crypto: adiantum - add Adiantum support

Add support for the Adiantum encryption mode.  Adiantum was designed by
Paul Crowley and is specified by our paper:

    Adiantum: length-preserving encryption for entry-level processors
    (https://eprint.iacr.org/2018/720.pdf)

See our paper for full details; this patch only provides an overview.

Adiantum is a tweakable, length-preserving encryption mode designed for
fast and secure disk encryption, especially on CPUs without dedicated
crypto instructions.  Adiantum encrypts each sector using the XChaCha12
stream cipher, two passes of an ε-almost-∆-universal (εA∆U) hash
function, and an invocation of the AES-256 block cipher on a single
16-byte block.  On CPUs without AES instructions, Adiantum is much
faster than AES-XTS; for example, on ARM Cortex-A7, on 4096-byte sectors
Adiantum encryption is about 4 times faster than AES-256-XTS encryption,
and decryption about 5 times faster.

Adiantum is a specialization of the more general HBSH construction.  Our
earlier proposal, HPolyC, was also a HBSH specialization, but it used a
different εA∆U hash function, one based on Poly1305 only.  Adiantum's
εA∆U hash function, which is based primarily on the "NH" hash function
like that used in UMAC (RFC4418), is about twice as fast as HPolyC's;
consequently, Adiantum is about 20% faster than HPolyC.

This speed comes with no loss of security: Adiantum is provably just as
secure as HPolyC, in fact slightly *more* secure.  Like HPolyC,
Adiantum's security is reducible to that of XChaCha12 and AES-256,
subject to a security bound.  XChaCha12 itself has a security reduction
to ChaCha12.  Therefore, one need not "trust" Adiantum; one need only
trust ChaCha12 and AES-256.  Note that the εA∆U hash function is only
used for its proven combinatorical properties so cannot be "broken".

Adiantum is also a true wide-block encryption mode, so flipping any
plaintext bit in the sector scrambles the entire ciphertext, and vice
versa.  No other such mode is available in the kernel currently; doing
the same with XTS scrambles only 16 bytes.  Adiantum also supports
arbitrary-length tweaks and naturally supports any length input >= 16
bytes without needing "ciphertext stealing".

For the stream cipher, Adiantum uses XChaCha12 rather than XChaCha20 in
order to make encryption feasible on the widest range of devices.
Although the 20-round variant is quite popular, the best known attacks
on ChaCha are on only 7 rounds, so ChaCha12 still has a substantial
security margin; in fact, larger than AES-256's.  12-round Salsa20 is
also the eSTREAM recommendation.  For the block cipher, Adiantum uses
AES-256, despite it having a lower security margin than XChaCha12 and
needing table lookups, due to AES's extensive adoption and analysis
making it the obvious first choice.  Nevertheless, for flexibility this
patch also permits the "adiantum" template to be instantiated with
XChaCha20 and/or with an alternate block cipher.

We need Adiantum support in the kernel for use in dm-crypt and fscrypt,
where currently the only other suitable options are block cipher modes
such as AES-XTS.  A big problem with this is that many low-end mobile
devices (e.g. Android Go phones sold primarily in developing countries,
as well as some smartwatches) still have CPUs that lack AES
instructions, e.g. ARM Cortex-A7.  Sadly, AES-XTS encryption is much too
slow to be viable on these devices.  We did find that some "lightweight"
block ciphers are fast enough, but these suffer from problems such as
not having much cryptanalysis or being too controversial.

The ChaCha stream cipher has excellent performance but is insecure to
use directly for disk encryption, since each sector's IV is reused each
time it is overwritten.  Even restricting the threat model to offline
attacks only isn't enough, since modern flash storage devices don't
guarantee that "overwrites" are really overwrites, due to wear-leveling.
Adiantum avoids this problem by constructing a
"tweakable super-pseudorandom permutation"; this is the strongest
possible security model for length-preserving encryption.

