Tags: crypto
Rating:
# *Caesar cipher 1*
## Information
| Points |Category | Level|
|--|--|--|
| 150 | Cryptography |Easy |
## Challenge
> This is one of the older ciphers in the books, can you decrypt the [message](https://2018shell.picoctf.com/static/8b8d9e1fd4c9cd66facc3794d9c69175/ciphertext)? You can find the ciphertext in /problems/caesar-cipher-1_2_73ab1c3e92ea50396ad143ca48039b86 on the shell server
### Hint
> caesar cipher [tutorial](https://learncryptography.com/classical-encryption/caesar-cipher)
## Solution
So the ciphertext we got is:
picoCTF{payzgmuujurjigkygxiovnkxlcgihubb}
we need to decrypt this, and all we know that the encryption is **Caesar cipher**.
So after searching it in google I found that ROT13 we [discuss earlier](https://github.com/zomry1/picoCTF_2018_Writeup/tree/master/Cryptography/Crypto%20Warmup%202)
it's a private case of Caesar cipher when the key is 13.
In Caesar cipher the key can be between 0 to 25 and for each letter we replace each letter with the key'th letter after it.
In more formal representation the encryption is:
**x** - the letter we want to encrypt
**n** - the key
**En(x)** - the ciphertext for n and x
And the decryption is just the reverse of this operation:
So I wrote a python script to try each valid key value for the ciphertext we got:
```python
#For comments on this function look at "Crypto Warmup 2" challenge
def decryptor(key, ciphertext):
plaintext = ""
for c in ciphertext:
newChar = ord(c)-key
if newChar < ord("a"):
newChar = newChar + 26
plaintext += chr(newChar)
return plaintext
ciphertext = "payzgmuujurjigkygxiovnkxlcgihubb"
#For each valid key value
for i in range(1,26):
plaintext = decryptor(i,ciphertext)
print "key is: ", i ,"text is: ", plaintext
```
*The results:*
>key is: 1 text is: ozxyflttitqihfjxfwhnumjwkbfhgtaa
>key is: 2 text is: nywxeksshsphgeiwevgmtlivjaegfszz
>key is: 3 text is: mxvwdjrrgrogfdhvduflskhuizdferyy
>key is: 4 text is: lwuvciqqfqnfecguctekrjgthycedqxx
>key is: 5 text is: kvtubhppepmedbftbsdjqifsgxbdcpww
>**key is: 6 text is: justagoodoldcaesarcipherfwacbovv**
>key is: 7 text is: itrszfnncnkcbzdrzqbhogdqevzbanuu
>key is: 8 text is: hsqryemmbmjbaycqypagnfcpduyazmtt
>key is: 9 text is: grpqxdllaliazxbpxozfmeboctxzylss
>key is: 10 text is: fqopwckkzkhzywaownyeldanbswyxkrr
>key is: 11 text is: epnovbjjyjgyxvznvmxdkczmarvxwjqq
>key is: 12 text is: domnuaiixifxwuymulwcjbylzquwvipp
>key is: 13 text is: cnlmtzhhwhewvtxltkvbiaxkyptvuhoo
>key is: 14 text is: bmklsyggvgdvuswksjuahzwjxosutgnn
>key is: 15 text is: aljkrxffufcutrvjritzgyviwnrtsfmm
>key is: 16 text is: zkijqweetebtsquiqhsyfxuhvmqsrell
>key is: 17 text is: yjhipvddsdasrpthpgrxewtgulprqdkk
>key is: 18 text is: xighouccrczrqosgofqwdvsftkoqpcjj
>key is: 19 text is: whfgntbbqbyqpnrfnepvcuresjnpobii
>key is: 20 text is: vgefmsaapaxpomqemdoubtqdrimonahh
>key is: 21 text is: ufdelrzzozwonlpdlcntaspcqhlnmzgg
>key is: 22 text is: tecdkqyynyvnmkockbmszrobpgkmlyff
>key is: 23 text is: sdbcjpxxmxumljnbjalryqnaofjlkxee
>key is: 24 text is: rcabiowwlwtlkimaizkqxpmzneikjwdd
>key is: 25 text is: qbzahnvvkvskjhlzhyjpwolymdhjivcc
And as we can see, only the key 6 has a meaning so it's the flag
## Flag
> `picoCTF{justagoodoldcaesarcipherfwacbovv}
`
