ROT13 is its own inverse. Run the output through again and the original comes back.
Show the steps
- Move every letter 13 places along the alphabet, going round from Z back to A.
- M + 13 = Z
- e + 13 = r
- e + 13 = r
- t + 13 = g, going past Z and round to the start
- m + 13 = z
- e + 13 = r
- a + 13 = n
- t + 13 = g, going past Z and round to the start
- The other 8 letters follow the same rule.
- Spaces, digits and punctuation stay as they are.
Choose a conversion
Every cipher
53 converters in 5 groups
- Caesar cipherHELLObecomesKHOORAny shift from 1 to 25, with automatic shift detection.
- ROT13HELLObecomesURYYBRotate every letter by 13. The same step encodes and decodes.
- AtbashHELLObecomesSVOOLReverse the alphabet: A becomes Z, B becomes Y.
- Affine cipherHELLObecomesRCLLAMultiply the letter's position, add a shift, and wrap at 26.
- Vigenère cipherATTACKbecomesIHSQIRA shift that changes with every letter, set by a keyword.
- Playfair cipherHELLObecomesCFSUPMEncrypt letters in pairs, using a 5x5 square built from a key phrase.
- Hill cipherACTbecomesPOHMultiply blocks of letters by a matrix, modulo 26.
- Rail fence cipherHELLObecomesHOELLWrite the message in a zigzag, then read it off one rail at a time.
- Beaufort cipherHELLObecomesYKGIUVigenère turned round. Key minus message, so one step encodes and decodes.
- Porta cipherHELLObecomesOYTUBThirteen reciprocal alphabets, one for each pair of key letters.
- Nihilist cipherHIbecomes45 83A keyed Polybius square with a key word added to every number.
- Bifid cipherHELLObecomesFNNVDDelastelle's square that splits rows from columns and mixes them.
- Trifid cipherHELLObecomesSRFRYThe bifid in three dimensions, on a 27-symbol cube.
- Four-square cipherHELPbecomesFYGMTwo keyed squares and two plain ones, worked a pair of letters at a time.
- ScytaleHELLOWORLDbecomesHWEOL…The Spartan rod. Wind a strip round it and write along it.
- Columnar transpositionZEBRASbecomesEVLNA…Rows under a keyword, read off a column at a time. Single or double.
- Pigpen cipherAbecomesdrawnThe Freemasons' grid cipher, drawn as symbols you can download.
- Enigma machineHELLObecomesILBDAThe WW2 rotor machine: rotors, rings, plugboard, M3 and M4.
- Keyword cipherZEBRASbecomesHELLO = DAIILA keyword starts the cipher alphabet; encode and decode.
- Substitution cipher solverCrack a substitution cipher with no key.
- Cipher identifierURYYBbecomeswhich cipher?Paste something unreadable and we work out what it is.
- Frequency analysisCount the letters against English and see which cipher the counts point to.
- Chain decoderBase64becomesMorseUndo several layers in a row, and see each one.
- Steganography decoderimagebecomestextHidden text in an image's low bits, metadata or tail.
- A1Z26 cipherHIbecomes8-9A is 1 and Z is 26. The simplest way to write letters as numbers.
- Tap codeAbecomes. .Two runs of taps per letter, one for the row and one for the column.
- Polybius squareHIbecomes23 24Turn each letter into the row and column it occupies in a 5x5 grid.
- Bacon's cipherAbecomesAAAAAFive As and Bs per letter. A binary alphabet, published in 1623.
- Pig LatinhellobecomesellohayMove the first sound to the end and add “ay”.
- Invisible texthibecomes(nothing)Hide a message in characters that draw nothing, or find one.
- Codon chartAUGbecomesMetAll 64 codons, and DNA or mRNA to the protein it makes.
- The Voynich manuscriptA 15th-century book nobody can read, and why every solution has failed.
- KryptosEMUFPHZbecomesBETWEENDecrypt the CIA sculpture's first passage, and where K4 stands.
- Zodiac ciphersZ408, Z340, Z13 and Z32, and the transposition that hid Z340 for 51 years.
