crypto: xctr - Add XCTR support

Add a generic implementation of XCTR mode as a template.  XCTR is a
blockcipher mode similar to CTR mode.  XCTR uses XORs and little-endian
addition rather than big-endian arithmetic which has two advantages:  It
is slightly faster on little-endian CPUs and it is less likely to be
implemented incorrect since integer overflows are not possible on
practical input sizes.  XCTR is used as a component to implement HCTR2.

More information on XCTR mode can be found in the HCTR2 paper:
https://eprint.iacr.org/2021/1441.pdf

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/Kconfig

@@ -460,6 +460,15 @@ config CRYPTO_PCBC
 	  PCBC: Propagating Cipher Block Chaining mode
 	  This block cipher algorithm is required for RxRPC.
 
+config CRYPTO_XCTR
+	tristate
+	select CRYPTO_SKCIPHER
+	select CRYPTO_MANAGER
+	help
+	  XCTR: XOR Counter mode. This blockcipher mode is a variant of CTR mode
+	  using XORs and little-endian addition rather than big-endian arithmetic.
+	  XCTR mode is used to implement HCTR2.
+
 config CRYPTO_XTS
 	tristate "XTS support"
 	select CRYPTO_SKCIPHER

crypto/Makefile

@@ -94,6 +94,7 @@ obj-$(CONFIG_CRYPTO_CTS) += cts.o
 obj-$(CONFIG_CRYPTO_LRW) += lrw.o
 obj-$(CONFIG_CRYPTO_XTS) += xts.o
 obj-$(CONFIG_CRYPTO_CTR) += ctr.o
+obj-$(CONFIG_CRYPTO_XCTR) += xctr.o
 obj-$(CONFIG_CRYPTO_KEYWRAP) += keywrap.o
 obj-$(CONFIG_CRYPTO_ADIANTUM) += adiantum.o
 obj-$(CONFIG_CRYPTO_NHPOLY1305) += nhpoly1305.o

crypto/tcrypt.c

@@ -1556,6 +1556,7 @@ static int do_test(const char *alg, u32 type, u32 mask, int m, u32 num_mb)
 		ret += tcrypt_test("rfc3686(ctr(aes))");
 		ret += tcrypt_test("ofb(aes)");
 		ret += tcrypt_test("cfb(aes)");
+		ret += tcrypt_test("xctr(aes)");
 		break;
 
 	case 11:

crypto/testmgr.c

@@ -5548,6 +5548,12 @@ static const struct alg_test_desc alg_test_descs[] = {
 		.suite = {
 			.cipher = __VECS(xchacha20_tv_template)
 		},
+	}, {
+		.alg = "xctr(aes)",
+		.test = alg_test_skcipher,
+		.suite = {
+			.cipher = __VECS(aes_xctr_tv_template)
+		}
 	}, {
 		.alg = "xts(aes)",
 		.generic_driver = "xts(ecb(aes-generic))",

crypto/xctr.c

@@ -0,0 +1,191 @@
+// 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);