Of course, storing random nonces along with the ciphertext would be the
ideal solution.  But doing that with existing hardware and filesystems
runs into major practical problems; in most cases it would require data
journaling (like dm-integrity) which severely degrades performance.
Thus, for now length-preserving encryption is still needed.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 658 insertions, 0 deletions

diff --git a/crypto/adiantum.c b/crypto/adiantum.c
new file mode 100644
index 000000000000..2dfcf12fd452
--- /dev/null
+++ b/crypto/adiantum.c
@@ -0,0 +1,658 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Adiantum length-preserving encryption mode
+ *
+ * Copyright 2018 Google LLC
+ */
+
+/*
+ * Adiantum is a tweakable, length-preserving encryption mode designed for fast
+ * and secure disk encryption, especially on CPUs without dedicated crypto
+ * instructions.  Adiantum encrypts each sector using the XChaCha12 stream
+ * cipher, two passes of an ε-almost-∆-universal (εA∆U) hash function based on
+ * NH and Poly1305, and an invocation of the AES-256 block cipher on a single
+ * 16-byte block.  See the paper for details:
+ *
+ *	Adiantum: length-preserving encryption for entry-level processors
+ *      (https://eprint.iacr.org/2018/720.pdf)
+ *
+ * For flexibility, this implementation also allows other ciphers:
+ *
+ *	- Stream cipher: XChaCha12 or XChaCha20
+ *	- Block cipher: any with a 128-bit block size and 256-bit key
+ *
+ * This implementation doesn't currently allow other εA∆U hash functions, i.e.
+ * HPolyC is not supported.  This is because Adiantum is ~20% faster than HPolyC
+ * but still provably as secure, and also the εA∆U hash function of HBSH is
+ * formally defined to take two inputs (tweak, message) which makes it difficult
+ * to wrap with the crypto_shash API.  Rather, some details need to be handled
+ * here.  Nevertheless, if needed in the future, support for other εA∆U hash
+ * functions could be added here.
+ */
+
+#include <crypto/b128ops.h>
+#include <crypto/chacha.h>
+#include <crypto/internal/hash.h>
+#include <crypto/internal/skcipher.h>
+#include <crypto/nhpoly1305.h>
+#include <crypto/scatterwalk.h>
+#include <linux/module.h>
+
+#include "internal.h"
+
+/*
+ * Size of right-hand block of input data, in bytes; also the size of the block
+ * cipher's block size and the hash function's output.
+ */
+#define BLOCKCIPHER_BLOCK_SIZE		16
+
+/* Size of the block cipher key (K_E) in bytes */
+#define BLOCKCIPHER_KEY_SIZE		32
+
+/* Size of the hash key (K_H) in bytes */
+#define HASH_KEY_SIZE		(POLY1305_BLOCK_SIZE + NHPOLY1305_KEY_SIZE)
+
+/*
+ * 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 Adiantum is 16, since that fits into one Poly1305 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 adiantum_instance_ctx {
+	struct crypto_skcipher_spawn streamcipher_spawn;
+	struct crypto_spawn blockcipher_spawn;
+	struct crypto_shash_spawn hash_spawn;
+};
+
+struct adiantum_tfm_ctx {
+	struct crypto_skcipher *streamcipher;
+	struct crypto_cipher *blockcipher;
+	struct crypto_shash *hash;
+	struct poly1305_key header_hash_key;
+};
+
+struct adiantum_request_ctx {
+
+	/*
+	 * Buffer for right-hand block of data, i.e.
+	 *
+	 *    P_L => P_M => C_M => C_R when encrypting, or
+	 *    C_R => C_M => P_M => P_L when decrypting.
+	 *
+	 * Also used to build the IV for the stream cipher.
+	 */
+	union {
+		u8 bytes[XCHACHA_IV_SIZE];
+		__le32 words[XCHACHA_IV_SIZE / sizeof(__le32)];
+		le128 bignum;	/* interpret as element of Z/(2^{128}Z) */
+	} rbuf;
+
+	bool enc; /* true if encrypting, false if decrypting */
+
+	/*
+	 * The result of the Poly1305 εA∆U hash function applied to
+	 * (message length, tweak).
