Secure Password Generator
パスワード生成ツール — Generate strong random passwords instantly
Generate cryptographically secure random passwords using your browser's built-in CSPRNG.
Customize length, character types, and generate up to 10 at once.
Everything happens in your browser — nothing is ever sent to any server. Free, no login required.
Password Strength Guide
| Entropy | Strength | Crack Time (est.) | Recommended For |
|---|
| < 40 bits | ⚠️ Weak | Seconds to hours | Not recommended |
| 40-59 bits | 🟡 Fair | Days to months | Low-risk services |
| 60-79 bits | 🟢 Strong | Years to decades | General services |
| ≥ 80 bits | 💪 Very Strong | Practically uncrackable | Banking, email, social media |
Common Password Configuration Examples
| Configuration | Length | Entropy | Strength |
|---|
| Numbers only | 8 chars | ~27 bits | ⚠️ Weak |
| Alphanumeric | 8 chars | ~48 bits | 🟡 Fair |
| Alphanumeric + symbols | 12 chars | ~79 bits | 🟢 Strong |
| Alphanumeric + symbols | 16 chars | ~105 bits | 💪 Very Strong |
| Alphanumeric + symbols | 20 chars | ~131 bits | 💪 Very Strong |
How to Create a Strong Password
A strong password is your first line of defense against unauthorized access to your online accounts. Whether you use Japanese services like Rakuten, Amazon Japan, LINE, or Yahoo! JAPAN, each account should have a unique, strong password. Japan's Ministry of Internal Affairs and Communications recommends passwords of at least 10 characters combining letters, numbers, and symbols.
Password strength is measured by "entropy" — the number of possible combinations expressed in bits. Higher entropy means more resistance to brute-force attacks. Generally, 80 bits or more of entropy is considered practically uncrackable with current computing technology. This tool automatically calculates and displays the entropy of every password it generates, so you can verify its strength at a glance.
Why Browser-Based Generation Is Secure
This password generator uses JavaScript's crypto.getRandomValues(), a cryptographically secure pseudo-random number generator (CSPRNG) built into every modern browser. Unlike Math.random(), which produces predictable patterns, CSPRNG output is suitable for security-critical applications.
Crucially, generated passwords never leave your browser. No server requests are made, eliminating the risk of network-based interception. When you close the page, passwords are cleared from memory. We recommend using a password manager (1Password, Bitwarden, LastPass, etc.) to securely store your generated passwords. Never reuse the same password across multiple services.
Frequently Asked Questions
How long should a secure password be?
At least 16 characters is recommended. A 16-character password with uppercase, lowercase, and numbers has about 95 bits of entropy, making it very strong. For critical accounts, aim for 20+ characters.
What characters should I include?
Use a mix of uppercase, lowercase, numbers, and symbols for maximum strength. If a service doesn't allow symbols, increase the length to compensate.
Are generated passwords stored on any server?
No. Everything is processed in your browser using crypto.getRandomValues(). Nothing is sent to any server. Your passwords never leave your browser.
What is password entropy?
Entropy measures password unpredictability in bits. Formula: log2(charset_size ^ length). Under 40 bits is weak, 60-79 is strong, 80+ is very strong.
Can I generate multiple passwords at once?
Yes — up to 10 at once. Set the count and click Generate. Each password is unique and independently generated.