cryptography - How does this message splitting work? -

i have been trying reverse engineer various encryption algorithms in compiled code recently, , came upon code. part of rsa algorithm. i've noted key size small encrypt/decrypt data it's supposed (in case int), code splits message 2 pieces, , encrypt/decrypt each, sum them together. i've pulled segments of code splits , joins message, , experimented it. appears numerical values uses dependent on n modulus. so, scheme, , how work?

uint n = 32437; uint origval = 12345; uint newval = 0;  (int = 0; < 2; ++i) {     ulong num = (ulong)origval * 43827549;     //uint num2 = ((origval - (uint)(num >> 32)) / 2 + (uint)(num >> 32)) >> 14;     uint num2 = (origval + (uint)(num >> 32)) / 32768;     origval -= num2 * n;                     // rsa encrypt/decrypt here     newval *= n;     newval += origval;     origval = num2; }  // put newval origval, reverse origval = newval; newval = 0;  (int = 0; < 2; ++i) {     ulong num = (ulong)origval * 43827549;     //uint num2 = ((origval - (uint)(num >> 32)) / 2 + (uint)(num >> 32)) >> 14;     uint num2 = (origval + (uint)(num >> 32)) / 32768;     origval -= num2 * n;                     // rsa encrypt/decrypt here     newval *= n;     newval += origval;     origval = num2; } 

note: seems operations applied symmetric.

after using various values origval, i've found out first 3 lines after for loop division, line after modulo operation. lines

ulong num = (ulong)origval * 43827549; //uint num2 = ((origval - (uint)(num >> 32)) / 2 + (uint)(num >> 32)) >> 14; uint num2 = (origval + (uint)(num >> 32)) / 32768; 

translates into

uint valdivn = origval / n; 

and

origval -= num2 * n; 

into

origval = origval % n; 

final code inside for loop looks this:

uint valdivn = origval / n; origval = origval % n; // rsa encrypt/decrypt here newval*= n; newval+= origval; origval = valdivn; 

analysis

this code splits values taking modulo of original value, transforming it, multiplying n, , tacking transformation of previous quotient onto result. lines uint valdivn = origval / n; , newval*= n; form inverse operations. can think of input message having 2 "boxes". after loop has run through, transformed value put in opposite "boxes". when message decrypted, 2 values in "boxes" reverse transformed, , put in original spots in "boxes". reason divisor n keep value being encrypted/decrypted under n, maximum value can encrypt rsa no larger n. there no possibility of wrong value being decrypted, code processes packed message , extracts part should decrypted prior decrypting. loop runs twice because there no chance quotient exceed size of int (since input int).