[Security Bug] Vulnerable to Timing Attack Against PIN Validation

[terryschmidt]

Describe the bug
A cursory review of the PFLockScreen-Android revealed that the PIN check was vulnerable to timing attack due to not being time constant.

To Reproduce
Steps to reproduce the behavior:
See checkPin() function here:

PFLockScreen-Android/pflockscreen/src/main/java/com/beautycoder/pflockscreen/security/PFFingerprintPinCodeHelper.java

Line 63 in ff5b7cd

public void checkPin(Context context, String encodedPin, String pin, PFPinCodeHelperCallback<Boolean> callback) {

The clear-text PIN will be checked against the decrypted PIN using String.equals(), which is vulnerable to timing attack. The equals function stops comparing the strings as soon as one-character mismatches.

Expected behavior
Remediation: First, PFLockScreen-Android should use hashing instead of encrypting the PIN of the user. For instance, the library could be using bcrypt or argon2id to create a strong hash of the PIN combined with a random unique salt. A timing attack will not reveal any information when comparing two hashes.
Additionally, the checkPin function could implement a time-constant comparison check using a XOR sum. Below is the implementation from OpenJDK:
// OpenJDK: https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/s⌋ 􏰀→ rc/share/classes/java/security/MessageDigest.java#L430
public static boolean isEqual(byte[] digesta, byte[] digestb) { if (digesta.length != digestb.length) {
return false;
int result = 0;
// time-constant comparison
for (int i = 0; i < digesta.length; i++) {
result |= digesta[i] ^ digestb[i];
}
return result == 0; }