Base64 エンコード／デコード

What is Base64?

Base64 is an encoding method that uses 64 printable characters to represent binary data. Since 2⁶=64, every 6 bits form a unit mapped to one printable character. Base64 is commonly used to transmit binary data in text-based protocols, such as Data URI in HTML/CSS and HTTP Basic Auth tokens.

How It Works

During encoding, every 3 bytes (24 bits) are split into 4 groups of 6 bits each, mapped to a Base64 table character. If data is less than 3 bytes, = padding is used. Decoding reverses this process, converting each Base64 character back to 6 bits and reassembling the original data.

Base64 Encoding Notes:

• Encoded data is about 33% larger than original (3 bytes → 4 chars)
• Printable characters: A-Z, a-z, 0-9, +, /
• May include 1 or 2 = padding characters at the end
• Fully reversible encoding (not encryption)

Common Use Cases

• Data URI — Embed images, fonts and other binaries directly in HTML/CSS
• HTTP Basic Auth — Username and password transmitted via Base64 encoding
• JWT Token — Header and Payload of JSON Web Tokens use Base64 URL Safe encoding
• API Token — Many API keys use Base64 encoding for storage and transmission
• Email Attachments — MIME protocol uses Base64 for binary attachment transfer

Base64 Encoding Table