Updated July 01, 2025
WHAT IS CRYPTOGRAPHY?
Cryptography is the science that studies methods used to hide messages in a way that the messages’ content can only be revealed to the intended recipient. The word ‘cryptography’ originated from two Greek words ‘krypto’ which means hidden and ‘graphene’ which means writing.
Encryption is the method by which an original text or “plaintext”, today known as “data”, is transformed into a “ciphertext”, that prevents third parties from discerning its content.
Decryption is the process of returning ciphertext back into the original plaintext.
A cipher is a secret or disguised way of writing, or today referred to as “code”.
Ciphertext is the text that has been written in code.
CRYPTOGRAPHY’S BEGINNINGS
The first discovered use of encryption can be traced back to 1,900 BC ancient Egypt, almost 4,000 years ago, when the tomb of nobleman Khnumhotep II was discovered to contain a script recording his life’s accomplishments. The scribe used some unusual hieroglyphic symbols in place of more ordinarily used symbols, which seemed to obscure the original meaning of the text. The script was not a form of secret writing, but it contained a transformation of the original text, and is the oldest known text to do so.
It is believed that the use of encryption in ancient Egypt was mainly to protect knowledge, as education was a privilege, and was limited to the highest circles of society and was also a way to show one’s skills in writing. It was also used for religious reasons, for example, to discuss taboos. So, perhaps the purpose of the Khnumhotep II script was to demonstrate his superior intellect. [1]
Around 400 BC, over 2,400 years ago, the ancient Greek Spartans used the first cipher device, called a “scytale”, for secret communications between military commanders in the field. This device was a small cylinder, around which an inscribed strip of parchment or leather could be wrapped. The intended recipient would receive a strip of parchment or leather with various jumbled letters inscribed on it, and when it was wrapped around a scytale, it would reveal the original plaintext. If the sender and recipient had the same length and diameter scytale, the message could be quickly read. If the message got into the wrong hands, it just looked like jumbled up letters. This is the first example of a transposition cipher. [2]
In 100 BC, 2,000 years ago, Julius Caesar was known to use a form of encryption called the Caesar Cipher, to secretly communicate with his army generals in the battle fields. The cipher was known as a shift cipher or substitution cipher, because it simply shifted letters by three spaces, substituting one letter for another. For example: A=D, B=E, C=F, D=G, etc. The characters at the end would start again at A, so ‘X’ would be replaced by ‘A’, etc. It is also considered a monoalphabetic cipher, because it used only one alphabet. This may seem like a simple cipher to break, but most peopleduring these times were illiterate, so any level of encryption was much stronger in Caesar’s time. [3]
VIGENÈRE CIPHER DISK
The Vigenère cipher disk was named for Blaise de Vigenère, even though it was invented in 1467 by Leon Battista Alberti, 56 years before Vigenère was even born. This seems to be the beginning of a repeating tradition of cipher devices being credited to the wrong inventor.
Alberti was considered the Father of Western Cryptology. He not only invented the Vigenère cipher disk method, he also invented polyalphabetic or polymath ciphers, which he called “worthy of kings” and claimed were unbreakable. This was also the start of another tradition of cipher inventors, claiming the invincibility of their invention, only to be proven wrong in every case except the One-Time Pad, which you will read more about below.
The Alberti cipher used a single mixed alphabet and a cipher disk device for encryption and decryption. Vigenère’s improvement introduced the use of a repeating keyword or phrase to determine shifts, making it a more secure polyalphabetic cipher compared to monoalphabetic ciphers like the Caesar cipher. A polyalphabetic cipher uses more than one monoalphabetic cipher arrangement within each message. The more cipher arrangements used, the stronger the cipher. This may seem to be a straightforward solution to thwart letter frequency analysis, so why did this solution take 500 years? The problem with polyalphabetic ciphers is the complexity of using multiple cipher arrangements and the high probability of introducing human errors. The beauty and power of the Vigenère cipher disk was the ability to make polyalphabetic ciphers user friendly.
Two other methods to make polyalphabetic ciphers easier to use are the “Playfair” cipher and the electro-mechanical “Rotor” ciphers, which will both be described below.
The Vigenère disk is made up of two concentric disks each with the 26-letter alphabet written on them. The outer disk in the normal sequence and the inner disk has the alphabet usually in reverse order but could be in any order. The inner disk rotates. The outer disk usually represents the plaintext and the inner disk represents the enciphered text.
The original disks used individual letter substitutions and the sender and receiver had to agree to an index letter as the starting position of the cipher disk. A later method of using the Vigenère disk used a keyword or phrase, so that the disk is rotated after each letter of a message based on that keyword or phrase.
The significance of polyalphabetic ciphers is that each letter is represented by several different letters in the ciphered message. Every time the disk is rotated during the encipherment of a single message, a new cipher alphabet is created. Letter frequency analysis will no longer work. This was an astounding insight and solution, which can be baffling to decipher, but the cipher is not unbreakable. The method used to break the cipher is to decipher the keyword along with the ciphertext and to exploit the recurrence of the keywords with letter frequency analysis. If several messages are sent with the same keyword, then letter frequency analysis will work on the first letter in each message, the second letter, and so on.
Alberti made a third remarkable invention in cryptology, “Enciphered” code. The difference between a code and a cipher is the cipher changes a message on a letter-by-letter basis while a code will substitute a few numbers or a word for different words or phrases.
