What is the Caesar cipher?

The Caesar cipher is a substitution cipher where each letter is replaced by a letter a fixed number of positions away in the alphabet. For example, with shift 3, A becomes D, B becomes E, and so on.

ROT13 is a special case of the Caesar cipher with a shift of 13. Because 26 ÷ 2 = 13, encoding and decoding use the same operation — applying ROT13 twice returns the original text.

Is the Caesar cipher secure?

No. The Caesar cipher is extremely weak by modern standards — there are only 25 non-trivial shifts, so it can be broken by hand or instantly by a computer. It is useful for learning cryptography basics but should never be used to protect real data.

Does this tool handle numbers and punctuation?

Yes. Numbers, spaces, and punctuation characters are left unchanged. Only letters (A–Z and a–z) are shifted.

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Caesar Cipher

Encode or decode text using the classic Caesar cipher shift algorithm.

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The Caesar cipher is one of the oldest and simplest encryption techniques, named after Julius Caesar who reportedly used it to communicate with his generals. It works by shifting each letter in the plaintext by a fixed number of positions in the alphabet. This tool lets you encode or decode any text with a shift value from 0 to 25, view an alphabet mapping visualization, and explore all 26 possible shifts at once in a brute-force table — perfect for solving puzzles or learning the basics of cryptography.

使い方

1 Type or paste your text into the input field.
2 Choose Encode to encrypt or Decode to decrypt.
3 Drag the shift slider or type a number between 0 and 25.
4 The result appears instantly below.
5 View the alphabet mapping to see which letters correspond.
6 Use the brute-force table to see all 26 possible decoded outputs.

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