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Subject[RFC PATCH 1/9] crypto: chacha20-generic - add HChaCha20 library function
Date
From: Eric Biggers <ebiggers@google.com>

Refactor the unkeyed permutation part of chacha20_block() into its own
function, then add hchacha20_block() which is the ChaCha equivalent of
HSalsa20 and is an intermediate step towards XChaCha20 (see
https://cr.yp.to/snuffle/xsalsa-20081128.pdf). HChaCha20 skips the
final addition of the initial state, and outputs only certain words of
the state. It should not be used for streaming directly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
include/crypto/chacha20.h | 2 ++
lib/chacha20.c | 52 +++++++++++++++++++++++++++++++++------
2 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/include/crypto/chacha20.h b/include/crypto/chacha20.h
index b83d66073db0..f00052137942 100644
--- a/include/crypto/chacha20.h
+++ b/include/crypto/chacha20.h
@@ -20,6 +20,8 @@ struct chacha20_ctx {
};

void chacha20_block(u32 *state, u32 *stream);
+void hchacha20_block(const u32 *in, u32 *out);
+
void crypto_chacha20_init(u32 *state, struct chacha20_ctx *ctx, u8 *iv);
int crypto_chacha20_setkey(struct crypto_skcipher *tfm, const u8 *key,
unsigned int keysize);
diff --git a/lib/chacha20.c b/lib/chacha20.c
index c1cc50fb68c9..13a0bdcb1604 100644
--- a/lib/chacha20.c
+++ b/lib/chacha20.c
@@ -1,5 +1,5 @@
/*
- * ChaCha20 256-bit cipher algorithm, RFC7539
+ * The "hash function" used as the core of the ChaCha20 stream cipher (RFC7539)
*
* Copyright (C) 2015 Martin Willi
*
@@ -16,14 +16,10 @@
#include <asm/unaligned.h>
#include <crypto/chacha20.h>

-void chacha20_block(u32 *state, u32 *stream)
+static void chacha20_permute(u32 *x)
{
- u32 x[16], *out = stream;
int i;

- for (i = 0; i < ARRAY_SIZE(x); i++)
- x[i] = state[i];
-
for (i = 0; i < 20; i += 2) {
x[0] += x[4]; x[12] = rol32(x[12] ^ x[0], 16);
x[1] += x[5]; x[13] = rol32(x[13] ^ x[1], 16);
@@ -65,10 +61,52 @@ void chacha20_block(u32 *state, u32 *stream)
x[8] += x[13]; x[7] = rol32(x[7] ^ x[8], 7);
x[9] += x[14]; x[4] = rol32(x[4] ^ x[9], 7);
}
+}
+
+/**
+ * chacha20_block - generate one keystream block and increment block counter
+ * @state: input state matrix (16 32-bit words)
+ * @stream: output keystream block (64 bytes)
+ *
+ * This is the ChaCha20 core, a function from 64-byte strings to 64-byte
+ * strings. The caller has already converted the endianness of the input. This
+ * function also handles incrementing the block counter in the input matrix.
+ */
+void chacha20_block(u32 *state, u32 *stream)
+{
+ u32 x[16];
+ int i;
+
+ memcpy(x, state, 64);
+
+ chacha20_permute(x);

for (i = 0; i < ARRAY_SIZE(x); i++)
- out[i] = cpu_to_le32(x[i] + state[i]);
+ stream[i] = cpu_to_le32(x[i] + state[i]);

state[12]++;
}
EXPORT_SYMBOL(chacha20_block);
+
+/**
+ * hchacha20_block - abbreviated ChaCha20 core, for XChaCha20
+ * @in: input state matrix (16 32-bit words)
+ * @out: output (8 32-bit words)
+ *
+ * HChaCha20 is the ChaCha equivalent of HSalsa20 and is an intermediate step
+ * towards XChaCha20 (see https://cr.yp.to/snuffle/xsalsa-20081128.pdf).
+ * HChaCha20 skips the final addition of the initial state, and outputs only
+ * certain words of the state. It should not be used for streaming directly.
+ */
+void hchacha20_block(const u32 *in, u32 *out)
+{
+ u32 x[16];
+
+ memcpy(x, in, 64);
+
+ chacha20_permute(x);
+
+ memcpy(&out[0], &x[0], 16);
+ memcpy(&out[4], &x[12], 16);
+}
+EXPORT_SYMBOL(hchacha20_block);
--
2.18.0.597.ga71716f1ad-goog
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