Ascii85（Base85）エンコーディングを使う方法

问题

バイナリデータのテキスト表現として Base64 よりコンパクトなものが必要で、対象システムがより大きな文字セットに対応している状況です。

前提条件

• Base64 の理解
• バイナリデータへの習熟

ステップ

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.

   入力

   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.

   入力

   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.

   入力

   5EWac

   输出

   test

期待される結果

元のバイナリデータに正確にデコードできる、コンパクトな ASCII 文字列。

よくあるエラー

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.

トラブルシューティング指引

• 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'.

セキュリティ上の制限

• Ascii85 is a reversible encoding with no security properties.