From 7226aee256fae278f07edb49f97ffa8074036830 Mon Sep 17 00:00:00 2001
From: "Kasper S. Nielsen" <kasper.s.nielsen@partisia.com>
Date: Tue, 19 May 2026 12:55:09 +0200
Subject: [PATCH] Fix truncate negative numbers

---
 .../lib/common/math/integer/binary/Truncate.java  | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/lib/common/src/main/java/dk/alexandra/fresco/lib/common/math/integer/binary/Truncate.java b/lib/common/src/main/java/dk/alexandra/fresco/lib/common/math/integer/binary/Truncate.java
index f0efeb0c4..f4a7136b5 100644
--- a/lib/common/src/main/java/dk/alexandra/fresco/lib/common/math/integer/binary/Truncate.java
+++ b/lib/common/src/main/java/dk/alexandra/fresco/lib/common/math/integer/binary/Truncate.java
@@ -49,13 +49,15 @@ public DRes<SInt> buildComputation(ProtocolBuilderNumeric builder) {
 
     }).seq((seq, randomAdditiveMask) -> {
 
-      DRes<SInt> result = seq.numeric().add(input, randomAdditiveMask.value);
+      DRes<SInt> b = seq.numeric().add(BigInteger.ONE.shiftLeft(maxBitLength - 1), input);
+      DRes<SInt> result = seq.numeric().add(b, randomAdditiveMask.value);
       DRes<BigInteger> open = seq.numeric().open(result);
       return Pair.lazy(open, randomAdditiveMask);
 
     }).seq((seq, maskedInput) -> {
 
       BigInteger masked = maskedInput.getFirst().out();
+      BigInteger maskedPrime = masked.mod(BigInteger.ONE.shiftLeft(shifts));
       RandomAdditiveMask mask = maskedInput.getSecond();
 
       /*
@@ -66,15 +68,10 @@ public DRes<SInt> buildComputation(ProtocolBuilderNumeric builder) {
 
       BigInteger inverse =
           BigInteger.ONE.shiftLeft(shifts).modInverse(seq.getBasicNumericContext().getModulus());
-      DRes<SInt> rTop = seq.numeric().sub(mask.value, rBottom);
 
-      /*
-       * rTop is r with the last shifts bits set to zero, and it is hence divisible by 2^shifts, so
-       * multiplying with the inverse in the field corresponds to shifting.
-       */
-      DRes<SInt> rShifted = seq.numeric().mult(inverse, rTop);
-      BigInteger mShifted = masked.shiftRight(shifts);
-      return seq.numeric().sub(mShifted, rShifted);
+      DRes<SInt> aPrime = seq.numeric().sub(maskedPrime, rBottom);
+      DRes<SInt> d = seq.numeric().sub(input, aPrime);
+      return seq.numeric().mult(inverse, d);
     });
   }
 }