Base64 vs Base64URL: RFC 4648 Standards Explained
Discover why standard Base64 breaks in URLs and query strings, and how the RFC 4648 Base64URL standard fixes it with hyphens and underscores.
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Standard Base64 (defined in RFC 4648 Section 4) was engineered for email and general text transport. It uses the plus sign "+" for character index 62, the forward slash "/" for character index 63, and the equal sign "=" for trailing padding.
Why Standard Base64 Breaks URLs
When standard Base64 strings are embedded inside URL paths or query parameters, severe parsing errors can occur: in URLs, the "+" character is interpreted as a space ("%20"), the "/" character is reserved as a directory path delimiter, and "=" signifies key-value parameter separation.
The Base64URL Solution (RFC 4648 Section 5)
To make Base64 safe for web addresses, JSON Web Tokens (JWT), and OAuth authorization codes, the IETF defined Base64URL:
- Replace "+" with "-" (hyphen / minus sign)
- Replace "/" with "_" (underscore)
- Remove trailing "=" padding characters completely
Because Base64URL strings contain only alphanumeric characters, hyphens, and underscores, they require no URL percent-encoding and can be safely transmitted inside HTTP GET query parameters, cookies, and HTTP header values.
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