Re: [PATCH] crypto: ghash - add comment and improve help text

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Sat, 20 Jul 2019 at 09:10, Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> 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>

> ---
>  arch/arm/crypto/ghash-ce-glue.c            |  2 +-
>  arch/s390/crypto/ghash_s390.c              |  2 +-
>  arch/x86/crypto/ghash-clmulni-intel_glue.c |  3 +--
>  crypto/Kconfig                             | 11 ++++----
>  crypto/ghash-generic.c                     | 31 +++++++++++++++++++---
>  drivers/crypto/Kconfig                     |  6 ++---
>  include/crypto/ghash.h                     |  2 +-
>  7 files changed, 41 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm/crypto/ghash-ce-glue.c b/arch/arm/crypto/ghash-ce-glue.c
> index 52d472a050e6a..bfdc557dc031c 100644
> --- a/arch/arm/crypto/ghash-ce-glue.c
> +++ b/arch/arm/crypto/ghash-ce-glue.c
> @@ -17,7 +17,7 @@
>  #include <linux/crypto.h>
>  #include <linux/module.h>
>
> -MODULE_DESCRIPTION("GHASH secure hash using ARMv8 Crypto Extensions");
> +MODULE_DESCRIPTION("GHASH hash function using ARMv8 Crypto Extensions");
>  MODULE_AUTHOR("Ard Biesheuvel <ard.biesheuvel@linaro.org>");
>  MODULE_LICENSE("GPL v2");
>  MODULE_ALIAS_CRYPTO("ghash");
> diff --git a/arch/s390/crypto/ghash_s390.c b/arch/s390/crypto/ghash_s390.c
> index eeeb6a7737a4a..a3e7400e031ca 100644
> --- a/arch/s390/crypto/ghash_s390.c
> +++ b/arch/s390/crypto/ghash_s390.c
> @@ -153,4 +153,4 @@ module_exit(ghash_mod_exit);
>  MODULE_ALIAS_CRYPTO("ghash");
>
>  MODULE_LICENSE("GPL");
> -MODULE_DESCRIPTION("GHASH Message Digest Algorithm, s390 implementation");
> +MODULE_DESCRIPTION("GHASH hash function, s390 implementation");
> diff --git a/arch/x86/crypto/ghash-clmulni-intel_glue.c b/arch/x86/crypto/ghash-clmulni-intel_glue.c
> index ac76fe88ac4fd..04d72a5a8ce98 100644
> --- a/arch/x86/crypto/ghash-clmulni-intel_glue.c
> +++ b/arch/x86/crypto/ghash-clmulni-intel_glue.c
> @@ -357,6 +357,5 @@ module_init(ghash_pclmulqdqni_mod_init);
>  module_exit(ghash_pclmulqdqni_mod_exit);
>
>  MODULE_LICENSE("GPL");
> -MODULE_DESCRIPTION("GHASH Message Digest Algorithm, "
> -                  "accelerated by PCLMULQDQ-NI");
> +MODULE_DESCRIPTION("GHASH hash function, accelerated by PCLMULQDQ-NI");
>  MODULE_ALIAS_CRYPTO("ghash");
> diff --git a/crypto/Kconfig b/crypto/Kconfig
> index e801450bcb1cf..f14c457183c55 100644
> --- a/crypto/Kconfig
> +++ b/crypto/Kconfig
> @@ -728,11 +728,12 @@ config CRYPTO_VPMSUM_TESTER
>           Unless you are testing these algorithms, you don't need this.
>
>  config CRYPTO_GHASH
> -       tristate "GHASH digest algorithm"
> +       tristate "GHASH hash function"
>         select CRYPTO_GF128MUL
>         select CRYPTO_HASH
>         help
> -         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"
> @@ -1057,12 +1058,12 @@ config CRYPTO_WP512
>           <http://www.larc.usp.br/~pbarreto/WhirlpoolPage.html>
>
>  config CRYPTO_GHASH_CLMUL_NI_INTEL
> -       tristate "GHASH digest algorithm (CLMUL-NI accelerated)"
> +       tristate "GHASH hash function (CLMUL-NI accelerated)"
>         depends on X86 && 64BIT
>         select CRYPTO_CRYPTD
>         help
> -         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 dad9e1f91a783..5027b3461c921 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=10.1.1.694.695&rep=rep1&type=pdf)
> + * [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_exit(ghash_mod_exit);
>
>  MODULE_LICENSE("GPL");
> -MODULE_DESCRIPTION("GHASH Message Digest Algorithm");
> +MODULE_DESCRIPTION("GHASH hash function");
>  MODULE_ALIAS_CRYPTO("ghash");
>  MODULE_ALIAS_CRYPTO("ghash-generic");
> diff --git a/drivers/crypto/Kconfig b/drivers/crypto/Kconfig
> index 603413f28fa35..43c36533322f1 100644
> --- a/drivers/crypto/Kconfig
> +++ b/drivers/crypto/Kconfig
> @@ -189,12 +189,12 @@ config S390_PRNG
>           It is available as of z9.
>
>  config CRYPTO_GHASH_S390
> -       tristate "GHASH digest algorithm"
> +       tristate "GHASH hash function"
>         depends on S390
>         select CRYPTO_HASH
>         help
> -         This is the s390 hardware accelerated implementation of the
> -         GHASH message digest algorithm for GCM (Galois/Counter Mode).
> +         This is the s390 hardware accelerated implementation of GHASH,
> +         the hash function used in GCM (Galois/Counter mode).
>
>           It is available as of z196.
>
> diff --git a/include/crypto/ghash.h b/include/crypto/ghash.h
> index 9136301062a5c..f832c9f2aca30 100644
> --- a/include/crypto/ghash.h
> +++ b/include/crypto/ghash.h
> @@ -1,6 +1,6 @@
>  /* SPDX-License-Identifier: GPL-2.0 */
>  /*
> - * Common values for GHASH algorithms
> + * Common values for the GHASH hash function
>   */
>
>  #ifndef __CRYPTO_GHASH_H__
> --
> 2.22.0
>