Remove pessimizing lookup tables

kimci86 · Aug 17, 2024 · 7b4ea52 · 7b4ea52
1 parent cc50681
commit 7b4ea52
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Showing 4 changed files with 6 additions and 24 deletions.
diff --git a/include/MultTab.hpp b/include/MultTab.hpp
@@ -7,18 +7,6 @@
 class MultTab
 {
 public:
-    /// \return mult * x using a lookup table
-    static auto getMult(std::uint8_t x) -> std::uint32_t
-    {
-        return instance.multtab[x];
-    }
-
-    /// \return mult^-1 * x using a lookup table
-    static auto getMultinv(std::uint8_t x) -> std::uint32_t
-    {
-        return instance.multinvtab[x];
-    }
-
     /// \return a vector of bytes x such that
     /// msb(x*mult^-1) is equal to msbprod or msbprod-1
     static auto getMsbProdFiber2(std::uint8_t msbprodinv) -> const std::vector<std::uint8_t>&
@@ -45,8 +33,6 @@ class MultTab
     MultTab();
 
     // lookup tables
-    std::array<std::uint32_t, 256>             multtab;
-    std::array<std::uint32_t, 256>             multinvtab;
     std::array<std::vector<std::uint8_t>, 256> msbprodfiber2;
     std::array<std::vector<std::uint8_t>, 256> msbprodfiber3;
 

diff --git a/src/Attack.cpp b/src/Attack.cpp
@@ -53,12 +53,12 @@ void Attack::exploreZlists(int i)
     else // the Z-list is complete so iterate over possible Y values
     {
         // guess Y7[8,24) and keep prod == (Y7[8,32) - 1) * mult^-1
-        for (auto y7_8_24 = std::uint32_t{}, prod = (MultTab::getMultinv(msb(ylist[7])) << 24) - MultTab::multInv;
+        for (auto y7_8_24 = std::uint32_t{}, prod = (MultTab::multInv * msb(ylist[7]) << 24) - MultTab::multInv;
              y7_8_24 < 1 << 24; y7_8_24 += 1 << 8, prod += MultTab::multInv << 8)
             // get possible Y7[0,8) values
             for (const auto y7_0_8 : MultTab::getMsbProdFiber3(msb(ylist[6]) - msb(prod)))
                 // filter Y7[0,8) using Y6[24,32)
-                if (prod + MultTab::getMultinv(y7_0_8) - (ylist[6] & mask<24, 32>) <= maxdiff<24>)
+                if (prod + MultTab::multInv * y7_0_8 - (ylist[6] & mask<24, 32>) <= maxdiff<24>)
                 {
                     ylist[7] = y7_0_8 | y7_8_24 | (ylist[7] & mask<24, 32>);
                     exploreYlists(7);
@@ -80,7 +80,7 @@ void Attack::exploreYlists(int i)
             const auto yim1 = fy - xi_0_8;
 
             // filter values with Y{i-2}[24,32)
-            if (ffy - MultTab::getMultinv(xi_0_8) - (ylist[i - 2] & mask<24, 32>) <= maxdiff<24> &&
+            if (ffy - MultTab::multInv * xi_0_8 - (ylist[i - 2] & mask<24, 32>) <= maxdiff<24> &&
                 msb(yim1) == msb(ylist[i - 1]))
             {
                 // add Y{i-1} to the Y-list

diff --git a/src/MultTab.cpp b/src/MultTab.cpp
@@ -4,13 +4,9 @@ const MultTab MultTab::instance;
 
 MultTab::MultTab()
 {
-    auto prod    = std::uint32_t{}; // x * mult
     auto prodinv = std::uint32_t{}; // x * mult^-1
-    for (auto x = 0; x < 256; x++, prod += mult, prodinv += multInv)
+    for (auto x = 0; x < 256; x++, prodinv += multInv)
     {
-        multtab[x]    = prod;
-        multinvtab[x] = prodinv;
-
         msbprodfiber2[msb(prodinv)].push_back(x);
         msbprodfiber2[(msb(prodinv) + 1) % 256].push_back(x);
 

diff --git a/src/password.cpp b/src/password.cpp
@@ -88,7 +88,7 @@ class SixCharactersRecovery
                 const auto yim1 = fy - xi_0_8;
 
                 // filter values with Y{i-2}[24,32)
-                if (ffy - MultTab::getMultinv(xi_0_8) - (y[i - 2] & mask<24, 32>) <= maxdiff<24> &&
+                if (ffy - MultTab::multInv * xi_0_8 - (y[i - 2] & mask<24, 32>) <= maxdiff<24> &&
                     msb(yim1) == msb(y[i - 1]))
                 {
                     // add Y{i-1} to the Y-list
@@ -255,7 +255,7 @@ class BruteforceRecovery : public SixCharactersRecovery<BruteforceRecovery>
             {
                 // finish to update the cipher state
                 const auto x0 = x0_partial ^ Crc32Tab::crc32(0, pi);
-                const auto y0 = y0_partial + MultTab::getMult(lsb(x0));
+                const auto y0 = y0_partial + MultTab::mult * lsb(x0);
                 const auto z0 = z0_partial ^ Crc32Tab::crc32(0, msb(y0));
 
                 // SixCharactersRecovery::search is inlined below for performance