cra_name) >= CRYPTO_MAX_ALG_NAME)
	goto err_free_inst;
	if (snprintf(inst->alg.base.cra_driver_name, CRYPTO_MAX_ALG_NAME,
	"hctr2_base(%s,polyval-lib)",
	xctr_alg->base.cra_driver_name) >= CRYPTO_MAX_ALG_NAME)
	goto err_free_inst;

	inst->alg.base.cra_blocksize = BLOCKCIPHER_BLOCK_SIZE;
	inst->alg.base.cra_ctxsize = sizeof(struct hctr2_tfm_ctx);
	inst->alg.base.cra_alignmask = xctr_alg->base.cra_alignmask;
	inst->alg.base.cra_priority = (2 * xctr_alg->base.cra_priority +
	blockcipher_alg->cra_priority) /
	3;

	inst->alg.setkey = hctr2_setkey;
	inst->alg.encrypt = hctr2_encrypt;
	inst->alg.decrypt = hctr2_decrypt;
	inst->alg.init = hctr2_init_tfm;
	inst->alg.exit = hctr2_exit_tfm;
	inst->alg.min_keysize = xctr_alg->min_keysize;
	inst->alg.max_keysize = xctr_alg->max_keysize;
	inst->alg.ivsize = TWEAK_SIZE;

	inst->free = hctr2_free_instance;

	err = skcipher_register_instance(tmpl, inst);
	if (err) {
	err_free_inst:
	hctr2_free_instance(inst);
	}
	return err;
	}

	static int hctr2_create_base(struct crypto_template tmpl, struct rtattr *tb)
	{
	const char *xctr_name;
	const char *polyval_name;

	xctr_name = crypto_attr_alg_name(tb[1]);
	if (IS_ERR(xctr_name))
	return PTR_ERR(xctr_name);

	polyval_name = crypto_attr_alg_name(tb[2]);
	if (IS_ERR(polyval_name))
	return PTR_ERR(polyval_name);
	if (strcmp(polyval_name, "polyval") != 0 &&
	strcmp(polyval_name, "polyval-lib") != 0)
	return -ENOENT;

	return hctr2_create_common(tmpl, tb, xctr_name);
	}

	static int hctr2_create(struct crypto_template tmpl, struct rtattr *tb)
	{
	const char *blockcipher_name;
	char xctr_name[CRYPTO_MAX_ALG_NAME];

	blockcipher_name = crypto_attr_alg_name(tb[1]);
	if (IS_ERR(blockcipher_name))
	return PTR_ERR(blockcipher_name);

	if (snprintf(xctr_name, CRYPTO_MAX_ALG_NAME, "xctr(%s)",
	blockcipher_name) >= CRYPTO_MAX_ALG_NAME)
	return -ENAMETOOLONG;

	return hctr2_create_common(tmpl, tb, xctr_name);
	}

	static struct crypto_template hctr2_tmpls[] = {
	{
	/* hctr2_base(xctr_name, polyval_name) */
	.name = "hctr2_base",
	.create = hctr2_create_base,
	.module = THIS_MODULE,
	}, {
	/* hctr2(blockcipher_name) */
	.name = "hctr2",
	.create = hctr2_create,
	.module = THIS_MODULE,
	}
	};

	static int __init hctr2_module_init(void)
	{
	return crypto_register_templates(hctr2_tmpls, ARRAY_SIZE(hctr2_tmpls));
	}

	static void __exit hctr2_module_exit(void)
	{
	return crypto_unregister_templates(hctr2_tmpls,
	ARRAY_SIZE(hctr2_tmpls));
	}

	module_init(hctr2_module_init);
	module_exit(hctr2_module_exit);

	MODULE_DESCRIPTION("HCTR2 length-preserving encryption mode");
	MODULE_LICENSE("GPL v2");
	MODULE_ALIAS_CRYPTO("hctr2");
	MODULE_IMPORT_NS("CRYPTO_INTERNAL");