Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
Search NOVA Teachers

Back to Teachers Home

Secrets, Lies, and Atomic Spies

Classroom Activity

To break a code and explore the skills required in this form of analytical thinking.

Materials for each team
  • copy of "Puzzling Messages" student handout (PDF or HTML)
  1. Tell students they will be working to crack two coded messages. Organize students into teams and provide each student with a copy of the "Puzzling Messages" student handout.

  2. Have students do Part I of the activity by unscrambling the five words. The first word, TODAY, was chosen because it's easy to unscramble.

  3. Tell students that the unscrambled words hold the clue to the code. Have students look at the unscrambled words in relation to the code name and see if they can determine the pattern used to scramble the words.

  4. Once they have figured out the code, students can move onto decoding the second message in Part II of the activity. This message uses the same strategy, but a different code, and is more complicated because the words are not all the same length and more letters are exchanged.

  5. After students have decoded both messages, discuss the results. What about decoding the messages was easiest? What was the most difficult?

  6. To close the activity, ask students to describe what skills they believe were helpful in breaking the codes. List these on the chalkboard. From this listing, construct a questionnaire and have students present it to another class before students in the other class try the 1245 code task. After the code is broken, identify the codebreakers. Did the questionnaire successfully classify who would be the best at breaking the code?

  7. As an extension, have students code their own messages and exchange them with other students to decode.

Activity Answer

Codes are symbols, letters, or letter groups that represent whole words or concepts. Ciphers are messages in which letters or symbols replace real letters. Encrypting a cipher is done one letter at a time using a cipher code. Codes that are particularly difficult to break usually contain an additive. The process of stripping off the additive is known as cryptanalysis.

The following are some common terms in codebreaking, with an example of how each applies to coding.



Plain text: The text of the message.

attack at dawn

Base code: A list of number codes, usually five numbers long, that represent each word of text.

03960 (a)
74903 (at)
55148 (attack)
10263 (attacker)

Encoded message: A message that has been assigned base code numbers.

55148 (attack)
74903 (at)
69042 (dawn)
63012 (period)

Additive (or key): A number from a separate page of numbers is added to the code; this is called an additive or key.

87743 (additive)

Enciphered message: A completed message that includes an additive or key.

55148 (attack)
74903 (at)
69042 (dawn)
63012 (period)
87743 (additive)

Indicator: A number encoded in each message that tells the recipient which page of additive numbers to use from her key pad to decipher the message.

73112 (indicator for page in code book)
55148 (attack)
74903 (at)
69042 (dawn)
63012 (period)
87743 (additive)

The first message in the activity reads:

The 1245 code that is used to encode and decode consists of pairs of numbers that govern how the message is encoded and decoded, as well as information on how many letters make up each code word.

1/2 = The first and second letters are transposed.
4/5 = The fourth and fifth letters are transposed.
5 = The last digit in the code indicates the length of the letter group; the plain text message is broken into five-letter increments. The missing number 3 denotes that the third letter remains unchanged.

To make the message easy to decode, the letter group length is equal to the length of all of the words. Usually the words are not all the same length and the letter group may be larger or smaller than five.

The second message reads:

The 132547 code also consists of pairs of numbers that govern how the message is encoded and decoded; it just has one more set of numbers than the previous code:

1/3 = The first and third letters are transposed.
2/5 = The second and fifth letters are transposed.
4/7 = The fourth and seventh letters are transposed.
7 = The plain text message is broken into seven-letter increments. The missing number 6 denotes that the sixth letter remains unchanged.

The message is decoded by reversing the steps. The order of exchanges is also reversed; fourth and seventh, second and fifth, and finally, first and third. A Ø is added to fill the last group to seven letters. In the second message, words are different lengths so students will have to regroup the letters to see the plain text message.

In the second message, code groups are divided into arbitrary groups, not words. This code is much harder to break by inspection because students will have to regroup the letters to see the plain text message.

Links and Books


Budiansky, Stephen. Battle of Wits: The Complete Story of Codebreaking in World War II. New York: Free Press, 2000.
Tells how the United States and the United Kingdom broke Japanese and German codes.

Web Sites

NOVA Online—Secrets, Lies, and Atomic Spies
Provides program-related articles, interviews, interactive activities, resources, and more on the VENONA project.

VENONA—Soviet Espionage and the American Response 1939-1957
Provides access to American documents revealing key intelligence decisions of the period, as well as selected VENONA messages in translation.


The "Puzzling Messages" activity aligns with the following National Science Education Standards:

Grades 5-8

Science as inquiry

Science Standard G:
History and Nature of Science

Science as a human endeavor

  • Women and men of various social and ethnic backgrounds—and with diverse interests, talents, qualities, and motivations—engage in the activities of science, engineering, and related fields such as health professions. Some scientists work in teams, and some work alone, but all communicate extensively with others.

Grades 9-12

Science as inquiry

Science Standard G:
History and Nature of Science

Science as a human endeavor

  • Individuals and teams have contributed and will continue to contribute to the scientific enterprise. Doing science or engineering can be as simple as an individual conducting field studies or as complex as hundreds of people working on a major scientific question or technological problem. Pursuing science as a career or as a hobby can be both fascinating and intellectually rewarding.

Teacher's Guide
Secrets, Lies, and Atomic Spies

Video is not required for this activity

Support provided by