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List of Changes

Version 3.3.0 (current)

Features

In this version, we have significantly improved the usability of the CKKS scheme in Microsoft SEAL and many of these improvements apply to the BFV scheme as well. Homomorphic operations that are based on key switching, i.e., relinearization and rotation, do not consume any noise budget (BFV) or impact accuracy (CKKS). The implementations of these operations are significantly simplified and unified, and no longer use bit decomposition, so decomposition bit count is gone. Moreover, fresh ciphertexts now have a larger noise budget. These changes have an effect on the API and it will be especially worthwhile for users of older versions of the library to study the examples and comments in native/examples/3_levels.cpp (C++) or dotnet/examples/3_Levels.cs (C#).

The setup of EncryptionParameters has been made both easier and safer (see API Changes below).

The examples in native/examples/ and dotnet/examples/ have been redesigned to better teach the multiple technical concepts required to use Microsoft SEAL correctly and efficiently, and more compactly demonstrate the API.

API Changes

Deleted header files:

  • native/defaultparameters.h

New header files:

  • kswitchkeys.h: new base class for RelinKeys and GaloisKeys)
  • modulus.h: static helper functions for parameter selection
  • valcheck.h: object validity check functionality
  • util/rlwe.h

In class SEALContext:

  • Replaced context_data(parms_id_type) with get_context_data(parms_id_type);
  • Removed context_data();
  • Added key_context_data(), key_parms_id(), first_context_data(), and last_context_data();
  • Added using_keyswitching() that indicates whether key switching is upported in this SEALContext;
  • Create(...) in C++, and constructor in C#, now accepts an optional security level based on HomomorphicEncryption.org security standard, causing it to enforce the specified security level. By default a 128-bit security level is used.
  • Added prev_context_data() method to class ContextData (doubly linked modulus switching chain);
  • In C# SEALContext now has a public constructor.

Parameter selection:

  • Removed the DefaultParams class;
  • Default coeff_modulus for the BFV scheme are now accessed through the function CoeffModulus::BFVDefault(...). These moduli are not recommended for the CKKS scheme;
  • Customized coeff_modulus for the CKKS scheme can be created using CoeffModulus::Create(...) which takes the poly_modulus_degree and a vector of bit-lengths of the prime factors as arguments. It samples suitable primes close to 2^bit_length and returns a vector of SmallModulus elements.
  • PlainModulus::Batching(...) can be used to sample a prime for plain_modulus that supports BatchEncoder for the BFV scheme.

Other important changes:

  • Removed size_capacity function and data members from Ciphertext class;
  • Moved all validation methods such as is_valid_for and is_metadata_valid_for to valcheck.h;
  • Removed argument decomposition_bit_count from methods relin_keys(...) and galois_keys(...) in class KeyGenerator;
  • It is no longer possible to create more than one relinearization key. This is to simplify the API and reduce confusion. We have never seen a real use-case where more relinearization keys would be a good idea;
  • Added methods to generate an encryption of zero to Encryptor;
  • Added comparison methods and primality check for SmallModulus;
  • Classes RelinKeys and GaloisKeys are now derived from a common base class KSwitchKeys;
  • GoogleTest framework is now included as a Git submodule;
  • Numerous bugs have been fixed, particularly in the .NET wrappers.

Version 3.2.0