Rephrase statement on the security of two-key 3DES.

  [Chosen plaintext attack: R. Merkle, M. Hellman: "On the Security of
  Multiple Encryption", CACM 24 (1981) pp. 465-467, p. 776.

  Known plaintext angriff: P.C. van Oorschot, M. Wiener: "A
  known-plaintext attack on two-key triple encryption", EUROCRYPT '90.]