Alberti had the numbers 1-4 on his disk and used codes from 11 to 4444 to replace entire phrases. For example, 324 may mean “the ships are ready to sail.” Rather than send 324 as part of the message, he recommended using the cipher disk to encipher the code. This represents a very strong cipher system, which would be difficult to decipher even if you captured the codebook. Codes became popular, but it was 400 years later, at the end of the 19th century, before the practice of enciphering codes was widely adopted.
Blaise de Vigenère had nothing to do with the famous cipher disk that bears his name. He did, however, make a major contribution to cryptology by inventing “Autokey”. Like Alberti, he recommended the sender and receiver agree to an index letter as the starting position of the cipher disk. After the first letter is enciphered, he then proposed using the first plaintext letter as the next letter to align as the index letter, and continuing in this manner through the entire message, changing the disk rotation after every letter of the message.
This is an advantage over repeating keywords, because a cryptanalyst would not be able to exploit the cyclic nature of the keyword. Also, this avoids the problem of changing and distributing keywords. Autokey is a cipher strategy still in use during modern times. It is ironic that even though Vigenère does not deserve credit for his namesake, the credit for his stronger method of encipherment is largely ignored. [4]
CIPHER WHEEL
US President Thomas Jefferson invented the Wheel Cypher in 1795. It was so far ahead of its time, that this type of cipher was still in active use in the US military 150 years later at the beginning of WWII. Only one original Jefferson cipher wheel is known to exist, which is in the NSA cipher museum in Ft. Meade, Maryland, USA.
The Jefferson cipher had 36 wheels arranged around an axle with a different random alphabet printed on the outside of each wheel. This is a polyalphabetic cipher and at first pass it may seem to be similar in function to a Vigenère disk, with a 36-character keyword, but it is not. Each of the 36-character Jefferson wheels has a different substitution alphabet, unlike the Vigenère wheel, which has one 26 letter moving wheel and one 26 letter stationary wheel. Also, the Jefferson wheels were arranged simultaneously instead of serially, so this type of device is usually called a multiplex cipher. You could keep up secure correspondence with many others by having a different wheel arrangement or completely different wheels with each person.
The key is not a repeating keyword, but the order of the 36 wheels on the axle, each wheel having a different cipher. There are 36 x 35, x 34, x 33, x 32, x 31, … x 2 x 1 (= 36 or 36 factorial) ways to arrange 36 wheels on an axle, which is 3.72 x 1041. Jefferson calculated this number exactly, calling it “372 with 39 cyphers [zeroes] added to it.” This compares favorably to the three-rotor Enigma machine, to will be discussed below, which has a key space of 1023.
In 1802, then-President Jefferson sent a cipher to the US Ambassador to France Robert R. Livingston, writing: “I send you a cipher to be used between us, which will give you some trouble to understand, but, once understood, is the easiest to use.” Jefferson also sent Merriweather Lewis a square table cipher in 1803 to use on his expedition with William Clark,
to explore land for the Louisiana Purchase. [5]
In 1891, just before WWI the Cipher Wheel was invented by French Lt. Etienne Bazeries and then independently invented by US Army Lt. Parker Hitt in 1912. Neither inventor realized that the original inventor of this technology was the third US President, Thomas Jefferson, who invented this in 1795, 96 years and 117 years prior, respectively.
Parker Hitt transformed his cipher wheel into a tablet form with sliding strips of the mixed alphabet to replace the wheels. Joseph Mauborgne’s wheel version, which likely saw limited use by the military attachés in WWI, was finally officially accepted into the military in 1922 and was used until 1943. [6]
PLAYFAIR CIPHER
In 1854, Sir Charles Wheatstone invented the Playfair Cipher, named for his friend Lyon Playfair, first Baron Playfair of St. Andrews, UK, who popularized and promoted the cipher. Its simplicity and its cryptographic strength compared to simple substitution and Vigènere cipher made it an immediate success as a field cipher, used by the British in the Boer War and by allied forces in WWI, and by several armed forces as an emergency back-up cipher in WWII.
Wheatstone’s cipher was considered one of the more sophisticated battlefield ciphers used during WWI, and was a diagraphic cipher. This means that the encipherment is performed on pairs of letters instead of one letter at a time. The advantage is that frequently used letters are thus hidden from decryption using letter frequency analysis. For instance, “TE” may encipher to “HN” and “TH” may encipher to “JZ”, so the frequently used letter “T” is coded into two different letters.
In WWII, the cipher was famously used when US Navy Lt. John F. Kennedy’s PT-109 was sunk by a Japanese cruiser in the Solomon Islands. Kennedy made it to shore on Japanese-controlled Plum Pudding Island and was able to send an emergency message in Playfair from an allied coast-watcher’s hut to arrange the rescue of the survivors from his crew.
The Playfair Cipher is susceptible to being broken by using frequency analysis on pairs of letters, however, there are 600 pairs of letters to analyze instead of the 26 letters of the alphabet. A book was written in 1914, by Lt. Joseph O. Maubourgne of the US Army on the method to decrypt the Playfair Cipher. This was the first book published by the US military on cryptology. Mauborgne would go on to fame as the US Chief Signals Officer and a Major General. [7] [8]
ONE-TIME PAD
One-Time Pad (OTP) was originally invented in 1882, and then reinvented several times throughout history. It was named such originally because it involved secret keys printed onto small sheets of paper that were then padded. The sender of the decoded message could be assured that the intended recipient would be able to decode the message so long as the recipient had the corresponding “key” in his padded book of keys. After each successful decoding, the recipient would then tear off and destroy the used key, limiting its use to just a single instance and at the same time revealing the key to be used for the next message. As long as the intended recipient had the corresponding key or pad, the message could be decoded.
