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protect mac comparisons from timing attacks

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This commit is contained in:
Kyle Spearrin 2017-04-27 12:00:32 -04:00
parent b3e94b13f7
commit 7d0a34fceb
1 changed files with 13 additions and 11 deletions
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24
src/app/services/cryptoService.js
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@ -358,7 +358,7 @@ angular
if (key.macKey && encPieces.length > 2) {
var macBytes = forge.util.decode64(encPieces[2]);
var computedMacBytes = computeMac(ctBytes, ivBytes, key.macKey, false);
if (!bytesAreEqual(macBytes, computedMacBytes)) {
if (!macsEqual(key.macKey, macBytes, computedMacBytes)) {
console.error('MAC failed.');
return null;
}
@ -431,18 +431,20 @@ angular
return b64Output ? forge.util.encode64(mac.getBytes()) : mac.getBytes();
}
// Constant time comparison. This removes the early-out optimizations of normal equality checks.
function bytesAreEqual(a, b) {
if (a.length !== b.length) {
return false;
}
// Safely compare two MACs in a way that protects against timing attacks.
// ref: https://www.nccgroup.trust/us/about-us/newsroom-and-events/blog/2011/february/double-hmac-verification/
function macsEqual(macKey, mac1, mac2) {
var hmac = forge.hmac.create();
var result = 0;
for (var i = 0; i < a.length; i++) {
result |= a[i] ^ b[i];
}
hmac.start('sha256', macKey);
hmac.update(mac1);
mac1 = hmac.digest().getBytes();
return result === 0;
hmac.start(null, null);
hmac.update(mac2);
mac2 = hmac.digest().getBytes();
return mac1 === mac2;
}
function SymmetricCryptoKey(keyBytes, b64KeyBytes, encType) {