Maximum Security Advisory
For maximum security, save this page as an HTML file and run it on an air-gapped, offline computer that has never connected to the internet. Write your seed phrase on paper immediately. Never store it digitally, photograph it, or type it into any device connected to the internet.
🗝 Seed Phrase Generator
Generated Seed Phrase
Critical Notice: This tool is provided for educational and research purposes. Never generate seed phrases for high-value wallets on a network-connected computer. Test your recovery phrase with a small balance before committing significant funds. The developers assume no liability for loss of funds. This tool embeds the BIP39 English wordlist - verify against the official specification at github.com/trezor/python-mnemonic before production use.
📚 Key Terms Explained
BIP39
Bitcoin Improvement Proposal 39. The standard that defines how a random sequence of words (a mnemonic) encodes the master seed for a hierarchical deterministic cryptocurrency wallet.
Mnemonic Seed Phrase
A human-readable sequence of 12 or 24 words that encodes a cryptographic seed. Used to back up and restore cryptocurrency wallets. Also called a recovery phrase or backup phrase.
Entropy
The raw randomness used to generate a seed phrase. Measured in bits: 128 bits for 12 words, 256 bits for 24 words. Generated by your operating system's CSPRNG via window.crypto.getRandomValues().
SHA-256
A cryptographic hash function that produces a 256-bit digest. BIP39 uses it to compute a checksum appended to the entropy, enabling wallets to detect transcription errors when you enter a recovery phrase.
Checksum
In BIP39, the first N bits of the SHA-256 hash of your entropy (4 bits for 12-word phrases, 8 bits for 24-word phrases). Appended to entropy before word selection, allowing error detection during wallet recovery.
Air-Gapped Computer
A computer that has never been connected to the internet or any network. Because there is no network path for malware to exfiltrate data, it is the most secure environment for generating cryptocurrency seed phrases.
Wallet Recovery
The process of restoring access to a cryptocurrency wallet using a seed phrase. Any BIP39-compatible hardware or software wallet can regenerate your complete wallet - including all keys and accounts - from your 12 or 24-word phrase.
CSPRNG
Cryptographically Secure Pseudo-Random Number Generator. window.crypto.getRandomValues() taps your OS-level CSPRNG, which uses hardware entropy sources. Never use Math.random() for cryptographic applications - it is not cryptographically secure.
The Complete Guide to BIP39 Mnemonic Seed Phrases
If you hold cryptocurrency in a self-custody wallet, your seed phrase is the single most important piece of information in your financial life. Understanding how it is generated, what it encodes, and how to store it safely is not optional - it is the difference between permanent access to your funds and permanent loss.
How BIP39 Generation Works
This tool follows the BIP39 specification exactly. When you click Generate, the following cryptographic steps execute entirely within your browser:
- Step 1 - Entropy: Your OS generates 128 bits (12 words) or 256 bits (24 words) of cryptographically secure random data using window.crypto.getRandomValues(). This is the same entropy source used by your operating system for TLS keys and other critical security functions.
- Step 2 - Checksum: The SHA-256 hash of the entropy bytes is computed using the browser's Web Crypto API. The first 4 bits (12-word) or 8 bits (24-word) of this hash are appended to the entropy.
- Step 3 - Word mapping: The combined bit string is split into 11-bit chunks. Each 11-bit value (0 to 2047) indexes into the BIP39 English word list of 2048 words, producing 12 or 24 human-readable words.
Why You Must Write It Down
The seed phrase is not stored anywhere. It is computed on-the-fly and displayed once. Your browser, this website, and your device retain no copy. When you close this page, the phrase exists only where you have written or saved it. Treat it with the same care as cash. Anyone who obtains your seed phrase has complete, irrevocable access to every wallet and account derived from it.
Secure Storage Best Practices
- Write the phrase on paper with a pen. Never type it on a networked device.
- Store multiple physical copies in geographically separate locations.
- Consider fireproof and waterproof storage such as a metal backup plate.
- Never photograph your seed phrase. Phone cameras auto-upload to cloud services.
- Never email, text, or message your seed phrase under any circumstances.
- Consider a passphrase (BIP39 optional extension) for an extra layer of security for large holdings.
Frequently Asked Questions
What is an air-gapped computer and why should I use one?
An air-gapped computer is a device that has never been connected to the internet or any network. Because no network connection exists, malware cannot exfiltrate your seed phrase after it is generated. For maximum security, save this HTML page to a USB drive, transfer it to a computer that has never been online, generate your seed phrase there, write it down on paper, and then power off the machine. This eliminates virtually all digital attack vectors.
What happens if I lose my seed phrase?
If you lose your BIP39 seed phrase and have no backup, your cryptocurrency is permanently inaccessible. There is no recovery mechanism, no customer support, and no reset option. The seed phrase is the only key to your funds. Store multiple physical copies in separate secure locations such as a fireproof safe, a safety deposit box, or with a trusted person. Never store it digitally or photograph it.
Why is 24 words more secure than 12 words?
A 12-word phrase encodes 128 bits of entropy, and a 24-word phrase encodes 256 bits. An attacker brute-forcing a 12-word phrase must try up to 2 to the power of 128 combinations, which is computationally infeasible with current technology. A 24-word phrase doubles the exponent to 256 bits, providing security against future advances including theoretical quantum computing attacks. For long-term cold storage of significant value, 24 words is the recommended standard.
Is my seed phrase ever sent to a server?
No. This tool generates your seed phrase entirely within your browser using JavaScript. The entropy comes from window.crypto.getRandomValues(), which is your operating system's cryptographically secure random number generator. The SHA-256 checksum is computed using the browser's built-in Web Crypto API (SubtleCrypto). Nothing is sent to any server, logged, stored, or transmitted. You can verify this by disconnecting from the internet before generating a phrase.
What is a BIP39 checksum and why does it matter?
BIP39 appends a checksum to the entropy before splitting it into 11-bit word indices. For a 12-word phrase, the first 4 bits of the SHA-256 hash of the 128-bit entropy are appended, giving 132 bits total (12 times 11). For a 24-word phrase, the first 8 bits of the SHA-256 hash of 256-bit entropy are appended, giving 264 bits (24 times 11). This checksum lets hardware wallets detect typos when you enter a recovery phrase - an invalid checksum means at least one word was entered incorrectly.