When theruleswere strictly adhered to, the One-Time Pad ciphering method was successfully used during WW1, WWII and even during the Cold War, for the passing of critically important diplomatic and military communication.
In 1940, Claude Shannon, known as the “Father of Information Theory” proved mathematically that if the following four requirements of an OTP are met the key is unbreakable:
- the key must be truly random;
- the key must be as large as the plaintext;
- the key may never be reused in whole or part;
- the key must be kept secret.
Shannon called it “perfect secrecy” and further stated in the Bell System Technical Journal in 1949, “If properly used, One-Time Pads are secure in the sense even against adversaries with infinite computational power.”
ONE-TIME-TAPE TELETYPE SYSTEM
During WWI, in 1917, Gilbert S. Vernam, an engineer at Bell Telephone Laboratories invented the One-Time Tape Teletype System (OTT). This was a teletype-based cipher system in which a reel of perforated paper tape representing random letters was added to a plaintext message to create the ciphertext. On the receiving end, a duplicate reel of random letters was used to subtract from the ciphertext to re-create the plaintext message. The entire end-to-end functions were handled automatically, without the need for human intervention. This insured efficient and error free functions. It was not more widely used because of the cumbersome distribution and destruction of random letter tape reel keys. Cipher tapes were to be used only one time and then destroyed. Vernam did receive a patent for his invention which he called a Secret Signaling System.
TWENTIETH CENTURY ROTOR MACHINE
As is common in history, the rotor machine was invented more or less simultaneously in different parts of the world. In 1915, Theo A. van Hengel and Rudolf Spengler, produced working rotor-based cipher machines for the Dutch War Ministry (Ministerie van Oorlog). And in 1917 and 1918, there were inventions from Edward Hebern in the US, Arvid Damm in Sweden, and Hugo Koch in the Netherlands and Arthur Scherbius in Germany.
In 1915, two Dutch naval officers, Theo van Hengel and Rudolf Spengler came up with the idea of a wheel cipher while working in the Dutch East Indies. They built a prototype but the Dutch Navy decided not to adopt the cipher. Hengel and Spengler tried to patent the device but were prohibited by the Dutch Navy from publicizing their invention for fear of the technology being used by the enemy. The patent attorney they hired was the brother-in-law of another electric rotor inventor, Hugo Koch of the Netherlands, who did get his machine patented in 1919, but never manufactured a machine and ultimately gave his patents to Arthur Scherbius to use.
In 1918, German Arthur Scherbius patented and began manufacturing his mechanical rotor machine under the brand name Enigma, as a means to protect communications in the banking industry. However, at the onset of WWII the Enigma became the cipher machine of choice for the Axis Alliance which included Germany, Japan and Italy. Decades later, Germany produced thousands of Enigma machines.
The Enigma had sets of rotors, each rotor connecting to a set of wires. It required two people to operate, one person to enter text on the Enigma’s keyboard and the second person to write down which of the 26 lights above the keyboard illuminated at each key stroke. When plaintext is entered, the illuminated letters are the ciphertext. Entering ciphertext transforms it back into readable plaintext. This is another example of a substitute cipher, where one letter is substituted by another letter. Each additional rotor used produced a different encoding scheme making decoding more complex. The key to decoding lay in learning the wiring pattern for each rotor. Knowing that this was a weak point, the Germans changed the settings on the machine each evening at midnight.
In 1917, American Edward Hebern, designed several cipher-related inventions, including an electro-mechanical device, that he coined Hebern. The Hebern rotor machine combined parts of a standard typewriter with parts of an electric typewriter. When a letter was typed, the rotor would rotate one space giving a new scrambling sequence. This scrambling sequence repeats after 26 letters of the message, which seems like a relatively weak cipher, but it was equivalent to other types of ciphers in use at the time. To decipher a message, the rotor is taken out of the machine and simply put back in backward. Hebern developed machines with three and 5 rotors, each rotor adding additional ciphering ability.
In 1925, Hebern actually sold a small number of his five-rotor devices to the US Army and Navy for evaluation. Over the following 6 years, the Navy purchased an additional 36 machines and tried but failed to convince the Army to purchase the same machines so that the Navy and Army could secretly exchange information.
The US Army never purchased additional Hebern machines and, in fact, disallowed these devices for use by the US military. Unbeknownst to Hebern, the reason for this strategic move, was that legendary US cryptanalyst, William F. Friedman had broken the code for the 5 rotor Hebern, because of its use of odometer-style rotor movements. This knowledge was then secretly exploited and used by other cryptologists to break similar machines, and was eventually useful in cracking the Enigma.
Eventually Friedman and fellow cryptanalyst Frank Rowlett invented a more complex cipher called the SIGABA, which had irregular rotor movements, rather than odometer-style movements and used 10 rotors to scramble the alphabet with another 5 to cause irregular stepping of the 10 rotors. This was used for high-level messages in WWII and was never broken by the enemy.
