In cryptography, Ladder-DES is a block cipher designed in 1994 by Terry Ritter. It is a 4-round Feistel cipher with a block size of 128 bits, using DES as the round function. It has no actual key schedule, so the total key size is 4×56=224 bits.

In 1997, Eli Biham found two forms of cryptanalysis for Ladder-DES that depend on the birthday paradox; the key is deduced from the presence or absence of collisions, plaintexts that give equal intermediate values in the encryption process. He presented both a chosen-plaintext attack and a known-plaintext attack; each uses about 2³⁶ plaintexts and 2⁹⁰ work, but the known-plaintext attack requires much more memory.

• Terry Ritter (22 February 1994). "Ladder DES". Newsgroup: sci.crypt. Usenet: 1994Feb22.083353.26012@cactus.org. Retrieved 30 January 2007.
• Eli Biham (1997). Cryptanalysis of Ladder-DES (PDF). 4th International Workshop on Fast Software Encryption (FSE '97). Haifa: Springer-Verlag. pp. 134–138. Retrieved 30 January 2007.