WIDE

TL; DR

The decryption key can be located in the decompiled binary. The flag is revealed as soon as the correct decryption key is entered.

Solve

Very straightforward binary possibly meant to be welcoming for beginners. The binary is very lenient in letting you know that you must pass db.ex as an argument when executing it. Once you successfully launch it, you are given a list of dimensions to examine. The flag can be unveiled if we enter 6 as our choice and then enter the decryption key.

To find the correct decryption key we must examine the decompiled binary. We notice that the string that prompts the input for the decryption key is present in menu(long, int). Here, the binary accepts the key as a string and then later converts this into a wide string. After the conversion, the inputted wide string is compared to the expected key sup3rs3cr3tw1d3. If the keys match, the binary prints out the flag HTB{str1ngs_4r3nt_4lw4ys_4sc11}.

The fact that the keys exist as wide strings is irrelevant to solving the problem. At best they serve as a nod to the name of the problem.

printf("[X] That entry is encrypted - please enter your WIDE decryption key: ");
fgets(local_c8,0x10,stdin);
mbstowcs(local_1c8,local_c8,0x10);
iVar1 = wcscmp(local_1c8,L"sup3rs3cr3tw1d3");

The lines above are an excerpt from menu(long, int) where the expected key can be seen written in-line in the source itself.