Merge pull request #428 from JesperBC/Par-14148_LT_takes_bitlength

Changed compareLT and LessThanZero to take bitlength

lib/common/src/main/java/dk/alexandra/fresco/lib/common/compare/Comparison.java

@@ -72,20 +72,29 @@ default DRes<SInt> equals(DRes<SInt> x, DRes<SInt> y, int bitlength) {
   /**
    * Computes if x &lt; y.
    *
-   * @param x the first input
-   * @param y the second input
+   * @param x the first input. Must be less than 2^{@code bitlength}
+   * @param y the second input. Must be less than 2^{@code bitlength}
+   * @param bitlength the amount of bits to do the comparison on. Must be less than or equal to
+   * the max bitlength allowed
    * @param algorithm the algorithm to use
-   * @return A deferred result computing x &lt; y. Result will be either [1] (true) or [0] (false).
+   * @return A deferred result computing x' &lt; y'. Where x' and y' represent the {@code bitlength}
+   * least significant bits of x, respectively y. Result will be either [1] (true) or [0] (false).
    */
-  DRes<SInt> compareLT(DRes<SInt> x, DRes<SInt> y, Algorithm algorithm);
+  DRes<SInt> compareLT(DRes<SInt> x, DRes<SInt> y, int bitlength, Algorithm algorithm);
 
   /**
-   * Call to {@link #compareLT(DRes, DRes, Algorithm)} with default comparison algorithm.
+   * Call to {@link #compareLT(DRes, DRes, int, Algorithm)} with default comparison algorithm.
    */
-  default DRes<SInt> compareLT(DRes<SInt> x, DRes<SInt> y) {
-    return compareLT(x, y, Algorithm.LOG_ROUNDS);
+  default DRes<SInt> compareLT(DRes<SInt> x, DRes<SInt> y, int bitlength) {
+    return compareLT(x, y, bitlength, Algorithm.LOG_ROUNDS);
   }
 
+  /**
+   * Call to {@link #compareLT(DRes, DRes, int, Algorithm)} with default comparison algorithm,
+   * comparing all bits.
+   */
+  DRes<SInt> compareLT(DRes<SInt> x, DRes<SInt> y);
+
   /**
    * Computes if the bit decomposition of an open value is less than the bit decomposition of a
    * secret value.

lib/common/src/main/java/dk/alexandra/fresco/lib/common/compare/DefaultComparison.java

@@ -59,21 +59,29 @@ public DRes<SInt> compareLEQ(DRes<SInt> x, DRes<SInt> y) {
   }
 
   @Override
-  public DRes<SInt> compareLT(DRes<SInt> x, DRes<SInt> y, Algorithm algorithm) {
+  public DRes<SInt> compareLT(DRes<SInt> x, DRes<SInt> y, int bitlength, Algorithm algorithm) {
+    if (bitlength > builder.getBasicNumericContext().getMaxBitLength()) {
+      throw new IllegalArgumentException("The bitlength is more than allowed for elements.");
+    }
     if (algorithm == Algorithm.LOG_ROUNDS) {
-      if (builder.getBasicNumericContext().getStatisticalSecurityParam() + builder
-          .getBasicNumericContext().getMaxBitLength() > builder.getBasicNumericContext()
-          .getModulus().bitLength()) {
+      if (builder.getBasicNumericContext().getStatisticalSecurityParam() + bitlength
+          > builder.getBasicNumericContext().getModulus().bitLength()) {
         throw new IllegalArgumentException(
             "The max bitlength plus the statistical security parameter overflows the size of the modulus.");
       }
       DRes<SInt> difference = builder.numeric().sub(x, y);
-      return builder.seq(new LessThanZero(difference));
+      return builder.seq(new LessThanZero(difference, bitlength));
     } else {
       throw new UnsupportedOperationException("Not implemented yet");
     }
   }
 
+  @Override
+  public DRes<SInt> compareLT(DRes<SInt> x, DRes<SInt> y) {
+    int maxBitLength = builder.getBasicNumericContext().getMaxBitLength();
+    return compareLT(x, y, maxBitLength);
+  }
+
   @Override
   public DRes<SInt> compareLTBits(BigInteger openValue, DRes<List<DRes<SInt>>> secretBits) {
     return builder.seq(new BitLessThanOpen(openValue, secretBits));

lib/common/src/main/java/dk/alexandra/fresco/lib/common/compare/lt/LessThanZero.java

