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Free online Caesar cipher encryption and decryption tool. Supports custom alphabets and shift offsets. Ideal for cryptography learning and fun.
Caesar cipher only works on English letters, numbers, punctuation and other characters will remain unchanged
Click generate button to start
Shift every letter in "HELLO" forward by 3 positions, and it becomes "KHOOR"—this is the core shifting concept of the Caesar cipher. Legend has it that Julius Caesar of ancient Rome used this substitution encryption to protect the contents of his military communications. It is a type of substitution cipher in classical cryptography: each letter in the plaintext is cyclically shifted along the alphabet by a fixed offset to produce the ciphertext. Our calculator operates around this "shift and substitute" principle. You can choose the standard alphabet (A-Z, a-z, 0-9) or customize any character sequence.
Although the Caesar cipher is simple, it is an indispensable starting point in cryptography education and frequently appears in fun puzzles and beginner cybersecurity challenges (CTFs).
During encryption, assuming we encode the letters A to Z sequentially as 0 to 25, with a shift value of n (0 ≤ n < 26), the ciphertext y corresponding to the plaintext letter x is:y = (x + n) mod 26
For decryption, simply shift in the reverse direction:x = (y - n) mod 26
"mod" represents the modulo operation (remainder), which ensures that shifting past the end of the alphabet loops back to the beginning. For example, shifting Z (encoded as 25) by 1 → (25+1
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