Proton Mail employs several methods to protect your privacy and your security. One method of protecting data from being read by outsiders is encryption. However, encryption alone does not guarantee the authenticity of the data. For that, Proton Mail uses a technology called digital signatures.
Digital signatures are like a physical signature on a specific piece of data, such as a message body or contact.
The next step is to sign this hash. The signature of this hash is computed with the signer’s private key using a signature algorithm (e.g. RSA or DSA).
Successful verification of the digital signature on a message body ensures that the author really did send the message and that the message has not been tampered with or otherwise changed. In the case of your contacts, they are signed with your private keys, and verification of a contact’s digital signature ensures that no one but you has modified the contact.
The next step is to generate a hash of the data using the hash function chosen by the signature type. The verification algorithm will then verify the signature in a method dependent on the algorithm.
After this step, the algorithm completes verification by ensuring that the hash of the received data is equal to the hash included in the digital signature.