@@ -10,24 +10,26 @@
 
 /**
  * Given secret value a, computes a <? 0.
+ * https://www.researchgate.net/publication/225092133_Improved_Primitives_for_Secure_Multiparty_Integer_Computation
  */
 public class LessThanZero implements Computation<SInt, ProtocolBuilderNumeric> {
-  // TODO add reference to protocol description
 
   private final DRes<SInt> input;
+  private final int maxBitlength;
 
   /**
    * Constructs new {@link LessThanZero}.
    *
    * @param input input to compare to 0
+   * @param maxBitlength number of bits to compare
    */
-  public LessThanZero(DRes<SInt> input) {
+  public LessThanZero(DRes<SInt> input, int maxBitlength) {
     this.input = input;
+    this.maxBitlength = maxBitlength;
   }
 
   @Override
   public DRes<SInt> buildComputation(ProtocolBuilderNumeric builder) {
-    final int maxBitlength = builder.getBasicNumericContext().getMaxBitLength();
     final int statisticalSecurity = builder.getBasicNumericContext()
         .getStatisticalSecurityParam();
     return builder.seq(seq -> {

lib/common/src/main/java/dk/alexandra/fresco/lib/common/math/integer/binary/NormalizeSInt.java

@@ -36,7 +36,9 @@ public DRes<Pair<DRes<SInt>, DRes<SInt>>> buildComputation(ProtocolBuilderNumeri
       DRes<List<DRes<SInt>>> bits = advancedNumeric.toBits(input, l);
 
       // Sign bit (0 or 1)
-      DRes<SInt> signBit = new LessThanZero(input).buildComputation(par);
+      DRes<SInt> signBit =
+          new LessThanZero(input, par.getBasicNumericContext().getMaxBitLength())
+              .buildComputation(par);
 
       return Pair.lazy(bits, signBit);
     }).seq((seq, params) -> {

lib/common/src/test/java/dk/alexandra/fresco/lib/common/compare/CompareTests.java

@@ -331,7 +331,7 @@ public void test() {
             List<DRes<SInt>> actualInner = new ArrayList<>(left.size());
             for (int i = 0; i < left.size(); i++) {
               actualInner.add(Comparison.using(builder).compareLT(left.get(i), right.get(i),
-                  Comparison.Algorithm.LOG_ROUNDS));
+                  builder.getBasicNumericContext().getMaxBitLength(), Comparison.Algorithm.LOG_ROUNDS));
             }
             DRes<List<DRes<BigInteger>>> opened = Collections.using(builder).openList(() -> actualInner);
             return () -> opened.out().stream().map(DRes::out).collect(Collectors.toList());
@@ -437,7 +437,7 @@ public void test() throws Exception {
             DRes<SInt> x = input.known(BigInteger.valueOf(3));
             DRes<SInt> y = input.known(BigInteger.valueOf(5));
             Comparison comparison = Comparison.using(builder);
-            DRes<SInt> result = comparison.compareLT(x, y, Algorithm.CONST_ROUNDS);
+            DRes<SInt> result = comparison.compareLT(x, y, 3, Algorithm.CONST_ROUNDS);
             return input.open(result);
           };
           runApplication(app);

lib/common/src/test/java/dk/alexandra/fresco/lib/common/compare/lt/LessThanZeroTests.java

@@ -53,10 +53,11 @@ public TestThread<ResourcePoolT, ProtocolBuilderNumeric> next() {
         public void test() {
           Application<List<BigInteger>, ProtocolBuilderNumeric> app = builder -> {
             Numeric numeric = builder.numeric();
-            List<DRes<SInt>> inputs = openInputs.stream().map(numeric::known).collect(Collectors.toList());;
+            List<DRes<SInt>> inputs = openInputs.stream().map(numeric::known).collect(Collectors.toList());
             List<DRes<SInt>> actualInner = new ArrayList<>(inputs.size());
             for (DRes<SInt> input : inputs) {
-              actualInner.add(builder.seq(new LessThanZero(input)));
+              actualInner.add(builder.seq(
+                  new LessThanZero(input, builder.getBasicNumericContext().getMaxBitLength())));
             }
             DRes<List<DRes<BigInteger>>> opened = Collections.using(builder).openList(DRes.of(actualInner));
             return () -> opened.out().stream().map(DRes::out).collect(Collectors.toList());