write in 30M

hc

2024-02-20 ea08eeccae9297f7aabd2ef7f0c2517ac4549acc

 kernel/crypto/ghash-generic.c
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@@ -1,15 +1,37 @@
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+// SPDX-License-Identifier: GPL-2.0-only
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 /*
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- * GHASH: digest algorithm for GCM (Galois/Counter Mode).
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+ * GHASH: hash function for GCM (Galois/Counter Mode).
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  *
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  * Copyright (c) 2007 Nokia Siemens Networks - Mikko Herranen <mh1@iki.fi>
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  * Copyright (c) 2009 Intel Corp.
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  *   Author: Huang Ying <ying.huang@intel.com>
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+ */
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+
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+/*
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+ * GHASH is a keyed hash function used in GCM authentication tag generation.
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  *
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- * The algorithm implementation is copied from gcm.c.
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+ * The original GCM paper [1] presents GHASH as a function GHASH(H, A, C) which
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+ * takes a 16-byte hash key H, additional authenticated data A, and a ciphertext
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+ * C.  It formats A and C into a single byte string X, interprets X as a
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+ * polynomial over GF(2^128), and evaluates this polynomial at the point H.
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  *
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- * This program is free software; you can redistribute it and/or modify it
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- * under the terms of the GNU General Public License version 2 as published
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- * by the Free Software Foundation.
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+ * However, the NIST standard for GCM [2] presents GHASH as GHASH(H, X) where X
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+ * is the already-formatted byte string containing both A and C.
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+ *
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+ * "ghash" in the Linux crypto API uses the 'X' (pre-formatted) convention,
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+ * since the API supports only a single data stream per hash.  Thus, the
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+ * formatting of 'A' and 'C' is done in the "gcm" template, not in "ghash".
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+ *
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+ * The reason "ghash" is separate from "gcm" is to allow "gcm" to use an
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+ * accelerated "ghash" when a standalone accelerated "gcm(aes)" is unavailable.
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+ * It is generally inappropriate to use "ghash" for other purposes, since it is
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+ * an "ε-almost-XOR-universal hash function", not a cryptographic hash function.
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+ * It can only be used securely in crypto modes specially designed to use it.
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+ *
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+ * [1] The Galois/Counter Mode of Operation (GCM)
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+ *     (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.694.695&rep=rep1&type=pdf)
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+ * [2] Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC
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+ *     (https://csrc.nist.gov/publications/detail/sp/800-38d/final)
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  */
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 #include <crypto/algapi.h>
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@@ -36,10 +58,8 @@
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 	struct ghash_ctx *ctx = crypto_shash_ctx(tfm);
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 	be128 k;
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-	if (keylen != GHASH_BLOCK_SIZE) {
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-		crypto_shash_set_flags(tfm, CRYPTO_TFM_RES_BAD_KEY_LEN);
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+	if (keylen != GHASH_BLOCK_SIZE)
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 		return -EINVAL;
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-	}
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 	if (ctx->gf128)
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 		gf128mul_free_4k(ctx->gf128);
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@@ -155,10 +175,10 @@
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 	crypto_unregister_shash(&ghash_alg);
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 }
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-module_init(ghash_mod_init);
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+subsys_initcall(ghash_mod_init);
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 module_exit(ghash_mod_exit);
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 MODULE_LICENSE("GPL");
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-MODULE_DESCRIPTION("GHASH Message Digest Algorithm");
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+MODULE_DESCRIPTION("GHASH hash function");
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 MODULE_ALIAS_CRYPTO("ghash");
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 MODULE_ALIAS_CRYPTO("ghash-generic");