Tips for Breaking the Take-Home Cryptogram
Computing I.C.
1. The frequencies f (subscript i) used in computing the I.C. are the frequencies of each letter,
e.g., A, B, C, etc. in the SUBSET of letters from the cryptogram that you are counting.
For example, if in counting frequencies among the first letter in every group of 5, you find that
there are 3 E's, then f (subscript E) = 3. Also, N is the total number of letters in the SUBSET you
counting.
2. DO NOT compute the I.C. of the complete message, as the textbook suggests. Only compute
it for the first letter in every group of 4 letters, first letter in every group of 5 letters, and first
letter in every group of 6 letters.
3. When one of the three groups tallied in 2. gives a high I.C., say the first in every 6 letters,
then build frequency counts for the 2nd letter in every group of 6 letter, for the 3rd letter
in every group of 6, then for the 4th, then for the 5th and then for the 6th. Compute the I.C. for
each of the other 5 frequency distributions and make sure that the average of the 6 IC's is over .06.
Aligning Alphabets
1. Be sure to have two complete copies of the alphabet (52 letters) on each of the frequency strips.
2. Be sure that the letters are evenly spaced on the strips; e.g., DO NOT use a font like Times
that creates letters of varying width.
3. Focus on matching voids (letters with zero frequency) more than matching high frequencies.
4. The second keyword will form in some column of the aligned alphabet strips with the
alphabet strips in order, i.e., the first strip on top, then the second strip, then the third strip, etc..
However, when you start to match your first two alphabet strips, you need not be restricted to
matching just strip 1 with strip 2, or 2 with 3, or 3 with 4, etc. You can also try 1 with, say, 4.
This gives you more chances to find a good match. Once all, or almost all, the alphabet strips
are matched, then put the strips in order so that you can look for the second keyword.
Collapsing Back to a Single Alphabet
1. If your second keyword in the aligned alphabet strips is VERTEX, then each V in the first
alphabet should be rewritten as an A, each W in the first alphabet rewritten as a B, each X
rewritten as a C, each Y rewritten as a D, Z as E, A as F, B as G, etc. In the second alphabet
each E is rewritten as A, F as B, G as C, etc.. In the third alphabet, each R is rewritten as A,
S as B, T as C, etc. Similarly for the fourth, fifth and sixth alphabets.
2. To check that you rewrote each letter correctly, when you start to decode the collapsed
cryptogram, the number of A's should equal the sum of the frequencies of the letters that were
collapsed back to A. If the 2nd keyword was VERTEX, then the frequencies of: (i) V in the
first alphabet, (ii) E in the 2nd alphabet, (iii) R in the 3rd alphabet, T in the 4th, E in the 5th,
and X in the 6th should together sum up to the frequency of A in the collapsed alphabet.
The same check should be done for every other letter's frequency in the collapsed alphabet.
3. REMMEBER in decoding the collapsed message that the encoding was reversed with the
scrambled alphabet representing the plan text, not the code text as in homework problems.
For example, in the handout example with MORNING, the plan letter M is decoded as an A,
not A decoded as M.
GOOD LUCK