What is Base64 encoding?

Base64 is a binary-to-text encoding scheme that represents binary data using 64 printable ASCII characters (A–Z, a–z, 0–9, +, /). It is used to safely transmit binary data over systems that only handle text, such as email, HTTP headers, and JSON payloads.

What is URL-safe Base64?

URL-safe Base64 replaces the characters + with - and / with _ so the encoded string can be safely included in URLs and filenames without percent-encoding. It also omits the trailing = padding. It is used in JWTs (JSON Web Tokens) and OAuth tokens.

Does Base64 encoding encrypt my data?

No. Base64 is an encoding scheme, not encryption. Encoded data can be decoded instantly by anyone. Use it only for format conversion, not for securing sensitive information.

How much larger is Base64 output compared to the original?

Base64-encoded data is approximately 33% larger than the original binary. Every 3 bytes of input become 4 characters of Base64 output (4/3 ≈ 1.333...).

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Invalid Base64 — contains characters outside the Base64 alphabet.

URL-safe (- _ instead of + /) Wrap lines (76 chars)

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Base64 Output

What Is Base64?

Base64 is a binary-to-text encoding scheme that represents binary data using 64 printable ASCII characters: A–Z, a–z, 0–9, +, and /. The name comes from the 64-character alphabet.

It was designed to safely transmit binary data (like images or executables) over channels that only handle plain text — such as email (MIME), HTTP headers, and JSON APIs. Base64 is an encoding, not encryption: anyone can decode it instantly.

Common Use Cases

🔑

API Tokens & Auth Headers

HTTP Basic Auth encodes username:password as Base64 in the Authorization header. Many API keys are Base64-encoded byte sequences.

🪙

JWT Tokens

JSON Web Tokens (JWTs) use URL-safe Base64 to encode the header, payload, and signature. The three parts are separated by dots.

🖼️

Data URIs (Inline Images)

CSS and HTML can embed images directly using data:image/png;base64,… — no separate HTTP request needed for small icons and logos.

📧

Email Attachments (MIME)

Email protocols (SMTP) are text-only. MIME encodes binary attachments as Base64 so they survive transmission through mail servers.

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JSON Payloads

APIs that need to include binary data (certificates, keys, file bytes) in a JSON body Base64-encode the binary into a string field.

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Cryptographic Keys

PEM certificates and SSH public keys are Base64-encoded DER structures wrapped in -----BEGIN … ----- headers.

Standard vs. URL-safe Base64

Standard Base64 uses + and / as the 62nd and 63rd characters, and = for padding. These characters have special meaning in URLs, so URL-safe Base64 (RFC 4648 §5) swaps them:

+ → - (hyphen)
/ → _ (underscore)
= padding is omitted

URL-safe Base64 is used in JWTs, OAuth tokens, and any situation where the encoded string appears in a URL or filename.

How Base64 Works

Base64 groups input bytes into 3-byte chunks and converts each chunk into 4 Base64 characters (each representing 6 bits). This is why the output is always 4/3 ≈ 33% larger than the input.

When the input length is not a multiple of 3, one or two = padding characters are appended to keep the output length a multiple of 4.

How to Use Base64 in Code

JavaScript

// Encode (handles ASCII only — for Unicode use TextEncoder) const encoded = btoa('Hello, World!'); // "SGVsbG8sIFdvcmxkIQ==" // Decode const decoded = atob('SGVsbG8sIFdvcmxkIQ=='); // "Hello, World!" // Unicode-safe encode function encodeUnicode(str) { const bytes = new TextEncoder().encode(str); let binary = ''; bytes.forEach(b => binary += String.fromCharCode(b)); return btoa(binary); }

Python

import base64 # Encode encoded = base64.b64encode(b'Hello, World!').decode('utf-8') # 'SGVsbG8sIFdvcmxkIQ==' # Decode decoded = base64.b64decode('SGVsbG8sIFdvcmxkIQ==').decode('utf-8') # 'Hello, World!' # URL-safe variant url_encoded = base64.urlsafe_b64encode(b'Hello!').decode().rstrip('=')

PHP

// Encode $encoded = base64_encode('Hello, World!'); // 'SGVsbG8sIFdvcmxkIQ==' // Decode $decoded = base64_decode('SGVsbG8sIFdvcmxkIQ=='); // 'Hello, World!' // URL-safe encode $urlSafe = rtrim(strtr(base64_encode($data), '+/', '-_'), '=');

Command Line (Linux / macOS)

# Encode a string echo -n 'Hello, World!' | base64 # SGVsbG8sIFdvcmxkIQ== # Decode echo 'SGVsbG8sIFdvcmxkIQ==' | base64 --decode # Encode a file base64 image.png > image.b64