Hackthissite/Basic/Level6

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• Level: Basic::6
• URL: http://www.hackthissite.org/missions/basic/6/
• Exercise: An encryption system has been set up, which uses an unknown algorithm to change the text given. Requirements: Persistence, some general cryptography knowledge. Network Security Sam has encrypted his password. The encryption system is publically available and can be accessed with this form. Please enter a string to have it encrypted. You have recovered his encrypted password. It is:a7e34:l=. Decrypt the password and enter it below to advance to the next level.
• Solution:

We have to understand how the encryption works. Enter some letters: a, b, c, ... and some numbers: 1, 2, 3. You will notice that the increment seems to be the same. By using an ASCII conversion table (http://www.asciitable.com/), you will notice that we have following scheme:

• Each position is converted in ASCII number.
• Each position is then transformed with: +0 for the first position, +1 for the second one, +2 for the third one, etc.
• New ASCII after transform are then converted into characters.

To get the solution, we apply the reverse scheme. Here is the scheme:

 Position Encrypted ASCII Transform Result after transform ASCII (after transform) 1 2 3 4 5 6 7 8 a 7 e 3 4 : l = 97 55 101 51 52 58 108 61 -0 -1 -2 -3 -4 -5 -6 -7 97 54 99 48 48 53 102 54 a 6 c 0 0 5 f 6