Friedman and Rowlett would also gain fame for breaking the Japanese PURPLE cipher, a 4- wheel rotor machine that was used by Japan during WWII to code diplomatic messages to various embassies throughout the world, without having the advantage of seeing the machine or speaking Japanese. Friedman would also go on to further cryptologic intrigue by travelling to Switzerland in the 1950s to make a deal with Crypto AG, a Swiss code making machine builder, so that the US could have access to the secrets of their cipher machines. Crypto AG’s machines were in use by over 100 nations and exploited by the US for the next four and a half decades. [9]
Interestingly, it was discovered in 1970 that Crypto AG had secretly been purchased by the German BND and the American CIA, in a project known as Operation THESAURUS, later renamed RUBICON. In 1994, the CIA became the sole owner, and in 2019 the company was dissolved. [10]
POLAND’S BOMBA AND BRITAIN’S BOMBE – BREAKING THE ENIGMA
In 1932, Polish mathematician Marian Rejewski deduced the wiring pattern inside the wheels of the Enigma, allowing him to draw up plans so that a military Enigma machine could be manufactured and used to better understand exactly what was needed to break Enigma’s code. Fellow Polish mathematicians Zygalski and Rozyki, discovered how to determine the German Enigma’s daily keys, which also helped in breaking the code. Their discoveries were based on pure mathematical analysis, some guess work and interestingly, the addition of a set of Enigma operating manuals and German Enigma settings for September and October 1932, that the French Secret Service received from a high-ranking German Cryptologist willing to sell secrets for cash. The French, realizing their cryptographers needed help, shared the information with the Polish. [11]
Eventually, the Poles could, through brute force, work through more than 17,000 possible solutions with a machine they built called the Bomba. Even though they now knew the wiring pattern, they still had to determine what the three letter key was. The key was the first letter position on each of the three rotors, which was set by the German cipher clerk, and it was changed with each message. The key was transmitted to the decipher clerk “in the clear”, or unenciphered two times. The decipher clerk could then set his rotors, followed by entering the next 6 letters in the cipher. The cipher clerks often used QWE, the first three letters on the keyboard, or the first three letters of names familiar to them. They often used HIT and the next 6 letters in sequence would be LERLER = HITLER. Or BER was often followed by LINLIN = BERLIN. One particular clerk often used his girlfriend’s name CIL followed by LIELIE = CELLIE. So, these easy to guess indicators became known as Cillies.
From 1932 until 1939, Poland was successfully deciphering messages sent from Germany’s the Enigma, but they kept their cryptanalytic secrets to themselves. In 1939, the Germans added two additional rotors to their machine, totaling 5, that they could rotate in at will. Although they were still only using three rotors, knowing which of the 5 rotors were being used, exponentially increased ciphering time and cipher possibilities. This overwhelmed the Polish cryptologists resources. In 1939, after reaching a ceiling in their ability to further encrypt German messages, the Poles shared their cryptographic findings with the British, who set up Ultra, a secret code-breaking group, headed by renowned mathematician Alan M. Turning and his Bletchley Park team.
Turing, playing a key role, along with fellow mathematician Gordon Welchman, invented a code breaking machine known as the Bombe. The Bombe was like having 36 Enigma machines linked together, which could simulate several Enigma machines at the same time, offering billions of possible decoding solutions. To help with the massive amount of time needed to decrypt the ever-increasing German messages, hundreds of Women’s Royal Naval Service (Wrens), were trained to operate the Bombe machines.
From mid-1940, German Air Force signals were being read at Bletchley Park, and the intelligence gained was making a considerable contribution to the war effort. The Bombe uncovered intelligence before the battle of El Alamein in 1942, that contributed to victory in this Egyptian campaign, which proved to be a turning point of the war. In 1943, German battleship Scharnhorst was located and sunk as a result of the Bombe’s deciphered messages. In 1944, the Bombe is credited with providing the details of German defensive preparations for and reaction to the D Day invasion. The Axis never did realize their codes had been broken. It is said that the Bombe saved millions of lives and ended the war at least two years earlier than if it had not been invented. [12]
BRITAIN’S COLOSSUS – BREAKING GERMAN LORENZ MESSAGES
The Colossus, considered to be the first digital, programmable, electronic computer ever built, was designed in 1943, by engineer Tommy Flowers, specifically to decipher encrypted messages transmitted by German Lorenz cipher teleprinters, which were Hitler’s message delivery system of choice between his generals during WWII.
Flowers delivered his machine to Bletchley Park in late December 1943, where it was assembled there by Harry Fensom and Don Horwood, and was operational in February 1944. Colossus greatly reduced the time to break the Lorenz settings and enabled more messages to be deciphered in a shorter period of time, changing the entire dynamic of Britain’s ability to break German code. By the end of the war, 63 million characters of high-grade German communications had been decrypted by 550 people helped by the ten Colossus computers. [13]Again, the Women’s Royal Naval Service operated many of Britain’s Colossus code-breaking machines. [14]
Colossus reduced the time to break Lorenz messages from weeks to hours. Most historians believe that the use of Colossus machines significantly shortened the war by providing evidence of enemy intentions and beliefs. The machine’s existence was not made public until the 1970s. [15]
BRITAIN’S TYPEX CIPHER MACHINE
British cipher machines, known as Typex were developed as early as 1934, by Wing Commander Oswyn G.W. Lywood together with Flight Lieutenant Coulson, Mr. E. W. Smith and Sergeant Albert Lemon, at the RAF’s wireless establishment at Kidbrooke, Southeast London. It was an adaptation of the commercial German Enigma. Similar to the Enigma and SIGABA, the Typex was also a rotor machine, but had 5 rotors that could be deployed. It is believed that the Typex was never broken, as it would have required the Germans to replicate and build both Britain’s Bombe and Colossus machines. The Typex rotors had detachable rotor cores which were known as ‘inserts’ allowing them to be switched between different rotors, and they could also be inserted in two different ways which doubled the mathematical output.
