International Association
for Cryptologic Research

Gentry\'s bootstrapping technique is still the only known method of obtaining fully homomorphic encryption where the system\'s parameters do not depend on the complexity of the evaluated functions. Bootstrapping involves a recryption procedure where the scheme decryption is evaluated homomorphically. So far, there have been precious few implementations of recryption, and none that could handle

\"packed ciphertexts\" that encrypt vectors of elements.

In the current work we implemented recryption of fully-packed ciphertexts using the HElib library for somewhat-homomorphic encryption. This required extending the recryption algorithms from the literature, as well as many aspects of the HElib library.

Our implementation supports bootstrapping of packed ciphertexts over

many extension fields/rings. One example that we tested involves

ciphertexts that encrypt vectors of 1024 elements from GF(2^{16}). In that setting, the recryption procedure takes under 5.5 minutes (at security-level ~ 76) on a single core, and allows a depth-9

computation before the next recryption is needed.