How does the basic encryption model work, and what are $K_E$ and $K_D$?

$K_E$ is the encryption key and $K_D$ is the decryption key — in symmetric crypto they are the same, in asymmetric crypto they differ.

* Encrypt with K_E, decrypt with K_D; symmetric means K_E = K_D, asymmetric means they differ. *

The encryption model:

$$m \xrightarrow{E(K_E, m)} c \xrightarrow{D(K_D, c)} m$$

Where:

• $m$ = plaintext message
• $c$ = ciphertext
• $E$ = encryption function
• $D$ = decryption function
• $K_E$ = encryption key
• $K_D$ = decryption key

Key relationship:

• Symmetric: $K_E = K_D$ (same secret key shared between Alice and Bob)
• Asymmetric: $K_E \neq K_D$ (public key encrypts, private key decrypts)

The fundamental requirement is: $D(K_D, E(K_E, m)) = m$ — decryption must perfectly reverse encryption.

Go deeper:

• Symmetric-key algorithm — the same-key case.
• Public-key cryptography — the different-key case.