Numerous prototypes were made and rejected for various reasons, until early 1937, when the “Typex Mark I” was finally accepted and machines were manufactured. This model was used to equip the main RAF headquarters in London.
A much improved Typex Mark II was produced in May of 1937, and was by far the most widely used Typex model. The British Cipher Committee ordered 350 machines which were delivered in the summer of 1939 and were in operation in Admiralty and at Malta and Gibraltar by the end of the year. Typex Mark II was equipped with two printers for printing the plaintext and ciphertext version of each message. It was this model that was built in large numbers and the first contract for 350 machines was signed in 1938. Typex production increased rapidly, with 500 machines built by June 1940, 2,300 by the end of 1942, 4,078 by December 1943 and 5,016 by May 1944. By the summer of 1945 about 11,000 were produced, 8,200 of Mark II and 3,000 of the improved Mark VI. [16]
It became apparent in 1943, that the British and Americans required a secure method to communicate with each other, via cipher messages. To accomplish this goal, a set of conversion kits were produced for each machine and exchanged to allow a common standard cipher between Britain’s Typex and the US’ SIGABA. The overall converted Typex / SIGABA system was known as the CCM or Combined Cipher Machine. Britain’s new Typex with CCM adaptor fitted was known as the Typex Mark 23. The CCM machines each had five SIGABA cipher wheels, and were fully interoperable. The modified machines were used from November 1943 onwards. As far as we know CCM was never broken. [17]
UNBREAKABLE US SIGABA CODE MACHINE
The US Army’s SIGABA, called the ECM (Electric Cipher Machine) in the Navy, was the only machine system used during WWII to remain completely unbroken by an enemy. The Germans referred to the US machine, SIGABA, as the “Big” machine. It utilized the same principle of rotating, removable, wired rotor wheels that the German Enigma employed. However, unlike the stepping motion of the Enigma, the SIGABA/ECM’s motion appeared to be random. It wasn’t, but it was so complicated, the German’s never broke it, and the Japanese gave up trying. Frank Rowlett of the Army’s Signal Intelligence Service developed the complicated stepping motion. [18]
Before, during and after WWII, the US Army and Navy each developed their own cryptographic systems. Neither service shared their knowledge with the other one, except for limited cooperation in the field of enemy signals intelligence. The first exception to this rule was the development of SIGABA. It combined the knowledge of top cryptographers William F. Friedman and Frank B. Rowlett (Army) and Lieutenant Commander Laurence Safford (Navy).
In 1935, prior to WWII, Army cryptologists designed a basic code machine, which they then shared with the Navy. Jointly, in 1940, the SIGABA code machine was adopted by both US services and by 1941, the machine was operational. By 1943, more than 10,000 machines were in use. Unlike the Enigma, the superior SIGABA encryption machine was never broken.
“SIGABA” is not an acronym and does not stand for anything; it is simply a code word. SIGABA was used extensively by the US Navy, from submarines to battleships, but also by the rest of the American armed forces. According to the NSA, over 5,000 machines were deployed by 1943, and well over 10,000 by the end of the war. This is far less than the number of Enigma Machines that were used by the Germans (more than 20,000), but still a significant quantity.
Like the Enigma, the SIGABA was an electric rotor machine which used a wired rotor system, with the major exception that SIGABA used a massive 15 rotors to encrypt messages, that were separated into 5 ciphering rotors, 5 control rotors and 5 indexing rotors.
SIGABA was easier to use than Enigma. The German machine needed two people to operate; one to type the message, and another to copy down the resulting lighted letters, which became the cipher. SIGABA printed the ciphered letters on a strip of paper tape, eliminating the need for a two-person operation.
The SIGABA was used until 1959, when modern communications demanded new and more high-tech equipment. However, its patent was kept classified until 1996, when it was finally deemed too obsolete to compete with modern encryption technologies. [19]
AMERICAN SIGSALY – SECURE VOICE COMMUNICATION
SIGSALY was developed in the US at Bell Telephone Laboratories, and built by Western Electric in 1943. The project involved some of the most prominent thinkers of the time. It was led by the future head of R&D at the National Security Agency (NSA), AB Clark, with assistance from famed English codebreaker, Alan Turing and America’s renowned mathematician, Claude Shannon.
It was a digital speech encryption system that allowed confidential talks between British Prime Minister Winston Churchill, US Presidents Roosevelt and Truman, and other high-ranking military officers and strategic local civilian leaders. It was vital in allowing safe discussions involving significant war tactics as well as important administrative details needed to run a smooth military operation. Over 3,000 top-secret conferences were held using SIGSALY. It was the first electronic voice encryption system to use One-Time Pad encryption.
