Diagraphic Substitution
Diagraphic substitution is a slightly more complex way of enciphering plain text letters
with two letters found within a matrix. However, the matrix used is actually four
matrices. There are two matrices used to identify two plain text letters at a time. The other two matrices are used to identify the corresponding cipher text values.
Each individual matrix has all the letters of the alphabet:
The dashed line above was drawn to identify letters in the plain text matrices labeled
P1 and P2. The letter "H" is identified in P1 and "I" in P2. Thus, the plain text
letters spell out the word "HI." The corresponding cipher values can be found in
C1 and C2 matrices. In this case, "H" in P1 corresponds to "A" in C1. "I" in P2 corresponds
with "T" in C2. Thus, the enciphered letters for "HI" are "AT."
To decipher "AT" you simply find the corresponding plain text value in the same row,
opposite plain text matrix.
Use the matrices above to encipher the message: Keep him alive. Start by writing
out the two letter blocks of the message to form the rectangle for each encipher.
After drawing the rectangle for each pair of letters find the corresponding cipher
values:
Plain: KE EP HI MA LI VE
Cipher: NC BL
Can make the matrices differently than shown above? What must all matrices have in
common? Does the order of letters in a matrix make a difference?