+	 */
+	le128 header_hash;
+
+	/* Sub-requests, must be last */
+	union {
+		struct shash_desc hash_desc;
+		struct skcipher_request streamcipher_req;
+	} u;
+};
+
+/*
+ * Given the XChaCha stream key K_S, derive the block cipher key K_E and the
+ * hash key K_H as follows:
+ *
+ *     K_E || K_H || ... = XChaCha(key=K_S, nonce=1||0^191)
+ *
+ * Note that this denotes using bits from the XChaCha keystream, which here we
+ * get indirectly by encrypting a buffer containing all 0's.
+ */
+static int adiantum_setkey(struct crypto_skcipher *tfm, const u8 *key,
+			   unsigned int keylen)
+{
+	struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
+	struct {
+		u8 iv[XCHACHA_IV_SIZE];
+		u8 derived_keys[BLOCKCIPHER_KEY_SIZE + HASH_KEY_SIZE];
+		struct scatterlist sg;
+		struct crypto_wait wait;
+		struct skcipher_request req; /* must be last */
+	} *data;
+	u8 *keyp;
+	int err;
+
+	/* Set the stream cipher key (K_S) */
+	crypto_skcipher_clear_flags(tctx->streamcipher, CRYPTO_TFM_REQ_MASK);
+	crypto_skcipher_set_flags(tctx->streamcipher,
+				  crypto_skcipher_get_flags(tfm) &
+				  CRYPTO_TFM_REQ_MASK);
+	err = crypto_skcipher_setkey(tctx->streamcipher, key, keylen);
+	crypto_skcipher_set_flags(tfm,
+				crypto_skcipher_get_flags(tctx->streamcipher) &
+				CRYPTO_TFM_RES_MASK);
+	if (err)
+		return err;
+
+	/* Derive the subkeys */
+	data = kzalloc(sizeof(*data) +
+		       crypto_skcipher_reqsize(tctx->streamcipher), GFP_KERNEL);
+	if (!data)
+		return -ENOMEM;
+	data->iv[0] = 1;
+	sg_init_one(&data->sg, data->derived_keys, sizeof(data->derived_keys));
+	crypto_init_wait(&data->wait);
+	skcipher_request_set_tfm(&data->req, tctx->streamcipher);
+	skcipher_request_set_callback(&data->req, CRYPTO_TFM_REQ_MAY_SLEEP |
+						  CRYPTO_TFM_REQ_MAY_BACKLOG,
+				      crypto_req_done, &data->wait);
+	skcipher_request_set_crypt(&data->req, &data->sg, &data->sg,
+				   sizeof(data->derived_keys), data->iv);
+	err = crypto_wait_req(crypto_skcipher_encrypt(&data->req), &data->wait);
+	if (err)
+		goto out;
+	keyp = data->derived_keys;
+
+	/* Set the block cipher key (K_E) */
+	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, keyp,
+				   BLOCKCIPHER_KEY_SIZE);
+	crypto_skcipher_set_flags(tfm,
+				  crypto_cipher_get_flags(tctx->blockcipher) &
+				  CRYPTO_TFM_RES_MASK);
+	if (err)
+		goto out;
+	keyp += BLOCKCIPHER_KEY_SIZE;
+
+	/* Set the hash key (K_H) */
+	poly1305_core_setkey(&tctx->header_hash_key, keyp);
+	keyp += POLY1305_BLOCK_SIZE;
+
+	crypto_shash_clear_flags(tctx->hash, CRYPTO_TFM_REQ_MASK);
+	crypto_shash_set_flags(tctx->hash, crypto_skcipher_get_flags(tfm) &
+					   CRYPTO_TFM_REQ_MASK);
+	err = crypto_shash_setkey(tctx->hash, keyp, NHPOLY1305_KEY_SIZE);
+	crypto_skcipher_set_flags(tfm, crypto_shash_get_flags(tctx->hash) &
+				       CRYPTO_TFM_RES_MASK);
+	keyp += NHPOLY1305_KEY_SIZE;
+	WARN_ON(keyp != &data->derived_keys[ARRAY_SIZE(data->derived_keys)]);
+out:
+	kzfree(data);
+	return err;
+}
+
