E2EE is an end to end encryption engine providing basic functionality with a user-friendly interface. Encryption is based on Diffie-Hellman key exchange algorithm, using 2048 bit prime number. Random values are generated with extreme value distribution, and the location of distribution is chosen empirically to escape both too short and too long calculations. The main Engine.h header file defines a singleton class called E2EE::Engine. It provides 6 functions:

• prepareToPairWith
• getKeyToSend
• setReceivedKey
• getHash
• serialize
• deserialize

bool prepareToPairWith(const std::string& user);

Takes a username as an input parameter, generates a random number for user, stores it in temporary key storage. Returns false if the temporary storage already has a key for user or permanent storage has a hash for user. Otherwise, everything is fine and it returns true.

BigInteger getKeyToSend(const std::string& user) const;

Takes a username as an input parameter, returns corresponding (random) key from the temporary storage. If there is no key generated for user, a default constructed BigInteger object is returned, which is equal to 0.

void setReceivedKey(const std::string& user, const BigInteger& key);

Takes a username and a BigInteger key as input parameters, returns nothing. If there is a generated key for user in temporary storage, it is removed from there, encrypted with key and the result is stored in the permanent storage. If there is no key for user in temporary storage, function does nothing.

BigInteger getHash(const std::string& user) const;

Takes a username as an input parameter, returns corresponding hash from the permanent storage. If there is no hash for user, a default constructed BigInteger object is returned, which is equal to 0.

ByteArray serialize() const;

Returns a byte array of temporary and permanent storages. It's purpose is to save and restore the state of engine.

bool deserialize(const ByteArray& data);

Takes a byte array (normally returned by serialize() function) and restores the state of engine.