Base64, URL Encoding, and Hashing Explained: What Every Web Developer Should Know

Encoding vs. Encryption vs. Hashing: The Critical Distinction

Before we dive into specific algorithms, you need to understand the fundamental difference between these three operations. Confusing them is not just a terminology issue — it leads to real security vulnerabilities. Developers who treat Base64 as "encryption" or SHA-256 as a password storage solution are building systems with exploitable flaws.

Encoding transforms data from one format to another for compatibility or transport purposes. It uses a publicly known scheme and requires no key. Anyone can encode and decode data freely. Encoding provides zero security. Its purpose is purely to make data safe for a particular transmission channel — Base64 makes binary data safe for text-based protocols, and URL encoding makes special characters safe for URLs.

Encryption transforms data to make it unreadable without a specific key. It is reversible, but only by someone who possesses the correct decryption key. AES-256, RSA, and ChaCha20 are encryption algorithms. The purpose is confidentiality — preventing unauthorized parties from reading the data.

Hashing transforms data into a fixed-size digest (fingerprint) that cannot be reversed. There is no key, no decryption, and no way to recover the original input from the hash. SHA-256, bcrypt, and Argon2 are hash functions. The purpose is integrity verification and secure storage of secrets like passwords.

Property	Encoding	Encryption	Hashing
Reversible?	Yes, by anyone	Yes, with the key	No, never
Key required?	No	Yes	No
Purpose	Data compatibility	Confidentiality	Integrity / fingerprint
Output size	Varies (~33% larger)	Varies (similar to input)	Fixed (e.g., 256 bits)
Examples	Base64, URL encoding	AES-256, RSA	SHA-256, bcrypt

Base64 Encoding: How It Works and When to Use It

Base64 is an encoding scheme that converts binary data into a string of 64 ASCII characters (A-Z, a-z, 0-9, +, /, and = for padding). It was designed to solve a specific problem: many transport protocols (email, JSON, XML, HTML) only support text, not raw binary data. Base64 bridges that gap.

How Base64 Actually Works

The algorithm is straightforward. It takes every 3 bytes (24 bits) of input and splits them into four 6-bit groups. Each 6-bit value (0-63) maps to a character in the Base64 alphabet. If the input length is not a multiple of 3, the output is padded with = characters to maintain alignment.

Input:    "Hi"
ASCII:    72, 105
Binary:   01001000 01101001

Split into 6-bit groups:
010010 | 000110 | 1001xx

Pad the last group with zeros:
010010 | 000110 | 100100

Map to Base64 alphabet:
010010 = 18 = S
000110 = 6  = G
100100 = 36 = k

Add padding (input was 2 bytes, not 3):
Result: "SGk="

This is why Base64 always increases the data size by approximately 33% — three bytes of input become four bytes of output. That overhead is the cost of text-safe representation.

When to Use Base64

Base64 is the right choice in these specific situations:

• Data URIs — Embedding small images, fonts, or files directly in HTML or CSS. For example, <img src="data:image/png;base64,iVBORw0KGgo..."> eliminates an HTTP request at the cost of a larger HTML document. This is beneficial for icons under 2-3 KB but counterproductive for larger assets.
• JSON Web Tokens (JWT) — The header and payload of every JWT are Base64URL-encoded. This allows the structured JSON data to be safely embedded in HTTP headers, cookies, and URLs.
• Email attachments (MIME) — The SMTP protocol only supports 7-bit ASCII. Base64 encoding allows binary files (images, PDFs, executables) to be transmitted as text within email bodies.
• API payloads — When you need to send binary data (file contents, images, certificates) inside a JSON request body, Base64 encoding the data is the standard approach since JSON does not support raw binary.
• HTTP Basic Authentication — The credentials are encoded as Base64(username:password) and sent in the Authorization header. Note: this is encoding, not encryption. The credentials are readable by anyone who intercepts the header. Always use HTTPS.

// Browser: btoa() encodes to Base64, atob() decodes
const encoded = btoa("Hello, World!");
console.log(encoded); // "SGVsbG8sIFdvcmxkIQ=="

const decoded = atob("SGVsbG8sIFdvcmxkIQ==");
console.log(decoded); // "Hello, World!"

// Handling Unicode characters (btoa only supports Latin-1)
function base64Encode(str) {
  return btoa(encodeURIComponent(str).replace(
    /%([0-9A-F]{2})/g,
    (_, p1) => String.fromCharCode(parseInt(p1, 16))
  ));
}

function base64Decode(b64) {
  return decodeURIComponent(
    atob(b64).split("").map(
      c => "%" + c.charCodeAt(0).toString(16).padStart(2, "0")
    ).join("")
  );
}

console.log(base64Encode("Hallo Welt!")); // Works with any Unicode
console.log(base64Decode(base64Encode("Hallo Welt!"))); // "Hallo Welt!"

