M-94
Encyclopedia
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. Officially adopted in 1922, it remained in use until 1945, replaced by more complex and secure electromechanical rotor machine
s, particularly the M-209
. The M-94 was also employed by the US Navy under the name CSP 488.
The device consisted of 25 aluminium
discs attached to a four-and-a-half inch long rod, each disc containing the 26 letters of the Roman alphabet in scrambled order around its circumference
(with the exception of the 17th disc, which began with the letters "ARMY OF THE US"). Each wheel had a different arrangement of the alphabet, and was stamped with an identifying number and letter; wheels were identified according to the letter following "A" on that wheel, from "B 1" to "Z 25". The wheels could be assembled on the rod in any order; the ordering used during encoding comprised the key
. There were 25! (25 factorial
) = 15,511,210,043,330,985,984,000,000 (more than 15 septillion) possible keys, which can be expressed as about an 84-bit key size
.
Messages were encrypted 25 letters at a time. Turning the discs individually, the operator aligned the letters in the message horizontally. Then, any one of the remaining lines around the circumference of the cylinder was sent as the ciphertext
. To decrypt, the wheels were turned until one line matched a 25 letter block of ciphertext. The plaintext
would then appear on one of the other lines, which could be visually located easily, as it would be the only one likely to "read."
A variant, called a strip cipher, had each scrambled alphabet, repeated twice, printed on a metal strip that could be slid back and forth in a frame (see photo).
The principle upon which the M-94/CSP-488 is based is at least as old as the 15th century, when the revolving wheel cryptograph was envisioned by Leone Battista Alberti
. Thomas Jefferson
independently invented a similar device in 1790, which had 36 disks.
Wheel ciphers could be broken, even in World War II
, if enough ciphertext was intercepted. However, this took time and specialized skills, so the M-94 was still good enough for tactical communications. The DRYAD cipher currently in use by the U.S. military is not much more sophisticated.
United States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...
, consisting of several lettered discs arranged as a cylinder
Cylinder (geometry)
A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given line segment, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...
. The idea for the device was conceived by Colonel Parker Hitt and then developed by Major Joseph Mauborgne
Joseph Mauborgne
In the history of cryptography, Joseph Oswald Mauborgne co-invented the one-time pad with Gilbert Vernam of Bell Labs. In 1914 he published the first recorded solution of the Playfair cipher...
in 1917. Officially adopted in 1922, it remained in use until 1945, replaced by more complex and secure electromechanical rotor machine
Rotor machine
In cryptography, a rotor machine is an electro-mechanical device used for encrypting and decrypting secret messages. Rotor machines were the cryptographic state-of-the-art for a prominent period of history; they were in widespread use in the 1920s–1970s...
s, particularly the M-209
M-209
In cryptography, the M-209, designated CSP-1500 by the Navy is a portable, mechanical cipher machine used by the US military primarily in World War II, though it remained in active use through the Korean War...
. The M-94 was also employed by the US Navy under the name CSP 488.
The device consisted of 25 aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
discs attached to a four-and-a-half inch long rod, each disc containing the 26 letters of the Roman alphabet in scrambled order around its circumference
Circumference
The circumference is the distance around a closed curve. Circumference is a special perimeter.-Circumference of a circle:The circumference of a circle is the length around it....
(with the exception of the 17th disc, which began with the letters "ARMY OF THE US"). Each wheel had a different arrangement of the alphabet, and was stamped with an identifying number and letter; wheels were identified according to the letter following "A" on that wheel, from "B 1" to "Z 25". The wheels could be assembled on the rod in any order; the ordering used during encoding comprised the key
Key (cryptography)
In cryptography, a key is a piece of information that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa...
. There were 25! (25 factorial
Factorial
In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n...
) = 15,511,210,043,330,985,984,000,000 (more than 15 septillion) possible keys, which can be expressed as about an 84-bit key size
Key size
In cryptography, key size or key length is the size measured in bits of the key used in a cryptographic algorithm . An algorithm's key length is distinct from its cryptographic security, which is a logarithmic measure of the fastest known computational attack on the algorithm, also measured in bits...
.
Messages were encrypted 25 letters at a time. Turning the discs individually, the operator aligned the letters in the message horizontally. Then, any one of the remaining lines around the circumference of the cylinder was sent as the 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...
. To decrypt, the wheels were turned until one line matched a 25 letter block of ciphertext. The plaintext
Plaintext
In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties....
would then appear on one of the other lines, which could be visually located easily, as it would be the only one likely to "read."
A variant, called a strip cipher, had each scrambled alphabet, repeated twice, printed on a metal strip that could be slid back and forth in a frame (see photo).
The principle upon which the M-94/CSP-488 is based is at least as old as the 15th century, when the revolving wheel cryptograph was envisioned by Leone Battista Alberti
Leone Battista Alberti
Leon Battista Alberti was an Italian author, artist, architect, poet, priest, linguist, philosopher, cryptographer and general Renaissance humanist polymath...
. Thomas Jefferson
Thomas Jefferson
Thomas Jefferson was the principal author of the United States Declaration of Independence and the Statute of Virginia for Religious Freedom , the third President of the United States and founder of the University of Virginia...
independently invented a similar device in 1790, which had 36 disks.
Wheel ciphers could be broken, even in World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, if enough ciphertext was intercepted. However, this took time and specialized skills, so the M-94 was still good enough for tactical communications. The DRYAD cipher currently in use by the U.S. military is not much more sophisticated.