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Use a constant-time MAX function

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Bug: 146520538
Test: atest HadamardTest
Change-Id: Ife1012c14d697141e6ee0c583dc32eaacdb72b73
This commit is contained in:
Paul Crowley
2020-03-11 16:30:18 -07:00
parent e648f7ce42
commit b0d2062abe
1 changed files with 32 additions and 14 deletions
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46
rebootescrow/aidl/default/HadamardUtils.cpp
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@@ -16,8 +16,6 @@
#include <HadamardUtils.h>
#include <limits>
#include <android-base/logging.h>
namespace aidl {
@@ -92,6 +90,31 @@ std::vector<uint8_t> EncodeKey(const std::vector<uint8_t>& input) {
return result;
}
// Constant-time conditional copy, to fix b/146520538
// ctl must be 0 or 1; we do the copy if it's 1.
static void CondCopy(uint32_t ctl, void* dest, const void* src, size_t len) {
const auto cdest = reinterpret_cast<uint8_t*>(dest);
const auto csrc = reinterpret_cast<const uint8_t*>(src);
for (size_t i = 0; i < len; i++) {
const uint32_t d = cdest[i];
const uint32_t s = csrc[i];
cdest[i] = d ^ (-ctl & (s ^ d));
}
}
struct CodewordWinner {
uint16_t codeword;
int32_t score;
};
// Replace dest with src if it has a higher score
static void CopyWinner(CodewordWinner* dest, const CodewordWinner& src) {
// Scores are between - 2^15 and 2^15, so taking the difference won't
// overflow; we use the sign bit of the difference here.
CondCopy(static_cast<uint32_t>(dest->score - src.score) >> 31, dest, &src,
sizeof(CodewordWinner));
}
// Decode a single codeword. Because of the way codewords are striped together
// this takes the entire input, plus an offset telling it which word to decode.
static uint16_t DecodeWord(size_t word, const std::vector<uint8_t>& encoded) {
@@ -118,20 +141,15 @@ static uint16_t DecodeWord(size_t word, const std::vector<uint8_t>& encoded) {
}
}
}
auto hiscore = std::numeric_limits<int32_t>::min();
uint16_t winner;
// TODO(b/146520538): this needs to be constant time
// -ENCODE_LENGTH is least possible score, so start one less than that
auto best = CodewordWinner{0, -static_cast<int32_t>(ENCODE_LENGTH + 1)};
// For every possible codeword value, look at its score, and replace best if it's higher,
// in constant time.
for (size_t i = 0; i < ENCODE_LENGTH; i++) {
if (scores[i] > hiscore) {
winner = i;
hiscore = scores[i];
} else if (-scores[i] > hiscore) {
winner = i | (1 << CODE_K);
hiscore = -scores[i];
}
CopyWinner(&best, CodewordWinner{static_cast<uint16_t>(i), scores[i]});
CopyWinner(&best, CodewordWinner{static_cast<uint16_t>(i | (1 << CODE_K)), -scores[i]});
}
return winner;
return best.codeword;
}
std::vector<uint8_t> DecodeKey(const std::vector<uint8_t>& shuffled) {