Enhanced PLC Encryption Standard

Crypto - Points: 600

Our IT department wants us to encrypt our PLC traffic. So we created our own encryption scheme, called the Enhanced PLC Encryption Standard.

The idea is simple, and brilliant:

1.) Every PLC gets the shared secret password – this is so long, nobody can brute force it.

2.) If two devices want to communicate, one of them (A) sends a unique challenge to the other device (B).

3.) B gets the challenge, and hashes each character of the password with the challenge: response = hash(char + challenge).

4.) For security purposes, we use SHA-256 here (no insecure MD5 or SHA-1!). We also hash each character separately, so the full password can’t be leaked if an attacker records the responses.

5.) B sends a lot of responses back to A. A knows the length of the password, and the password itself. So A can terminate the connection if responses are missing, or if there are too many responses.

6.) A also conducts hash(char + challenge), and compares every response. If there is any mismatch, A terminates the connection.

7.) If every response matches, A and B start to communicate using the shared secret password as the key. We use 3DES in CBC mode here, because our PLCs don’t support military-grade AES.

We sent a sample log to IT. For us, this looks clearly encrypted and secure. Our key is 24 bytes strong. One website says it takes 76 SEXTILLION YEARS to crack this.

P.S. We didn’t find a possibility to implement an IV, so it’s 8 times 0.

Since every character of the key is hashed separately, we can calculate the hashes of all printable characters and compare with the responses to get the key and finally decrypt the DES3 encrypted messages.

flag: {never-roll-your-own-crypto}