Ascii85 Encoder / Decoder

How it is calculated

Encoding reads the text as bytes and takes them in groups of 4. Each group is read as one 32-bit number n = b0·256³ + b1·256² + b2·256 + b3, which is written in base 85 as five digits; each digit d becomes the character with code d + 33, so the output uses the printable characters from ! to u. A last group of r bytes (fewer than 4) is padded with zero bytes and only the first r + 1 characters are kept. Decoding reverses this: whitespace is removed, characters are read in groups of 5, and a short last group is padded with u. This tool has no <~ ~> wrapper and no z shortcut for four zero bytes; input containing them, or any character outside ! to u, returns no result. Non-ASCII letters take several bytes, so they give more characters.

Common mistakes

• Pasting Ascii85 with the <~ and ~> markers from a PDF or PostScript file; this tool does not strip them, so remove them first.
• Expecting the z abbreviation for four zero bytes. It is not produced when encoding and is not accepted when decoding.
• Leaving the mode on encode when the text is already Ascii85. It is then encoded a second time, giving longer, different text.
• Copying only part of an encoded string. A missing character changes the group alignment, and the output is garbled or empty.
• Treating Ascii85 as encryption. It is only a reversible encoding that anyone can decode.