+/* Addition in Z/(2^{128}Z) */
+static inline void le128_add(le128 *r, const le128 *v1, const le128 *v2)
+{
+	u64 x = le64_to_cpu(v1->b);
+	u64 y = le64_to_cpu(v2->b);
+
+	r->b = cpu_to_le64(x + y);
+	r->a = cpu_to_le64(le64_to_cpu(v1->a) + le64_to_cpu(v2->a) +
+			   (x + y < x));
+}
+
+/* Subtraction in Z/(2^{128}Z) */
+static inline void le128_sub(le128 *r, const le128 *v1, const le128 *v2)
+{
+	u64 x = le64_to_cpu(v1->b);
+	u64 y = le64_to_cpu(v2->b);
+
+	r->b = cpu_to_le64(x - y);
+	r->a = cpu_to_le64(le64_to_cpu(v1->a) - le64_to_cpu(v2->a) -
+			   (x - y > x));
+}
+
+/*
+ * Apply the Poly1305 εA∆U hash function to (message length, tweak) and save the
+ * result to rctx->header_hash.
+ *
+ * This value is reused in both the first and second hash steps.  Specifically,
+ * it's added to the result of an independently keyed εA∆U hash function (for
+ * equal length inputs only) taken over the message.  This gives the overall
+ * Adiantum hash of the (tweak, message) pair.
+ */
+static void adiantum_hash_header(struct skcipher_request *req)
+{
+	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
+	const struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
+	struct adiantum_request_ctx *rctx = skcipher_request_ctx(req);
+	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
+	struct {
+		__le64 message_bits;
+		__le64 padding;
+	} header = {
+		.message_bits = cpu_to_le64((u64)bulk_len * 8)
+	};
+	struct poly1305_state state;
+
+	poly1305_core_init(&state);
+
+	BUILD_BUG_ON(sizeof(header) % POLY1305_BLOCK_SIZE != 0);
+	poly1305_core_blocks(&state, &tctx->header_hash_key,
+			     &header, sizeof(header) / POLY1305_BLOCK_SIZE);
+
+	BUILD_BUG_ON(TWEAK_SIZE % POLY1305_BLOCK_SIZE != 0);
+	poly1305_core_blocks(&state, &tctx->header_hash_key, req->iv,
+			     TWEAK_SIZE / POLY1305_BLOCK_SIZE);
+
+	poly1305_core_emit(&state, &rctx->header_hash);
+}
+
+/* Hash the left-hand block (the "bulk") of the message using NHPoly1305 */
+static int adiantum_hash_message(struct skcipher_request *req,
+				 struct scatterlist *sgl, le128 *digest)
+{
+	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
+	const struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
+	struct adiantum_request_ctx *rctx = skcipher_request_ctx(req);
+	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
+	struct shash_desc *hash_desc = &rctx->u.hash_desc;
+	struct sg_mapping_iter miter;
+	unsigned int i, n;
+	int err;
+
+	hash_desc->tfm = tctx->hash;
+	hash_desc->flags = 0;
+
+	err = crypto_shash_init(hash_desc);
+	if (err)
+		return err;
+
+	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);
+		err = crypto_shash_update(hash_desc, miter.addr, n);
+		if (err)
+			break;
+	}
+	sg_miter_stop(&miter);
+	if (err)
+		return err;
+
+	return crypto_shash_final(hash_desc, (u8 *)digest);
+}
+
+/* Continue Adiantum encryption/decryption after the stream cipher step */
+static int adiantum_finish(struct skcipher_request *req)
+{
+	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
