path: root/crypto
diff options
authorEric Biggers <ebiggers@google.com>2019-07-19 23:09:18 -0700
committerHerbert Xu <herbert@gondor.apana.org.au>2019-07-27 21:08:38 +1000
commit8dfa20fcfbeb245642dfe3a43f8a3735d9aed42a (patch)
tree887e9ffb1793ff6f754ab839a528a170177f219d /crypto
parentpadata: purge get_cpu and reorder_via_wq from padata_do_serial (diff)
crypto: ghash - add comment and improve help text
To help avoid confusion, add a comment to ghash-generic.c which explains the convention that the kernel's implementation of GHASH uses. Also update the Kconfig help text and module descriptions to call GHASH a "hash function" rather than a "message digest", since the latter normally means a real cryptographic hash function, which GHASH is not. Cc: Pascal Van Leeuwen <pvanleeuwen@verimatrix.com> Signed-off-by: Eric Biggers <ebiggers@google.com> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Acked-by: Pascal Van Leeuwen <pvanleeuwen@verimatrix.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Diffstat (limited to 'crypto')
2 files changed, 34 insertions, 8 deletions
diff --git a/crypto/Kconfig b/crypto/Kconfig
index 2e7f08ba0675..455a3354e291 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -647,11 +647,12 @@ config CRYPTO_VPMSUM_TESTER
Unless you are testing these algorithms, you don't need this.
- tristate "GHASH digest algorithm"
+ tristate "GHASH hash function"
select CRYPTO_GF128MUL
- GHASH is message digest algorithm for GCM (Galois/Counter Mode).
+ GHASH is the hash function used in GCM (Galois/Counter Mode).
+ It is not a general-purpose cryptographic hash function.
config CRYPTO_POLY1305
tristate "Poly1305 authenticator algorithm"
@@ -976,12 +977,12 @@ config CRYPTO_WP512
- tristate "GHASH digest algorithm (CLMUL-NI accelerated)"
+ tristate "GHASH hash function (CLMUL-NI accelerated)"
depends on X86 && 64BIT
- GHASH is message digest algorithm for GCM (Galois/Counter Mode).
- The implementation is accelerated by CLMUL-NI of Intel.
+ This is the x86_64 CLMUL-NI accelerated implementation of
+ GHASH, the hash function used in GCM (Galois/Counter mode).
comment "Ciphers"
diff --git a/crypto/ghash-generic.c b/crypto/ghash-generic.c
index dad9e1f91a78..5027b3461c92 100644
--- a/crypto/ghash-generic.c
+++ b/crypto/ghash-generic.c
@@ -1,12 +1,37 @@
// SPDX-License-Identifier: GPL-2.0-only
- * GHASH: digest algorithm for GCM (Galois/Counter Mode).
+ * GHASH: hash function for GCM (Galois/Counter Mode).
* Copyright (c) 2007 Nokia Siemens Networks - Mikko Herranen <mh1@iki.fi>
* Copyright (c) 2009 Intel Corp.
* Author: Huang Ying <ying.huang@intel.com>
+ */
+ * GHASH is a keyed hash function used in GCM authentication tag generation.
+ *
+ * The original GCM paper [1] presents GHASH as a function GHASH(H, A, C) which
+ * takes a 16-byte hash key H, additional authenticated data A, and a ciphertext
+ * C. It formats A and C into a single byte string X, interprets X as a
+ * polynomial over GF(2^128), and evaluates this polynomial at the point H.
+ *
+ * However, the NIST standard for GCM [2] presents GHASH as GHASH(H, X) where X
+ * is the already-formatted byte string containing both A and C.
+ *
+ * "ghash" in the Linux crypto API uses the 'X' (pre-formatted) convention,
+ * since the API supports only a single data stream per hash. Thus, the
+ * formatting of 'A' and 'C' is done in the "gcm" template, not in "ghash".
+ *
+ * The reason "ghash" is separate from "gcm" is to allow "gcm" to use an
+ * accelerated "ghash" when a standalone accelerated "gcm(aes)" is unavailable.
+ * It is generally inappropriate to use "ghash" for other purposes, since it is
+ * an "ε-almost-XOR-universal hash function", not a cryptographic hash function.
+ * It can only be used securely in crypto modes specially designed to use it.
- * The algorithm implementation is copied from gcm.c.
+ * [1] The Galois/Counter Mode of Operation (GCM)
+ * (http://citeseerx.ist.psu.edu/viewdoc/download?doi=
+ * [2] Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC
+ * (https://csrc.nist.gov/publications/detail/sp/800-38d/final)
#include <crypto/algapi.h>
@@ -156,6 +181,6 @@ subsys_initcall(ghash_mod_init);
-MODULE_DESCRIPTION("GHASH Message Digest Algorithm");