emp-toolkit
shift operation will change a secure negative integer from signed to unsigned
Hello, sorry for disturbing you. I have observed that shift operations can be conducted on secure integers successfully on bit-representation. However, it will turn a negative signed integer into an unsigned one. Here is an example.
Integer a(32, -44, ALICE);
cout<< "a: "<< a.reveal<int32_t>()<<endl;
a = (a >> 2);
cout<< "a: "<< a.reveal<int32_t>()<<endl;
The output is:
a: -44
a: 1073741813
From the view of bit representation, the 11111111 11111111 11111111 11010100 is shifted by 2 bits to 11111111 11111111 11111111 11110101 successfully. However, it is now converted to an unsigned form. In comparison, if the operation is conducted on a public variable
int a = -44;
cout<< "a: "<< a<<endl;
a = a >> 2;
cout<< "a: "<< a<<endl;
The output is:
a: -44
a: -11
I am curious whether this is caused by my misuse of shift or if I did not define the type of variable correctly. Or maybe, this behavior is special in the MPC scenario so I should avoid using it. May you help me by explaining this?
