Integer

tfhe::integer is dedicated to integers smaller than 256 bits. The steps to homomorphically evaluate an integer circuit are described here.

Key Types

integer provides 3 basic key types:

• ClientKey

• ServerKey

• PublicKey

The ClientKey is the key that encrypts and decrypts messages, thus this key is meant to be kept private and should never be shared. This key is created from parameter values that will dictate both the security and efficiency of computations. The parameters also set the maximum number of bits of message encrypted in a ciphertext.

The ServerKey is the key that is used to actually do the FHE computations. It contains a bootstrapping key and a keyswitching key. This key is created from a ClientKey that needs to be shared to the server, so it is not meant to be kept private. A user with a ServerKey can compute on the encrypted data sent by the owner of the associated ClientKey.

To reflect this, computation/operation methods are tied to the ServerKey type.

The PublicKey is a key used to encrypt messages. It can be publicly shared to allow users to encrypt data such that only the ClientKey holder will be able to decrypt. Encrypting with the PublicKey does not alter the homomorphic capabilities associated to the ServerKey.

1. Key Generation

To generate the keys, a user needs two parameters:

• A set of shortint cryptographic parameters.

• The number of ciphertexts used to encrypt an integer (we call them "shortint blocks").

We are now going to build a pair of keys that can encrypt 8-bit integers (signed or unsigned) by using 4 shortint blocks that store 2 bits of message each.

2. Encrypting values

Once we have our keys, we can encrypt values:

3. Encrypting values with the public key

Once the client key is generated, the public key can be derived and used to encrypt data.

4. Computing and decrypting

With our server_key, and encrypted values, we can now do an addition and then decrypt the result.