+	const struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
+	struct adiantum_request_ctx *rctx = skcipher_request_ctx(req);
+	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
+	le128 digest;
+	int err;
+
+	/* If decrypting, decrypt C_M with the block cipher to get P_M */
+	if (!rctx->enc)
+		crypto_cipher_decrypt_one(tctx->blockcipher, rctx->rbuf.bytes,
+					  rctx->rbuf.bytes);
+
+	/*
+	 * Second hash step
+	 *	enc: C_R = C_M - H_{K_H}(T, C_L)
+	 *	dec: P_R = P_M - H_{K_H}(T, P_L)
+	 */
+	err = adiantum_hash_message(req, req->dst, &digest);
+	if (err)
+		return err;
+	le128_add(&digest, &digest, &rctx->header_hash);
+	le128_sub(&rctx->rbuf.bignum, &rctx->rbuf.bignum, &digest);
+	scatterwalk_map_and_copy(&rctx->rbuf.bignum, req->dst,
+				 bulk_len, BLOCKCIPHER_BLOCK_SIZE, 1);
+	return 0;
+}
+
+static void adiantum_streamcipher_done(struct crypto_async_request *areq,
+				       int err)
+{
+	struct skcipher_request *req = areq->data;
+
+	if (!err)
+		err = adiantum_finish(req);
+
+	skcipher_request_complete(req, err);
+}
+
+static int adiantum_crypt(struct skcipher_request *req, bool enc)
+{
+	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
+	const struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
+	struct adiantum_request_ctx *rctx = skcipher_request_ctx(req);
+	const unsigned int bulk_len = req->cryptlen - BLOCKCIPHER_BLOCK_SIZE;
+	unsigned int stream_len;
+	le128 digest;
+	int err;
+
+	if (req->cryptlen < BLOCKCIPHER_BLOCK_SIZE)
+		return -EINVAL;
+
+	rctx->enc = enc;
+
+	/*
+	 * First hash step
+	 *	enc: P_M = P_R + H_{K_H}(T, P_L)
+	 *	dec: C_M = C_R + H_{K_H}(T, C_L)
+	 */
+	adiantum_hash_header(req);
+	err = adiantum_hash_message(req, req->src, &digest);
+	if (err)
+		return err;
+	le128_add(&digest, &digest, &rctx->header_hash);
+	scatterwalk_map_and_copy(&rctx->rbuf.bignum, req->src,
+				 bulk_len, BLOCKCIPHER_BLOCK_SIZE, 0);
+	le128_add(&rctx->rbuf.bignum, &rctx->rbuf.bignum, &digest);
+
+	/* If encrypting, encrypt P_M with the block cipher to get C_M */
+	if (enc)
+		crypto_cipher_encrypt_one(tctx->blockcipher, rctx->rbuf.bytes,
+					  rctx->rbuf.bytes);
+
+	/* Initialize the rest of the XChaCha IV (first part is C_M) */
+	BUILD_BUG_ON(BLOCKCIPHER_BLOCK_SIZE != 16);
+	BUILD_BUG_ON(XCHACHA_IV_SIZE != 32);	/* nonce || stream position */
+	rctx->rbuf.words[4] = cpu_to_le32(1);
+	rctx->rbuf.words[5] = 0;
+	rctx->rbuf.words[6] = 0;
+	rctx->rbuf.words[7] = 0;
+
+	/*
+	 * XChaCha needs to be done on all the data except the last 16 bytes;
+	 * for disk encryption that usually means 4080 or 496 bytes.  But ChaCha
+	 * implementations tend to be most efficient when passed a whole number
+	 * of 64-byte ChaCha blocks, or sometimes even a multiple of 256 bytes.
+	 * And here it doesn't matter whether the last 16 bytes are written to,
+	 * as the second hash step will overwrite them.  Thus, round the XChaCha
+	 * length up to the next 64-byte boundary if possible.
