Symmetric Key Cryptography

A "symmetric" encryption scheme uses the same secret key for both encryption and decryption.

The need to share that key safely is the main weakness of this approach — this is known as the Key Distribution Problem.

Modern schemes use long, random keys and heavy maths. When the key is protected, they are extremely hard to break.

Interactive demo
Try sending a message and see what Eve can learn.
Step 1

Sender (Alice)

Generate and share a secret key, then encrypt a message for Bob.

Shared Key

Plaintext for Bob

Cyphertext for Bob

Frequency Analysis

Compare how often each symbol appears in the plaintext and in the ciphertext.

Visual attack surface
Plaintext Symbol Frequency
Cyphertext Symbol Frequency
Step 2

Recipient (Bob)

Receive the shared key and decrypt Alice's ciphertext back to plaintext.

Shared Key

Cyphertext from Alice

Plaintext from Alice

Attacker

Eavesdropper (Eve)

Eve can steal the key or intercept the ciphertext to try to read the message.

Stolen Key

Cyphertext from Alice

Plaintext from Alice