# Base64 Canonical URL: https://offlineutils.com/glossary/base64 Also known as: Base-64, Base 64 encoding Published: 2026-06-24 Publisher: offlineutils.com > Base64 is a way of encoding arbitrary binary data as a string of 64 ASCII characters (A–Z, a–z, 0–9, '+', '/') so it can travel through systems that only handle plain text — like email, URLs, JSON, or HTML data URIs. Base64 takes three bytes of input at a time (24 bits) and emits four output characters (6 bits each). The output is exactly 4/3 the size of the input, padded with '=' characters when the input length is not a multiple of three. Because every output character is a printable ASCII letter, digit, or punctuation mark, the encoded string survives transport through text-only channels without escaping or corruption. Base64URL is a URL-safe variant that swaps '+' and '/' for '-' and '_' — the encoding JWTs use for their three segments. Base64 is not encryption. Anyone who sees a Base64 string can decode it back to the original bytes in milliseconds — there is no key, no secret. Treat it strictly as a transport encoding. ## Frequently asked questions ### Is Base64 encryption? No. Base64 is a reversible encoding with no key — anyone who sees the encoded string can decode it back instantly. Never use Base64 to 'hide' passwords or secrets; it provides no security at all. ### Why is the output longer than the input? Base64 encodes three bytes (24 bits) of input as four ASCII characters (6 bits each), so the output is exactly 4/3 the size of the input plus '=' padding when the input length is not a multiple of three. ### What's the difference between Base64 and Base64URL? Standard Base64 uses '+' and '/' as the last two characters of its alphabet, which have special meaning in URLs. Base64URL replaces them with '-' and '_' so the encoded string is safe to drop into a URL or filename without escaping. JWTs use Base64URL. ## Related tools - https://offlineutils.com/base64 - https://offlineutils.com/base32 - https://offlineutils.com/base58 - https://offlineutils.com/base85 - https://offlineutils.com/jwt-decoder ## References - https://en.wikipedia.org/wiki/Base64 - https://datatracker.ietf.org/doc/html/rfc4648