Tags: crypto python
Rating: 3.0
<span># seccon2016-vigenere-writeup
Seccon 2016 Vigenere Writeup
The question was like following:
Vigenere
k: ????????????
p: SECCON{???????????????????????????????????}
c: LMIG}RPEDOEEWKJIQIWKJWMNDTSR}TFVUFWYOCBAJBQ
k=key, p=plain, c=cipher, md5(p)=f528a6ab914c1ecf856a1d93103948fe
|ABCDEFGHIJKLMNOPQRSTUVWXYZ{}
-+----------------------------
A|ABCDEFGHIJKLMNOPQRSTUVWXYZ{}
B|BCDEFGHIJKLMNOPQRSTUVWXYZ{}A
C|CDEFGHIJKLMNOPQRSTUVWXYZ{}AB
D|DEFGHIJKLMNOPQRSTUVWXYZ{}ABC
E|EFGHIJKLMNOPQRSTUVWXYZ{}ABCD
F|FGHIJKLMNOPQRSTUVWXYZ{}ABCDE
G|GHIJKLMNOPQRSTUVWXYZ{}ABCDEF
H|HIJKLMNOPQRSTUVWXYZ{}ABCDEFG
I|IJKLMNOPQRSTUVWXYZ{}ABCDEFGH
J|JKLMNOPQRSTUVWXYZ{}ABCDEFGHI
K|KLMNOPQRSTUVWXYZ{}ABCDEFGHIJ
L|LMNOPQRSTUVWXYZ{}ABCDEFGHIJK
M|MNOPQRSTUVWXYZ{}ABCDEFGHIJKL
N|NOPQRSTUVWXYZ{}ABCDEFGHIJKLM
O|OPQRSTUVWXYZ{}ABCDEFGHIJKLMN
P|PQRSTUVWXYZ{}ABCDEFGHIJKLMNO
Q|QRSTUVWXYZ{}ABCDEFGHIJKLMNOP
R|RSTUVWXYZ{}ABCDEFGHIJKLMNOPQ
S|STUVWXYZ{}ABCDEFGHIJKLMNOPQR
T|TUVWXYZ{}ABCDEFGHIJKLMNOPQRS
U|UVWXYZ{}ABCDEFGHIJKLMNOPQRST
V|VWXYZ{}ABCDEFGHIJKLMNOPQRSTU
W|WXYZ{}ABCDEFGHIJKLMNOPQRSTUV
X|XYZ{}ABCDEFGHIJKLMNOPQRSTUVW
Y|YZ{}ABCDEFGHIJKLMNOPQRSTUVWX
Z|Z{}ABCDEFGHIJKLMNOPQRSTUVWXY
{|{}ABCDEFGHIJKLMNOPQRSTUVWXYZ
}|}ABCDEFGHIJKLMNOPQRSTUVWXYZ{
Vigenere cipher
https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher
First, we have to derive key to decrypt the ciphertext.
As given some of the plaintext is : SECCON{ and }
So we derived the key from first 7 characters of ciphertext: LMIG}RP
The derived key is: VIGENERxxxxx
Since the algorithm is Vigenere, we guessed that the 8th character is E and the current key value is:
VIGENERExxxx
In Vigenere algorithm the key is repeated by the length of plaintext.
So we divided plaintext by 12 which is keys length.
key: VIGENERExxxxVIGENERExxxxVIGENERExxxxVIGENER
cipher: LMIG}RPEDOEEWKJIQIWKJWMNDTSR}TFVUFWYOCBAJBQ
plain: SECCON{AxxxxBCDEDEFGxxxxKLMNOPQRxxxxVWXYYZ}
We saw that the missing parts of plain text are relevant to neighbor characters.
* In the first chunk the characters will be some sequence of A,B,C
* In the second chunk the characters will be some sequence of F,G,H,I,J,K,L
* In the third chunk the characters will be some sequence of Q,R,S,T,U,V,W
Since we have md5 of plaintext, we write a code that tries the md5 of following sequences to derive the key.
Flag is: SECCON{ABABABCDEDEFGHIJJKLMNOPQRSTTUVWXYYZ}</span>
