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Symmetric-Key Cryptography
The core mechanism for high-speed data protection, built on a single secret key shared by sender and receiver.
I. Overview
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A["Plaintext"] -- "High-speed computation with a single secret key" --> B["Ciphertext"]
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style B fill:#e1f5fe,stroke:#01579b,stroke-width:3px
Definition: A cryptographic scheme in which sender and receiver use a pre-shared “single secret key” to transform plaintext into ciphertext, and back again into plaintext.
Features:
( High-Speed Computation ) Computation is far faster than asymmetric-key cryptography
( Simple Structure ) The algorithm structure is simple, making software and hardware implementation easy
( Efficiency ) High computational efficiency makes it well suited to processing large volumes of data and real-time encryption
II. Mechanism & Components
A. Structure of Symmetric-Key Cryptography
- Encryption:
C = E(K, P)(P: plaintext, K: shared key, C: ciphertext) - Decryption:
P = D(K, C)(the same key K is used for both encryption and decryption)
B. Classification by Data Processing Unit
| Category | Stream Cipher | Block Cipher |
|---|---|---|
| Processing Unit | 1 bit or 1 byte at a time | Fixed-size blocks (64/128/256 bits) |
| Core Principle | Pseudorandom stream XORed with the data | Repeated substitution (S) and permutation (P) |
| Computation Speed | Very fast (real-time capable) | Relatively slower than a stream cipher |
| Examples | RC4, A5/1, Salsa20 | DES, AES, SEED, ARIA |
| Trade-offs | Easy hardware implementation, low diffusion | Higher security, but requires padding |
III. Advanced Topics & Comparison
| Limitation | Details | Solution |
|---|---|---|
| Key Distribution Problem | Safely delivering the key to the recipient is difficult | Diffie-Hellman, hybrid cryptography (envelope) |
| Key Management Burden | n(n-1)/2 keys are required among n users | KDC (key distribution center), integration with accredited certification systems |
| Lack of Integrity | Exposure of the key threatens not just confidentiality but also tampering | Use authenticated encryption such as HMAC or GCM mode |
Last updated 18 Aug 2026, 00:00 UTC.