Base64 をテキストにデコードする方法

问题

Base64 エンコードされた文字列を受け取り、元の平文やバイナリデータを復元する必要があります。

前提条件

• Base64 アルファベットの理解
• パディング文字を識別できること

ステップ

1. 1. Inspect the Base64 string

   Confirm the string only contains valid Base64 characters (A-Z, a-z, 0-9, +, /) and optional = padding. Remove any whitespace or line breaks that may have been inserted for readability.

   入力

   SGVsbG8sIFdvcmxkIQ==

   输出

   clean string

2. 2. Decode the string

   Pass the cleaned string to a Base64 decoder. The decoder maps each character back to a 6-bit value, concatenates the bits, and splits them into 8-bit bytes.

   入力

   SGVsbG8sIFdvcmxkIQ==

   输出

   Hello, World!

3. 3. Interpret the result

   If the original data was text, display it as text using the correct character set. If it was binary, save it as a file or inspect the bytes. A garbled result usually means the wrong charset was assumed.

   入力

   SGVsbG8sIFdvcmxkIQ==

   输出

   Hello, World!

期待される結果

エンコード前とまったく同じ、元のテキストまたはバイナリデータ。

よくあるエラー

Decoding fails with 'invalid length'

The string is not a multiple of 4. Add the missing = padding characters or re-encode the source.

Output is garbled or shows replacement characters

The original data was binary or used a different character set. Decode as bytes and inspect the raw output.

トラブルシューティング指引

• Strip all whitespace before decoding.
• If the string came from a URL, replace - with + and _ with / (URL-safe Base64) first.

セキュリティ上の制限

• Decoding Base64 reveals the original data instantly. Do not treat Base64 as a security barrier.