Cscamp CTF Quals - Exploitation 200 [Team xbios]

For expliotation 200 we were given a ELF 64-bit LSB executable. The binary takes a password as input, if we get the password right, setuid() and system() functions are called which will eventually print the flag from a file. Disassembly shows a series of movb instruction, writing some value into the stack at address range between $rbp-0x12 to $rbp-0x20
data = [0x64, 0x36, 0x34, 0x38, 0x37, 0x65, 0x36, 0x39, 0x62, 0x65, 0x38, 0x64, 0x38, 0x35, 0x31 ]
Then there is a loop which checks user input, against this value. By reading the value from $rbp-0x20 we get the password. Input the password and get the flag

// This is the loop
   0x0000000000400783 <+177>: movl   $0x0,-0x38(%rbp)
   0x000000000040078a <+184>: jmp    0x4007c0 <main+238>
   0x000000000040078c <+186>: lea    -0x20(%rbp),%rdx
   0x0000000000400790 <+190>: mov    -0x38(%rbp),%eax
   0x0000000000400793 <+193>: cltq   
   0x0000000000400795 <+195>: lea    (%rdx,%rax,1),%rax
   0x0000000000400799 <+199>: movzbl (%rax),%edx
   0x000000000040079c <+202>: lea    -0x30(%rbp),%rcx
   0x00000000004007a0 <+206>: mov    -0x38(%rbp),%eax
   0x00000000004007a3 <+209>: cltq   
   0x00000000004007a5 <+211>: lea    (%rcx,%rax,1),%rax
   0x00000000004007a9 <+215>: movzbl (%rax),%eax
   0x00000000004007ac <+218>: cmp    %al,%dl
   0x00000000004007ae <+220>: je     0x4007bc <main+234>
   0x00000000004007b0 <+222>: mov    $0x400920,%edi
   0x00000000004007b5 <+227>: callq  0x400568 <puts@plt>
   0x00000000004007ba <+232>: jmp    0x4007f5 <main+291>
   0x00000000004007bc <+234>: addl   $0x1,-0x38(%rbp)
   0x00000000004007c0 <+238>: cmpl   $0xf,-0x38(%rbp)
   0x00000000004007c4 <+242>: jle    0x40078c <main+186>

Breakpoint 1, 0x000000000040078c in main ()
(gdb) x/i $rip
=> 0x40078c <main+186>: lea    -0x20(%rbp),%rdx
(gdb) x/s $rbp-0x20
0x7fffffffe060:  "d6487e69be8d851"

$ ./level100 d6487e69be8d851
Congratulation, let me grab you content of key.txt
YOUR KEY IS : e4783253f92332ddb7d30a24cd9d1541