lib: fix unsigned conversion of 4-byte RSA publicExponent

`bigIntArrayToUnsignedInt` used the signed `<<` operator, so when the
most significant byte of a 4-byte input had its top bit set (e.g.
`[0x80, 0x00, 0x00, 0x01]`) the result was a negative Int32 instead of
the intended unsigned 32-bit value. This caused any RSA `publicExponent`
exactly 4 bytes long with the top bit set to be parsed incorrectly.
Coerce the final value with `>>> 0` and add a unit test.

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: DeepView Autofix <276251120+deepview-autofix@users.noreply.github.com>
Co-Authored-By: Nikita Skovoroda <chalkerx@gmail.com>
Signed-off-by: Nikita Skovoroda <chalkerx@gmail.com>

Author: 3 people

lib/internal/crypto/util.js

@@ -696,7 +696,10 @@ function bigIntArrayToUnsignedInt(input) {
     result |= input[n] << 8 * n_reversed;
   }
 
-  return result;
+  // Coerce the result to an unsigned 32-bit integer. Without this, a most
+  // significant byte with the top bit set (e.g. [0x80, 0x00, 0x00, 0x01])
+  // would be returned as a negative Int32 because of the signed `<<` operator.
+  return result >>> 0;
 }
 
 function bigIntArrayToUnsignedBigInt(input) {

test/parallel/test-webcrypto-util.js

@@ -8,9 +8,28 @@ if (!common.hasCrypto)
 const assert = require('assert');
 
 const {
+  bigIntArrayToUnsignedInt,
   normalizeAlgorithm,
 } = require('internal/crypto/util');
 
+// bigIntArrayToUnsignedInt must return an unsigned 32-bit value even when
+// the most significant byte has its top bit set. Otherwise the signed `<<`
+// operator yields a negative Int32 for inputs like [0x80, 0x00, 0x00, 0x01].
+{
+  assert.strictEqual(
+    bigIntArrayToUnsignedInt(new Uint8Array([0x80, 0x00, 0x00, 0x01])),
+    0x80000001);
+  assert.strictEqual(
+    bigIntArrayToUnsignedInt(new Uint8Array([0xff, 0xff, 0xff, 0xff])),
+    0xffffffff);
+  assert.strictEqual(
+    bigIntArrayToUnsignedInt(new Uint8Array([1, 0, 1])),
+    65537);
+  assert.strictEqual(
+    bigIntArrayToUnsignedInt(new Uint8Array([1, 0, 0, 0, 0])),
+    undefined);
+}
+
 {
   // Check that normalizeAlgorithm does not mutate object inputs.
   const algorithm = { name: 'ECDSA', hash: 'SHA-256' };