Language: C

/!\ Task was changed a bit: previously it accepted even a part of the flag. Source code was changed. If you use the binaries, please re-download.

Source code:

#include <stdio.h> #include <string.h> #include <stdint.h> #include <stdlib.h> #define EMAIL_LEN 256 #define SERIAL_LEN 256 #define is_alpha(c) ({ char _c = (c) | 0x20; _c >= 'a' && _c <= 'z'; }) #define is_digit(c) ({ (c) >= '0' && (c) <= '9'; }) #define is_valid(c) ({ is_alpha(c) || is_digit(c); }) #define A 7 #define MODULE 26 int check_email(const char *email) { char c; int at = 0, dot = 0; int bdlen = 0; int adbdlen = 0; int adadlen = 0; while ((c = *email++) != 0) { if (!at) { if (is_valid(c)) bdlen++; if (c == '@') at = 1; } if (at) { if (!dot) { if (is_valid(c)) adbdlen++; if (c == '.') dot = 1; } if (dot && is_valid(c)) adadlen++; } } if (at && dot && bdlen && adbdlen && adadlen) return 1; else return 0; } int check_serial(char *email, char *serial) { char *em, *sr; uint8_t val_em, val_sr; if (!email) return 0; if (!serial) return 0; em = email; sr = serial; while (*em != 0) { if (is_alpha(*em)) { val_em = ((uint8_t)*em | 0x20) - 'a'; val_em = ((A * val_em) % MODULE) + 'a'; val_sr = (uint8_t)(*sr) | 0x20; if (val_sr != val_em) return 0; else sr++; } em++; } return 1; } int main() { char email[EMAIL_LEN]; char serial[SERIAL_LEN]; char email_format[32]; char serial_format[32]; sprintf(email_format, "%%%ds", EMAIL_LEN-1); sprintf(serial_format, "%%%ds", SERIAL_LEN-1); memset(email, 0, EMAIL_LEN); memset(serial, 0, SERIAL_LEN); printf("Email: "); scanf(email_format, email); printf("Serial: "); scanf(serial_format, serial); if (check_email(email) && check_serial(email, serial)) puts("Right"); else puts("Wrong"); return 0; }

Flag is the right serial for email spbctf2017@forkbomb.ru

Compiled Linux binary: spbctf_0
Compiled Windows binary: spbctf_0.exe