Prior to SIGSALY’s inception, some technical progress had been made in the field of secure voice systems, but none had been made that provided complete security. In fact, during early WWII voice encryption called the A-3 Scrambler, were used by both sides and each party is known to have intercepted and broken the other’s communication.
In the early 1940s Bell Telephone Laboratories became aware of the need for a new and safe method of Allied communication. Given the go, A.B. Clark immediately headed up two teams of experts to start the research and engineering processes. The voice transmission research team was led by R.C. Mathes, and included earlier voice recording work done by Eugene Paterson and Homer Dudley on a device they called the Vocoder, which they demonstrated at the 1939 New York World’s fair. The Vocoder was capable of taking a synthesized voice recording and digitizing it at a remarkable compression ratio of 10 to 1. Signals digitized in the telegraph range could be transmitted over short-wave radio channels.
SIGSALY’s encryption was based on the One-Time Pad method. Recorded voice messages were analyzed and divided into small intervals. Sound frequencies in the voice message were then spread out and digitized. Digitized voice messages were mixed with a “key” which consisted of prerecorded noise, made from an electric tube, which was thought to simulate a random sound. The coding technician mixed a noise key in with the digitized voice recording and sent the coded message across the ocean. The decoding technician had to utilize the matching key for successful message decoding, allowing him to remove the noise, leaving an audible plaintext message. A potential complication with using the One-Time Pad system, was the necessity of numerous duplicate keys on both ends. To overcome this noise was prerecorded onto phonograph records en masse, and delivered via special courier. As long as the sender and receiver used the corresponding key all was well. Used keys would be destroyed so that they could never be used again.
The completed machine went into service in 1943, just before the invasion of Italy. Because of the buzzing sound an eaves dropper would hear when attempting to listen in, it was nicknamed the “Green Hornet”, after the theme song of a popular serial radio show.
The SIGSALY was an expensive machine that required complex sets of circuits and equipment, including thousands of vacuum tubes and special air conditioning. It was also massive, consisting of over 40 racks of equipment and weighing over 50 tons. It featured two turntables which were synchronized on the sending and receiving ends. The cost for one machine was US$1 million.
Running the machine required teams of specially trained operators that were familiar with electronics and telephony. At the height of its commission, almost 360 expertly trained individuals were working out of 12 locations. Each location operated 24 hours a day.
Notably, at one-point, mathematician Alan M. Turning was asked to inspect SIGSALY to determine whether the encryption was strong enough to withstand German decoding forces, which he confirmed it was. At another point during early development, Claude Shannon was given permission to view the machine, but it is questionable as to whether he contributed anything to its progress. In 1949 however, Shannon would go on to prove mathematically that if all of the requirements of One-Time Pad encryption were met, that it was an unbreakable encryption. Shannon called it “perfect secrecy”.
During WWII, there were a dozen SIGSALYs distributed around the globe. The first three machines were strategically installed in the newly constructed US Pentagon, a second in London in Churchill’s War Rooms in the basement of a building in Whitehall, and the third on a ship that followed US General MacArthur during his South Pacific campaigns. Others wound up in Algiers, Australia, Hawaii, California, Paris (after its liberation), Guam, and after VE Day, in Frankfurt and Berlin.
The SIGSALY system was credited with helping to secure the Allied victory in WWII, by preventing the interception of critical communications by the Axis powers. The SIGSALY was never broken.[20]
NAVAJO CODE -TALKERS
American Navajo Indians had their own languages and dialects that few outside their tribes understood. Realizing a unique opportunity, US Marine Corps, in an effort to find quicker and more secure ways to send and receive code, enlisted Navajos as code talkers. After being given a list of terms and phrases, a team of Navajo made up substitute words and phrases that only a trained Navajo code talker could encrypt and decode. Navajo encrypted messages were used on a limited basis in WWI, but were more widely used in WWII. Over 400 Navajo Indians were deployed in the Pacific theater, and were successful at quickly sending and receiving messages, usually while being in active battle. This allowed for almost real time descriptions of Japanese enemy locations and artillery and for the safe deliverance of US Marine’s immediate assistance needs. Even after the capture and torture of a non-code talker Navajo Indian, the Japanese were never able to decipher the code-talker’s language. Multiple WWII historians have stated that the Navajo code talkers helped expedite the end of WWII and, undoubtedly, saved thousands of lives. “Were it not for the Navajos, the Marines would never have taken Iwo Jima,” said Major Howard Connor, the signal officer of the Navajos at Iwo Jima in 1945. [21]
HEDY LAMARR
Better known as “Hollywood’s most beautiful woman”, during her career in the 1930s and 1940s, Hedy Lamarr, along with friend and musical composer George Antheil, invented a signal blocking technology they patented as Frequency Hopping. Had her technology been taken seriously, it is possible the duration of WWII could have been shortened, and the jamming of numerous Allied torpedoes could have been stopped.
Hedy was driven to invent and said it came to her naturally. As a Jewish woman born in Vienna Austria, she was greatly affected by WWII, especially after Austria was taken under Nazi control. And she desperately wanted to help the war effort. In addition to helping raise millions in US War Bonds, she along with friend and composer, and George Antheil, who had been experimenting with the automated control of musical instruments, came up with the idea of Frequency Hopping.
