Thank you for showing interest! Will try and answer as much as I can
The purpose of a tumbler is privacy and anonymity. How can this be achieved if you are maintaining backups? You state that logs are deleted nightly which sounds good. However, you go on to state that hourly backups are made and stored in an encrypted Truecrypt container. Now, given that Truecrypt is obsolete, I hope you are using at least Veracrypt, and even better, dmcrypt, but it seems that you may be running this on a Windows box based on several statements you've made.
- How is this 'container' copied to another server? FTP?
- Is this secondary server bare-metal and do you have full control over the the so-called 'redundant discs'?
- How do you guarantee the purging of the hourly backups that get moved to 'redundant discs'?
- How is the container managed? Are you creating a new container every hour and sending this off, each with a unique key?
- You state that you destroy logs on your primary server, but do you destroy these containers?
- Could you please state officially each item that is in the log that is purged?
I need backups to be sure I don't lose users coins, their current balance etc. The message you quote is old and things have changed since then. I take backups every minute now, and they are copied using rsync over Tor to a backup server that is kept as a copy of the original server. In case the original server hardware fails I can quickly change over to the backup server as its always ready. Both are bare metal and are installed by me (Not Windows) and uses full disk encryption. Old backups are of course also removed.
Old log files are removed. In the database old withdraws, deposits, deposit addresses, support messages, quickmixes, login attempts are removed. Nothing is saved. Each row in the DB is timestamped when added and then later removed. I like to keep the database as small and fast as possible.
When logged in, under the 'WITHDRAW' tab:Bitcoin Blender is ready! If you withdraw 1.12345678 you will receive new coins!
I'd like some clarification on the context of this message. I understand that the tumbler is ready, but I don't understand what you mean to imply by stating that if the user is to withdrawal the 'entire' and 'full' amount at one time, the user will receive new coins. This is how I have interpreted this message and it's left me flabbergasted.
- What are 'new coins' in this context?
- Is receiving these 'new coins' solely dependent on withdrawing the entire amount at one time?
When you open the withdraw page the mixer checks the coins currently in the withdraw wallet and if there is enough in there for you to withdraw your whole balance or not.
You either get a message that it is ready, or a message that it is not ready. Usually you never get the other message where it explains you have deposited too much because you would need to deposit around 800-900 BTC at once (i don't like keeping too much on hot wallets). That message explains that you can either wait for the mixer to catch up so you can withdraw or wait for me to move in coins from cold storage, and recommends the user to contact me through support.
I will change the message as it is misleading, of course you can withdraw less than your whole balance.
Deposited coins are sent and mixed in different ways on the way to the withdraw wallet and thats where withdraws are sent from. The system keeps track of the coins original owner and will never send coins originaly deposited by the same user.
Under the same tab, but further down below 'Automatic Withdrawal':Automatic Withdraw runs four times per day and will at these times take your accounts balance and create withdraws spread across your specified addresses with random delays and amounts.
Are these 'four times per day' at the exact same time each and every day, or are these four times randomised between days? I'm concerned that if they are not at randomised time, this could be a vulnerability in your 'QUICK MIX' feature that could lead to fingerprinting.
Quickmix and automatic withdraw is not the same thing. Automatic withdraw is a feature registered users can choose to activate, the withdraws created by automatic withdraw have a forced randomized delay between 1-18 hours, it has a high randomized delay to prevent that. The problem with the automatic withdraw feature is that users set their addresses there once and then don't seem to change them often, so it encourages address re-use which is bad for anonymity.
While NOT logged in and under the 'QUICK MIX' tab:If you have a Quick Mix ID from previous mixes enter it here, or if you want to check the status of a Quick Mix in progress enter it and use the button on the bottom of the page.
I scoured this entire thread and all the documentation available on your website, and no where is the function of the 'Quick Mix ID' explained anywhere.
- How does the 'Quick Mix ID' function? Does it ensure that I can continue to reuse the 'QUICK MIX' function without ever mixing in with previously mixed in coins?
- Does this ID function track previously tumbled in coins? If so, to what degree?
- Is the 'Quick Mix ID' used only for tracking the tumble function while in-progress and immediately after for support?
Correct, the quickmix id is used instead of a username to keep track of your coins. So if you come back later and you enter the same quickmix id it wont send you any of your old coins on withdraw. If you don't mix more often than maybe once a week your old coins would have been sent out from the mixer by then anyways.
And finally just one more question. If a user were to create a single account and use your service over the course of a single year, making 100 deposits and 50 tumble operations to various wallet addresses, how can the user be sure that they will never have the unfortunate issue of receiving one of their previously entered coins from months earlier?
Let's say that the user deposited Coin A in January, Coin B in February, requested a withdrawal for the total value of Coin A and Coin B and receiving this withdrawal and now have an account balance of 0.0 BTC and then later Coins C and D are deposited in August and a subsequent withdrawal is made resulting in a part of the earlier Coin A being tumbled into the mix with Coins C and D.
Have you protected against this?
As long as a part of your coins are still in the mixer wallet I keep track of them, each UTXO is marked as belonging to a specific user or several users. Usually coins are only in the mixer for a few days so its not a big issue to keep track of them. That will prevent your example above from happening, if I understood you correct?
But there are other interesting things that can happen. Hope I can explain this so it makes sense with this example..
User A deposits 100 BTC to Address 1A. Some part of the coins deposited on 1A is eventually sent out of the mixer to another bitcoin service by some random Blender user.
User B also have an account on the same bitcoin service and decides to withdraw his coins to Bitcoin Blender and receives parts of the same coins the other user deposited on that service. Now there are coins in the mixer which has taint to Address 1A but is deposited by User B... The coins once deposited on Address 1A could of course be sent between several different bitcoin services before ending up on Bitcoin Blender again, but there could still be some small taint % to Address 1A there.
Thats why I have built my own taint analysis tool into the mixer, it is similar to the one blockchain.info used to have.
So on withdraw the mixer checks that the UTXO it is about to spend does not belong to you, and it also checks the taint on that UTXO. It checks that any of your deposit addresses is not in the taint report of that UTXO. Once you remove a deposit address it is kept for 7 days before the link to the user is completely removed, after that this taint check wont work for that address.