Terminal device performing homomorphic encryption, server device processing ciphertext and methods thereof

Patent metadata

Field	Value
Publication number	US-10778409-B2
Application number	US-201816478596-A
Country	US
Kind code	B2
Filing date	Dec 7, 2018
Priority date	Dec 15, 2017
Publication date	Sep 15, 2020
Grant date	Sep 15, 2020

Abstract

An encryption method is provided. According to the encryption method, a scaling factor may be reflected in a message and then, a homomorphic ciphertext may be generated using a public key. The generated ciphertext is, when decryption is performed, generated in a form that a result value obtained by adding an error value to a value obtained by reflecting the scaling factor in the message is restored. Accordingly, a homomorphic ciphertext capable of being computed in a ciphertext state can be effectively generated.

First claim

The invention claimed is:

1. An encryption method of a terminal device, the encryption method comprising:
setting a ring which is a set including elements between which addition and multiplication are defined, the ring including a plaintext space that is a real number from among sets that are closed to addition and multiplication;
calculating a secret key from the ring;
calculating a public key corresponding to the calculated secret key;
applying the public key to a message and generating a homomorphic ciphertext;
receiving a pre-generated homomorphic ciphertext from another terminal device;
performing at least one computation from among addition or multiplication with respect to the generated homomorphic ciphertext and the received pre-generated homomorphic ciphertext;
transmitting a result of the at least one computation to the other terminal device, wherein performing the at least one computation comprises:
performing a first function process with respect to each of the generated homomorphic ciphertext and the received pre-generated homomorphic ciphertext;
performing a computation between the homomorphic ciphertext for which the first function process is performed; and
performing a second function process, which is an inverse function of the first function, with respect to a homomorphic ciphertext for which the computation is performed.

2. The encryption method as claimed in claim 1 , wherein the ring satisfies the following equation:
a ( X )= a ( X −1 ) where a ( X )∈ R, X −1 :−X n−1

wherein X is a variable, n is a natural number, a(X) is a polynomial corresponding to the ring, and the R is a ring including a real space that is a complex number.

3. The encryption method as claimed in claim 2 , wherein the homomorphic ciphertext has a complexity of o(n log n) with respect to an addition or multiplication computation where n is a natural number.

4. The encryption method as claimed in claim 1 , further comprising:
calculating a first random polynomial from the ring;
extracting an error; and
modulating the error in the first random polynomial and the secret key, and calculating a second random polynomial,
wherein the calculating the public key comprises:
calculating the public key using the first and second random polynomials.

5. The encryption method as claimed in claim 1 , wherein the generating the homomorphic ciphertext comprises:
calculating the message as a polynomial corresponding to the ring, and applying the public key to the calculated polynomial and generating a homomorphic ciphertext.

6. The encryption method as claimed in claim 1 , wherein the first function is a Number Theoretic Transform (NTT) function.

7. The encryption method as claimed in claim 1 , wherein the homomorphic ciphertext is, based on a decryption being performed, in a form that a result value obtained by adding an error value to the message is restored.

Assignees

• Crypto Lab Inc

Inventors

• Cheon Jung Hee
• Kim Duhyeong
• Song Yongsoo
• Han Kyoohyung

Classifications

• H04L9/008 Primary

  involving homomorphic encryption · CPC title

• H04L9/3093

  involving Lattices or polynomial equations, e.g. NTRU scheme · CPC title

• H04L9/304

  based on error correction codes, e.g. McEliece · CPC title