# US 11,115,183 B2 · Terminal device performing homomorphic encryption, server device processing ciphertext and methods thereof

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- Number: US 11,115,183 B2
- Document ID: US11115183B2
- Status: Granted
- Kind: B2
- Application number: 16/855,262
- Filed: 2020-04-22
- Published: 2020-08-06
- Granted: 2021-09-07
- Earliest claimed: 2017-12-15
- Assignees: Crypto Lab Inc.
- Applicants: Crypto Lab Inc.
- Inventors: Jung Hee Cheon; Duhyeong Kim; Yongsoo Song; Kyoohyung Han
- Web page: https://patentgrep.com/patent/US11115183B2

## CPC

- H04L 9/008: {involving homomorphic encryption}
- H04L 9/304: {based on error correction codes, e.g. McEliece}
- H04L 9/3093: {involving Lattices or polynomial equations, e.g. NTRU scheme}
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

A decryption method includes: receiving a homomorphic ciphertext; and obtaining a result value added an error value at a message from the received homomorphic ciphertext. The error is disposed on the least significant bit (LSB) side in the homogeneous ciphertext, and the message is disposed at a position adjacent to the error.

## Claims

### Claim 1

A decryption method, the method comprising:
receiving a homomorphic ciphertext; and
obtaining a result value added an error value at a message from the received homomorphic ciphertext,
wherein the error is disposed on the least significant bit (LSB) side in the result value, and the message is disposed at a position adjacent to the error,
wherein the homomorphic ciphertext is generated by reflecting a scaling factor in a message, and
wherein the result value is obtained by multiplying an error value by a value in which the scaling factor is reflected in the message, and
wherein a modulus of the homomorphic ciphertext is a value obtained by multiplying a plurality of different scaling factors by each other, and the plurality of different scaling factors are set as values which are disjoint from each other.

### Claim 2 (depends on 1)

The decryption method as claimed in claim 1,
wherein a plaintext space of the homomorphic ciphertext is a complex number, and
wherein the obtaining a result value comprises:
obtaining a result value from a message of a real number part of the homomorphic ciphertext.

### Claim 3 (depends on 1)

The decryption method as claimed in claim 1, further comprising:
processing a rounding process by multiplying the result value by the inverse of the scaling factor.

### Claim 4 (depends on 1)

The decryption method as claimed in claim 1,
wherein the obtaining a result value comprises:
generating a deciphertext as a polynomial form by applying a secret key to the received homomorphic ciphertext; and
decoding the generated deciphertext.

### Claim 5 (depends on 1)

The decryption method as claimed in claim 1,
wherein the homomorphic ciphertext is expressed as a plurality of bases, and
wherein the obtaining a result value comprises:
performing a decrypting operation for each basis of the homomorphic ciphertext.

### Claim 6

A terminal device, comprising:
a memory configured to store a homomorphic ciphertext; and
a processor configured to obtain a result value added an error value at a message from the homomorphic ciphertext,
wherein the error is disposed on the least significant bit (LSB) side in the result value, and the message is disposed at a position adjacent to the error,
wherein the homomorphic ciphertext is generated by reflecting a scaling factor in a message, and
wherein the result value is obtained by multiplying an error value by a value in which the scaling factor is reflected in the message, and
wherein a modulus of the homomorphic ciphertext is a value obtained by multiplying a plurality of different scaling factors by each other, and the plurality of different scaling factors are set as values which are disjoint from each other.

### Claim 7 (depends on 6)

The terminal device as claimed in claim 6,
wherein a plaintext space of the homomorphic ciphertext is a complex number, and
wherein the processor obtains a result value from a message of a real number part of the homomorphic ciphertext.

### Claim 8 (depends on 6)

The terminal device as claimed in claim 6, wherein the processor processes a rounding process by multiplying the result value by the inverse of the scaling factor.

### Claim 9 (depends on 6)

The terminal device as claimed in claim 6, wherein the processor generates a deciphertext as a polynomial form by applying a secret key to the received homomorphic ciphertext, and decodes the generated deciphertext.

### Claim 10 (depends on 6)

The terminal device as claimed in claim 6,
wherein the homomorphic ciphertext is expressed as a plurality of bases, and
wherein the processor performs a decrypting operation for each basis of the homomorphic ciphertext.

### Claim 11

A non-transitory computer readable recording medium including a program for executing a decryption method,
wherein the method comprises:
receiving a homomorphic ciphertext; and
obtaining a result value added an error value at a message from the received homomorphic ciphertext, and
wherein the error is disposed on the least significant bit (LSB) side in the result value, and the message is disposed at a position adjacent to the error,
wherein the homomorphic ciphertext is generated by reflecting a scaling factor in a message, and
wherein the result value is obtained by multiplying an error value by a value in which the scaling factor is reflected in the message, and
wherein a modulus of the homomorphic ciphertext is a value obtained by multiplying a plurality of different scaling factors by each other, and the plurality of different scaling factors are set as values which are disjoint from each other.