GC1XZB8 THE NIHILIST CIPHER (Unknown Cache) in Tennessee, United States created by KNOW FUTURE

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GC1XZB8 ▼

A cache by KNOW FUTURE Message this owner

Hidden : 8/30/2009

Difficulty:

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Size: Size: micro (micro)

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Geocache Description:

Joint Intelligence Training Center
(J.I.T.C.)

Cryptography 305: Field Ciphers

Laboratory Session 9: THE NIHILIST CIPHER

I. Objective

This exercise demonstrates how an ancient cipher (the Polybius Square) was modified to create a classic field cipher -- the Nihilist cipher. In this exercise the student will learn to encrypt and decrypt messages using both the Polybius Square and the Nihilist ciphers.

II. Definitions

Ciphertext (CT): the encrypted version of a message that the sender wishes to transmit to the receiver.

Decipher (decrypt, decode): to convert a ciphertext message to plain text.

Encipher (encrypt, encode): to convert a plaintext message to ciphertext.

Fractionate: to divide or separate into parts; break up

Key (keyword, keytext): in cryptography, the key is a piece of information (a parameter) that determines the functional output of a cryptographic algorithm or cipher. Without the key, an algorithm will have no result. In encryption, the key specifies a particular transformation of plaintext into ciphertext, or vice versa during decryption.

Plaintext (PT): In cryptography, plaintext (also called cleartext) is the information which the sender wishes to transmit to the receiver. The plaintext is used as input to an encryption algorithm. The output is termed ciphertext.

IV. The Polybius Square Cipher

Polybius was an ancient Greek historian and scholar. He designed the cipher bearing his name to fractionate plaintext characters so that they can be represented by a smaller set of symbols. The 6 x 6 grid in Figure 1, for example, represents all the letters of the alphabet plus all the numbers using two-letter combinations of the numbers 1 through 6. Each letter and number is represented by its co-ordinates in the grid. Thus "MEET" is enciphered as 31-15-15-42.

Figure 1: 6 x 6 Polybius Square grid

Polybius did not originally conceive of his device as a cipher, but rather as an aid to telegraphy. He suggested the symbols could be used as visual signals by holding pairs of flags in various arrangements. Polybius used a 5 x 5 grid because the Greek alphabet had 24 letters.

Figure 2: Message enciphered with Polybius Square

V. The Nihilist Cipher

The Nihilists were a group of anti-czarists who, in the 1880s, sought to overthrow the Russian monarchy by means of sabotage, terrorism and assassination. Lacking the ideology of Marx and the iron discipline of Lenin, they were never as well organized as the Bolesheviks. To protect themselves from the Okhrana, the Czar's secret police, the Nihilists developed their own cipher based on the Polybius Square and a keyword. The term Nihilist cipher is sometimes extended to include several improved algorithms used much later for communication by the Soviet First Chief Directorate (KGB) with its spies.

The original Nihilist cipher uses the Polybius square to convert both the plaintext and a keyword to a series of two-digit numbers. These numbers are then added (Figure 3) to generate the ciphertext. The key is repeated as required.

VI. Enciphering a Nihilist Message

With any cipher, encryption and decryption is easiest using a worksheet designed especially for the purpose. You can view and print an encryption worksheet for this exercise by clicking here. Follow the steps listed below to encrypt a message.

Figure 3: Message encrypted using the Nihilist cipher

1. Enter the first letter of the plaintext message (M) in the first box of line 1.
2. Enter the Polybius equivalent of M (31) in first box of line 2.
3. Enter the first letter of the keyword (F) in the first box of line 3.
4. Enter the Polybius equivalent (or Additive) of F (16) in first box of line 4.
5. Sum the Polybius and Additive values (31 + 16) to determine the ciphertext value (47).
6. Repeat with the next letter of the plaintext message.

VII. Deciphering a Nihilist Message

To decipher a Nihilist message follow the steps listed above, except in reverse order. You can view and print a decryption worksheet for this exercise by clicking here. Follow the steps listed below to decrypt a message.

Figure 4: Message decrypted using the Nihilist cipher

1. Enter the first number of the ciphertext message (47) in the first box of line 1.
2. Enter the first letter of the keyword (F) in the first box of line 2.
3. Enter the Polybius equivalent (or Additive) of F (16) in first box of line 3.
4. Subtract the Additive from the Ciphertext (47-16) to determine the Polybius value (31) in first box of line 4.
5. Use the Polybius square to determine the plaintext equivalent of 31 (M) in first box of line 5.
6. Repeat with the next letter of the ciphertext message.

VIII. Things to Think About

The Polybius Square has obvious weaknesses. First, it is susceptible to < span style="font-style: italic;"> frequency analysis. Second, no number in an encrypted message will exceed 66. This betrays that the cipher is based on a 6 x 6 grid.

By adding a keyword the Nihilist cipher addresses both weaknesses of the Polybius Square. First, it defeats the frequency analysis since, for example, the E-E in 'MEET' enciphers as two different numbers. Second, adding the key disguises the 6 x 6 grid by producing numbers that will occasionally exceed 100.

IX. Laboratory Exercise

Your fellow agent SUPREME sends you the following Nihilist enciphered message:

Figure 4: Secret message from SUPREME

Follow these steps to complete the assignment:

1. Discover the keyword. It is clearly visible on a sign attached to the building at the posted coordinates (latitude 36 degrees 27.883 minutes north, longitude 86 degrees 39.546 minutes west).

2. Decipher the message in Figure 4 by applying the keyword to the 6 x 6 Polybius square in Figure 1.

3. The deciphered message reveals the location of a small metallic capsule. The capsule contains only a logsheet.

4. Sign the logsheet and return the capsule to its original hiding place. Be sure to bring your own pen or pencil.

5. Log your find at the Geocaching.com website.

Please note that it is not necessary to visit the posted coordinates to view the keyword. It may be helpful to click here to view a short video that will help you interpret the message.

Figure 5: A final note

Additional Hints (Decrypt)

TBBTYR ZNCF FGERRG IVRJ

Decryption Key

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

(letter above equals below, and vice versa)

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THE NIHILIST CIPHER Mystery Cache

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