Decrypting, a simple lesson (From hackthissite.org)

how do we decrypt 5278f:il if we know the encryption key?
Lucky network security sam has left the encryption public. In lesson 6 we look at this.

Let's try to see what 2 strings are so we know what to do.
aaaaaaaa = abcdefgh
00000000 = 01234567

abcdefgh
========= #this is the string counting up
01234567
5278f:il

---

5155b5:ce

How do we get this answer?
5 - 0 = 5
2 - 1 = 1
7 - 2 = 5
8 - 3 = 5
f - 4 = b
: - 5
Now for the : we actually need the ASCII table. But an educated guess can work too.
But let's use the table

We need to go back 6 from : in the table which makes : -5 = 5

Let's put that back together below

01234567
5278f:il

---

5155b5:ce

5 - 0 = 5
2 - 1 = 1
7 - 2 = 5
8 - 3 = 5
f - 4 = b
: - 5 = 5
f - 6 = c
l - 7 = e

You could either use the table or just count and guessed that : = 5 this is not a strong password. It is predictable.
The owner of the password clearly has something for the number 5.