This exercise considers the security of key establishment with the aid of a KDC.

Question

This exercise considers the security of key establishment with the aid of a
KDC. Assume that a hacker performs a successful attack against the KDC at the
point of time tx, where all keys are compromised. The attack is detected.
1. Which (practical) measures have to be taken in order to prevent decryption of
future communication between the network nodes?

2. Which steps did the attacker have to take in order to decipher data transmissions
which occurred at an earlier time (t < tx)? Does such a KDC system provide
Perfect Forward Secrecy (PFS) or not?

Explanation / Answer

1. To prevent decryption of future communication, all keys need to be replaced. Also, the strength of the keys and vulnerabilities in the key generation process needs to be addressed before replacing all the keys with new keys. The key generation process must not reveal any pattern in the keys which the attackers can use. The key should be strong enough such that it cannot be compromised even if the plaintext and corresponding ciphertext is available.

2. The attacker needs to intercept the data transmission and look for patterns in the transmitted data. Using other information like timing, frequency of patterns, the attacker can guess or use brute force to decrypt part of the data. Once he is able to identify a plaintext along with it's ciphertext, he can recover the key in a weak encryption system.

Such KDC will not provide PFS as the attacker can again recover the keys from plaintext and ciphertext. The key generation and method of encryption needs to be stronger to provide PFS.

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