What Is a Letter of Guarantee and Why It Matters

Key Takeaways

• A Letter of Guarantee is a short statement about one mix, signed with the platform's own key.
• It names the date, the coin, the one-time address to send to and the address that receives the payout.
• Anyone can check the signature in a standard Bitcoin wallet in about a minute, with no account and no special software.
• One changed character, or a signature from another address, makes the check fail. That is what makes the letter worth keeping.

What Is a Letter of Guarantee?

A Letter of Guarantee is the written promise a crypto mixer gives for a single mix. When you start a mix, the service creates a one-time address for your coins and records where the fresh coins should go. The letter states exactly that, then seals it with a cryptographic signature.

The signature is the important part. A screenshot or a confirmation email can be edited by anyone. A signed letter cannot: if a single digit of an address changes, the signature no longer matches. So the letter is not just a note that something was agreed. It is proof of what was agreed. Anyone can verify it on their own computer.

You get a Letter of Guarantee with every mix on the Bitcoin mixer, the Ethereum mixer, the BNB mixer and the Solana mixer. It appears on the private mix page as soon as the mix is created, before you send any coins.

What the Letter Contains

The signed part of the letter is five short lines. Nothing else is signed, which keeps it easy to read and easy to check.

Under those lines come two more fields that are not part of the signed text: the signing address and the signature itself.

The letter deliberately leaves out the private mix link. That way you can show the letter to someone as proof without handing over access to your mix.

Why a Letter of Guarantee Matters

It Proves the Terms

The letter fixes the two addresses that matter most. If there were ever a question about where coins were supposed to go, the signed letter settles it. It cannot be rewritten later, not even by the service, because any change breaks the signature.

It Protects You From Copycat Sites

Fake mixer sites copy the look of real ones and swap in their own addresses. A copycat can sign letters with its own key, but never with the real signing address. That address never changes, so note it once from the site you trust and bookmark. Before you send coins, check that the letter verifies against that same address. If the address differs, you are on a copy.

It Makes Support Fast

If you ever need help with a mix, the letter tells support everything they need: which addresses, which coin, which day. There is no guessing and no back and forth.

It Needs No Trust in a Database

Verification happens in your own wallet, against public math. You do not need to log in anywhere or trust any server to confirm what the letter says.

How the Signature Works

Every letter is signed as a standard Bitcoin signed message. It is the same method wallets have used for years to prove control of an address. The service signs the five lines with the private key of its signing address. Your wallet takes the same five lines, the signature and the signing address, then confirms the key behind that address produced that exact text.

The signing address is a legacy Bitcoin address that starts with 1. Legacy addresses are used because every wallet can verify them, including older ones that do not support newer address types. The signing address never holds coins. It exists only to sign letters.

Letters for Ethereum, BNB and Solana mixes are signed the same way. The signature proves who wrote the letter, whichever coin the mix is in.

How to Check Your Letter, Step by Step

On the mix page, open the Letter of Guarantee. It shows three boxes, each with its own copy button: Signing address, Message and Signature. Any message verifier asks for exactly these three things.

1. Copy the signing address and paste it into the verifier's address field.
2. Copy the message and paste it into the message field. It holds exactly the five signed lines, with nothing before or after them.
3. Copy the signature and paste it into the signature field.
4. Press Verify.

Use the copy buttons rather than selecting the text by hand. A single extra space or line break makes the check fail, even on a perfectly valid letter.

In Electrum

Open Tools, then Sign/verify message. Paste the signing address, the message and the signature into the matching fields, then press Verify. A valid letter shows a confirmation that the signature is correct.

In Sparrow

Open Tools, then Sign/Verify Message. Fill in the address, message and signature, then choose Verify. Sparrow reports whether the signature matches.

In Bitcoin Core

Open File, then Verify message. Paste the three parts and press Verify Message. A green line confirms the message was signed by that address.

With an Online Verifier

Any website that verifies Bitcoin signed messages works too. Some take the three parts separately. Others take the whole letter in one block, in the standard layout that starts with a "BEGIN BITCOIN SIGNED MESSAGE" line, which is the format of the downloadable .txt version.

What the Result Means

If you get invalid, first rule out a copy mistake: copy all three parts again with the copy buttons, then retry. If it still fails, do not send coins to that address. Contact support with the letter.

What to Check Beyond the Signature

A valid signature proves the letter is genuine. A few more seconds make the check complete:

• The signing address is the one you noted from the real site. It never changes, so a different address means a different issuer.
• The output address is your own address, exactly as you entered it.
• The receiving address is the address shown on the mix page, the one you are about to send to.
• The coin is the network you are using.
• The domain in the first line is the site you are on.

Keep Your Letter Safe

Save the letter until the payout has arrived. The downloadable .txt file is the simplest way to keep it. It contains your addresses, so store it as privately as the mix link itself: do not post it in public chats or forums. Share it only with support when you need help.

Frequently Asked Questions

Is a Letter of Guarantee the Same as a Receipt?

No. A receipt says a payment happened. A Letter of Guarantee is issued before you send anything: it states where your coins go and where the payout goes, then signs it so it can never be changed.

Can Someone Fake a Letter of Guarantee?

They can copy the text, but they cannot produce a valid signature from the real signing address without its private key. A fake letter either fails verification or verifies only against a different address, which is why it pays to know the real signing address.

Why Does an Ethereum Mix Have a Bitcoin Signature?

The signature proves who wrote the letter, not which coin is mixed. Bitcoin signed messages are the most widely supported signing standard, so one method works for every coin and every wallet.

What If the Verifier Says the Signature Is Invalid?

Most often it is a copy mistake, such as an extra line break or part of the signature missing. Copy the three parts again with the copy buttons. If it still fails, do not send coins and contact support.

Do I Need to Check Every Letter?

Checking takes about a minute and is the surest way to know you are on the real site. It is most important the first time you use a site, or when you arrive from a link you did not type yourself.

Should I Share My Letter With Support?

Yes, if you need help with a mix. It gives support every detail at once. It does not include your private mix link, so sharing it does not give anyone access to your mix.

How Long Should I Keep the Letter?

At least until the payout has arrived and confirmed. Many people keep it for good as a record of the mix.