http://bitmixeqlpdbdrw2xvquvaxgjkscuzvt4rwovhgcsnqrgnueko4j4jad.onion/faqs.html
If attacker knows service fee, he can analyse blockchain to find exact sum transferred and discover your destination account. For example if you send 100 BTC with a fixed fee 0.5% + 0.0005 BTC, you should receive 99.4995 BTC. It is not so difficult to check blockchain after 24 hours and find all exact transactions.
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