A mixer like Samourai that does not take custody of the cryptocurrency by possessing the private keys would strongly suggest that Samourai is not acting as an MSB.
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SDNY Withholding Of Evidence Undermines Charges Against Samourai Wallet
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Samourai Wallet is a non-custodial crypto mixing service designed to obscure the origin of cryptocurrency transactions by blending funds from multiple users.
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FinCEN told prosecutors that Samourai Wallet’s non-custodial model meant it likely did not meet the legal definition of a money transmitter.
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