From 2ceda579893ceb23c5eb0d56df47dc235644e0f4 Mon Sep 17 00:00:00 2001
From: Paul Bakker <p.j.bakker@polarssl.org>
Date: Thu, 6 Feb 2014 15:55:25 +0100
Subject: [PATCH] Ability to force the Entropy module to use SHA-256 as its
basis
By default the SHA-512 module is used if both are available. On some
systems, SHA-256 is the better choice.
Contributed by: Gergely Budai
---
ChangeLog | 2 ++
include/polarssl/config.h | 22 +++++++++++++++++++++-
include/polarssl/ctr_drbg.h | 2 +-
include/polarssl/entropy.h | 2 +-
4 files changed, 25 insertions(+), 3 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index baf67fee3f..2e2ecb87a9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -11,6 +11,8 @@ Features
and CRLs
* Single Platform compatilibity layer (for memory / printf / fprintf)
* Ability to provide alternate timing implementation
+ * Ability to force the entropy module to use SHA-256 as its basis
+ (POLARSSL_ENTROPY_FORCE_SHA256)
Changes
* Deprecated the Memory layer
diff --git a/include/polarssl/config.h b/include/polarssl/config.h
index 5c36ae6b3c..db34e6a298 100644
--- a/include/polarssl/config.h
+++ b/include/polarssl/config.h
@@ -665,6 +665,22 @@
*/
//#define POLARSSL_NO_PLATFORM_ENTROPY
+/**
+ * \def POLARSSL_ENTROPY_FORCE_SHA256
+ *
+ * Force the entropy accumulator to use a SHA-256 accumulator instead of the
+ * default SHA-512 based one (if both are available).
+ *
+ * Requires: POLARSSL_SHA256_C
+ *
+ * On 32-bit systems SHA-256 can be much faster than SHA-512. Use this option
+ * if you have performance concerns.
+ *
+ * This option is only useful if both POLARSSL_SHA256_C and
+ * POLARSSL_SHA512_C are defined. Otherwise the available hash module is used.
+ */
+//#define POLARSSL_ENTROPY_FORCE_SHA256
+
/**
* \def POLARSSL_MEMORY_DEBUG
*
@@ -1334,7 +1350,7 @@
* Module: library/entropy.c
* Caller:
*
- * Requires: POLARSSL_SHA512_C
+ * Requires: POLARSSL_SHA512_C or POLARSSL_SHA256_C
*
* This module provides a generic entropy pool
*/
@@ -2103,6 +2119,10 @@
defined(POLARSSL_CONFIG_OPTIONS) && (CTR_DRBG_ENTROPY_LEN > 32)
#error "CTR_DRBG_ENTROPY_LEN value too high"
#endif
+#if defined(POLARSSL_ENTROPY_C) && \
+ defined(POLARSSL_ENTROPY_FORCE_SHA256) && !defined(POLARSSL_SHA256_C)
+#error "POLARSSL_ENTROPY_FORCE_SHA256 defined, but not all prerequisites"
+#endif
#if defined(POLARSSL_GCM_C) && ( \
!defined(POLARSSL_AES_C) && !defined(POLARSSL_CAMELLIA_C) )
diff --git a/include/polarssl/ctr_drbg.h b/include/polarssl/ctr_drbg.h
index 756b5a3261..8b0f38a159 100644
--- a/include/polarssl/ctr_drbg.h
+++ b/include/polarssl/ctr_drbg.h
@@ -43,7 +43,7 @@
/**< The seed length (counter + AES key) */
#if !defined(POLARSSL_CONFIG_OPTIONS)
-#if defined(POLARSSL_SHA512_C)
+#if defined(POLARSSL_SHA512_C) && !defined(POLARSSL_ENTROPY_FORCE_SHA256)
#define CTR_DRBG_ENTROPY_LEN 48 /**< Amount of entropy used per seed by default (48 with SHA-512, 32 with SHA-256) */
#else
#define CTR_DRBG_ENTROPY_LEN 32 /**< Amount of entropy used per seed by default (48 with SHA-512, 32 with SHA-256) */
diff --git a/include/polarssl/entropy.h b/include/polarssl/entropy.h
index 2b824ef6dd..c4d49556f6 100644
--- a/include/polarssl/entropy.h
+++ b/include/polarssl/entropy.h
@@ -31,7 +31,7 @@
#include "config.h"
-#if defined(POLARSSL_SHA512_C)
+#if defined(POLARSSL_SHA512_C) && !defined(POLARSSL_FORCE_ENTROPY_SHA256)
#include "sha512.h"
#define POLARSSL_ENTROPY_SHA512_ACCUMULATOR
#else