ROT13 decoder
Apply ROT13 (and ROT47) instantly. ROT13 is its own inverse, so the same operation encodes and decodes.
Open in ctfpalROT13 is the Caesar cipher with a shift of 13. Because 13 is exactly half of 26, applying it twice returns the original - it is its own inverse, which is why it became the standard way to hide spoilers on Usenet and why it is the most common single-step obfuscation in beginner CTF challenges.
ROT47, the version that hides punctuation too
ROT13 leaves digits, braces, and underscores alone, so a ROT13'd flag still visibly looks like a flag. ROT47 rotates the entire printable ASCII range from ! to ~ by 47 positions, which scrambles the punctuation as well and makes the result look like line noise. If a blob is dense printable garbage with no recognisable structure, ROT47 is worth one click.
Worked example
Classic ROT13
Input
cvpbPGS{ebg13_vf_abg_pelcgb}Result
picoCTF{rot13_is_not_crypto}Go deeper
- Chi-squared, index of coincidence, and why classical ciphers fallCaesar, Vigenere, and substitution ciphers do not need guesswork - they need two statistics. How chi-squared scores a candidate plaintext, how the index of coincidence recovers a key length, and how to combine them into an attack that runs in milliseconds.
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Vigenere cipher solver with automatic key recovery
Decrypt Vigenere with a known key, or recover the key from ciphertext alone using index-of-coincidence period detection and per-column chi-squared.