NSA made a claim that Skipjack couldn't be extended past 80 bits of key. Most plausible explanation to my mind is that they're lying. Second is that there is an attack against a class of Skipjack-like ciphers that requires only a few plaintexts and 2^80 operations. Third is that some common key-lengthening tricks like those for 2-key-3DES, DES-X, and DEAL fail when applied to Skipjack. I can hardly fathom one resistant to all three, but I guess it's possible with NSA. Seems to me that you could always figure out some construct so that no practically-secure cipher with Skipjack's observable properties could evade having its key lengthened with much probability. Or maybe not. IANAC. Besides, it's impossible to make a cipher that can't be used to construct constructs with bigger key lengths: Skipjack(cryptovariable, IDEA(key, plaintext)) -- terminology jab intended -- provably has an effective key length as long as IDEA's. Even if that is cheating...