Symmetric ciphersBroken
Data Encryption Standard
Symmetric block cipher (obsolete)
How it works
DES is a Feistel cipher standardised in the 1970s. It splits each 64-bit block in half and runs 16 rounds of substitution and permutation driven by a 56-bit key. The algorithm itself held up well to analysis, but the key is far too short for modern computing.
Sizes: 64-bit block; 56-bit effective key (64 bits with parity); 16 rounds
Status: The 56-bit key can be brute forced; the FIPS standard was withdrawn in 2005.
Strengths
- Historically important: the design ideas (Feistel networks, S-boxes) appear throughout the exam syllabus
Weaknesses and known attacks
- Exhaustive key search is practical: purpose-built hardware recovered DES keys in days in the late 1990s, and it is far faster today.
- 64-bit block, so it shares the birthday-bound problems of 3DES.
- Double DES does not help because of the meet-in-the-middle attack, which is why Triple DES was used instead.
Use it for
- Nothing new; recognise it in legacy systems and exam questions
Don't use it for
- Any encryption
Exam tip
DES has a 56-bit key and 64-bit block. Remember why 2DES was skipped: meet-in-the-middle makes it barely stronger than DES.