Click flag, set "letter shift" as you wish, enter text you want to encrypt, see encrypted text. Do the same to decrypt to original but use the negative of the original "letter shift" then enter encrypted text. The cipher shifts each letter of the original text by a fixed number down the alphabet. For example, with a shift of 1, letter A would be replaced by B, letter B would be replaced by C, and so on (see example text under notes and credits).
Works both to encrypt or decrypt, e.g. a message encrypted with a shift of +7, decrypts with shift of -7. Set letter shift to -4, then paste this text in "XLI QSPI LEH FIIR ASVOMRK ZIVC LEVH EPP XLI QSVRMRK, WTVMRK-GPIERMRK LMW PMXXPI LSQI" Hit return to decode. Julius Caesar used this type of cipher to secretly communicate with his generals, so it's known as the "Caesar Shift." If someone sends you a message encrypted with this "Caesar Shift" and doesn't tell you the shift number, use the letter frequency to figure out 1 or 2 common letters, such as "e" or find a lone "I". Once you have one letter see what shift produces it, or try all shifts. In the background is a chart of letter frequency. For example, you could enter this Caesar Shift encrypted text (copy and paste it in, even with shift 0, just to see which of it's letter is most frequent): YMJ QTSLJW F RJXXFLJ NX, YMJ RTWJ FHHZWFYJ NX DTZW KWJVZJSHD FSFQDXNX, RFPNSL IJHWDUYNSL JFXNJW. XT ZXJ XMTWYJW RJXXFLJX NK DTZ BFSY YT UWTANIJ QJXX HQZJX KTW IJHWDUYNSL. The frequency of letters shows J to be the most frequent letter in the encrypted text. Then note that the bar chart shows "E" or "e" is the most common letter. So to get the J to E use a shift of -5 (count FGHIJ, five letters). Use shift of -5 to decrypt that message (try it). The chart of letter frequency comes from: https://pi.math.cornell.edu/~mec/2003-2004/cryptography/subs/frequencies.html Also see: https://www.csfieldguide.org.nz/en/chapters/coding-encryption/substitution-ciphers/
