Base91 encoder and decoder

How to use it

1. Choose Encode or Decode.
2. Set Bytes as: Text (UTF-8) for ordinary text, or Hex bytes for binary data written as hex.
3. Type or paste the input; copy the result.

How it works

basE91 keeps a bit queue. Each input byte is added to it; when more than 13 bits are waiting, the encoder looks at the lowest 13 bits:

• if their value is above 88, it takes those 13 bits;
• otherwise it takes 14 bits (a 14-bit value whose low 13 bits are 88 or less is still below 91 × 91 = 8281).

The chunk v is written as two characters: v mod 91, then v div 91. At the end, the remaining bits become one or two more characters. The decoder reverses this: it reads characters in pairs, rebuilds v and knows from v whether it carried 13 or 14 bits.

Worked example

Encoding the text test (bytes 74 65 73 74):

• the first two bytes give the bit queue 0x6574; its low 13 bits are 0x0574 = 1396, which is above 88, so 13 bits are taken;
• 1396 mod 91 = 31 → f, 1396 div 91 = 15 → P;
• the rest of the bytes continue the same way and the result is fPNKd.

Hello, World! encodes to >OwJh>}AQ;r@@Y?F.

Tips

• basE91 output contains “ and other symbols — put it in single quotes in a shell, or escape it in JSON.
• There is no padding and no checksum: a cut-off string decodes to fewer bytes without an error.
• For a URL-safe format, use Base64 (URL variant) or Base58.