Electric device and key switching method thereof

1 . An electronic device, comprising:

a memory in which an input encrypted message with dimension N is stored; and

a processor configured to transform the input encrypted message with dimension N into an encrypted message with dimension N′,

perform linear transformation on the encrypted message with dimension N′, and transform the encrypted message corresponding to a result value of the linear transformation into an output encrypted message with dimension N,

wherein the N is smaller than N′, and

wherein the processor is configured to perform the linear transformation by performing a rotate-sum operation on the encrypted message with dimension N′.

2 . The electronic device as claimed in claim 1 , wherein the input encrypted message is a ring learning with error (RLWE) encrypted message, and

the processor is configured to use module learning with error (MLWE) key switching to transform the RLWE encrypted message with dimension N into an MLWE encrypted message with dimension N′; and wherein the processor is configured to perform the rotate-sum operation on the MLWE encrypted message with dimension N′.

3 . The electronic device as claimed in claim 2 , wherein the processor is configured to transform the MLWE encrypted message corresponding to the result value of the linear transformation into the RLWE encrypted message with dimension N using RLWE key switching.

4 . The electronic device as claimed in claim 1 , wherein the input encrypted message is a multi-secret ring learning with error (MSRLWE) encrypted message, and

the processor is configured to transform the MSRLWE encrypted message with dimension N into a MSRLWE encrypted message with dimension N′ using homomorphism and MSRLWE key switching.

5 . The electronic device as claimed in claim 4 , wherein the processor is configured to perform a rotate-sum operation on the MSRLWE encrypted message with dimension N′.

6 . The electronic device as claimed in claim 5 , wherein the processor is configured to transform the MSRLWE encrypted message corresponding to the result value of the linear transformation into the MSRLWE encrypted message with dimension N using MSRLWE key switching.

7 . A key switching method of an electronic device, comprising:

transforming an input encrypted message with dimension N into an encrypted message with dimension N′;

performing linear transformation on the encrypted message with dimension N′; and

transforming the encrypted message corresponding to a result value of the linear transformation into an output encrypted message with dimension N,

wherein the N is smaller than N′, and

wherein the performing the linear transformation comprises performing a rotate-sum operation on the encrypted message with dimension N′.

8 . The key switching method as claimed in claim 7 , wherein the input encrypted message is an RLWE encrypted message, and

in the transforming into the encrypted message with dimension N′, the RLWE encrypted message with dimension N is transformed into an MLWE encrypted message with dimension N′ using MLWE key switching; and wherein in the performing, the rotate-sum operation is performed on the MLWE encrypted message with dimension N′.

9 . The key switching method as claimed in claim 8 , wherein in the transforming into the output encrypted message with dimension N, the MLWE encrypted message corresponding to the result value of the linear transformation is transformed into the RLWE encrypted message with dimension N using the RLWE key switching.

10 . The key switching method as claimed in claim 7 , wherein the input encrypted message is an MSRLWE encrypted message, and

in the transforming into the encrypted message with dimension N′, the MSRLWE encrypted message with dimension N is transformed into a MSRLWE encrypted message with dimension N′ using homomorphism and MSRLWE key switching.

11 . The key switching method as claimed in claim 10 , wherein in the performing, a rotate-sum operation is performed on the MSRLWE encrypted message with dimension N′.

12 . The key switching method as claimed in claim 11 , wherein in the transforming into the output encrypted message with dimension N, the MSRLWE encrypted message corresponding to the result value of the linear transformation is transformed into the MSRLWE encrypted message with dimension N using the MSRLWE key switching.