WWII radio-controlled torpedo frequencies between the launching ship and torpedo were easily detected and guidance signals could be jammed, taking the torpedo off course. Lamarr’s Frequency Hopping technology was designed to prevent interference with guidance controls. By rapidly jumping from one radio frequency to another and under the control of a secret key, only the intended receiver with a shared key could find the transmission. Her technology created secure radio communications. By manipulating radio frequencies, hopping from one frequency to another, at irregular intervals between transmission and reception, the invention formed an unbreakable code that prevented secret messages from being intercepted, or torpedoes from being taken off course.
During WWII, the US Navy rejected the importance of the technology, showing no interest, even though they took control of the patent, considering her an enemy alien. She was never compensated for her invention, which some have estimated to be worth US$30 billion.
During the Cold War, the Navy could no longer ignore the importance of Lamarr’s invention and integrated her technology into their communications systems, and it was used extensively during the 1962 Cuban Missile Crisis to maintain secure radio communications between American Navy ships enforcing the blockade of Cuba.
Although Lamarr’s technology was not directly used in cryptography during the war, her invention laid the groundwork for future secure wireless communications. Frequency Hopping became what is known today as Spread Spectrum. Much of our digital technology that we use each and every day, with Bluetooth and WIFI being the most similar, is a result of Lamarr’s invention. [22] [23]
POST WWII ENCRYPTION
Until the end of WWII, cryptography was mainly designed and used for military purposes. However, post-war businesses around the world started to realize its value and demanded the ability to keep company secrets, secret.
In the early 1970s, IBM’s customers were demanding some form of encryption, so IBM formed a “crypto group” headed by Horst-Feistel. They designed a cipher called Lucifer.
In 1973, the US’ National Bureau of Standards, now known as the National Institute of Standards and Technology (NIST), put out a request for proposals for a block cipher which would become a national standard. Lucifer was eventually accepted and was named the Data Encryption Standard or DES.
DES was designed with a small size encryption key. As computing power increased it became easy to brute force all different combinations of the key to obtain a possible plain text message. By 1999, DES had been broken.
When the World Wide Web was invented in 1989, and computers became available to the mainstream, it became inevitable that the masses would soon have access to affordable encryption. Not surprisingly, financial services were some of the first to utilize cryptography to secure electronic transactions. Soon after, other businesses wanted to secure their digitally stored trade secrets and finally, the public demanded the same security.
In 1997, Ron Rivest, Adi Shamir and Leonard Adleman of MIT, described the first encryption system to use a public and a private key generated using prime number factoring. They coined it “RSA”, using the first letter in each inventor’s name. RSA is widely used in web browsers, email chats, VPNs, and other communication channels. It bears mentioning that in 1973 (24 years earlier), British mathematician Clifford Cocks, invented a public key algorithm that was kept secret by the British government until coincidentally after the public introduction in 1997, of the RSA two key algorithm.
In 1997, NIST initiated the first world-wide public competition calling for submissions of a 128-bit block cipher with three key length options: 128, 192, and 256 bits. The winning cipher would be named the Advanced Encryption Standard (AES).
In 2001, NIST announced the Rijndael block cipher was to become the AES standard cipher. AES remains the cipher of choice for the US government, including the NSA and many other departments across the government.
Today, the advancements in quantum computers have forced us to think about post-quantum cryptography solutions. At the behest of the US government, in 2016, NIST put out another request for proposals, looking for a new type of encryption that could withstand the power and lightning computational speed of the quantum computer. In 2020, NIST announced four finalists. Many of the early contender’s code was broken and one of the finalist’s algorithms was broken in less than two hours.
PRE-QUANTUM CYBERSECURITY
With the government’s realization that cybersecurity was a top national security concern, in 2013, Executive Order 13636 mandated that NIST develop a Cybersecurity Framework.
One year after the release of Executive Order 13636, NIST released version 1.0 of the Framework for improving critical infrastructure cybersecurity. The Framework was released as voluntary guidance, based on existing standards, guidelines, and practices, for critical infrastructure organizations to better manage and reduce cybersecurity risk.
In 2017, a draft version of the framework, version 1.1, was circulated for public comment.
POST-QUANTUM CYBERSECURITY
In December 2016, NIST issued a public call for submissions to the PQC Standardization Process.
Below is a timeline of candidate selection:
- First Round 2017 – 69 candidates chosen
- Second Round 2019 – 26 surviving candidates
- Third Round 2020 – 7 finalists, 8 alternates
- Fourth Round 2022 – 3 finalists and 1 alternate selected as standards. [24]
In July 2023, NIST announced the first four winners from its six-year competition. This first group of encryption tools chosen are “designed to withstand the assault of a future quantum computer, which could potentially crack the security used to protect privacy in the digital systems we rely on every day, such as online banking and email software. The four selected encryption algorithms will become part of NIST’s post-quantum cryptographic standard, expected to be finalized and ready for public release for use, in about two years.”
NIST went on to specify that for general encryption, used when we access secure websites, NIST selected the CRYSTALS-Kyber algorithm. Among its advantages are comparatively small encryption keys that two parties can exchange easily, as well as its speed of operation.
