add explanations about modulo by power of 2

erik-krogh · erik-krogh · commit 16ec405724f2 · 2020-06-10T10:38:47.000+02:00
diff --git a/javascript/ql/src/Security/CWE-327/BadRandomness.ql b/javascript/ql/src/Security/CWE-327/BadRandomness.ql
@@ -99,6 +99,7 @@ DataFlow::Node badCrypto(string description) {
     description = "modulo" and
     goodRandom() = random and
     random.flowsToExpr(mod.getLeftOperand()) and
+    // division by a power of 2 is OK. E.g. if `x` is uniformly random is in the range [0..255] then `x % 32` is uniformly random in the range [0..31]. 
     not mod.getRightOperand().getIntValue() = [2, 4, 8, 16, 32, 64, 128] and
     // not exists a comparison that checks if the result is potentially biased.
     not exists(BinaryExpr comparison | comparison.getOperator() = [">", "<", "<=", ">="] |
diff --git a/javascript/ql/test/query-tests/Security/CWE-327/bad-random.js b/javascript/ql/test/query-tests/Security/CWE-327/bad-random.js
@@ -7,7 +7,7 @@ const buffer = crypto.randomBytes(bytes);
 const digits = [];
 for (let i = 0; i < buffer.length; ++i) {
     digits.push(Math.floor(buffer[i] / 25.6)); // NOT OK
-    digits.push(buffer[i] % 8); // OK - 8 is a power of 2, so the result is unbiased.
+    digits.push(buffer[i] % 8); // OK - input is a random byte, so the output is a uniformly random number between 0 and 7. 
     digits.push(buffer[i] % 100); // NOT OK
 }
 

Original file line number	Diff line number	Diff line change
`@@ -7,7 +7,7 @@ const buffer = crypto.randomBytes(bytes);`
`7`	`7`	`const digits = [];`
`8`	`8`	`for (let i = 0; i < buffer.length; ++i) {`
`9`	`9`	`digits.push(Math.floor(buffer[i] / 25.6)); // NOT OK`
`10`		`- digits.push(buffer[i] % 8); // OK - 8 is a power of 2, so the result is unbiased.`
	`10`	`+ digits.push(buffer[i] % 8); // OK - input is a random byte, so the output is a uniformly random number between 0 and 7.`
`11`	`11`	`digits.push(buffer[i] % 100); // NOT OK`
`12`	`12`	`}`
`13`	`13`