// Node.js: Use Buffer
const b64 = Buffer.from("Hello, World!").toString("base64");
const text = Buffer.from(b64, "base64").toString("utf-8");

// Base64URL variant (used in JWTs) — no +, /, or = characters
function toBase64Url(base64) {
  return base64.replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "");
}

function fromBase64Url(base64url) {
  let b64 = base64url.replace(/-/g, "+").replace(/_/g, "/");
  while (b64.length % 4 !== 0) b64 += "=";
  return b64;
}

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URL Encoding: Percent-Encoding for the Web

URLs have a strictly defined set of characters they can contain. Letters, digits, and a few special characters (-, _, ., ~) are allowed as-is. Everything else — spaces, accented characters, emoji, symbols like & and = that have special meaning in query strings — must be percent-encoded before being placed in a URL.

How Percent-Encoding Works

Percent-encoding is simple: take the UTF-8 byte representation of a character and express each byte as % followed by two hexadecimal digits. A space becomes %20, an ampersand becomes %26, and the euro sign (€) becomes %E2%82%AC because its UTF-8 representation is three bytes: 0xE2, 0x82, 0xAC.

Space     → %20  (or + in form data)
!         → %21
#         → %23
$         → %24
&         → %26
+         → %2B
/         → %2F
:         → %3A
=         → %3D
?         → %3F
@         → %40
[         → %5B
]         → %5D

encodeURI vs. encodeURIComponent

JavaScript provides two functions for URL encoding, and choosing the wrong one is a frequent source of bugs.

encodeURI encodes a complete URL. It preserves characters that have structural meaning in URLs: : / ? # [ ] @ ! $ & ' ( ) * + , ; =. Use it when you have a full URL string and want to make it safe without breaking its structure.

encodeURIComponent encodes a URL component (a query parameter value, a path segment). It encodes everything except A-Z a-z 0-9 - _ . ~. Use it when you are building a URL piece by piece and need to encode the individual values.

const url = "https://example.com/search?q=hello world&lang=en";

// encodeURI: preserves URL structure characters
console.log(encodeURI(url));
// "https://example.com/search?q=hello%20world&lang=en"
// Note: & and = are preserved (they are part of the URL structure)

// encodeURIComponent: encodes EVERYTHING except unreserved chars
console.log(encodeURIComponent(url));
// "https%3A%2F%2Fexample.com%2Fsearch%3Fq%3Dhello%20world%26lang%3Den"
// Note: This BREAKS the URL — it encoded the structure characters too

// CORRECT usage: Build URLs with encodeURIComponent for values
const query = "price >= 100 & category = tools";
const safeUrl = `https://example.com/search?q=${encodeURIComponent(query)}`;
console.log(safeUrl);
// "https://example.com/search?q=price%20%3E%3D%20100%20%26%20category%20%3D%20tools"

// Modern alternative: URLSearchParams handles encoding automatically
const params = new URLSearchParams({
  q: "hello world",
  category: "dev tools & utilities",
  page: "1"
});
console.log(params.toString());
// "q=hello+world&category=dev+tools+%26+utilities&page=1"

const fullUrl = `https://example.com/search?${params}`;
console.log(fullUrl);
// Clean, correctly encoded URL

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Common URL Encoding Pitfalls

• Double encoding — If you encode a value that is already encoded, %20 becomes %2520. This happens when frameworks or libraries encode values automatically and you encode them manually as well. Always know whether your framework handles encoding for you.
• Forgetting to encode path segments — File names with spaces or special characters in URL paths break if not encoded. Use encodeURIComponent for each path segment individually.
• Using encodeURI for query values — If a query value contains & or =, encodeURI will not encode them, which breaks the query string parsing. Always use encodeURIComponent for parameter values.
• The + sign ambiguity — In query strings (application/x-www-form-urlencoded), + means space. In path segments, + means a literal plus. This distinction catches many developers off guard when parsing URLs manually.

Hashing: One-Way Functions for Integrity and Security

A hash function takes an input of any size and produces a fixed-size output (the hash, or digest) that serves as a unique fingerprint of the input. Change even one bit of the input, and the entire hash changes unpredictably. Critically, hashing is a one-way operation — you cannot reverse a hash to recover the original input.

