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Fix truncate negative numbers #438

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Merged
quackzar merged 1 commit into from
May 19, 2026
Merged

Fix truncate negative numbers #438

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15 changes: 6 additions & 9 deletions lib/common/src/main/java/dk/alexandra/fresco/lib/common/math/integer/binary/Truncate.java
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Original file line number Diff line number Diff line change
Expand Up @@ -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();

/*
Expand All @@ -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);
});
}
}
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