We use Stripe, an industry-leading payment processor, for secure credit card transactions over SSL. Stripe has been certified as a PCI Level 1 Service Provider, the most stringent level of certification available. Stripe is also a participant in the PCI Security Standards Council.
We also use Stripe to securely process bank transfers (ACH). If you are using your bank login to add a bank account, the most sensitive data is heavily encrypted and stored with Plaid, which integrates with Stripe specifically for this. Their data is encrypted with 2048-bit RSA and decryption is only possible using dedicated machines on their private network.
If you are using your account number and micro-deposits to add your bank account, then your bank account is verified with Stripe. As noted above, Stripe has been certified as a PCI Level 1 Service Provider.
We've partnered with Coinbase to securely transact bitcoins. 98% of Coinbase funds are stored offline and sensitive data normally stored on servers are disconnected entirely from the internet. Drives and paper backups are distributed geographically in safe deposit boxes and vaults around the world.