Time-Constant String Comparison

When comparing strings in a security context (e.g. comparing keys), you should avoid leaking timing information.

Standard string comparison algorithms are optimized to stop comparing characters as soon as a difference is found. An attacker can exploit this by making many requests with strings that all differ in the first character. The strings that begin with an incorrect first character will make a single character comparison and stop. However, the strings that begin with a correct first character will need to make additional string comparisons, and thus take more time before they stop. Sophisticated attackers can measure this difference and use it to deduce the characters that their input is being compared to.

Time-Constant String Comparison

using System.Security.Cryptography;

// Simulated sensitive data (e.g., a secure token or password hash)
var storedHash = Convert.FromBase64String("HJG3+eXAIoQsNI1ASD2i+If7xhQAEjZLefBWo5pcuDE=");

// Incoming hash to validate (e.g., provided by the user)
var providedHash = Convert.FromBase64String("HJG3+eXAIoQsNI1ASD2i+If7xhQAEjZLefBWo5pcuDE=");

// Compare the two byte sequences using FixedTimeEquals
var isEqual = CryptographicOperations.FixedTimeEquals(storedHash, providedHash);

var result = isEqual
    ? "the hashes match!"
    : "the hashes do not match!";

Console.WriteLine(result);

TimeConstantComparer

The TimeConstantComparer class defends against these timing attacks by implementing a constant-time string comparison. The string comparison is a constant-time operation in the sense that comparing strings of equal length always performs the same amount of work.

Usage example:

using Duende.IdentityModel;

// Simulated sensitive data (e.g., a secure token or password hash)
var storedHash = "HJG3+eXAIoQsNI1ASD2i+If7xhQAEjZLefBWo5pcuDE=";

// Incoming hash to validate (e.g., provided by the user)
var providedHash = "HJG3+eXAIoQsNI1ASD2i+If7xhQAEjZLefBWo5pcuDE=";

// Compare the two byte sequences using FixedTimeEquals
var isEqual = TimeConstantComparer.IsEqual(storedHash, providedHash);
var result = isEqual
    ? "the hashes match!"
    : "the hashes do not match!";

Console.WriteLine(result);