This Fortran program appears to function as a simple challenge...

This Fortran program appears to function as a simple challenge that verifies if a user can input a correct "flag" to gain "access." Here's an explanation of what each part of the code does:

1. Constants and Variables:

• N is a constant set to 13.
  • This represents the length of the character arrays doop2 and foo1.
• data is an integer array of size N initialized with specific values: (/ 70, 112, 98, 99, 106, 108, 114, 63, 88, 99, 94, 98, 90 /).
  • This array holds some data that will be used to generate a reference string (foo1).
• foo1 and doop2 are character arrays of length N.
  • foo1 will store a generated string based on data.
  • doop2 will store user input.

2. String Generation Logic:

The program generates a string (foo1) using the following loop:

do i = 1, N
    foo1(i:i) = achar(mod(data(i) + i, 128))
end do

• For each index i from 1 to 13:
  • data(i) is added to the current index i.
  • The result of the addition is taken modulo 128 using mod.
  • The modulo result is converted to its corresponding ASCII character using the Fortran intrinsic function achar.
  • The character is assigned to the ith position of the string foo1.

This produces a specific "secret" string stored in foo1.

3. User Input:

The program asks the user to "Enter the flag:". It reads the user's input into doop2.

4. Flag Validation:

The user's input (doop2) is compared to the generated string (foo1) in this if statement:

if (doop2 == foo1) then
    print *, "Access Granted!"
    print *, "The flag is: uactf{", foo1, "}"
else
    print *, "Access Denied!"
end if

• If the input string exactly matches foo1, the program prints "Access Granted!" and reveals the "flag" in the format uactf{<foo1>}.
• If the input does not match foo1, the program prints "Access Denied!".

Summary:

The program creates a secret string (foo1) based on a predefined integer array (data) and ASCII computations. It then asks the user to input a string. If the user's input matches the generated secret string (foo1), it grants access and reveals the flag in the format uactf{<foo1>}. Otherwise, access is denied.

This is likely a simple CTF (Capture the Flag) style challenge designed to reverse-engineer the flag by understanding the logic used to generate foo1.