+	 */
+	stream_len = bulk_len;
+	if (round_up(stream_len, CHACHA_BLOCK_SIZE) <= req->cryptlen)
+		stream_len = round_up(stream_len, CHACHA_BLOCK_SIZE);
+
+	skcipher_request_set_tfm(&rctx->u.streamcipher_req, tctx->streamcipher);
+	skcipher_request_set_crypt(&rctx->u.streamcipher_req, req->src,
+				   req->dst, stream_len, &rctx->rbuf);
+	skcipher_request_set_callback(&rctx->u.streamcipher_req,
+				      req->base.flags,
+				      adiantum_streamcipher_done, req);
+	return crypto_skcipher_encrypt(&rctx->u.streamcipher_req) ?:
+		adiantum_finish(req);
+}
+
+static int adiantum_encrypt(struct skcipher_request *req)
+{
+	return adiantum_crypt(req, true);
+}
+
+static int adiantum_decrypt(struct skcipher_request *req)
+{
+	return adiantum_crypt(req, false);
+}
+
+static int adiantum_init_tfm(struct crypto_skcipher *tfm)
+{
+	struct skcipher_instance *inst = skcipher_alg_instance(tfm);
+	struct adiantum_instance_ctx *ictx = skcipher_instance_ctx(inst);
+	struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
+	struct crypto_skcipher *streamcipher;
+	struct crypto_cipher *blockcipher;
+	struct crypto_shash *hash;
+	unsigned int subreq_size;
+	int err;
+
+	streamcipher = crypto_spawn_skcipher(&ictx->streamcipher_spawn);
+	if (IS_ERR(streamcipher))
+		return PTR_ERR(streamcipher);
+
+	blockcipher = crypto_spawn_cipher(&ictx->blockcipher_spawn);
+	if (IS_ERR(blockcipher)) {
+		err = PTR_ERR(blockcipher);
+		goto err_free_streamcipher;
+	}
+
+	hash = crypto_spawn_shash(&ictx->hash_spawn);
+	if (IS_ERR(hash)) {
+		err = PTR_ERR(hash);
+		goto err_free_blockcipher;
+	}
+
+	tctx->streamcipher = streamcipher;
+	tctx->blockcipher = blockcipher;
+	tctx->hash = hash;
+
+	BUILD_BUG_ON(offsetofend(struct adiantum_request_ctx, u) !=
+		     sizeof(struct adiantum_request_ctx));
+	subreq_size = max(FIELD_SIZEOF(struct adiantum_request_ctx,
+				       u.hash_desc) +
+			  crypto_shash_descsize(hash),
+			  FIELD_SIZEOF(struct adiantum_request_ctx,
+				       u.streamcipher_req) +
+			  crypto_skcipher_reqsize(streamcipher));
+
+	crypto_skcipher_set_reqsize(tfm,
+				    offsetof(struct adiantum_request_ctx, u) +
+				    subreq_size);
+	return 0;
+
+err_free_blockcipher:
+	crypto_free_cipher(blockcipher);
+err_free_streamcipher:
+	crypto_free_skcipher(streamcipher);
+	return err;
+}
+
+static void adiantum_exit_tfm(struct crypto_skcipher *tfm)
+{
+	struct adiantum_tfm_ctx *tctx = crypto_skcipher_ctx(tfm);
+
+	crypto_free_skcipher(tctx->streamcipher);
+	crypto_free_cipher(tctx->blockcipher);
+	crypto_free_shash(tctx->hash);
+}
+
+static void adiantum_free_instance(struct skcipher_instance *inst)
+{
+	struct adiantum_instance_ctx *ictx = skcipher_instance_ctx(inst);
+
+	crypto_drop_skcipher(&ictx->streamcipher_spawn);
+	crypto_drop_spawn(&ictx->blockcipher_spawn);
+	crypto_drop_shash(&ictx->hash_spawn);
+	kfree(inst);
+}
+
+/*
+ * Check for a supported set of inner algorithms.
+ * See the comment at the beginning of this file.
