Codebusters - Xenocrypt
On this page(11 sections)
A Xenocrypt is a Spanish message encrypted with a monoalphabetic substitution. The substitution alphabet comes from an English keyword: a K1 alphabet in Division B, and a K1 or K2 alphabet in Division C. Every plaintext letter always becomes the same cipher letter and the word spaces stay in place, so the solver's job is the same as for an Aristocrat with one difference: the plaintext is Spanish, so the letter frequencies, short words and word endings you rely on are different. Xenocrypts are on the 2027 list in both divisions, at every level.
What the rules cover
Both 2027 rules manuals list the Xenocrypt as its own code type, separate from Aristocrats and Patristocrats.
| Item | Division B (2027) | Division C (2027) |
|---|---|---|
| Regional | At most 1 Xenocrypt (3.e.iii) | At most 2 Xenocrypts (3.e.iv) |
| State and National | At least 1 and at most 2 (3.f.ii) | At least 2 and at most 3 (3.f.ii) |
| Alphabet | K1 English keyword alphabet | K1 or K2 English keyword alphabet |
| No self-decryption | No letter decrypts to itself (3.g) | Same (3.g) |
| Timed first question | Never; question 1 is always an Aristocrat (3.h) | Same (3.h) |
| Special bonus | Never eligible (3.i) | Never eligible (3.i) |
| Crib | None promised; Xenocrypts are not in the crib list (3.j) | Same (3.j) |
| Scoring | Error-based: 2 or fewer errors is full credit (4.d) | Same (4.d) |
Event format. Teams of up to 3 get about 50 minutes on a written or computer-based exam. Under the 2027 rules you bring writing utensils only: no notes, no binder and no calculator of any class. The packet is printed single-sided so the team can pull the pages apart, and each member gets a resource sheet that includes English and Spanish letter frequency tables (2.d). Those Spanish frequencies are the only reference you get for a Xenocrypt.
How the K1 cipher is built
Build the keyed row (a rearranged alphabet) from the keyword in three steps:
- Write the keyword from left to right. Skip any letter already written, so only the first copy of a repeated letter counts.
- Write the unused letters of A to Z in normal alphabetical order after the keyword.
- Number the 26 letters of the keyed row from 0 to 25.
In a K1 alphabet (the American Cryptogram Association definition the rules cite) the keyed row is the plaintext row and A to Z is the ciphertext row. Let K be the keyed row:
The no-self rule (3.g) comes from a mechanism you need to check. A letter encrypts to itself whenever the keyed row has the same letter as the normal alphabet at the same position. A short keyword leaves a long alphabetical tail, and that tail can sit exactly where the normal alphabet puts it. The keyword CASTLE makes the keyed row end in U V W X Y Z, in the same six positions as in A to Z, so U through Z would encrypt to themselves and the key is illegal. A keyword such as WIZARD moves every letter, so no letter maps to itself.
Encryption step by step
- Build the keyed row from the keyword.
- Write the Spanish plaintext in capitals and keep the word breaks.
- Find each plaintext letter in the keyed row and note its position.
- Write the A–Z letter at that position.
- Check that no cipher letter equals its plaintext letter.
Worked example. Keyword WIZARD, plaintext QUE EL MAR ES AZUL ("that the sea is blue").
Cipher 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
Plain W I Z A R D B C E F G H J K L M N O P Q S T U V X Y
| Plain letter | Position in keyed row | Cipher letter (A–Z at that position) |
|---|---|---|
| Q | 19 | T |
| U | 22 | W |
| E | 8 | I |
| L | 14 | O |
| M | 15 | P |
| A | 3 | D |
| R | 4 | E |
| S | 20 | U |
| Z | 2 | C |
Encrypting word by word gives TWI IO PDE IU DCWO. The full keyed row has no position where its letter matches A to Z, so the key is legal.
Decryption step by step
- Find the cipher letter's position in A–Z.
- Write the keyed-row letter at that position.
- Keep the word breaks and read the result. A correct decryption reads as Spanish.
Worked example. Keyword WIZARD, ciphertext OD XBFD IU GIOOD.
| Cipher letter | Position in A–Z | Plain letter |
|---|---|---|
| O | 14 | L |
| D | 3 | A |
| X | 23 | V |
| B | 1 | I |
| F | 5 | D |
| I | 8 | E |
| U | 20 | S |
| G | 6 | B |
OD = LA, XBFD = VIDA, IU = ES and GIOOD = BELLA, so the message is LA VIDA ES BELLA.
K2 Xenocrypts (Division C)
A K2 alphabet flips which row carries the keyword. The plaintext row is the normal alphabet A to Z, and the keyed row (keyword, then the unused letters in order) is the cipher row written under it. The test writer may also slide the keyed row along by any amount, wrapping around the end. See the K2 Aristocrat guide for the full construction.
