The audit process is designed to allow the auditor – in this case me, Stefan Thomas – to verify that the total amount of bitcoins held by Kraken matches the amount required to cover an anonymized set of customer balances. I am attesting to is the root hash of a merkle tree containing all balances that were considered in the audit. … [If] you believe that I am trustworthy, then you can be confident that your balance was covered by 100% reserves at the time of the audit.
Kraken also made a customer-level audit available so users could verify their own balances.
Last month, Coinbase submitted itself to an audit by Blockchain.info’s Chief Security Officer Andreas M. Antonopoulos, who found that all customer bitcoins were accounted for. Earlier this month, BitStamp also revealed that an audit of their accounts confirmed all user funds in BTC and fiat were accounted for.