+ */
+static bool adiantum_supported_algorithms(struct skcipher_alg *streamcipher_alg,
+					  struct crypto_alg *blockcipher_alg,
+					  struct shash_alg *hash_alg)
+{
+	if (strcmp(streamcipher_alg->base.cra_name, "xchacha12") != 0 &&
+	    strcmp(streamcipher_alg->base.cra_name, "xchacha20") != 0)
+		return false;
+
+	if (blockcipher_alg->cra_cipher.cia_min_keysize > BLOCKCIPHER_KEY_SIZE ||
+	    blockcipher_alg->cra_cipher.cia_max_keysize < BLOCKCIPHER_KEY_SIZE)
+		return false;
+	if (blockcipher_alg->cra_blocksize != BLOCKCIPHER_BLOCK_SIZE)
+		return false;
+
+	if (strcmp(hash_alg->base.cra_name, "nhpoly1305") != 0)
+		return false;
+
+	return true;
+}
+
+static int adiantum_create(struct crypto_template *tmpl, struct rtattr **tb)
+{
+	struct crypto_attr_type *algt;
+	const char *streamcipher_name;
+	const char *blockcipher_name;
+	const char *nhpoly1305_name;
+	struct skcipher_instance *inst;
+	struct adiantum_instance_ctx *ictx;
+	struct skcipher_alg *streamcipher_alg;
+	struct crypto_alg *blockcipher_alg;
+	struct crypto_alg *_hash_alg;
+	struct shash_alg *hash_alg;
+	int err;
+
+	algt = crypto_get_attr_type(tb);
+	if (IS_ERR(algt))
+		return PTR_ERR(algt);
+
+	if ((algt->type ^ CRYPTO_ALG_TYPE_SKCIPHER) & algt->mask)
+		return -EINVAL;
+
+	streamcipher_name = crypto_attr_alg_name(tb[1]);
+	if (IS_ERR(streamcipher_name))
+		return PTR_ERR(streamcipher_name);
+
+	blockcipher_name = crypto_attr_alg_name(tb[2]);
+	if (IS_ERR(blockcipher_name))
+		return PTR_ERR(blockcipher_name);
+
+	nhpoly1305_name = crypto_attr_alg_name(tb[3]);
+	if (nhpoly1305_name == ERR_PTR(-ENOENT))
+		nhpoly1305_name = "nhpoly1305";
+	if (IS_ERR(nhpoly1305_name))
+		return PTR_ERR(nhpoly1305_name);
+
+	inst = kzalloc(sizeof(*inst) + sizeof(*ictx), GFP_KERNEL);
+	if (!inst)
+		return -ENOMEM;
+	ictx = skcipher_instance_ctx(inst);
+
+	/* Stream cipher, e.g. "xchacha12" */
+	err = crypto_grab_skcipher(&ictx->streamcipher_spawn, streamcipher_name,
+				   0, crypto_requires_sync(algt->type,
+							   algt->mask));
+	if (err)
+		goto out_free_inst;
+	streamcipher_alg = crypto_spawn_skcipher_alg(&ictx->streamcipher_spawn);
+
+	/* Block cipher, e.g. "aes" */
+	err = crypto_grab_spawn(&ictx->blockcipher_spawn, blockcipher_name,
+				CRYPTO_ALG_TYPE_CIPHER, CRYPTO_ALG_TYPE_MASK);
+	if (err)
+		goto out_drop_streamcipher;
+	blockcipher_alg = ictx->blockcipher_spawn.alg;
+
+	/* NHPoly1305 εA∆U hash function */
+	_hash_alg = crypto_alg_mod_lookup(nhpoly1305_name,
+					  CRYPTO_ALG_TYPE_SHASH,
+					  CRYPTO_ALG_TYPE_MASK);
+	if (IS_ERR(_hash_alg)) {
+		err = PTR_ERR(_hash_alg);
+		goto out_drop_blockcipher;
+	}
+	hash_alg = __crypto_shash_alg(_hash_alg);
+	err = crypto_init_shash_spawn(&ictx->hash_spawn, hash_alg,
+				      skcipher_crypto_instance(inst));
+	if (err) {
+		crypto_mod_put(_hash_alg);
+		goto out_drop_blockcipher;
+	}
+
+	/* Check the set of algorithms */
