SuppressionList is designed to protect your data in the following ways:
The data in your lists are stored only in hashed form. A hash is an irreversible method of encoding data such that it is not recognizable. Unlike encryption and decryption, hashing algorithms are designed to prevent de-encoding of the data.
When you add data to your List, SuppressionList does not keep the plain-text data, It hashes it and stores only the hashed value. later, when a query is made on the List, the query data is hashed and that hash compared with those stored in the List. If a match is found, then the original values must match.
A List's API Key is "key" to its use. You should never provide you API keys to an untrusted source. The QueryUrl feature of SuppressionList lets you create, control, and destroy publicly-distributable urls that automatically perform queries of the specific Lists you designate.
Additionally, all queries are performed using https encryption to prevent exposing the query data itself to sniffers.