BigInteger (fallback): add support for negative shift

Native BigInts support negative shifts, which are needed for noble-curves (secp256k1)
openpgpjs · Jul 21, 2023 · 79d1b86 · 79d1b86
1 parent ca8b544
commit 79d1b86
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Showing 2 changed files with 18 additions and 2 deletions.
diff --git a/src/biginteger/bn.interface.ts b/src/biginteger/bn.interface.ts
@@ -205,7 +205,10 @@ export default class BNBigInteger extends AbstractBigInteger {
    * Shift this to the left by x, in place
    * @param {BigInteger} x - Shift value
    */
-  ileftShift(x: BNBigInteger) {
+  ileftShift(x: BNBigInteger): this {
+    if (x.isNegative()) {
+      return this.irightShift(x.negate());
+    }
     this.value.ishln(x.value.toNumber());
     return this;
   }
@@ -223,7 +226,10 @@ export default class BNBigInteger extends AbstractBigInteger {
    * Shift this to the right by x, in place
    * @param {BigInteger} x - Shift value
    */
-  irightShift(x: BNBigInteger) {
+  irightShift(x: BNBigInteger): this {
+    if (x.isNegative()) {
+      return this.ileftShift(x.negate());
+    }
     this.value.ishrn(x.value.toNumber());
     return this;
   }

diff --git a/test/biginteger.test.js b/test/biginteger.test.js
@@ -141,6 +141,16 @@ function bigIntegerTests (implementation) {
     });
   });
 
+  should('support shifting by negative values', function() {
+    const four = BigInteger.new(4);
+    const two = BigInteger.new(2);
+    const eight = BigInteger.new(8);
+    const minusOne = BigInteger.new(-1);
+
+    ok(four.rightShift(minusOne).equal(eight));
+    ok(four.leftShift(minusOne).equal(two));
+  });
+
   should('unary operators are consistent', function() {
     const a = BigInteger.new(12);
     const zero = BigInteger.new(0);