Substitution ciphers · Cryptography 101

Secret Encoder

Apply a substitution cipher. Encrypt, decrypt, and see why this scheme broke.

A substitution cipher swaps each letter for another according to a fixed rule. It's the simplest form of encryption — and the first one ever broken systematically, by Arab cryptographers in the 9th century, using letter-frequency analysis. Modern cryptography is built on the lessons of why this scheme fails.

Secret Encoder

Scramble messages with a secret key — only you and your friend can read them

Your message

Shift key3

125

Encoded message

Your result will appear here

Alphabet mapping

A↓D

B↓E

C↓F

D↓G

E↓H

F↓I

G↓J

H↓K

I↓L

J↓M

K↓N

L↓O

M↓P

N↓Q

O↓R

P↓S

Q↓T

R↓U

S↓V

T↓W

U↓X

V↓Y

W↓Z

X↓A

Y↓B

Z↓C

What you just learned

You just used encryption — scrambling a message so only someone with the key can read it. Every time you see a lock icon in your browser, encryption is protecting your data.

Explore the activity to earn its passport stamp. Stamps record exploration, not mastery.

What’s happening under the hood

›Key: a permutation of the 26-letter alphabet. Encrypt = lookup table. Decrypt = reverse lookup. Trivial to implement, trivial to attack.
›Vulnerable to frequency analysis: in English, E appears ~12% of the time, T ~9%, A ~8%. The most common letter in the ciphertext is almost certainly E.
›Modern encryption does not preserve single-letter substitution patterns. AES is a block cipher used with an appropriate mode; ChaCha20 produces a keystream from a key, nonce, and counter. Neither requires a new secret key for every block. Correct nonce use and authentication are essential.

Dig deeper

Phase 8 · Security Practice

The concept you just explored is taught with full depth in the formal DURA curriculum.