Common Hash Algorithms

MD5 produces a 128-bit (32 hex character) hash. It is fast and widely supported but cryptographically broken — collisions (two different inputs producing the same hash) can be generated in seconds. Do not use MD5 for anything security-related. It is still acceptable for non-security checksums, like verifying file integrity during downloads where an attacker cannot modify the checksum.

SHA-1 produces a 160-bit (40 hex character) hash. Like MD5, it is considered broken for security purposes since practical collision attacks were demonstrated in 2017. Major browsers and certificate authorities stopped trusting SHA-1 certificates years ago. Do not use SHA-1 in new systems.

SHA-256 and SHA-512 are members of the SHA-2 family. SHA-256 produces a 256-bit (64 hex character) hash, and SHA-512 produces a 512-bit (128 hex character) hash. Both remain secure with no known practical attacks. SHA-256 is the standard choice for digital signatures, certificate chains, blockchain, and data integrity verification.

bcrypt, scrypt, and Argon2 are specialized password hashing functions. Unlike general-purpose hash functions that are designed to be fast, these are intentionally slow and memory-intensive. They include a built-in salt and a configurable cost factor that makes brute-force attacks impractical. Argon2id is the current state-of-the-art recommendation.

// SHA-256 hash in the browser (Web Crypto API)
async function sha256(message) {
  const encoder = new TextEncoder();
  const data = encoder.encode(message);
  const hashBuffer = await crypto.subtle.digest("SHA-256", data);
  const hashArray = Array.from(new Uint8Array(hashBuffer));
  return hashArray.map(b => b.toString(16).padStart(2, "0")).join("");
}

// Usage
const hash = await sha256("Hello, World!");
console.log(hash);
// "dffd6021bb2bd5b0af676290809ec3a53191dd81c7f70a4b28688a362182986f"

// SHA-512
async function sha512(message) {
  const data = new TextEncoder().encode(message);
  const hashBuffer = await crypto.subtle.digest("SHA-512", data);
  return Array.from(new Uint8Array(hashBuffer))
    .map(b => b.toString(16).padStart(2, "0")).join("");
}

// MD5 is NOT available in Web Crypto API (intentionally — it is broken)
// If you need MD5 for legacy compatibility, use a library like js-md5

import { createHash, randomBytes, timingSafeEqual } from "crypto";

// SHA-256
const hash = createHash("sha256").update("Hello, World!").digest("hex");
console.log(hash);
// "dffd6021bb2bd5b0af676290809ec3a53191dd81c7f70a4b28688a362182986f"

// SHA-512
const hash512 = createHash("sha512").update("Hello, World!").digest("hex");

// File integrity check — hash an entire file
import { createReadStream } from "fs";

function hashFile(filePath) {
  return new Promise((resolve, reject) => {
    const hash = createHash("sha256");
    const stream = createReadStream(filePath);
    stream.on("data", chunk => hash.update(chunk));
    stream.on("end", () => resolve(hash.digest("hex")));
    stream.on("error", reject);
  });
}

// HMAC — Hash-based Message Authentication Code
import { createHmac } from "crypto";
const hmac = createHmac("sha256", "your-secret-key")
  .update("message to authenticate")
  .digest("hex");

// bcrypt for passwords (use the 'bcrypt' npm package)
import bcrypt from "bcrypt";
const saltRounds = 12;

// Hash a password
const passwordHash = await bcrypt.hash("user-password-here", saltRounds);
// "$2b$12$LJ3m4ys8Lp.XHxZp2Kq.5OQ4H1Gq6e3mZf8Rj8NxK0hP1qE9Tv.C"

// Verify a password
const isValid = await bcrypt.compare("user-password-here", passwordHash);
console.log(isValid); // true

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When to Use Which Hash Algorithm

• Passwords: Always use bcrypt (cost 12+), scrypt, or Argon2id. Never SHA-256 or MD5. General-purpose hash functions are too fast — an attacker can try billions of guesses per second.
• Data integrity: Use SHA-256. Verifying file downloads, ensuring data has not been tampered with, generating checksums for caching.
• Digital signatures: Use SHA-256 or SHA-512 as part of RSA, ECDSA, or EdDSA signature schemes.
• API authentication: Use HMAC-SHA256 for signing API requests (like AWS Signature V4).
• Deduplication: SHA-256 for content-addressable storage where you need to detect duplicate files.
• Non-security checksums: MD5 or CRC32 is acceptable when you only need to detect accidental data corruption, not deliberate tampering.

Security Implications and Common Mistakes

Here are the mistakes that actually cause breaches, not theoretical vulnerabilities but patterns found in production systems every day.

