Add files via upload

Author: idekctf

aptenodytes-forsteri.md

@@ -0,0 +1,39 @@
+Writeup for crypto/aptenodytes-forsteri
+
+# Problem
+Here's a warmup cryptography challenge. Reverse the script, decrypt the output, submit the flag.
+[output.txt] [aptenodytes-forsteri.py]
+
+# Solution
+Let's start by downloading both and opening them
+-[output.txt]:
+IOWJLQMAGH
+
+-[aptenodytes-forsteri.py]:
+```
+flag = open('flag.txt','r').read() #open the flag
+assert flag[0:5]=="flag{" and flag[-1]=="}" #flag follows standard flag format
+letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+encoded = ""
+for character in flag[5:-1]:
+    encoded+=letters[(letters.index(character)+18)%26] #encode each character
+print(encoded)
+```
+
+We know that each letter in flag.txt is being indexed in letters, then +18, then %26 from: 
+`encoded+=letters[(letters.index(character)+18)%26] #encode each character`
+We can reverse this by changing the +18 to -18.
+`decoded+=letters[(letters.index(character)-18)%26] #decodes each character`
+Using this as our main solve scirpt we can write a new decoder:
+```
+enc_flag = "flag{QWERTYUIOP}"
+letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+decoded = ""
+for character in enc_flag[5:-1]:
+    decoded+=letters[(letters.index(character)+18)%26] #decodes each character
+print(decoded)
+```
+We get `QWERTYUIOP` which is our flag!
+
+# Flag
+flag{QWERTYUIOP}