Tags: crypto

Rating: 0

# *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:
python
ciphertext = "cvpbPGS{guvf_vf_pelcgb!}"
plaintext = ""

#Foreach char in the ciphertext
for char in ciphertext:
#If the char is not a letter
#=> Add it has is
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!}