[PATCH 2/3] crypto: add output feedback mode

[Gilad Ben-Yossef]

Add a generic version of output feedback mode. We already have support of
several hardware based transformations of this mode and the needed test
vectors but we somehow missed adding a generic software one. Fix this now.

Signed-off-by: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx>
---
 crypto/Kconfig  |  12 +++
 crypto/Makefile |   1 +
 crypto/ofb.c    | 225 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 238 insertions(+)
 create mode 100644 crypto/ofb.c

diff --git a/crypto/Kconfig b/crypto/Kconfig
index 90f2811..d8cb65b 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -456,6 +456,18 @@ config CRYPTO_LRW
 	  The first 128, 192 or 256 bits in the key are used for AES and the
 	  rest is used to tie each cipher block to its logical position.
 
+config CRYPTO_OFB
+	tristate "OFB support"
+	select CRYPTO_BLKCIPHER
+	select CRYPTO_MANAGER
+	help
+	  OFB: the Output Feedback mode makes a block cipher into a synchronous
+	  stream cipher. It generates keystream blocks, which are then XORed
+	  with the plaintext blocks to get the ciphertext. Flipping a bit in the
+	  ciphertext produces a flipped bit in the plaintext at the same
+	  location. This property allows many error correcting codes to function
+	  normally even when applied before encryption.
+
 config CRYPTO_PCBC
 	tristate "PCBC support"
 	select CRYPTO_BLKCIPHER
diff --git a/crypto/Makefile b/crypto/Makefile
index d719843..5cde057 100644
--- a/crypto/Makefile
+++ b/crypto/Makefile
@@ -141,6 +141,7 @@ obj-$(CONFIG_CRYPTO_USER_API_SKCIPHER) += algif_skcipher.o
 obj-$(CONFIG_CRYPTO_USER_API_RNG) += algif_rng.o
 obj-$(CONFIG_CRYPTO_USER_API_AEAD) += algif_aead.o
 obj-$(CONFIG_CRYPTO_ZSTD) += zstd.o
+obj-$(CONFIG_CRYPTO_OFB) += ofb.o
 
