# zomry1/picoCTF_2018_Writeup

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Solution

# Crypto Warmup 2

## Information

Points Category Level
75 Cryptography Easy

## Challenge

Cryptography doesn't have to be complicated, have you ever heard of something called rot13? `cvpbPGS{guvf_vf_pelcgb!}`

### Hint

This can be solved online if you don't want to do it by hand!

## Solution

So rot13 encryption? after google it, I found that it's really simple encryption,

``````For each letter in the plaintext:
replace this letter with the 13th letter after it
``````

*In order to decrypt the ciphertext, we need to replace each letter with the 13th letter before it.

So I wrote a python script for this:

```ciphertext = "cvpbPGS{guvf_vf_pelcgb!}"
plaintext = ""

#Foreach char in the ciphertext
for char in ciphertext:
#If the char is not a letter
if not char.isalpha():
plaintext += char
continue

#Get the 13th before letter
newLetter = ord(char) - 13

#If the letter is uppercase
if char.isupper():
#If now we below A
if newLetter < ord("A"):
#=> Add 26 to complete circle of letters
newLetter += 26
#Else it's a lowercase letter and below a
elif newLetter < ord("a"):
#=> Add 26 to complete circle of letters
newLetter += 26

#Add the new letter to the plaintext
plaintext += chr(newLetter)

print plaintext```

So the script return the flag!

## Flag

`picoCTF{this_is_crypto!} `

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