Base64 Encoder & Decoder

What Base64 is for

Base64 is a binary-to-text encoding scheme that maps every three bytes of binary input into four ASCII characters drawn from a fixed alphabet (A-Z, a-z, 0-9, +, /, with = as padding). It exists because many of the protocols and formats engineers work with — email, HTTP headers, JSON, URL parameters — are text-only and cannot safely carry arbitrary bytes.

Base64 is not encryption. The transformation is fully reversible without any key and adds about 33% overhead compared to the raw bytes. Use it when you need to transport binary data through a text channel, not when you need to keep it secret.

How the encoding works

Each character in the Base64 alphabet encodes 6 bits, so 4 characters carry exactly 24 bits — three 8-bit bytes. When the input length isn't a multiple of three, the encoder pads the output with one or two = characters to keep alignment. The decoder reverses this: it strips padding, maps each character back to 6 bits, and reassembles the bytes.

The original Base64 alphabet is unsafe for URLs (+ and / have special meaning in URLs and filenames). A variant called base64url uses - and _ instead and is the variant used inside JWTs.

Common use cases

• Embedding small images. The data: URI scheme uses Base64 to inline images in HTML and CSS.
• HTTP Basic authentication. The Authorization: Basic <b64> header encodes username:password as Base64.
• JWTs and signed tokens. Header, payload, and signature are each base64url-encoded.
• Email attachments. MIME uses Base64 to ship binary attachments inside text-only SMTP messages.
• API payloads carrying binary. APIs that exchange JSON often Base64-encode binary fields like keys, certificates, or images.

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