Tick-tock, tick-tock… Ah, the melodic passage of time. But did you perchance grasp the mischievous nature time conceals? Indeed, beyond the facade of clock hands and digital displays, lies a clandestine realm of timing attacks.

Allow me to furnish an example—the game of word guessing. Imagine, within the depths of your mind, you have conjured the word “hello world.” If I were to successfully divine this word, it would proclaim my victory. However, I would certainly not be so foolish as to attempt every conceivable word, for that would take an eternity.

In the preliminary stages, I would be utterly confounded and choose to employ arbitrary words like “abcdefg.” As we commence the comparison within your thoughts, starting from the first letter, when “a” does not equal “h,” you swiftly discern the fallacy of my response and duly inform me. Yet, when I come close with “hbcdefg,” the situation changes. Now, you begin by comparing the initial letters. Therein lies the distinction—we now compare two letters, necessitating a longer duration. Once you enlighten me with the answer, I know that my first letter was correct.

Now, an intriguing development unfolds. With the knowledge of “h” as the initial letter, I employ the same methodology to deduce the second letter, and so forth. The time required for comparing “hecdefgh” is undoubtedly shorter than that for “hello world.” And thus, I gradually arrive at the realization that the word is “hello world.”

Time Attack

Such, my dear interlocutor, is the essence of a time attack. A string comparison timing attack exemplifies a cryptographic assault that capitalizes on the temporal aspects of comparing two strings character by character. This technique allows the attacker to glean information about a confidential string—a password or an encryption key, perchance.

During a typical string comparison operation, each character in the two strings is sequentially compared. Upon detecting a disparity, the comparison halts, and the result is revealed. However, the duration for performing this comparison differs based on the position of the first differing character. This temporal discrepancy becomes a tool, wielded by the attacker to extract insights into the secret string.

Timing attacks can grow more intricate, incorporating statistical analysis to account for timing variations resulting from factors such as system load and network latency. Alleviating such attacks involves utilizing constant-time string comparison algorithms or implementing countermeasures such as introducing random delays or employing fixed-time comparisons.

In the course of my practical endeavors, I have discovered potential vulnerabilities to such sequential timing attacks. Concern arises when verifying if the input from the user corresponds to the expected value. Utilizing the Strings.equal method, as traditionally done, fosters susceptibility to timing attacks. To address this predicament optimally, one must embrace the usage of constant-time string comparison algorithms.

Much alike the aforementioned example, when ascertaining the correctness of an answer, you ought not to disclose the outcome instantly but rather share it after a fixed duration, regardless of my guessed word. This approach ensures uniformity, albeit the downside manifests in the time consumed, which equates to the worst-case performance of the function.

Constant-Time String Comparison

To implement a constant-time string comparison, we have employed the MessageDigest.isEqual function:

public static boolean isEqual(byte[] digesta, byte[] digestb) {  
    if (digesta == digestb)
        return true;  
    if (digesta == null || digestb == null)
        return false;  
    
    int lenA = digesta.length;  
    int lenB = digestb.length;  
    
    if (lenB == 0)
        return lenA == 0;  
    
    int result = 0;  
    result |= lenA - lenB;  
    
    // Time-constant comparison  
    for (int i = 0; i < lenA; i++) {  
        // If i >= lenB, set indexB as 0; otherwise, set it as i.  
        int indexB = ((i - lenB) >>> 31) * i;  
        result |= digesta[i] ^ digestb[indexB];  
    }  
    return result == 0;  
}

Let us delve into an elucidation of the code:

  1. In the opening line of the code, the == operator verifies if digesta and digestb refer to the same memory object. Should this hold true, the method promptly returns true, denoting equality. This check optimizes the code by evading superfluous comparisons.

  2. Subsequently, the code examines whether either digesta or digestb is null. If either array is indeed null, signaling the presence of one non-null array alongside one null array, the method returns false, signifying inequality. This safeguard ensures graceful handling of null arrays.

  3. The code proceeds to acquire the lengths of digesta and digestb, assigning them to variables lenA and lenB, respectively.

  4. Then, the code verifies if lenB is 0. If so, it signifies that digestb is an empty array. In this scenario, the code checks if lenA is also 0. If that holds true, it returns true as both arrays are empty and deemed equal. If not, it returns false since one array is empty while the other is not. This validation ensures proper treatment of empty arrays.

  5. An integer variable called result is initialized to 0. This variable accumulates the comparison result.

  6. The subsequent loop iterates over the elements of digesta, utilizing the index variable i. It compares each element of digesta with the corresponding element of digestb. However, a twist regarding the calculation of the index for digestb emerges.

  7. Within the loop, the code calculates the index for digestb based on the values of i and lenB. If i exceeds or equals lenB, the index indexB is set to 0. Otherwise, it is set to i. This calculation ensures that the code handles cases where digestb is shorter than digesta without divulging timing information.

  8. A bitwise XOR operation is then performed between the i-th element of digesta and the indexB-th element of digestb. The resulting value is bitwise OR-ed with result, accumulating the comparison outcome.

  9. Once the loop concludes, the code verifies if result is equal to 0. If so, it signifies that all elements of digesta and digestb are bitwise equal, leading to a return of true to indicate equality. If not, it returns false.

In essence, the isEqual method engages in a time-constant comparison of two byte arrays, digesta and digestb. This comparison encompasses checks for array equality, null arrays, and empty arrays. Additionally, it performs bitwise comparisons between the elements of the arrays.

Summary

By ensuring a constant execution duration regardless of the input, this implementation effectively mitigates timing attacks. Such attacks exploit execution time discrepancies to deduce concealed information about the compared strings. By eliminating these timing variations, the code forestalls malevolent actors from exploiting temporal insights to unravel confidential details regarding the strings subject to comparison.

When designing and implementing systems, it is crucial to pay special attention to mitigating the risks of time attacks, especially when it comes to comparing passwords, tokens, and signatures. We should review the string comparison logic in our code to ensure that it is not vulnerable to timing attacks. Employing constant-time comparison algorithms or implementing other defensive measures such as introducing random delays or using fixed-time comparisons can effectively reduce the risk of timing attacks.

Furthermore, it is essential to regularly review the security of our systems and follow best security practices. This includes using strong passwords, regularly changing passwords, employing secure encryption algorithms and protocols, and limiting access privileges to sensitive operations and data. By applying a comprehensive set of security measures, we can enhance the security of our systems, reduce the likelihood of being vulnerable to timing attacks, and protect users’ sensitive information from potential threats.