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.
If you have additional questions, please feel free to contact support.