DRYAD
Encyclopedia
The DRYAD Numeral Cipher/Authentication System (KTC 1400 D) is a simple, paper cryptographic system currently in use by the U.S.
military for authentication
and for encryption
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 cryptoperiod
.
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 permutation
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 analysis
more difficult. But much of the security comes from keeping the cryptoperiod short.
DRYAD can be used in two modes, authentication or encryption.
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 authentication
. 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).
Numbers are enciphered one digit at a time. A ciphertext
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.)
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
military for authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
and for encryption
Encryption
In 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 cryptoperiod
Cryptoperiod
A 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....
.
Permutation
In mathematics, the notion of permutation is used with several slightly different meanings, all related to the act of permuting objects or values. Informally, a permutation of a set of objects is an arrangement of those objects into a particular order...
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 analysis
Frequency analysis
In 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 authentication
Two-factor authentication
Two-factor authentication is an approach to authentication which requires the presentation of two different kinds of evidence that someone is who they say they are. It is a part of the broader family of multi-factor authentication, which is a defense in depth approach to security...
. 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 ciphertext
Ciphertext
In cryptography, ciphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher...
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-94M-94The 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 - BATCOBATCOBATCO, short for Battle Code, is a hand-held, paper-based encryption system used at a low, front line level in the British Army during the late Cold War period....
— similar paper based tactical cipher - DryadDryadDryads are tree nymphs 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 cipherPolyalphabetic cipherA 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 cipherSubstitution cipherIn 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
- Further explanation of the DRYAD Numeral Cipher/Authentication System
- U.S. Army Field Manual FM 24-35