And for digital signatures, often used when we need to verify identities during a digital transaction or to sign a document remotely, NIST selected the three algorithms CRYSTALS-Dilithium, FALCON and SPHINCS+. Reviewers noted the high efficiency of the first two, and NIST recommends CRYSTALS-Dilithium as the primary algorithm, with FALCON for applications that need smaller signatures than Dilithium can provide. The third, SPHINCS+, is somewhat larger and slower than the other two, but it is valuable as a backup for one chief reason: It is based on a different math approach than all three of NIST’s other selections. [25]
In August of 2024, NIST released a final set of encryption tools designed to withstand the attack of a quantum computer, stating that “these three post-quantum encryption standards secure a wide range of electronic information, from confidential email messages to e-commerce transactions that propel the modern economy.” NIST is encouraging computer system administrators to begin transitioning to the new standards as soon as possible.
While there have been no substantive changes made to the standards since the draft versions, NIST has changed the algorithms’ names to specify the versions that appear in the three finalized standards, which are:
- Federal Information Processing Standard (FIPS) 203, intended as the primary standard for general encryption. Among its advantages are comparatively small encryption keys that two parties can exchange easily, as well as its speed of operation. The standard is based on the CRYSTALS-Kyber algorithm, which has been renamed ML-KEM, short for Module-Lattice-Based Key-Encapsulation Mechanism.
- FIPS 204, intended as the primary standard for protecting digital signatures. The standard uses the CRYSTALS-Dilithium algorithm, which has been renamed ML-DSA, short for Module-Lattice-Based Digital Signature Algorithm.
- FIPS 205, also designed for digital signatures. The standard employs the Sphincs+ algorithm, which has been renamed SLH-DSA, short for Stateless Hash-Based Digital Signature Algorithm. The standard is based on a different math approach than ML-DSA, and it is intended as a backup method in case ML-DSA proves vulnerable.
Similarly, when the fourth draft FIPS 206 standard built around FALCON is released, the algorithm will be dubbed FN-DSA, short for FFT (fast-Fourier transform) over NTRU-Lattice-Based Digital Signature Algorithm. [26]
In March of 2025, NIST announced that it has chosen a new algorithm for post-quantum encryption called HQC, which will serve as a backup for ML-KEM, the main algorithm for general encryption. HQC is based on different math than ML-KEM, which could be important if a weakness were discovered in ML-KEM. NIST plans to issue a draft standard incorporating the HQC algorithm in about a year, with a finalized standard expected in 2027. [27]
Disclaimer: This document includes only a handful of the numerous and amazing stories that make up the entire history of cryptography and ciphering. There is mystery and intrigue, sheer genius and happenstance, all waiting for the curious mind to discover. A wealth of knowledge is available on the world wide web, thanks to some of the individuals mentioned in this paper and the work of the many great authors and articles referenced in my footnotes. We apologize if we have over simplified technical information, misinterpreted actual occurrences, or missed important milestones.
[1] https://www.redhat.com/en/blog/brief-history-cryptography
[2] https://www.britannica.com/topic/cipher#ref287635
[3] https://www.redhat.com/en/blog/brief-history-cryptography
[4] https://ciphermachines.com/vigenere (by Ralph Simpson)
[5] https://ciphermachines.com/jefferson (by Ralph Simpson)
[6] ww1cryptology_paper.pdf
[7] https://www.pbs.org/wgbh/nova/decoding/playfair.html
[8] ww1cryptology_paper.pdf
[9] https://ciphermachines.com/hebern
[10] https://www.nzz.ch/schweiz/crypto-international-entlaesst-fast-die-gesamte-belegschaft-als-folge-eines-fragwuerdigen-entscheids-des-bundesrats-ld.1564577?rflmnt=adnz%3B%3B%3Bbc&reduced=true
[11] ww1cryptology_paper.pdf
[12] Solving the Enigma – History of the Cryptanalytic Bombe.pdf
[13] tnmoc.org/colossus(The National Museumof Computing, Colossus)
[14] https://wrens.org.uk/world-war-two-code-breaker/
[15] https://www.computerhistory.org/timeline/1944/
[16] https://www.cryptomuseum.com/crypto/uk/typex/
[17] https://www.cryptomuseum.com/crypto/usa/ccm.htm
[18] https://www.nsa.gov/History/National-Cryptologic-Museum/Exhibits-Artifacts/Exhibit-View/Article/2719165/sigabaecm/
[19] https://www.cryptomuseum.com/crypto/usa/sigaba/
[20] https://www.discoveryuk.com/mysteries/what-was-sigsaly-and-how-did-it-help-win-wwii/
[21] Code Talker: by Chester Nez and Judith Schiess Avila
[22] https://www.rosieriveters.com/hedy_lamarr_hollywood_pinup_inventor_of_wi_fi
[23] https://www.thalesgroup.com/en/worldwide/digital-identity-and-security/magazine/women-technology-hedy-lamarr-mother-wi-fi
[24] theregister.com/2022/07/05/nist_quantum_resistant_algorithms/
[25] nist.gov/news-events/news/2022/07/nist-announces-first-four-quantum-resistant-cryptographic-algorithms#:~:text=The algorithms are designed for two main,experts collaborating from multiple countries and institutions.
[26] nist.gov/news-events/news/2024/08/nist-releases-first-3-finalized-post-quantum-encryption-standards#:~:text=GAITHERSBURG%2C Md. — The U.S. Department of,to withstand cyberattacks from a quantum computer.
[27] https://www.nist.gov/news-events/news/2025/03/nist-selects-hqc-fifth-algorithm-post-quantum-encryption
