Tag: statistics

Cryptanalysis of the Caesar Cipher

If you need a reminder on how the Caesar Cipher works click here.

The Caesar Cipher is a very easy to crack as there are only 25 unique keys so we can test all of them and score how English they are using either Chi-Squared Statistic or N-Gram Probability.

Example

Ciphertext of “RCZIOCZXGJXFNOMDFZNORZGQZVOOVXF”

Shift | Decrypted Text                 | Chi-Sq Score
1       QBYHNBYWFIWEMNLCEYMNQYFPYUNNUWE  201.327499
2       PAXGMAXVEHVDLMKBDXLMPXEOXTMMTVD  599.489345
3       OZWFLZWUDGUCKLJACWKLOWDNWSLLSUC  267.058510
4       NYVEKYVTCFTBJKIZBVJKNVCMVRKKRTB  325.267580
5       MXUDJXUSBESAIJHYAUIJMUBLUQJJQSA  775.163340
6       LWTCIWTRADRZHIGXZTHILTAKTPIIPRZ  434.880892
7       KVSBHVSQZCQYGHFWYSGHKSZJSOHHOQY  554.916606
8       JURAGURPYBPXFGEVXRFGJRYIRNGGNPX  340.923863
9       ITQZFTQOXAOWEFDUWQEFIQXHQMFFMOW  1012.384679
10      HSPYESPNWZNVDECTVPDEHPWGPLEELNV  115.358434
11      GROXDROMVYMUCDBSUOCDGOVFOKDDKMU  91.670467
12      FQNWCQNLUXLTBCARTNBCFNUENJCCJLT  283.701596
13      EPMVBPMKTWKSABZQSMABEMTDMIBBIKS  194.299832
14      DOLUAOLJSVJRZAYPRLZADLSCLHAAHJR  385.733449
15      CNKTZNKIRUIQYZXOQKYZCKRBKGZZGIQ  1520.292006
16      BMJSYMJHQTHPXYWNPJXYBJQAJFYYFHP  801.523128
17      ALIRXLIGPSGOWXVMOIWXAIPZIEXXEGO  603.683962
18      ZKHQWKHFORFNVWULNHVWZHOYHDWWDFN  280.874579
19      YJGPVJGENQEMUVTKMGUVYGNXGCVVCEM  269.610988
20      XIFOUIFDMPDLTUSJLFTUXFMWFBUUBDL  176.849244
21      WHENTHECLOCKSTRIKESTWELVEATTACK  51.921327
22      VGDMSGDBKNBJRSQHJDRSVDKUDZSSZBJ  460.236803
23      UFCLRFCAJMAIQRPGICQRUCJTCYRRYAI  262.108135
24      TEBKQEBZILZHPQOFHBPQTBISBXQQXZH  1373.411997
25      SDAJPDAYHKYGOPNEGAOPSAHRAWPPWYG  90.715517

As you can see the lowest Chi-Squared value is 51.921327, which was using a shift of 21. If you read the decrypted text for a shift of 21 you can indeed see that it is English. Hence cipher has been broken!

WIP

 

Index of Coincidence

Index of Coincidence is the probability that when selecting two letters from a text (without replacement), the two letters are the same. For a random piece of text with every letter having a chance of \frac{1}{26} of appearing, the Index of Coincidence is also \frac{1}{26} ({0.0385} ).

If the frequency of the letters are known and the sum of the frequencies is 1 then this formula can be used to calculate Index of Coincidence for a particular language.

I.C=\sum_{i=A}^{i=Z}(F_{i})^{2}
Fi is the frequency, in decimal form (10% = 0.1), of a letter in your text.

For for a generic piece of text written in English the Index of Coincidence is 0.0667, it is different for each language as the letter frequencies are different…

Language Index of Coincidence
English 0.0667
French 0.0694
German 0.0734
Spanish 0.0729
Portuguese 0.0824
Turkish 0.0701
Swedish 0.0681
Polish 0.0607
Danish 0.0672
Icelandic 0.0669
Finnish 0.0699
Czech 0.0510

Values for this tabled created from the frequencies from Wikipedia. The values are for letters A-Z other letters such as ‘á’ or ‘â’ are considered to be the same as ‘a’, ‘ü’ or ‘ú’ are considered to be the same as ‘u’ etc…

However if you want to figure out the index of coincidence for a particular piece of text this formula can be used.

