Cryptography & Security Newsletter

Two zeros bypass Java’s ECDSA signature check

28 April 2022

Feisty Duck’s Cryptography & Security Newsletter is a periodic dispatch bringing you commentary and news surrounding cryptography, security, privacy, SSL/TLS, and PKI. It's designed to keep you informed about the latest developments in this space. Enjoyed every month by more than 50,000 subscribers. Written by Hanno Böck.

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A security vulnerability in Java’s ECDSA implementation allows creating a signature that would always be evaluated as valid. Neil Madden, who discovered the flaw, called it a psychic signature.

ECDSA signatures consist of two values, usually called r and s. The issue is that ECDSA signature validation compares r to the product of the inverse of s multiplied with other values. Depending on how the inverse of s is calculated, the inverse of zero may also be evaluated to zero.

In Java this is the case, so zero gets compared to zero—independent of the message to be validated. Thus a signature where the r and s values are set to zero will always pass.

This is generally a known property of ECDSA. It is therefore required to check that the r and s values are not zero, which is supposed to be the first step of the signature verification algorithm. However, Java failed to implement that check.

Madden noted in a later blog post that two zeros are not the only way to bypass the signature check.

The vulnerability was introduced in Java version 15, where the previous ECDSA implementation written in C++ was replaced by one written in native Java. Oracle fixed the vulnerability in its April update, and the vulnerability was assigned the ID CVE-2022-21449. OpenJDK, the open-source distribution of Java, also published an advisory.

It is noteworthy that this vulnerability was not caught earlier. Neil Madden points out that Project Wycheproof, a cryptographic test suite published by Google, would have caught this vulnerability.

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