 ecdh_generic-y := ecc.o
 ecdh_generic-y += ecdh.o
diff --git a/crypto/ofb.c b/crypto/ofb.c
new file mode 100644
index 0000000..8866317
--- /dev/null
+++ b/crypto/ofb.c
@@ -0,0 +1,225 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * OFB: Output FeedBack mode
+ *
+ * Copyright (C) 2018 ARM Limited or its affiliates.
+ * All rights reserved.
+ *
+ * Based loosely on public domain code gleaned from libtomcrypt
+ * (https://github.com/libtom/libtomcrypt).
+ */
+
+#include <crypto/algapi.h>
+#include <crypto/internal/skcipher.h>
+#include <linux/err.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/scatterlist.h>
+#include <linux/slab.h>
+
+struct crypto_ofb_ctx {
+	struct crypto_cipher *child;
+	int cnt;
+};
+
+
+static int crypto_ofb_setkey(struct crypto_skcipher *parent, const u8 *key,
+			     unsigned int keylen)
+{
+	struct crypto_ofb_ctx *ctx = crypto_skcipher_ctx(parent);
+	struct crypto_cipher *child = ctx->child;
+	int err;
+
+	crypto_cipher_clear_flags(child, CRYPTO_TFM_REQ_MASK);
+	crypto_cipher_set_flags(child, crypto_skcipher_get_flags(parent) &
+				       CRYPTO_TFM_REQ_MASK);
+	err = crypto_cipher_setkey(child, key, keylen);
+	crypto_skcipher_set_flags(parent, crypto_cipher_get_flags(child) &
+				  CRYPTO_TFM_RES_MASK);
+	return err;
+}
+
+static int crypto_ofb_encrypt_segment(struct crypto_ofb_ctx *ctx,
+				      struct skcipher_walk *walk,
+				      struct crypto_cipher *tfm)
+{
+	int bsize = crypto_cipher_blocksize(tfm);
+	int nbytes = walk->nbytes;
+
+	u8 *src = walk->src.virt.addr;
+	u8 *dst = walk->dst.virt.addr;
+	u8 *iv = walk->iv;
+
+	do {
+		if (ctx->cnt == bsize) {
+			if (nbytes < bsize)
+				break;
+			crypto_cipher_encrypt_one(tfm, iv, iv);
+			ctx->cnt = 0;
+		}
+		*dst = *src ^ iv[ctx->cnt];
+		src++;
+		dst++;
+		ctx->cnt++;
+	} while (--nbytes);
+	return nbytes;
+}
+
+static int crypto_ofb_encrypt(struct skcipher_request *req)
+{
+	struct skcipher_walk walk;
+	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
+	unsigned int bsize;
+	struct crypto_ofb_ctx *ctx = crypto_skcipher_ctx(tfm);
+	struct crypto_cipher *child = ctx->child;
+	int ret = 0;
+
+	bsize =  crypto_cipher_blocksize(child);
+	ctx->cnt = bsize;
+
+	ret = skcipher_walk_virt(&walk, req, false);
+
+	while (walk.nbytes) {
+		ret = crypto_ofb_encrypt_segment(ctx, &walk, child);
+		ret = skcipher_walk_done(&walk, ret);
+	}
+
+	return ret;
+}
+
+/* OFB encrypt and decrypt are identical */
+static int crypto_ofb_decrypt(struct skcipher_request *req)
+{
+	return crypto_ofb_encrypt(req);
+}
+
+static int crypto_ofb_init_tfm(struct crypto_skcipher *tfm)
+{
+	struct skcipher_instance *inst = skcipher_alg_instance(tfm);
+	struct crypto_spawn *spawn = skcipher_instance_ctx(inst);
+	struct crypto_ofb_ctx *ctx = crypto_skcipher_ctx(tfm);
+	struct crypto_cipher *cipher;
+
+	cipher = crypto_spawn_cipher(spawn);
+	if (IS_ERR(cipher))
+		return PTR_ERR(cipher);
+
+	ctx->child = cipher;
+	return 0;
+}
+
+static void crypto_ofb_exit_tfm(struct crypto_skcipher *tfm)
+{
+	struct crypto_ofb_ctx *ctx = crypto_skcipher_ctx(tfm);
+
+	crypto_free_cipher(ctx->child);
+}
+
+static void crypto_ofb_free(struct skcipher_instance *inst)
+{
+	crypto_drop_skcipher(skcipher_instance_ctx(inst));
+	kfree(inst);
+}
+
+static int crypto_ofb_create(struct crypto_template *tmpl, struct rtattr **tb)
+{
+	struct skcipher_instance *inst;
+	struct crypto_attr_type *algt;
+	struct crypto_spawn *spawn;
+	struct crypto_alg *alg;
+	u32 mask;
+	int err;
+
+	err = crypto_check_attr_type(tb, CRYPTO_ALG_TYPE_SKCIPHER);
+	if (err)
+		return err;
+
+	inst = kzalloc(sizeof(*inst) + sizeof(*spawn), GFP_KERNEL);
+	if (!inst)
+		return -ENOMEM;
+
+	algt = crypto_get_attr_type(tb);
+	err = PTR_ERR(algt);
+	if (IS_ERR(algt))
+		goto err_free_inst;
+
+	mask = CRYPTO_ALG_TYPE_MASK |
+		crypto_requires_off(algt->type, algt->mask,
+				    CRYPTO_ALG_NEED_FALLBACK);
+
+	alg = crypto_get_attr_alg(tb, CRYPTO_ALG_TYPE_CIPHER, mask);
+	err = PTR_ERR(alg);
+	if (IS_ERR(alg))
+		goto err_free_inst;
+
+	spawn = skcipher_instance_ctx(inst);
+	err = crypto_init_spawn(spawn, alg, skcipher_crypto_instance(inst),
+				CRYPTO_ALG_TYPE_MASK);
+	crypto_mod_put(alg);
+	if (err)
+		goto err_free_inst;
+
+	err = crypto_inst_setname(skcipher_crypto_instance(inst), "ofb", alg);
+	if (err)
+		goto err_drop_spawn;
+
+	inst->alg.base.cra_priority = alg->cra_priority;
+	inst->alg.base.cra_blocksize = alg->cra_blocksize;
+	inst->alg.base.cra_alignmask = alg->cra_alignmask;
+
+	/* We access the data as u32s when xoring. */
+	inst->alg.base.cra_alignmask |= __alignof__(u32) - 1;
+
+	inst->alg.ivsize = alg->cra_blocksize;
+	inst->alg.min_keysize = alg->cra_cipher.cia_min_keysize;
+	inst->alg.max_keysize = alg->cra_cipher.cia_max_keysize;
+
+	inst->alg.base.cra_ctxsize = sizeof(struct crypto_ofb_ctx);
+
+	inst->alg.init = crypto_ofb_init_tfm;
+	inst->alg.exit = crypto_ofb_exit_tfm;
+
+	inst->alg.setkey = crypto_ofb_setkey;
+	inst->alg.encrypt = crypto_ofb_encrypt;
+	inst->alg.decrypt = crypto_ofb_decrypt;
+
+	inst->free = crypto_ofb_free;
+
+	err = skcipher_register_instance(tmpl, inst);
+	if (err)
+		goto err_drop_spawn;
+
+out:
+	return err;
+
+err_drop_spawn:
+	crypto_drop_spawn(spawn);
+err_free_inst:
+	kfree(inst);
+	goto out;
+}
+
+static struct crypto_template crypto_ofb_tmpl = {
+	.name = "ofb",
+	.create = crypto_ofb_create,
+	.module = THIS_MODULE,
+};
+
+static int __init crypto_ofb_module_init(void)
+{
+	return crypto_register_template(&crypto_ofb_tmpl);
+}
+
+static void __exit crypto_ofb_module_exit(void)
+{
+	crypto_unregister_template(&crypto_ofb_tmpl);
+}
+
+module_init(crypto_ofb_module_init);
+module_exit(crypto_ofb_module_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("OFB block cipher algorithm");
+MODULE_ALIAS_CRYPTO("ofb");
-- 
2.7.4