/* * Timing Attack Countermeasure Functions * (C) 2010 Falko Strenzke, Jack Lloyd * * Botan is released under the Simplified BSD License (see license.txt) */ #include <botan/internal/ta_utils.h> namespace Botan { namespace TA_CM { /* * We use volatile in these functions in an attempt to ensure that the * compiler doesn't optimize in a way that would create branching * operations. * * Note: this needs further testing; on at least x86-64 with GCC, * volatile is not required to get branch-free operations, it just * makes the functions much longer/slower. It may not be required * anywhere. */ namespace { template<typename T> T expand_mask(T x) { volatile T r = x; for(size_t i = 1; i != sizeof(T) * 8; i *= 2) r |= r >> i; r &= 1; r = ~(r - 1); return r; } } u32bit expand_mask_u32bit(u32bit in) { return expand_mask<u32bit>(in); } u16bit expand_mask_u16bit(u16bit in) { return expand_mask<u16bit>(in); } u32bit max_32(u32bit a, u32bit b) { const u32bit a_larger = b - a; /* negative if a larger */ const u32bit mask = expand_mask<u32bit>(a_larger >> 31); return (a & mask) | (b & ~mask); } u32bit min_32(u32bit a, u32bit b) { const u32bit a_larger = b - a; /* negative if a larger */ const u32bit mask = expand_mask<u32bit>(a_larger >> 31); return (a & ~mask) | (b & mask); } } }