Building Block: Symmetric Encryption

%%{init: {"flowchart": {"rankSpacing": 70}}}%%
flowchart LR
    m([m]) --> E["E"]
    k1([k]) --> E
    E -->|"E(k,m) = c"| Net((Network))
    Net -->|c| gap((" ")):::hidden
    gap --> D["D"]
    k2([k]) --> D
    D -->|"D(k,c) = m"| out([m])

    subgraph Alice
        E
        k1
    end

    subgraph Bob
        D
        k2
    end

    classDef hidden fill:none,stroke:none;

• E, D: cipher
• k: secret key (e.g. 128 bits (2¹²⁸ ≈ 3.4 × 10³⁸ possible keys) )
• m, c: plaintext, ciphertext

Encryption algorithm is publicly known

• Never use a proprietary cipher