IoC=\frac{\sum_{i=A}^{i=Z}C_{i}(C_{i}-1)}{L(L-1)} 
Ci is the count, of a letter in the text.
Li is the total number of letters in the text.

If a letter does not appear more than once then is does not need to be involved in the calculation as when Ci is 1 or 0, Ci x (Ci – 1) will equal 0;

Example: ‘WHENTHECLOCKSTRIKESTWELVEATTACK’ Text length = 31

Letter Count (Ci) Ci(Ci – 1)
A 2 2
C 3 6
E 5 20
H 2 2
K 3 6
L 2 2
M 0 0
S 2 2
T 5 20
W 2 2
Total 31 62

\frac{62}{31\times30}=0.0666

This value is reasonably close to the expected Index of Coincidence value of English (0.0667). It is also much higher than that the expected Index of Coincidence of random text (0.0385) suggesting that this text is not random.

The larger the Index of Coincidence the more likely that there is some sort of language structure behind text. For example the Vigenère Cipher has an average Index of Coincidence of 0.042 – suggesting that the text is not random, which it is not.

Chi-Squared Statistic

The Chi-Squared Statistic is a measure of how two categorical distributions differ from one another. So for 2 identical distributions the score would be 0 and as the distributions begin to diff the score will increase. The formula is…

X^{2}=\sum_{i=A}^{i=Z}\frac{(O_{i}-E_{i})^2}{E_{i}}
Oi is the observed count of that letter in your text.
Ei is the expected count of that letter in the length of your text.

Chi-Squared Statistic in words is, “the sum, of the squared difference between observed count and expected count divided by the expected count, of each letter.”

Example: ‘WHENTHECLOCKSTRIKESTWELVEATTACK’ Text length = 31

Letter Observed Count (Oi) Frequency in English Expected Count (Ei)* (Oi – Ei)2/Ei
A 2 8.17% 2.53177 0.11169
B 0 1.49% 0.46252 0.46252
C 3 2.78% 0.86242 5.29817
D 0 4.25% 1.31843 1.31843
E 5 12.70% 3.93762 0.28663
F 0 2.23% 0.69068 0.69068
G 0 2.02% 0.62465 0.62465
H 2 6.09% 1.88914 0.00651
I 1 7.00% 2.16876 0.62985
J 0 0.15% 0.04743 0.04743
K 3 0.77% 0.23932 31.84587
L 2 4.03% 1.24775 0.45352
M 0 2.41% 0.74586 0.74586
N 1 6.75% 2.09219 0.57016
O 1 7.51% 2.32717 0.75688
P 0 1.93% 0.59799 0.59799
Q 0 0.10% 0.02945 0.02945
R 1 5.99% 1.85597 0.39477
S 2 6.33% 1.96137 0.00076
T 5 9.06% 2.80736 1.71252
U 0 2.76% 0.85498 0.85498
V 1 0.98% 0.30318 1.60155
W 2 2.36% 0.73160 2.19907
X 0 0.15% 0.04650 0.04650
Y 0 1.97% 0.61194 0.61194
Z 0 0.07% 0.02294 0.02294
Total 31 1.00029 31.00899 51.92133

* Expected Count = FREQ / 100 × LEN

For English a Chi-Squared value of about 150 or less is expected anything above does likely does not resemble English.

WHENTHECLOCKSTRIKESTWELVEATTACK, X2 = 51.92133
THWKEEVIWTETSCANHKERCTTAKSCLLOE, X2 = 51.92133
ZDXPLXTDOWXSWCRSGPWVVOCWEOTTXOK, X2 = 425.59631

As you can see English text scores low however score is independent of letter order and a random text does not score highly.

I have created an Excel spreadsheet that can calculate Chi-Squared when given the frequencies of letters. It does not use macros. Chi-Squared Calculator