DES accepts 64 key bits, eight of which are parity bits: 56 effective key bits.
1970s → PRESENT
LAB 03 — DES TO AES
FROM 56-BIT KEYS TO THE MODERN BLOCK CIPHER
BLOCK CIPHER BASICS FIXED-SIZE DATA IN, FIXED-SIZE DATA OUT
PLAINTEXT BLOCK
+ KEY →"MUSEUM!!"BLOCK
CIPHER
→CIPHER
CIPHERTEXT BLOCK
85E813540F0AB405AES always has a 128-bit block; AES-128/192/256 names its key length.
INSIDE DES HISTORICAL / INSECURE FOR MODERN USE
A real DES teaching vector moves through an initial permutation, sixteen Feistel rounds, and a final permutation. In each round, the right half is expanded, mixed with a round key, passed through S-boxes and a permutation, then XORed with the left half.
64-BIT INPUTINITIAL PERMUTATION ↓16 FEISTEL ROUNDS ↓
EXPANSION → KEY MIXING → S-BOXES → P → XOR WITH LEFT
HALVES EXCHANGE ROLES
FINAL PERMUTATION ↓HALVES EXCHANGE ROLES
THE 56-BIT PROBLEM KEYSPACE IS A SCALE, NOT A CRACKER
25672,057,594,037,927,936POSSIBLE EFFECTIVE DES KEYS
Expected search is roughly half the space; a full exhaustive search is the worst case. In 1998, EFF’s DES Cracker and distributed Deep Crack work demonstrated practical exhaustive DES key search.
TRIPLE DES / TDEA EXTEND THE OLD DESIGN
DATADES
ENCRYPTDES
DECRYPTDES
ENCRYPTOUTPUT
ENCRYPTDES
DECRYPTDES
ENCRYPTOUTPUT
The common EDE construction retained compatibility with single DES when the same key was used. TDEA brought more keying material and more work, but retained DES’s 64-bit block limitation and was far slower than AES. Nominal key material is not the same as effective security strength; TDEA is withdrawn for new protection in current NIST guidance.
THE AES COMPETITION OPEN EVALUATION, 1997–2001
1997
NIST begins AES process2000
Rijndael selected2001
FIPS 197 standardizes AES
NIST begins AES process2000
Rijndael selected2001
FIPS 197 standardizes AES
MARSRC6RIJNDAELSELECTED FOR AESSERPENTTWOFISH
Joan Daemen and Vincent Rijmen designed Rijndael. AES standardized specific 128-bit-block parameter choices from that broader family after evaluation of security, performance, memory use, and hardware/software implementation characteristics.
INSIDE AES A 4 × 4 BYTE STATE · AES-128
INITIAL STATE — load the FIPS 197 example.
SHOW THE MATH: MIX COLUMNS
Each column is multiplied by a fixed matrix in the finite field GF(28). Byte arithmetic uses polynomial operations modulo x8 + x4 + x3 + x + 1—not ordinary decimal multiplication.
Remember XOR? Lab 01 introduced the operation AES uses in AddRoundKey. Rounds 1–9 have SubBytes, ShiftRows, MixColumns, AddRoundKey. Round 10 deliberately omits MixColumns.
DES vs AES DIFFERENT ARCHITECTURES, DIFFERENT ERA
DES
1970s · 64-bit block · 56 effective key bits · 16-round Feistel network
HISTORICAL / OBSOLETE FOR MODERN PROTECTIONAES
2001 · 128-bit block · 128 / 192 / 256-bit keys · substitution-permutation network · 10 / 12 / 14 rounds
WIDELY USED MODERN STANDARDAVALANCHE EFFECT
Small input changes tend to produce widespread, unpredictable output differences—not exactly 50% every time.
COMMON MISCONCEPTION: AES-256 means a 256-bit key. Its block remains 128 bits.
WHERE AES LIVES TODAY THE CIPHER IS ONLY ONE PART OF A SECURE SYSTEM
ENCRYPTED STORAGETLSVPNsWI-FI SECURITYHARDWARE ACCELERATION
AES alone is a block cipher, not a complete secure system. Real deployments need an appropriate construction such as authenticated encryption (for example AES-GCM), unique nonces, sound randomness, and key management. Strong algorithms can still fail through protocol mistakes, implementation bugs, or misuse—territory for Lab 07: Broken Crypto.