Cracking Codes: Part I

Since the dawn of time (or at least since humans have been communicating with each other), people have wanted to be able to transmit messages secretively. Over centuries of war, star-crossed loves, power-hungry reigns, conniving traitors and common crime, a number of ‘secret writing’ methods (i.e. cryptography) have been developed.

There are two major categories of cryptographic systems: ciphers and codes. Criminals frequently use both to conceal clandestine records, conversations and writings. Officially, cryptology is the scientific study of cryptography and includes cryptanalytics, which deals with methods of solving cryptographic systems. Over time, we’ll be sharing a variety of secret writing systems encountered by law enforcement officials. You’ll also get a sneak peak into some of the approaches groups like the FBI use to solve these deceptive systems.

Cipher Systems:

Ciphers involve the replacement of true letters or numbers (plain text) with different characters (cipher text) or the systematic rearrangement of the true letters without changing their identities to form a coded message.

Believe it or not, cipher systems have been common since antiquity, and some are more complex and sophisticated than others. During WWII, the Germans used the Enigma Cipher Machine, which they thought to be unbreakable. Only after the war did it come out that the Allies had figured out the cipher and had been intercepting secret German communications throughout the war.

Over the course of history, criminals like rum runners during Prohibition and the Zodiac Killer that terrorized the Bay Area in the 60s and 70s have used cipher systems. Criminals usually employ home-brewed, simple substitution cipher systems. Those likely to use such ciphers are criminals involved in secret activities that involve incriminating records (think drug trafficking, loansharking, and illegal bookmaking). Imprisoned criminals also use cipher systems to communicate with supporters both inside and outside of prison.

Lesson I: Simple Substitution Ciphers:

The Caesar cipher is relatively basic – a substitution cipher named for its Roman origins. It involves writing two alphabets, one above the other. The lower alphabet is shifted by one or more characters to the right or left and is used as the cipher text to represent the plain text letter in the alphabet above it.

Plain Text
A     B     C      D     E     F     G     H     I     J     K     L     M     N     O     P     Q     R     S     T     U     V     W     X     Y     Z
B     C     D     E     F     G     H     I     J     K     L     M     N     O     P     Q     R     S     T     U     V     W     X     Y     Z     A
Cipher Text

In this example, the plain text K is enciphered with the cipher text L. The name ‘Dan Gordon’ would be enciphered as follows:

Plain Text:       D     A     N     G     O     R     D     O     N
Cipher Text:     E     B     O     H     P     S     E     P     O

This cipher’s pretty easy to break, but they can be made far more challenging (and secure) by using a keyword to scramble one of the alphabets. Keywords can be placed in the plain text, the cipher text or both, and any word can be used as a key if repeated letters are dropped. Here the word SECRETLY (minus the second E) is used as the plain text keyword.

Plain Text
S     E     C     R     T     L     Y     A     B     D     F     G     H     I     J     K     M     N     O     P     Q     U     V     W     X     Z
A     B     C     D     E     F     G     H     I     J     K     L     M     N     O     P     Q     R     S     T     U     V     W     X     Y     Z
Cipher Text

Getting the hang of it? Don’t forget that the cipher text may utilize numbers, symbols or even letter combinations to represent plain text characters.

Solving Simple Substitution Ciphers:

If the cryptanalyst knows which language the cipher was written in and has enough cipher text to work with, simple substitution ciphers can often be solved quickly and easily. Cryptanalysts use the following procedures when cracking an unknown cipher:

• The cipher text message is identified from other cipher text or plain text on the document
• The number of different cipher text characters or combinations are counted to determine if the characters or combinations represent plain text letters, numbers or both
• Each cipher text character is counted to determine the frequency of usage
• The cipher text is examined for patterns, repeated series, and common combinations

After these analyses are complete, the cryptanalyst starts to replace cipher text characters with possible plain text equivalents using known language characteristics. For example:

• The English language is composed of 26 letters, but the nine high-frequency letters E, T, A, O, N, I, R, S, and H constitute 70 percent of plain text
• EN is the most common two-letter combination, followed by RE, ER, and NT
• Vowels, which constitute 40 percent of plain text, are often separated by consonants
• The letter A is often found in the beginning of a word or second from last
• The letter I is often third from the end of a word

Using these and many other known language characteristics, a cryptanalyst can often decipher a simple substitution cipher with little difficulty.

To learn more, stay tuned for Cracking Codes: Part II…