inline int64_t pmod(int64_t x){ int64_t v = x % 745018031; if(v < 0) return v + 745018031; else return v; }

intmain(int argc, char** argv){ int64_t x; scanf("%ld", &x); x = pmod(x); x = pmod(x + 544945418); x = pmod(x - 413537559); x = pmod(x + 742620753); x = pmod(x + 521496552); x = pmod(x * 217323223); x = pmod(x + 555632816); x = pmod(x * 150026081); x = pmod(x * 488682886); x = pmod(x * 438027182); x = pmod(x - 277961190); x = pmod(x + 733382750); x = pmod(x * 365221180); x = pmod(x * 206951831); x = pmod(x * 206787839); x = pmod(x + 466722489); x = pmod(x + 590127514); x = pmod(x + 729286233); x = pmod(x - 511866912); x = pmod(x * 62049651); x = pmod(x + 651782087); x = pmod(x * 84167258); x = pmod(x * 534841468); x = pmod(x - 152376404); x = pmod(x - 708647451); x = pmod(x * 402510031); x = pmod(x + 684952081); x = pmod(x + 157356094); x = pmod(x + 581217174); x = pmod(x * 494995445); x = pmod(x * 413245049); x = pmod(x - 465738152); x = pmod(x - 445076249); x = pmod(x * 634818015); x = pmod(x + 311971388); x = pmod(x * 395815216); x = pmod(x - 228527650); x = pmod(x - 390982625); x = pmod(x * 167525229); x = pmod(x - 67124720); x = pmod(x + 196498368); x = pmod(x - 389321179); x = pmod(x * 407769339); x = pmod(x - 668656066); x = pmod(x + 737252782); x = pmod(x * 563677494); x = pmod(x * 381498309); x = pmod(x - 162841870); x = pmod(x * 221177751); x = pmod(x * 186322169); x = pmod(x - 273857766); x = pmod(x * 714646597); x = pmod(x + 528983903); x = pmod(x + 161261126); x = pmod(x + 580072051); x = pmod(x + 73562406); x = pmod(x * 580917844); x = pmod(x * 333982766); x = pmod(x * 26464204); x = pmod(x + 444910125); x = pmod(x - 211161758); x = pmod(x + 857150); x = pmod(x + 151241230); x = pmod(x * 460103625); x = pmod(x + 349965546); x = pmod(x + 336647433); x = pmod(x - 651868703); x = pmod(x - 560457375); x = pmod(x - 310027822); x = pmod(x * 173984028); x = pmod(x * 743346575); x = pmod(x - 257680324); x = pmod(x - 193467970); x = pmod(x - 37222260); x = pmod(x - 719528370); x = pmod(x + 455833225); x = pmod(x - 694794023); x = pmod(x + 40785744); x = pmod(x + 485999702); x = pmod(x * 467215786); x = pmod(x + 143033673); x = pmod(x - 203472932); x = pmod(x * 509780859); x = pmod(x + 651949322); x = pmod(x * 178249781); x = pmod(x - 318931279); x = pmod(x + 402539290); x = pmod(x + 737886610); x = pmod(x * 206810936); x = pmod(x + 277968593); x = pmod(x * 210016962); x = pmod(x - 30673877); x = pmod(x * 385367916); x = pmod(x * 502168498); x = pmod(x - 450292458); x = pmod(x - 315721957); x = pmod(x * 555742689); x = pmod(x * 575672494); x = pmod(x + 678057901); x = pmod(x * 128401438); x = pmod(x + 277859141); if(x == 737645692) { printf("Success\n"); return0; } else { printf("Failure\n"); return1; } } ----

and will awaits for an input: the x that after all that operations is equal to 737645692. At each connection operations and numbers will are randomized so after parsing correctly each raw is time to reverse the pmod function.

The pmod constant (in the above example is 745018031) is always a prime number so, using the Fermat’s Little Theorem, is possible to reverse operation with * operator.

For example, if a = 2 and p = 7, then 27 = 128, and 128 − 2 = 126 = 7 × 18 is an integer multiple of 7.

# Connect and readall r = remote('bitpuzzler.wpictf.xyz', '31337')

# 82 Rounds for z in range(82): log.info("Round: {}".format(z)) line = r.recvuntil("}\n----").decode() log.info(line) # Read: int64_t v = x % 1403906831; pmod_val = int(re.findall("int64_t v = x % (\d*);", line)[0])

# First pmod skipped (is a prime number) => just get pmod_val # Read all: x = pmod(x * 1291394425); ops = re.findall("x = pmod\(x (.) (\d*)\);", line)