Historical Note
A compressed model of the real VENONA problem
This educational experience models the principal layers of the VENONA cryptographic problem. Processes that required teams of analysts and years of work have been compressed and simplified for gameplay.
Most Secret · Cipher Section
Prepare a transmission to Moscow
You are serving as a Soviet cipher clerk in New York during the summer of 1944. An intelligence officer has prepared a report that must be encoded and transmitted securely to Moscow.
You will prepare the text, convert it into numeric code groups and protect those groups using one-time-pad material.
Transmission rules
- Your report may contain up to 250 characters.
- Routing information is already supplied.
- Do not enter real personal, confidential or sensitive information.
- Names and unusual words will require special handling.
- You will complete part of the encryption process yourself.
This is a fictional scenario based on historical cryptographic methods.
Step 1 of 4 · Plaintext
Compose the intelligence report
Your assignment
A source using the covername FALCON reports unusual activity connected to a restricted American military project. Prepare a concise message describing what was observed, when it occurred and what Moscow should do next.
The event, source and intelligence in this scenario are fictional. You may write your own report or load the sample report.
Use ordinary English. Punctuation is allowed. The next stage will prepare the text for coding.
Step 2 of 4 · Message Preparation
Prepare the plaintext for coding
Before the numbers
The message cannot be enciphered exactly as written. The cipher clerk first standardizes the text, removes unsupported punctuation and determines which words have entries in the codebook.
Common terms receive a single four-digit code value. Words without a codebook entry must be transmitted using special spelling indicators.
Original report
Prepared plaintext
Codebook review
Each word has been checked against the game’s abbreviated operational codebook.
Step 3 of 4 · Codebook
Convert the words into code groups
The first layer of protection
A codebook replaces words and common phrases with unrelated numeric values. Someone who intercepts the numbers cannot simply read the message as ordinary language.
When a word is absent from the codebook, the clerk inserts a SPELL group, encodes each letter and then inserts an END SPELL group.
Clerk’s conversion worksheet
Review how each word was converted before the additive cipher is applied.
Complete coded text
This is still not the transmitted ciphertext. A one-time-pad additive must be applied next.
Step 4 of 4 · Additive Cipher
Apply the one-time pad
The second layer of protection
Each four-digit code group is combined with a four-digit group from a page of random pad material. The game uses digit-by-digit addition without carrying: add each pair of digits and keep only the final digit.
Example: 4827 + 3594 = 7311. The second digit is 8 + 5 = 13, so the result keeps only 3.
One-time additive page
Use once · destroy after encipherment
Complete the first three groups
Add each code group to its matching pad group. Enter four digits for each result.
Manual groups correct
You have demonstrated the procedure. The cipher section can now complete the remaining groups mechanically.
Transmitted ciphertext
The codebook values are now concealed beneath the pad additive.
Pad Destroyed
Phase One complete
Your encrypted message is ready for transmission to Moscow. From the cipher clerk’s perspective, every required security step has been completed correctly.
Transmission · 14 August 1944
Send the traffic
Circuit log
Encrypted groups are passed to the radio operator for transmission.
Receipt Confirmed
Moscow has acknowledged the message
The ciphertext has been received. The used pad page was destroyed in New York, and the matching copy in Moscow can be used to recover the coded text.
What the cipher clerk did not know
The pad was used once. The weakness was created elsewhere.
During pad production, part of the same random additive sequence was placed into another pad set. A second Soviet message was later enciphered with that duplicated material.
Arlington Hall now has two intercepted ciphertexts that overlap in key material. That does not reveal either message automatically—but it creates an opening that should not exist in a correctly produced one-time-pad system.
Phase Two Access · Arlington Hall
Report for cryptanalysis duty
Your next assignment
Continue as an Arlington Hall cryptanalyst. Compare the two intercepted messages, test possible readings and recover as much of your original report as the evidence supports.
Enter your email to unlock Phase Two and receive your final Arlington Hall Analyst File when the game is complete.
Unlock the duplicated-key cryptanalysis challenge.
Receive your score and recovered-message result.
Continue into the historical VENONA explanation.
Arlington Hall access record
Your email will be used to provide Phase Two access and deliver the Analyst File you request. Newsletter enrollment requires the separate optional selection above. See the Tradecraft Travel Privacy Policy.
Phase Two · Arlington Hall · 1946
The traffic has been paired
Analyst briefing
Welcome to Arlington Hall.
The two messages below were collected separately. Analysts now suspect that both were covered by the same additive sequence. Your task will be to exploit that relationship without pretending the evidence reveals more than it does.
The analytical rule
You are not solving a properly used one-time pad from one ciphertext. You are exploiting a production failure that caused two messages to share additive material. Even then, the result will be fragments, candidate readings and confidence judgments—not an instant full translation.
Phase Two · Case NY–742
Work the duplicated additive
Round 1 · Identify the opening
Why are these two messages worth pairing?
Several features are shared by the intercepts. Only one creates the cryptanalytic weakness that should not exist in a correctly produced one-time-pad system.
Round 2 · Test a crib
Recover the first additive groups
Formulaic language provides a foothold
Traffic analysis suggests that Message B begins with the routine phrase REPORT FROM SOURCE. Its reconstructed codebook values can be tested against the ciphertext.
Recover each additive group by subtracting the assumed code group from Message B, digit by digit, without borrowing. Example: 2 minus 7 becomes 5 because the arithmetic wraps through zero.
Additive recovered
The candidate phrase produces a consistent set of additive groups. Apply those groups to Message A to expose its underlying code values.
Round 3 · Interpret the groups
Assign the supported readings
The additive layer is only one obstacle. You have now exposed the first three underlying code values from Message A. Your task is to decide what each value represents in the partially reconstructed codebook.
Analyst’s frame of reference
These are no longer ciphertext or pad numbers. They are the recovered codebook-layer values in their original message order. Select the best supported meaning for each highlighted value. Indicators describe structure rather than ordinary words.
Example: a sequence beginning with 9901 is not read as a word. It tells the analyst to interpret the following letter groups until 9902 appears.
Opening fragment developed
Analysts can now extend the same formulaic crib across a limited run of groups. The result remains a partial recovery, not a complete translation.
Round 4 · Control the conclusion
What does the evidence actually support?
Arlington Hall partial recovery
Analyst File Complete