Counting Letter Contacts

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Counting Letter Contacts; Using a contact chart
Topic Started: Oct 11 2005, 01:19 PM (378 Views)
rot13	Oct 11 2005, 01:19 PM Post #1
Elite member Posts: 306 Group: Trusted Member Member #14 Joined: September 29, 2005	I don't know if anyone else has taken a look at Challenge 2 at http://www.cipher.maths.soton.ac.uk (the site mentioned by our new member, cows. The second part of the challenge is morse code that breaks down into a substitution cipher without word breaks. One of the things I occasionally find handy with these types of cryptograms is to count the frequency of contacts between letters. How often does X precede G? How often does it follow it? Here is a chart for the 10 most frequent letters, generated by the python program I use for simple substitutions: Code: H2 G2 B2 V3 W4 T4 M5 E5 A5 U9 C18 \|H\| U11 F8 B6 I6 S6 T5 E4 G3 M3 H2 L2 Y1 R1 S1 P1 O1 L1 I1 F1 \| \| O2 W2 A1 D1 J1 P1 R1 X1 Z1 O2 L2 G2 E2 C2 U5 B5 S6 T7 R7 I8 \|Z\| P9 S9 L7 U6 G4 O4 D3 I3 B2 F2 T2 X1 W1 M1 J1 K1 H1 \| \| W2 J1 I2 G2 Z3 T3 U3 O4 J4 B4 S6 H6 F8 \|I\| C24 Z8 J4 F3 I2 R2 U2 B1 E1 H1 M1 L1 D1 C1 A1 \| \| X1 W2 P2 O2 T3 S3 L3 I3 E3 C4 U5 H8 \|F\| I8 S7 T6 U5 E4 A3 B3 G2 R2 W2 H1 V1 R1 M1 J1 D1 B1 A1 Z2 \| \| K1 J1 L1 Y1 D1 A1 X2 P3 H6 F7 Z9 J11 \|S\| I6 Z6 A5 B3 F3 J3 O3 V3 T2 C1 H1 \| \| M1 L1 P1 R1 J1 G1 E1 A1 O2 L2 I2 P3 F5 Z6 H11 \|U\| H9 J6 F5 Z5 B3 I3 P2 R2 T2 L1 U1 V1 W1 T1 U1 \| \| T1 O1 H1 F1 C1 L2 S3 D3 I4 U6 B6 \|J\| S11 A4 I4 E3 B2 T2 F1 U1 W1 X1 Z1 Z1 X1 \| \| T1 S1 X1 I24 \|C\| H18 F4 Z2 E1 I1 J1 I1 G1 B1 Z2 P2 L2 J2 U3 S3 F3 H6 \|B\| J6 Z5 I4 P3 H2 X2 B1 D1 F1 O1 R1 \| \| R1 P1 G1 A1 Z2 U2 S2 J2 H5 F6 \|T\| Z7 H4 F3 I3 E2 C1 J1 U1 X1 One of the first things I always look for is the pattern you see here with I-C-H. I is fairly frequent, C is a good bit less frequent, and H is the most frequent. The pattern, you see that I is the letter that most often precedes C, and H is the letter that most often follows it. This pattern usually points towards THE. The next interesting one, which shows up more often in longer cryptograms than in short ones is that U follows H more than any other letter, and comes before H more than any other letter except C. ER and RE are two very common digraphs, so the fact that UH and HU occur so frequently, it is a good guess that U is R. Also, notice that H, Z and F tend to contact more letters than the others. This suggests that they are vowels. Noticing that S comes after H, Z and F fairly often, I might conclude that it is a consonant, and that might also suggest that J is also a vowel. Since I aready have a guess for what represents T, I might guess that S represents N.



insecure	Oct 11 2005, 02:21 PM Post #2
Elite member Posts: 381 Group: Trusted Member Member #10 Joined: September 9, 2005	As rot13 rightly points out, digraphs (and trigraphs, and tetragraphs, and ...) are extremely useful indicators in monoalphabetic substitution ciphers. Not only that, but they can also be handy in recognisers - that is, in modules whose task is to determine whether a given attempt at a key has resulted in a readable plaintext.



Donald	Oct 11 2005, 03:03 PM Post #3
Elite member Posts: 454 Group: Trusted Member Member #8 Joined: September 2, 2005	I've been trying to learn the "consonant-line short cut" as explained in Helen Fouche Gaines "Cryptanalysis". You can do some REALLY cool things with contact information. Gaines's book is not exactly user friendly, but it IS full of useful techniques. Donald



rot13	Oct 11 2005, 03:35 PM Post #4
Elite member Posts: 306 Group: Trusted Member Member #14 Joined: September 29, 2005	Donald Oct 11 2005, 03:03 PM I've been trying to learn the "consonant-line short cut" as explained in Helen Fouche Gaines "Cryptanalysis". You can do some REALLY cool things with contact information. Gaines's book is not exactly user friendly, but it IS full of useful techniques. Donald Now that you mention it, I think I need to go back and re-read that book. It has been a while and when I first read it, a lot of the techniques were new to me, so I probably missed a lot of the subtle tricks.



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