random_bytes()

Also Known As

TL;DR

Explanation

random_bytes(int $length) returns a string of $length cryptographically random bytes sourced from /dev/urandom (Linux/macOS) or CryptGenRandom (Windows). bin2hex(random_bytes(32)) produces a 64-character hex token with 256 bits of entropy — unguessable in practice. Use it for password reset tokens, API keys, CSRF tokens, and any other security-sensitive random value. Never use rand(), mt_rand(), or uniqid() for these purposes.

Common Misconception

Why It Matters

Common Mistakes

• Using rand(), mt_rand(), or array_rand() for security purposes — they are seeded by time and predictable.
• Using uniqid() for tokens — it is based on microtime and has insufficient entropy.
• Not hex-encoding or base64-encoding the output before storage — raw bytes may contain null bytes.
• Requesting fewer bytes than needed — 16 bytes (128 bits) is the minimum for security tokens; 32 bytes (256 bits) is better.

Code Examples

// Predictable token generation:
$token = md5(uniqid(mt_rand(), true)); // Guessable — time-based entropy
$token = sha1(rand()); // rand() is not cryptographically secure

// Correct:
$token = bin2hex(random_bytes(32)); // 256 bits of CSPRNG entropy

// Cryptographically secure random bytes (PHP 7+)
$token = random_bytes(32);                    // 32 bytes = 256 bits
$hex   = bin2hex($token);                     // 64 hex chars
$b64   = base64_encode($token);               // URL-safe: strtr(base64_encode($token), '+/', '-_')

// Secure random integer (e.g. OTP, random delay)
$otp   = random_int(100000, 999999);          // cryptographically secure

// NOT cryptographically secure — never use for tokens/secrets:
// mt_rand(), rand(), array_rand(), shuffle() — all predictable

// Password reset token
$resetToken = bin2hex(random_bytes(32)); // store hash in DB, send raw to user
$stored     = hash('sha256', $resetToken);

References

• ↗ https://www.php.net/manual/en/function.random-bytes.php

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