- The Beale ciphers115becomesIDecode cipher 2 with the Declaration, and why 1 and 3 never decode.
- Famous codes and ciphersSolved, unsolved and still argued over, with Elgar's Dorabella.
- Book cipher115becomesinstitutedWord, line and page numbers into any text, both ways.
- Caesar cipher wheelTurn a cipher wheel on screen, or print two discs to cut out and pin.
- Cryptogram solverPaste any letter-swap puzzle and get the answer, with your own hints kept.
- StringMash DailyA new cipher puzzle every day, easy on Monday, hardest on Sunday.
- CryptogramsFree cryptogram puzzles at Easy, Medium and Hard.
- Cryptogram makerAny sentence as a printable cryptogram, with hint letters.
- Escape room puzzle makerPrintable code puzzles from your own message, with an answer key.
- Puzzle lockanswerbecomessecretA link that reveals coordinates or a clue only for the right answer.
- Free printable puzzle packsA classroom escape room, maths code-breakers and Halloween codes.
- Hint decoderYbbx [Rock]becomesLook [Rock]ROT13 for cache hints, with [bracketed] words left alone.
- Steganography decoderimagebecomestextHidden text in an image's low bits, metadata or tail.
- GPS coordinate converter32.70205becomesN 32° 42.123DDM, decimal degrees and DMS from one paste, each with a copy button.
- Formula solver2A.B(C+1)becomes23.713Fill in a multi-cache's letters and brackets to get the final.
- Waypoint projection250 m at 70°becomesnew pointGo a distance on a bearing, or measure between two points, on the WGS84 ellipsoid.
- Word value calculatorGEOCACHEbecomes47Letter sums, digital roots, coordinate checksums and keypad digits.
- Puzzle lockanswerbecomessecretA link that reveals coordinates or a clue only for the right answer.
- Maidenhead locator41.71, -72.73becomesFN31PRHam radio grid squares from coordinates, and back.
- UTM converter40.7484, -73.9857becomes18T 585631Latitude and longitude to UTM zone, easting and northing, and back.
- MGRS converter4Q FJ 1234 6789becomes21.4098Military grid references to latitude and longitude, 1 m to 10 km.
Nothing here matches that. If you're holding text you can't read, the cipher identifier tries every converter on it.
Start with the identifier
Know which cipher you're looking at? Go straight to its page. If you don't, paste the message into the cipher identifier. It tries every shift, the Atbash mirror and the common text encodings, then ranks the results by how much each reads like English.
Puzzle messages usually turn out to be one of three things: a shifted alphabet, which the Caesar cipher page handles at any shift from 1 to 25; a mirrored alphabet, which is Atbash; or not a cipher at all, but an encoding like Base64 or binary in disguise.
Substitution or transposition?
Most classic ciphers substitute, swapping each letter for another, as in ROT13, Caesar and Atbash. Transposition ciphers keep every letter and scramble the order instead. A rail fence or columnar transposition message holds every letter of the original, rearranged.
That decides how a message breaks. Substitution leaves letter frequencies intact, so counting letters gives the game away. Counting won't help with a transposition, whose frequencies look like ordinary English even though the words don't read; it needs a different attack.
Questions
Which cipher is this message in?
Paste it into the cipher identifier. All letters with normal-looking word lengths suggests a shift or Atbash; digits and symbols suggest an encoding rather than a cipher.
Are these ciphers secure?
No. They're for puzzles and spoilers, and for escape rooms that need a lock anyone can pick. None of them would stop someone who knows the method. Most fall to letter counting, which al-Kindi described in the 9th century, and invisible text hides nothing from a tool that looks for it.
Why do some pages disable the swap button?
ROT13 and Atbash undo themselves, so there's nothing to swap. Each page says so where it happens.
Sources
- Folger Shakespeare Library: Decoding the Renaissance exhibition material
- Wikipedia: Pigpen cipher
- Wikipedia: Transposition cipher
- Wikipedia: Frequency analysis
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