Fundamentals of Information Encryption - Multilevel Security for Relational Databases - page 74

weakness of the double DES algorithm [54]. The meet-in-the-middle
attack requires that the attacker have both a known piece of plain-
text and its corresponding ciphertext. The attack needs to store 2 56
results when trying to discover a datum that has been encrypted with
the double DES. Merkle and Hellman presented a time-memory
trade-off, which could break this double-encryption scheme in 2 56+1
encryptions, rather than 2 112 encryptions.
4.5.3 Triple DES
The dangers of the Merkle-Hellman attack (meet-in-the-middle
attack) can be overcome by conducting three block encryption opera-
tions. This method is called triple DES and is performed by execut-
ing the DES three times, producing an effective key size of 168 bits.
In the triple DES, each 64-bit block of data is encrypted three times
with the DES algorithm.
The triple DES is performed as follows:
1. Use key 1 for encryption.
2. Use key 2 for decryption.
3. Use key 3 for encryption.
To decrypt, reverse the steps:
1. Use key 3 for decryption.
2. Use key 2 for encryption.
3. Use key 1 for decryption.
For several applications, both keys 1 and 3 can be the same without
creating a significant vulnerability. The choice among single, double,
and triple DES is a trade-off between performance and security
requirements [55].
4.5.4 International Data Encryption Algorithm (IDEA)
The IDEA cipher was first presented by Lai and Massey in 1990 under
the name of proposed encryption standard (PES). After Biham and
Shamir presented differential cryptanalysis, the authors named it the
improved proposed encryption standard (IPES). The IPES name was
changed to international data encryption algorithm (IDEA) in 1992.