The Problem Base64 Solves
Many communication systems were designed for 7-bit ASCII text and cannot reliably transmit arbitrary binary data. Bytes with values 0-31 and 127+ can trigger control behaviors or get corrupted in transit. Base64 converts arbitrary binary data into a safe subset of ASCII characters that survives any text-safe transmission channel.
The Encoding Alphabet
Base64 uses 64 characters: A-Z (26), a-z (26), 0-9 (10), and two symbols (+ and /, or - and _ in URL-safe variants). Every possible 6-bit value maps to exactly one of these characters. Since text transmission systems reliably transmit all these characters unchanged, Base64-encoded data arrives intact regardless of what the original binary contained.
The Size Trade-off
Base64 represents every 3 bytes of input as 4 ASCII characters — a 33% size increase. A 1MB binary file becomes 1.37MB when Base64-encoded. This overhead is acceptable for small-to-medium data where transmission safety is the priority. For large binary transfers, purpose-built binary protocols are more efficient.
Not Encryption
Base64 encoding is not encryption or security. Encoded data is fully readable by anyone who decodes it — Base64 is reversible with no key or secret. It's purely a data format transformation for transmission safety. Treating Base64-encoded data as secure is a common and dangerous mistake.
Base64 encodes binary data as ASCII text to survive transmission through text-only channels. It's ubiquitous, useful, and completely transparent to decode — never mistake it for security.