Information

Points	Category	Level
150	Cryptography	Easy

Challenge

This is one of the older ciphers in the books, can you decrypt the message? You can find the ciphertext in /problems/caesar-cipher-1_2_73ab1c3e92ea50396ad143ca48039b86 on the shell server

Hint

caesar cipher tutorial

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
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:

#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}

Downloads

• python script

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