On the security of multiple encryption

ABSTRACT

Double encryption has been suggested to strengthen the Federal Data Encryption Standard (DES). A recent proposal suggests that using two 56-bit keys but enciphering 3 times (encrypt with a first key, decrypt with a second key, then encrypt with the first key again) increases security over simple double encryption. This paper shows that although either technique significantly improves security over single encryption, the new technique does not significantly increase security over simple double encryption. Cryptanalysis of the 112-bit key requires about 256 operations and words of memory, using a chosen plaintext attack. While DES is used as an example, the technique is applicable to any similar cipher.

REFERENCES

• 1: Branstad, D.K., Gait, J., and Katzke, S. Report of the workshop on cryptography in support of computer security, National Bureau of Standards Rep. NBSIR 77-1291 (Sept. 21-22, 1976).
• 2: Dime, W., and Hellman, M. Exhaustive cryptanalysis of the NBS data encryption standard. Computer (June 1977), 74-84.
• 3: Diffie, W., and Hellman, M.E. New directions in cryptography. IEEE Trans. Info. IT-22, 6 (Nov. 1976), 644-654.
• 4: Hellman, M.E., An extension of the Shannon theory approach to cryptography, IEEE Trans. Info. IT-23, 3 (May 1977), 289-294.
• 5: Hellman, M., Merkle, R., Schroeppel, R., Washington, L., Diffie, W., Pohlig, S., and Schweitzer, P. Results of an initial attempt to cryptanalyze the NBS data encryption standard. Information Systems Laboratory SEL 76-042 (Sept. 9, 1976).
• 6: Kahn, D. The Codebreakers. Macmillan, New York, 1976.
• 7: Kolata, G.B, Computer encryption and the National Security Agency. Science 197 (July 29, 1977) 438--440.
• 8: Morris, R., Sloane, N.J.A., and Wyner, A.D. Assessment of the National Bureau of Standards proposed federal data encryption standard. Cryptologia 1 (July 1977), 281-291.
• 9: National Bureau of Standards. Federal Information Processing Standards Publication No. 46, Jan 1977.
• 10: Senate Select Committee on Intelligence. Involvement of the NSA in the development of the data encryption standard. News release (Apr. 12, 1978).
• 11: Shannon, C.E. Communication theory of secrecy systems. Bell. Syst. Tech. J. 28 (Oct. 1949), 656-715.
• 12: Tuchman, W.L. Talk presented at the Nat. Computer Conf., Anaheim, CA., June 1978.
• 13: Yasaki, E.K. Encryption algorithm: Key size is the thing. Datamation 22, 3 (Mar. 1976), 164-166.

INDEX TERMS

Primary Classification:
  E. Data
  E.3 DATA ENCRYPTION

Additional Classification:
  K. Computing Milieux
  K.6 MANAGEMENT OF COMPUTING AND INFORMATION SYSTEMS

General Terms:
Documentation, Security

Keywords:
DES, cipher, cryptanalysis, cryptography, data encryption standard, encipher, encrypt, encryption, multiple encryption