Shift every letter by the same amount: E(x) = x + k mod 26. Type a message to see it encrypt live, watch the alphabet mapping move, and use the brute-force panel to break a shift cipher without the key.
Every possible key, scored by how closely its letter frequencies match English (chi-squared — lower is better). The best candidate is highlighted. Click any row to load that shift.
| k | Decryption with that key | Score |
|---|
E(x) = (x + k) mod 26 D(y) = (y - k) mod 26
Number the letters A = 0, B = 1, …, Z = 25. Encryption adds the same key k to every letter and wraps around at Z. Decryption subtracts it.
Julius Caesar used k = 3, so A became D. ROT13 is the shift with k = 13 — and because 13 + 13 = 26, applying it twice returns the original text, which is why it is its own inverse.
The whole key space is just the 26 values of k, and k = 0 does nothing. So there are only 25 useful keys, and a computer tries all of them faster than you can read this sentence.
Two separate weaknesses combine, and either one alone would be fatal:
The tool shows the mechanism — the slides show why it is built that way.
The shift cipher is where cryptography starts and where cryptanalysis starts too. One-on-one tutoring walks you from frequency analysis all the way to RSA, with the reasoning intact at every step.