Base64 Encoder & Decoder

Base64 is a binary-to-text encoding that uses 64 ASCII characters (A–Z, a–z, 0–9, +, /) to represent any binary data. It is the standard way to embed binary content — images, fonts, signatures, certificates — inside text-only contexts like HTML, JSON, XML, or HTTP headers. The price of this portability is roughly 33% size inflation: every 3 bytes of binary become 4 ASCII characters, plus optional padding.

A critical thing to understand: Base64 is not encryption. It is trivially reversible by anyone with a decoder. If you need to keep something secret, encrypt it first and then Base64-encode the ciphertext if needed for transport. Utilify's encoder handles UTF-8 strings correctly — including accented characters and emoji — by encoding through TextEncoder before the Base64 transformation, so round-trips preserve every byte exactly.

Under the hood the algorithm is simple: the input bytes are taken three at a time (24 bits), split into four 6-bit groups, and each group maps to one character in the 64-character alphabet. When the input length is not a multiple of three, one or two "=" padding characters are appended so the output length stays a multiple of four. That padding is why so many Base64 strings end in "=" or "==".

There are two common alphabets. Standard Base64 uses "+" and "/" for the last two characters; Base64URL replaces them with "-" and "_" and usually drops the padding, because "+", "/", and "=" have special meanings in URLs and filenames. JWTs, for example, are Base64URL-encoded. If a token or string fails to decode here, a mismatch between these two variants — or missing padding — is the usual culprit.

Beyond JWTs and data URIs, Base64 shows up almost everywhere binary data has to survive a text-only channel intact: email attachments (MIME), embedded TLS and SSH keys (PEM files are Base64 wrapped in header lines), inline SVG and font assets in CSS, and binary fields stored in JSON APIs. Knowing how to encode and decode it quickly is a small but constant part of working with the web.