The slide matters because of the no-self rule. An unslid K2 row whose keyword has no Z always ends in Z, directly under plain Z, so Z would encrypt to itself. Expect K2 Xenocrypts to use a shifted row.
Worked example. Keyword JUNGLE gives the keyed row J U N G L E A B C D F H I K M O P Q R S T V W X Y Z. Slide it one place to the right, so Z wraps to the front, and write it under A to Z:
Plain 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 Z J U N G L E A B C D F H I K M O P Q R S T V W X Y
No column has the same letter twice, so the key is legal. To encrypt EL SOL SALE POR EL ESTE ("the sun rises in the east"), read straight down: E → G, L → F, S → Q, O → K, A → Z, P → M, R → P, T → R. The ciphertext is GF QKF QZFG MKP GF GQRG. To decrypt, find each cipher letter in the lower row and read the letter above it: G sits under E, F under L, and so on back to the Spanish.
Solving under tournament conditions
You have no keyword, so you solve from the ciphertext alone. The work is the same kind of cryptanalysis as an Aristocrat, aimed at Spanish.
Use the Spanish frequency table, not the English one
In Spanish text E and A are the most common letters, with O close behind, then S, R and N. Those letters carry most of the message, so the most frequent cipher letter is a good first guess for E or A. Count each cipher letter by hand (there is no calculator, and you do not need one), rank the counts, and treat the top letters as candidates. Confirm a guess with word shapes before you build on it.
Use Spanish words and patterns
Short words are the fastest way in. One-letter Spanish words are A, E, O, U and Y. Common two-letter words include EL, LA, DE, ES, SE, NO, UN and LO. Three-letter words to test first are DEL, LOS, LAS and QUE, and PERO, COMO and UNA appear constantly.
Q is almost always followed by U. If one cipher letter is always followed by the same second letter, test that pair as QU and check that a vowel comes next. Endings help too: -CION, -MENTE, -ADO and -IDO are regular enough to test quickly. Gender and number endings (-O, -A, -OS, -AS) link neighboring words, so a guess that makes two adjacent words agree is stronger than one that fixes a single word. The most common doubled letters are LL and RR; CC (as in ACCION) and EE (as in LEER) also occur.
Use the structure of the English keyword
The keyed row starts with the keyword and ends with the unused letters in alphabetical order. In a K1 key, once you know the plaintext for cipher A, B, C and onward, you are reading the keyword off the front of the row. In the tail, consecutive cipher letters decode to letters that climb alphabetically, skipping keyword letters. Once a few tail letters are confirmed, fill the gaps in alphabetical order and test the filled pairs against the message. If the front letters spell an English word, every remaining pair follows from it.
Apply the no-self rule to every guess
Before you write a guess into the table, check that it does not make a letter stand for itself. A guess that does is impossible, so erase it.
Work as a team
Keep one table of firm pairs and pencil tentative ones beside it. Pages can be separated (3.d), so on a State exam with two or three Xenocrypts, members can take different messages.
Scoring and time
Xenocrypts use the default error rule (4.d). Two or fewer errors in the decoded plaintext earn full credit, and each additional error costs 100 points, never going below 0.
Xenocrypts never earn the timing bonus, which belongs only to the first-question Aristocrat (3.h, 4.f), and they are never special bonus questions (3.i).
Common pitfalls
Using English frequencies on Spanish text
In English, T and H are common. In Spanish the common letters are different, so a guess built on English frequency fails quickly. Use the Spanish column on the resource sheet.
Assuming K1 in Division C
A Division C Xenocrypt may use K2. If your partial table never shows the keyword in the plaintext letters, sort it by plaintext instead and look in the cipher letters.
Breaking one-to-one consistency to force a word
Each cipher letter stands for one plaintext letter, and each plaintext letter has one cipher letter. A word that needs a second mapping for the same letter is wrong, even if it looks like Spanish.
Expecting a crib
The rules promise a placed crib only for certain cryptanalysis types, and Xenocrypts are not among them. Build the table from frequencies and word patterns.
How tournaments test this
The rules fix how many Spanish messages each exam holds. Division B has at most one Xenocrypt at Regional and one or two at State and National. Division C has up to two at Regional and two or three at State and National. At State and National these are minimums as well as maximums, so a team that cannot read Spanish ciphertext gives up at least one full question in B and at least two in C.
Practice
Solve these without the key first. The keyword and answer follow each ciphertext.
References and further reading
- Science Olympiad: the 2027 Codebusters rules manual for Divisions B and C (sections 2.d, 3.e to 3.j, 4.d to 4.g).
- SciOly Wiki Codebusters page: tournament history and past exams. Use it for context only.
- American Cryptogram Association: the source of the K1, K2 and K3 keyed alphabet conventions the rules refer to.
- Letter frequency, Wikipedia: background on English and other language frequency tables, for comparison with the Spanish column on the resource sheet.