Skipjack is a block cipher developed by the US National Security Agency in 1987, first reviewed in 1993 and declassified in 1998. Skipjack is a block cipher that is designed to perform the encryption functionality of the clipper chip whereas a separate Law Enforcement Access Field (LEAF) mechanism was used to provide the escrow functionality. The algorithm was generally regarded with suspicion as it was initially classified before a small number of security experts were allowed to review it in 1993, in 1998 the security community independently discovered the design principles of the cipher and it was declassified.
Technipages Explains Skipjack
Skipjack uses an 80-bit encryption key which is considered insecure in modern computing, a 128-bit key is considered a minimum for considerable future-proofed cryptographic security. Each extra bit of length of the encryption key exponentially increases the difficulty of a brute-force attack.
A number of attacks have been found that can break some of the rounds of encryption that skipjack uses although no attack has been found that breaks all 32 rounds to date. There have been no suggestions or indications that the algorithm contains any backdoors directly, beyond its use with the escrow functionality of the clipper chip.
The clipper chip’s failure was not really related to the skipjack cipher in particular, while skipjack has flaws there were more significant issues with the public opinion against the key escrow functionality of the clipper chip and later the security flaws identified in it.
Skipjack’s main modern counterpart as a block cipher is the Advanced Encryption Standard (AES) which was standardised in 2001. AES supports key sizes of 128, 192 and 256 bits making it significantly more secure. While a few attacks against AES are known, they either have a minimal effect on the security or have such high access requirements as to be infeasible.
Common Uses of Skipjack
- Skipjack was used as the encryption algorithm in the US government-sponsored Clipper chip.
- Skipjack uses an 80-bit key to encrypt or decrypt 64-bit data blocks over 32 rounds.
- Eli Biham and Adi Shamir discovered an attack against 16 of the 32 rounds within one day of Skipjack’s declassification
Common Misuses of Skipjack
- Skipjack was a backdoored encryption cipher developed by the NSA for use in the clipper chip.