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In cryptography, a timing attack is a side channel attack in which the attacker attempts to compromise a cryptosystem by analyzing the time taken to execute cryptographic algorithms. Every logical operation in a computer takes time to execute, and the time can differ based on the input; with precise measurements of the time for each operation, an attacker can work backwards to the input.

Information can leak from a system through measurement of the time it takes to respond to certain queries. How much such information can help an attacker depends on many variables: crypto system design, the CPU running the system, the algorithms used, assorted implementation details, timing attack countermeasures, the accuracy of the timing measurements, etc.

Timing attacks are often overlooked in the design phase because they are so dependent on the implementation.


The idea behind the attack

A timing attack is an example of an attack that exploits the implementation power of an algorithm rather than the algorithm itself. The same algorithm can always be reimplemented in a way that leaks little or no information to a timing attack: consider an implementation in which every call to a subroutine always returns in exactly x seconds, where x is the maximum time it ever takes to execute the routine. In such an implementation, timing gives an attacker no useful information; it also has the adverse effect of slower response times on average.

The practicality of the attack implies several things:

  • It is algorithm-independent. Notice that the theoretical security of such algorithms remains — it is mainly the need for a high-speed implementation of a particular algorithm, that introduces such vulnerabilities.
  • Finding timing information is more routine. While finding cryptographic errors in a crypto-primitive such as the DES may require deeper knowledge of mathematics, timing is relatively easy. And measuring response time for a specific query might give away relatively large amounts of information.


The execution time for the square-and-multiply algorithm used in modular exponentiation depends linearly on the number of '1' bits in the key. While the number of '1' bits alone is not nearly enough information to make finding the key trivially easy, repeated executions with the same key and different inputs can be used to perform statistical correlation analysis of timing information to recover the key completely, even by a passive attacker. Observed timing measurements often include noise (from such sources as network latency, or disk drive access differences from access to access, and the error correction techniques used to recover from transmission errors). Nevertheless, timing attacks are practical against a number of encryption algorithms, including RSA, ElGamal, and the Digital Signature Algorithm.

In 2003, Boneh and Brumley demonstrated a practical network-based timing attack on SSL-enabled web servers, based on a different vulnerability having to do with the use of RSA with Chinese Remainder Theorem optimizations. The actual network distance was small in their experiments, but the attack successfully recovered a server private key in a matter of hours. This demonstration led to the widespread deployment and use of blinding techniques in SSL implementations. In this context, blinding is intended to remove correlations between key and encryption time.

Some versions of Unix use a relatively expensive implementation of the crypt library function for hashing an 8-character password into an 11-character string. On older hardware, this computation took a deliberately and measurably long time: as much as two or three seconds in some cases. The login program in early versions of Unix executed the crypt function only when the login name was correct, which leaked information through timing that the login name itself was valid, even though the password was incorrect. Later versions of Unix fixed this leak by always executing the crypt function to avoid revealing the proper login name.

Two otherwise securely isolated processes running on a single system with either cache memory or virtual memory can communicate by deliberately causing page faults and/or cache misses in one process, then monitoring the resulting changes in access times from the other. Likewise, if an application is trusted, but its paging/caching is affected by branching logic, it may be possible for a second application to determine the values of the data compared to the branch condition by monitoring access time changes; in extreme examples, this can allow recovery of cryptographic key bits.[1]


Timing attacks are easier to mount if the adversary knows the internals of the hardware implementation, and even more so, the crypto system in use. Since cryptographic security should never depend on the obscurity of either (see security through obscurity, specifically both Shannon's Maxim and Kerchoff's Law), resistance to timing attacks should not either. If nothing else, an exemplar can be purchased and reverse engineered. Timing attacks and other side-channel attacks may also be useful in identifying, or possibly reverse-engineering, a cryptographic algorithm used by some device.


  1. ^ See Percival, Colin, Cache Missing for Fun and Profit, 2005; Bernstein, Daniel J., Cache-timing attacks on AES, 2005.

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