What Is Base64 Encoding? The Complete Guide to RFC 4648

What Is Base64?

Base64 is a binary-to-text encoding scheme defined in RFC 4648. It converts arbitrary binary data or text into an ASCII string format using a 64-character alphabet:

Alphabet: A-Z (26), a-z (26), 0-9 (10), + (1), / (1) = 64 characters

How the 6-Bit Algorithm Works

The encoder takes 3 bytes of input data (3 × 8 = 24 bits).
It divides the 24 bits into 4 groups of 6 bits each (4 × 6 = 24 bits).
Each 6-bit group represents a number from 0 to 63, which maps directly to an ASCII character in the Base64 index table.

Padding (=) Explained

If the input byte count is not evenly divisible by 3, padding characters (=) are appended:

If 1 byte remains (8 bits): Produces 2 Base64 characters and == padding.
If 2 bytes remain (16 bits): Produces 3 Base64 characters and = padding.

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