如何Use Ascii85 (Base85) Encoding

问题

你 need 更多 compact 文本 representation 的二进制数据 than Base64 offers, 和你的目标系统 supports larger 字符集合.

前置条件

• Understanding 的 Base64
• 熟悉二进制数据

步骤

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.

   输入

   4 bytes → 5 chars

   输出

   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.

   输入

   test

   输出

   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.

   输入

   5EWac

   输出

   test

预期结果

compact ASCII 字符串该 decodes 到 exact 原始二进制数据.

常见错误

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.

排障指引

• 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'.

安全限制

• Ascii85 is a reversible encoding with no security properties.