From 8123ce62094bf88a4107506d7acd3e8e2866bc1f Mon Sep 17 00:00:00 2001
From: Mark Haines <mark.haines@matrix.org>
Date: Tue, 3 Mar 2015 15:08:56 +0000
Subject: Constant time comparison for mac
---
include/axolotl/memory.hh | 11 +++++++++--
src/memory.cpp | 20 +++++++++++++++++---
src/ratchet.cpp | 2 +-
3 files changed, 27 insertions(+), 6 deletions(-)
diff --git a/include/axolotl/memory.hh b/include/axolotl/memory.hh
index 7749c54..8313a19 100644
--- a/include/axolotl/memory.hh
+++ b/include/axolotl/memory.hh
@@ -1,17 +1,24 @@
#include <cstddef>
+#include <cstdint>
namespace axolotl {
/** Clear the memory held in the buffer */
void unset(
- volatile void * buffer, std::size_t buffer_length
+ void volatile * buffer, std::size_t buffer_length
);
/** Clear the memory backing an object */
template<typename T>
void unset(T & value) {
- unset(reinterpret_cast<volatile void *>(&value), sizeof(T));
+ unset(reinterpret_cast<void volatile *>(&value), sizeof(T));
}
+/** Check if two buffers are equal in constant time. */
+bool is_equal(
+ std::uint8_t const * buffer_a,
+ std::uint8_t const * buffer_b,
+ std::size_t length
+);
} // namespace axolotl
diff --git a/src/memory.cpp b/src/memory.cpp
index 14c95dd..07e8de2 100644
--- a/src/memory.cpp
+++ b/src/memory.cpp
@@ -1,11 +1,25 @@
#include "axolotl/memory.hh"
+
void axolotl::unset(
- volatile void * buffer, std::size_t buffer_length
+ void volatile * buffer, std::size_t buffer_length
) {
- volatile char * pos = reinterpret_cast<volatile char *>(buffer);
- volatile char * end = pos + buffer_length;
+ char volatile * pos = reinterpret_cast<char volatile *>(buffer);
+ char volatile * end = pos + buffer_length;
while (pos != end) {
*(pos++) = 0;
}
}
+
+
+bool axolotl::is_equal(
+ std::uint8_t const * buffer_a,
+ std::uint8_t const * buffer_b,
+ std::size_t length
+) {
+ std::uint8_t volatile result = 0;
+ while (length--) {
+ result |= (*(buffer_a++)) ^ (*(buffer_b++));
+ }
+ return result == 0;
+}
diff --git a/src/ratchet.cpp b/src/ratchet.cpp
index d2903bb..ccb8e73 100644
--- a/src/ratchet.cpp
+++ b/src/ratchet.cpp
@@ -104,7 +104,7 @@ bool verify_mac(
mac
);
- bool result = std::memcmp(mac, reader.mac, MAC_LENGTH) == 0;
+ bool result = axolotl::is_equal(mac, reader.mac, MAC_LENGTH);
axolotl::unset(mac);
return result;
}
--
cgit v1.2.3-70-g09d2