ECDSA Signature Spoofing (CVE-2015-2730)

TL;DR

CVE-2015-2730 is a serious flaw in some ECDSA implementations that could allow an attacker to forge signatures. This happens because of incorrect handling of random number generation during signature creation. If your systems use ECDSA, especially older ones, you need to check if they are vulnerable and apply the necessary patches or updates.

Understanding the Problem

ECDSA (Elliptic Curve Digital Signature Algorithm) is used to prove that a message comes from a specific person. It relies on a secret key and a random number for each signature. The flaw in CVE-2015-2730 means that if the random number isn’t truly random, an attacker can calculate the secret key and then forge signatures.

How the Attack Works

1. Weak Random Number Generation: The core issue is a predictable or reused random number used when creating ECDSA signatures.
2. Key Recovery: An attacker can collect multiple signatures created with these weak random numbers. Using mathematical techniques, they can then calculate the private key associated with those signatures.
3. Signature Forgery: Once the attacker has the private key, they can create valid signatures for any message, effectively spoofing the original signer.

Checking if You’re Vulnerable

The vulnerability affects specific implementations of ECDSA, particularly those found in older versions of OpenSSL and other cryptographic libraries.

1. Identify ECDSA Usage: First, determine if your systems use ECDSA for signing data. This is common in SSH keys, code signing, and digital certificates.
2. Check Library Versions:
   • OpenSSL: If you’re using OpenSSL, check the version number. Vulnerable versions include those before 1.0.1e, 1.0.2b, and 0.9.8o. You can check your version with:

     openssl version

   • Other Libraries: Check the documentation for any other cryptographic libraries you use to see if they are affected by CVE-2015-2730.
3. Review Code: If you’re using a custom ECDSA implementation, carefully review the code that generates random numbers. Ensure it uses a cryptographically secure random number generator (CSPRNG).

How to Fix It

The best way to fix this vulnerability is to update your software.

1. Update OpenSSL: Upgrade to the latest version of OpenSSL. This will include the necessary patches to address CVE-2015-2730.

   # Example on Debian/Ubuntu
   sudo apt-get update
   sudo apt-get upgrade openssl

2. Update Other Libraries: Update any other cryptographic libraries you use to the latest versions.
3. Regenerate Keys (Important): After updating your software, it’s crucial to regenerate any ECDSA keys that were created using the vulnerable version of the library. This is because the private key may have been compromised if weak random numbers were used.

   # Example generating a new SSH key
   ssh-keygen -t ecdsa -b 256

4. Secure Random Number Generation: If you’re using a custom implementation, ensure your random number generator is properly seeded and uses a CSPRNG. Avoid predictable sources of randomness.
   • On Linux, use /dev/urandom for generating random numbers.

Further Resources

• CVE-2015-2730 Details
• OpenSSL Website