Base64とBase64URLの違い — 技術規格と変換解説

標準Base64がWeb URLで問題を起こす理由

Standard Base64 (RFC 4648 §4) uses the 64-character alphabet A-Z, a-z, 0-9, +, /, with = used as trailing byte padding.

However, when standard Base64 strings are passed in HTTP query parameters, URI path segments, or web cookies:

• The plus sign + is automatically interpreted by web servers as a space character (%20), corrupting the payload.
• The forward slash / is interpreted by web routers as a path delimiter, altering URL routing logic.
• The equals sign = is used as a key-value parameter separator in query strings.

アルファベット文字の置換ルール（RFC 4648 §5）

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