DES (Data Encryption Standard) is a symmetric-key block cipher developed by IBM and adopted by the U.S. government in the 1970s. It encrypts data in 64-bit blocks using a 56-bit key.
Key Features
| Feature | Description |
|---|---|
| Type | Symmetric Key (same key for encryption and decryption) |
| Block Size | 64 bits |
| Key Size | 56 bits (plus 8 parity bits, total 64 bits) |
| Rounds | 16 rounds of processing |
| Structure | Feistel Network |
High-Level DES Process
1. Initial Permutation (IP)
- A fixed initial permutation is applied to the 64-bit plaintext block.
2. 16 Rounds of Feistel Cipher
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The block is divided into two halves: Left (L) and Right (R) — each 32 bits.
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Each round performs the following:
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L[i] = R[i-1] -
R[i] = L[i-1] ⊕ f(R[i-1], K[i])
(Here,f()is a round function andK[i]is the subkey for round i)
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3. Function f (The Round Function)
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Takes a 32-bit input and a 48-bit subkey.
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Steps inside
f():-
Expansion (E-box): Expands 32-bit input to 48 bits.
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Key Mixing: XOR with 48-bit subkey.
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Substitution (S-boxes): Divides into 8 blocks of 6 bits, each passed through an S-box to get 4 bits. Output: 32 bits.
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Permutation (P-box): Rearranges the 32 bits.
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4. Final Permutation (FP or IP⁻¹)
- Inverse of the initial permutation is applied to the final output of the 16 rounds.