How to Use Ascii85 (Base85) Encoding

Problem

You need a more compact text representation of binary data than Base64 offers, and your target system supports a larger character set.

Prerequisites

• Understanding of Base64
• Familiarity with binary data

Steps

1. 1. Understand the 5-to-4 ratio

   Ascii85 encodes 4 bytes of binary into 5 ASCII characters, using an 85-character alphabet. This is more compact than Base64's 3-to-4 ratio, saving about 7% space.

   Input

   4 bytes → 5 chars

   Output

   vs Base64: 3 bytes → 4 chars

2. 2. Encode the data

   Split the input into 4-byte groups, interpret each as a 32-bit big-endian integer, and convert it to base 85 using 5 digits. Map each digit to a character starting from '!' (ASCII 33). Handle the remainder if the input length is not a multiple of 4.

   Input

   test

   Output

   5EWac

3. 3. Decode and verify

   Reverse the process: convert each 5-character group back to a 32-bit integer, then split into 4 bytes. Strip the wrapper delimiters (<~ and ~> for Adobe Ascii85) before decoding if present.

   Input

   5EWac

   Output

   test

Expected result

A compact ASCII string that decodes to the exact original binary data.

Common errors

Mixing Adobe Ascii85 and standard Ascii85

Adobe Ascii85 wraps the output in <~ and ~> and uses a special 'z' shorthand for all-zero groups. Know which variant your decoder expects.

Incorrect remainder handling

When the input is not a multiple of 4, pad with zero bytes, encode, then truncate the output to the correct length. The decoder must reverse this.

Troubleshooting

• If the decoded data is wrong, check for the Adobe 'z' shorthand which represents 4 zero bytes as a single character.
• Verify the alphabet starts at '!' (ASCII 33), not at '0' or 'A'.

Security limits

• Ascii85 is a reversible encoding with no security properties.