Cracking Java’s weak encryption – Nail in the JKS coffin

POC||GTFO journal edition 0x15 came out a while ago and I’m happy to have contributed the article “Nail in the JKS coffin”. You should really read the article, I’m not going to repeat myself here. I’ve also made the code available on my “JKS private key cracker hashcat” github repository.

For those who really need a TL;DR, the developed cracking technique relies on three main issues with the JKS format:

  1. Due the unusual design of JKS the key store password can be ignored and the private key password cracked directly.
  2. By exploiting a weakness of the Password Based Encryption scheme for the private key in JKS described by cryptosense, the effort to try a password is minimal (one SHA-1 calculation).
  3. As public keys are not encrypted in the JKS file format, we can determine the algorithm and key size of the public key to know the PKCS#8 encoded fingerprint we have to expect in step 2.

For a practical TL;DR, see the github repository on how JksPrivkPrepare.jar can be used together with the hashcat password cracking tool to crack passwords.

Not affected of the described issues are other key store file formats such as JCEKS, PKCS12 or BKS. It is recommended to use the PKCS12 format to store private keys and to store the files in a secure location. For example it is recommended to store Android app release JKS files somewhere else than a repository such as git.

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