Decompile it with IDA:

__int64 __fastcall sub_1230(unsigned int a1)
{
  int v1; // eax
  int i; // ecx
  int v3; // edi

  v1 = 0;
  for ( i = 0; i != 32; ++i )
  {
    v3 = __ROR4__(a1 * a1, i);
    a1 = v1 ^ (4919 * v3 + 69210935);
    v1 += 322376503;
  }
  return HIBYTE(a1) + a1 + HIWORD(a1) + (a1 >> 8);
}
__int64 __fastcall main(int a1, char **a2, char **a3)
{
  size_t v3; // rax
  __int64 v4; // rsi
  unsigned int *v5; // r8
  char v7[264]; // [rsp+0h] [rbp-108h] BYREF

  puts("Welcome to CTFd+!");
  puts("So far, we only have one challenge, which is one more than the number of databases we have.\n");
  puts("Very Doable Pwn - 500 points, 0 solves");
  puts("Can you help me pwn this program?");
  puts("#include <stdio.h>\nint main(void) {\n    puts(\"Bye!\");\n    return 0;\n}\n");
  puts("Enter the flag:");
  fgets(v7, 256, stdin);
  v3 = strcspn(v7, "\n");
  v4 = 0LL;
  v5 = (unsigned int *)&unk_4060;
  v7[v3] = 0;
  do
  {
    if ( (unsigned __int8)sub_1230(v5[v4]) != v7[v4] )
    {
      puts("Incorrect flag.");
      return 0LL;
    }
    ++v4;
  }
  while ( v4 != 47 );
  puts("You got the flag! Unfortunately we don't exactly have a database to store the solve in...");
  return 0LL;
}

then use dynamic debugging and set breakpoint at the if statement (patched return 0LL; to NOP). You can also use hook or re-compile the program to output the flag.

lactf{m4yb3_th3r3_1s_s0m3_m3r1t_t0_us1ng_4_db}

Original writeup (https://tiefsee5037008.github.io/posts/LA-CTF-Writeup/#ctfd-plus).