Mistake 1: Storing Passwords with SHA-256

SHA-256 is a good hash function, but it is a terrible password hash. Modern GPUs can compute billions of SHA-256 hashes per second, which means an attacker who obtains your hashed password database can brute-force most passwords in hours. Password hashing functions like bcrypt, scrypt, and Argon2 are intentionally slow (hundreds of milliseconds per hash) and include salts that prevent precomputed attacks.

Mistake 2: Treating Base64 as Security

Storing API keys, tokens, or sensitive configuration as Base64 strings and calling it "obfuscation" or "light encryption" is a recurring anti-pattern. Base64 decoding is a single function call. It provides no security whatsoever. If you need to protect secrets at rest, use proper encryption (AES-256-GCM) with a key management system.

Mistake 3: Using MD5 or SHA-1 for Security

Both MD5 and SHA-1 have known collision attacks. An attacker can create two different documents with the same hash. This undermines certificate verification, code signing, and any system that relies on the hash as proof of integrity. Migrate to SHA-256 for all security-critical applications.

Mistake 4: Not Salting Hashes

Without a salt (a random string appended to the input before hashing), identical passwords produce identical hashes. This enables rainbow table attacks — precomputed tables mapping common passwords to their hashes. A unique salt per user makes precomputed attacks useless because the attacker would need a separate rainbow table for every possible salt value. Bcrypt, scrypt, and Argon2 handle salting automatically.

Mistake 5: Comparing Hashes with ===

String comparison with === in most languages is not constant-time. It returns false as soon as it finds the first differing character. An attacker can measure the response time to determine how many characters of a hash match, gradually narrowing down the correct value. This is called a timing attack. Always use a constant-time comparison function like Node.js crypto.timingSafeEqual() when comparing hashes or signatures.

import { timingSafeEqual, createHmac } from "crypto";

function verifySignature(payload, signature, secret) {
  const expected = createHmac("sha256", secret)
    .update(payload)
    .digest();

  const received = Buffer.from(signature, "hex");

  // Prevent timing attacks: both buffers must be the same length
  if (expected.length !== received.length) return false;

  // Constant-time comparison — takes the same time regardless of where
  // the first difference occurs
  return timingSafeEqual(expected, received);
}

// WRONG: Vulnerable to timing attacks
// if (computedHash === receivedHash) { ... }

// CORRECT: Constant-time comparison
// if (timingSafeEqual(computedHash, receivedHash)) { ... }

The Connection to Binary and Hex

All encoding and hashing ultimately operates on binary data. Understanding how data flows between binary, hexadecimal, Base64, and text representations gives you a complete mental model of data transformation in web development.

When you hash a string with SHA-256, the internal computation works entirely on the binary representation of the input. The hex output you see (dffd6021bb...) is just one way to display those 256 bits as human-readable text. You could equally represent the same hash as Base64 (3/1gIbsr1bC...) or raw binary.

Our Binary Converter and Hex Converter tools let you see exactly how text maps to binary and hexadecimal representations. This is invaluable when debugging encoding issues, inspecting network traffic, or understanding how cryptographic functions process your data at the byte level.

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Practical Cheatsheet: What to Use When

Scenario	Use This	Never Use This
Storing user passwords	bcrypt / Argon2id	SHA-256, MD5, Base64
Embedding image in HTML	Base64 data URI	Any hash or encryption
URL query parameter value	encodeURIComponent	encodeURI, Base64
File integrity check	SHA-256	MD5 (for security), Base64
API request signing	HMAC-SHA256	Plain SHA-256, MD5
Sending binary in JSON	Base64	Raw binary, hex (too large)
Protecting sensitive data	AES-256-GCM encryption	Base64, any hash function
JWT header & payload	Base64URL	Standard Base64 (not URL-safe)

Essential Tools for Encoding, Decoding, and Hashing

Having the right tools at hand eliminates guesswork. These run entirely in your browser — no data is sent to any server, and no signup is needed.

• Base64 Encoder/Decoder — Encode text or binary to Base64, decode Base64 strings back to their original form. Supports standard Base64 and Base64URL. Essential for debugging JWTs, data URIs, and API payloads.
• URL Encoder/Decoder — Encode and decode URL components with proper percent-encoding. See exactly which characters get encoded and how. Invaluable when debugging query strings and API endpoints.
• Hash Generator — Generate MD5, SHA-1, SHA-256, and SHA-512 hashes of any input instantly. Compare hashes, verify file integrity, and understand how different algorithms produce different outputs.
• Binary Converter — Convert between text, binary, and decimal representations. See the raw binary data that underlies all encoding and hashing operations.
• Hex Converter — Convert between text and hexadecimal. Inspect hash outputs, debug byte sequences, and understand how hex notation maps to binary data.

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