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protect mac comparisons from timing attacks

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This commit is contained in:
Kyle Spearrin 2017-04-27 12:01:22 -04:00
parent 2cc2057f88
commit 585a603cf6
1 changed files with 22 additions and 5 deletions
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27
src/services/cryptoService.js
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@ -336,7 +336,7 @@ function initCryptoService(constantsService) {
var iv = forge.util.encode64(ivBytes);
var ctBytes = cipher.output.getBytes();
var ct = forge.util.encode64(ctBytes);
var mac = !key.macKey ? null : computeMac(ctBytes, ivBytes, key.macKey);
var mac = !key.macKey ? null : computeMac(ctBytes, ivBytes, key.macKey, true);
var cs = new CipherString(key.encType, iv, ct, mac);
deferred.resolve(cs);
@ -379,8 +379,9 @@ function initCryptoService(constantsService) {
var ctBytes = forge.util.decode64(cipherString.cipherText);
if (key.macKey && cipherString.mac) {
var computedMac = computeMac(ctBytes, ivBytes, key.macKey);
if (computedMac !== cipherString.mac) {
var macBytes = forge.util.decode64(cipherString.mac);
var computedMacBytes = computeMac(ctBytes, ivBytes, key.macKey, false);
if (!macsEqual(key.macKey, computedMacBytes, macBytes)) {
console.error('MAC failed.');
deferred.reject('MAC failed.');
}
@ -471,12 +472,28 @@ function initCryptoService(constantsService) {
});
};
function computeMac(ct, iv, macKey) {
function computeMac(ct, iv, macKey, b64Output) {
var hmac = forge.hmac.create();
hmac.start('sha256', macKey);
hmac.update(iv + ct);
var mac = hmac.digest();
return forge.util.encode64(mac.getBytes());
return b64Output ? forge.util.encode64(mac.getBytes()) : mac.getBytes();
}
// 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();
hmac.start('sha256', macKey);
hmac.update(mac1);
mac1 = hmac.digest().getBytes();
hmac.start(null, null);
hmac.update(mac2);
mac2 = hmac.digest().getBytes();
return mac1 === mac2;
}
function SymmetricCryptoKey(keyBytes, b64KeyBytes, encType) {