The
DRYAD Numeral Cipher/Authentication System (KTC 1400 D) is a simple, paper cryptographic system currently in use by the
U.S.The United States of America is a federal constitutional republic comprising fifty states and a federal district...
military for
authenticationAuthentication is the act of establishing or confirming something as authentic, that is, that claims made by or about the subject are true...
and for
encryptionIn cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
of short, numerical messages. Every unit with a radio is given a set of DRYAD code sheets. A single sheet is valid for a limited time (e.g. 6 hours), called a
cryptoperiodA cryptoperiod is the time span during which a specific cryptographic key is authorized for use. Common government guidelines range from 1 to 3 years for asymmetric cryptography, and 1 day to 7 days for symmetric cipher traffic keys....
.
A DRYAD code sheet contains 25 lines or rows indexed by the letters in a column on the left of the page.
The
DRYAD Numeral Cipher/Authentication System (KTC 1400 D) is a simple, paper cryptographic system currently in use by the
U.S.The United States of America is a federal constitutional republic comprising fifty states and a federal district...
military for
authenticationAuthentication is the act of establishing or confirming something as authentic, that is, that claims made by or about the subject are true...
and for
encryptionIn cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
of short, numerical messages. Every unit with a radio is given a set of DRYAD code sheets. A single sheet is valid for a limited time (e.g. 6 hours), called a
cryptoperiodA cryptoperiod is the time span during which a specific cryptographic key is authorized for use. Common government guidelines range from 1 to 3 years for asymmetric cryptography, and 1 day to 7 days for symmetric cipher traffic keys....
.
A DRYAD code sheet contains 25 lines or rows indexed by the letters in a column on the left of the page. Each row contains a random
permutationIn several fields of mathematics the term permutation is used with different but closely related meanings. They all relate to the notion of mapping the elements of a set to other elements of the same set, i.e., exchanging elements of a set.- Definitions :The general concept of permutation can be...
of the letters A through Y. The letters in each row are grouped into 10 columns labeled 0 through 9. The columns under 0, 1, 2 and 5 have more letters than the other digits, which have just two each.
While crude, the DRYAD Numeral Cipher/Authentication System has the advantage of being fast, relatively easy and requires no extra equipment (such as a pencil). The presence of more cipher-text columns under the digits 0, 1, 2 and 5, is apparently intended to make ciphertext
frequency analysisIn cryptanalysis, frequency analysis is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers....
more difficult. But much of the security comes from keeping the cryptoperiod short.
DRYAD can be used in two modes, authentication or encryption.
Authentication
For authentication, a challenging station selects a letter at random from the left most column followed by a second (randomly selected) letter in the row of the first chosen letter. The station being challenged would then authenticate by picking the letter directly below the row and position of the second letter selected.
For example, using the example cipher sheet to the right, Victor could challenge Peggy by transmitting the letters "Alpha" and "Bravo". Peggy's correct response would then be "Yankee".
Another form used involves selecting the third letter to the right of the second letter chosen by the challenging station (Victor's "Bravo" letter). Both the directional offset (up, down, left or right) and numeral offset can be different values then the examples given here; but must be agreed upon and understood by both parties before authentication. The offset number can even remain secret between Victor and Peggy creating a
Two-factor authenticationAn authentication factor is a piece of information and process used to authenticate or verify the identity of a person or other entity requesting access under security constraints. Two-factor authentication or is a system wherein two different factors are used in conjunction to authenticate...
. Authentication then would require something you have (the DRYAD cipher sheet) and something you know (the secret offset value). Oscar, who is attempting to penetrate the network by posing as Peggy, would both need a copy of the DRYAD cipher sheet as well as knowledge of secret offset value.
One problem presented is that Oscar has a one in 25 chance of guessing the correct response. A solution to this is for Victor to require Peggy to authenticate twice; lowering Oscar's odds of guessing the correct response to one in 625 possible guesses. The downside to this method is reduced longevity of the current DRYAD page (since the page is getting twice as much use as a single-authentication scheme).
Encryption
The second mode is used to encrypt short numeric information (such as map coordinates or a new radio frequency). The coder selects two letters at random. The first selects a row in the current active page. The second letter is used as in the authentication mode, except the adjacent letter to the right is the one selected; and is called the "SET LETTER."
Numbers are enciphered one digit at a time. A
ciphertextIn cryptography, ciphertext is the result of the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. This result is also known as encrypted information...
letter is chosen from the selected row in the column under the plain text digit. If the digit occurs more than once in the number, the coder is instructed to choose a different letter in the same column. All the digits in a single plaintext number are encoded from the same row. (There is also a provision for encoding letters associated with map grid coordinates.)
See also
- M-94
The M-94 was a piece of cryptographic equipment used by the United States army, consisting of several lettered discs arranged as a cylinder. The idea for the device was conceived by Colonel Parker Hitt and then developed by Major Joseph Mauborgne in 1917...
— tactical cipher used in WWII
- BATCO
BATCO, short for Battle Code, is a hand-held, paper-based One-time pad encryption system used at a low, front line level in the British Army during the late Cold War period....
— similar paper based tactical cipher
- Dryad
Dryads are tree spirits in Greek mythology. In Greek drys signifies 'oak,' from an Indo-European root *derew- 'tree' or 'wood'. Thus dryads are specifically the nymphs of oak trees, though the term has come to be used for all tree nymphs in general...
— original meaning in mythology
- Polyalphabetic cipher
A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. The Vigenère cipher is probably the best-known example of a polyalphabetic cipher, though it is a simplified special case...
- Substitution cipher
In cryptography, a substitution cipher is a method of encryption by which units of plaintext are replaced with ciphertext according to a regular system; the "units" may be single letters , pairs of letters, triplets of letters, mixtures of the above, and so forth...
Sources