crypto: fix randomInt bias

Co-authored-by: Andrey Pechkurov <[email protected]>

PR-URL: #36894
Refs: #34600
Reviewed-By: Andrey Pechkurov <[email protected]>
Reviewed-By: James M Snell <[email protected]>

Author: tniessen

lib/internal/crypto/random.js

@@ -176,30 +176,29 @@ function randomInt(min, max, callback) {
                                `<= ${RAND_MAX}`, range);
   }
 
-  const excess = RAND_MAX % range;
-  const randLimit = RAND_MAX - excess;
+  // For (x % range) to produce an unbiased value greater than or equal to 0 and
+  // less than range, x must be drawn randomly from the set of integers greater
+  // than or equal to 0 and less than randLimit.
+  const randLimit = RAND_MAX - (RAND_MAX % range);
 
   if (isSync) {
     // Sync API
     while (true) {
       const x = randomBytes(6).readUIntBE(0, 6);
-      // If x > (maxVal - (maxVal % range)), we will get "modulo bias"
-      if (x > randLimit) {
-        // Try again
+      if (x >= randLimit) {
+        // Try again.
         continue;
       }
-      const n = (x % range) + min;
-      return n;
+      return (x % range) + min;
     }
   } else {
     // Async API
     const pickAttempt = () => {
       randomBytes(6, (err, bytes) => {
         if (err) return callback(err);
         const x = bytes.readUIntBE(0, 6);
-        // If x > (maxVal - (maxVal % range)), we will get "modulo bias"
-        if (x > randLimit) {
-          // Try again
+        if (x >= randLimit) {
+          // Try again.
           return pickAttempt();
         }
         const n = (x % range) + min;