+	if (!adiantum_supported_algorithms(streamcipher_alg, blockcipher_alg,
+					   hash_alg)) {
+		pr_warn("Unsupported Adiantum instantiation: (%s,%s,%s)\n",
+			streamcipher_alg->base.cra_name,
+			blockcipher_alg->cra_name, hash_alg->base.cra_name);
+		err = -EINVAL;
+		goto out_drop_hash;
+	}
+
+	/* Instance fields */
+
+	err = -ENAMETOOLONG;
+	if (snprintf(inst->alg.base.cra_name, CRYPTO_MAX_ALG_NAME,
+		     "adiantum(%s,%s)", streamcipher_alg->base.cra_name,
+		     blockcipher_alg->cra_name) >= CRYPTO_MAX_ALG_NAME)
+		goto out_drop_hash;
+	if (snprintf(inst->alg.base.cra_driver_name, CRYPTO_MAX_ALG_NAME,
+		     "adiantum(%s,%s,%s)",
+		     streamcipher_alg->base.cra_driver_name,
+		     blockcipher_alg->cra_driver_name,
+		     hash_alg->base.cra_driver_name) >= CRYPTO_MAX_ALG_NAME)
+		goto out_drop_hash;
+
+	inst->alg.base.cra_blocksize = BLOCKCIPHER_BLOCK_SIZE;
+	inst->alg.base.cra_ctxsize = sizeof(struct adiantum_tfm_ctx);
+	inst->alg.base.cra_alignmask = streamcipher_alg->base.cra_alignmask |
+				       hash_alg->base.cra_alignmask;
+	/*
+	 * The block cipher is only invoked once per message, so for long
+	 * messages (e.g. sectors for disk encryption) its performance doesn't
+	 * matter as much as that of the stream cipher and hash function.  Thus,
+	 * weigh the block cipher's ->cra_priority less.
+	 */
+	inst->alg.base.cra_priority = (4 * streamcipher_alg->base.cra_priority +
+				       2 * hash_alg->base.cra_priority +
+				       blockcipher_alg->cra_priority) / 7;
+
+	inst->alg.setkey = adiantum_setkey;
+	inst->alg.encrypt = adiantum_encrypt;
+	inst->alg.decrypt = adiantum_decrypt;
+	inst->alg.init = adiantum_init_tfm;
+	inst->alg.exit = adiantum_exit_tfm;
+	inst->alg.min_keysize = crypto_skcipher_alg_min_keysize(streamcipher_alg);
+	inst->alg.max_keysize = crypto_skcipher_alg_max_keysize(streamcipher_alg);
+	inst->alg.ivsize = TWEAK_SIZE;
+
+	inst->free = adiantum_free_instance;
+
+	err = skcipher_register_instance(tmpl, inst);
+	if (err)
+		goto out_drop_hash;
+
+	return 0;
+
+out_drop_hash:
+	crypto_drop_shash(&ictx->hash_spawn);
+out_drop_blockcipher:
+	crypto_drop_spawn(&ictx->blockcipher_spawn);
+out_drop_streamcipher:
+	crypto_drop_skcipher(&ictx->streamcipher_spawn);
+out_free_inst:
+	kfree(inst);
+	return err;
+}
+
+/* adiantum(streamcipher_name, blockcipher_name [, nhpoly1305_name]) */
+static struct crypto_template adiantum_tmpl = {
+	.name = "adiantum",
+	.create = adiantum_create,
+	.module = THIS_MODULE,
+};
+
+static int __init adiantum_module_init(void)
+{
+	return crypto_register_template(&adiantum_tmpl);
+}
+
+static void __exit adiantum_module_exit(void)
+{
+	crypto_unregister_template(&adiantum_tmpl);
+}
+
+module_init(adiantum_module_init);
+module_exit(adiantum_module_exit);
+
+MODULE_DESCRIPTION("Adiantum length-preserving encryption mode");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("Eric Biggers <ebiggers@google.com>");
+MODULE_ALIAS_CRYPTO("adiantum");