Google open-sources test suite to find crypto bugs

Developers can use Project Wycheproof to test cryptographic algorithms against a library of known attacks to uncover potential weaknesses

Working with cryptographic libraries is hard, and a single implementation mistake can result in serious security problems. To help developers check their code for implementation errors and find weaknesses in cryptographic software libraries, Google has released a test suite as part of Project Wycheproof.

"In cryptography, subtle mistakes can have catastrophic consequences, and mistakes in open source cryptographic software libraries repeat too often and remain undiscovered for too long," Google security engineers Daniel Bleichenbacher and Thai Duong, wrote in a post announcing the project on the Google Security blog.

Named after Australia's Mount Wycheproof, the world's smallest mountain, Wycheproof provides developers with a collection of unit tests that detect known weaknesses in cryptographic algorithms and check for expected behaviors. The first set of tests is written in Java because Java has a common cryptographic interface and can be used to test multiple providers.

"We recognize that software engineers fix and prevent bugs with unit testing, and we found that many cryptographic issues can be resolved by the same means," Bleichenbacker and Duong wrote.

The suite can be used to test such cryptographic algorithms as RSA, elliptic curve cryptography, and authenticated encryption, among others. The project also has ready-to-use tools to check Java Cryptography Architecture providers, such as Bouncy Castle and the default providers in OpenJDK. The engineers said they are converting the tests into sets of test vectors to simplify the process of porting them to other languages.

The tests in this release are low-level and should not be used directly, but they still can be applied for testing the algorithms against publicly known attacks, the engineers said. For example, developers can use Wycheproof to verify whether algorithms are vulnerable to invalid curve attacks or biased nonces in digital signature schemes.

So far the project has been used to run more than 80 test cases and has identified 40-plus vulnerabilities, including one issue where the private key of DSA and ECDHC algorithms could be recovered under specific circumstances. The weakness in the algorithm was present because libraries were not checking the elliptic curve points they received from outside sources.

"Encodings of public keys typically contain the curve for the public key point. If such an encoding is used in the key exchange, then it is important to check that the public and secret key used to compute the shared ECDH secret are using the same curve. Some libraries fail to do this check," according to the available documentation.

Cryptographic libraries can be quite difficult to implement, and attackers frequently look for weak cryptographic implementations rather than trying to break the actual mathematics underlying the encryption. With Wycheproof, developers and users can check their libraries against a large number of known attacks without having to dig through academic papers to find out what kind of attacks they need to worry about.

The engineers looked through public cryptographic literature and implemented known attacks to build the test suite. However, developers should not consider the suite to be comprehensive or able to detect all weaknesses, because new weaknesses are always being discovered and disclosed.

"Project Wycheproof is by no means complete. Passing the tests does not imply that the library is secure, it just means that it is not vulnerable to the attacks that Project Wycheproof tries to detect," the engineers wrote.

Wycheproof comes two weeks after Google released a fuzzer to help developers discover programming errors in open source software. Like OSS-Fuzz, all the code for Wycheproof is available on GitHub. OSS-Fuzz is still in beta, but it has already worked through 4 trillion test cases and uncovered 150 bugs in open source projects since it was publicly announced.

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