From the days of ancient Greece and Rome, through the intrigues of medieval and renaissance courts, two long world wars, and beyond, military intelligence has routinely relied on codes and ciphers to hide sensitive information from enemy eyes.
As the tools for keeping secrets became ever more valuable, so too did the potential rewards for cracking them, sparking an arms race between the code-makers and the code-breakers, as nations engaged in clandestine, silent wars for information.
For the US Navy, that fight effectively began with the formation of the Communications Security Group (CSG), the first unified organisation to coordinate naval cryptographic operations. The CGS celebrated its 81st anniversary on 11 March this year, and though the Naval Security Group Command was formally disestablished in 2005 and integrated under Naval Network Warfare Command, the role of naval cryptography remains as vital as ever.
Between 1928 and 1941, a total of 176 radio operators - 150 of them Navy, 26 Marines - were trained to intercept and analyse foreign radio transmissions at a unique school on the roof of the old Navy Department Building in Washington DC. Their memorial plaque, now on the quarterdeck of the Command Headquarters at Fort George G. Meade, Maryland notes that the 'On The Roof Gang' as they were known "formed the vanguard of the US Naval Communications Intelligence efforts and laid the cornerstone of Naval Cryptology."
Since then both the numbers and profile of cryptologists have grown, and they now form part of nearly 52,000 professionals - along with intelligence analysts, meteorologists, oceanographers and space specialists - in the US Navy's Information Warfare Corps, formerly known as the Information Dominance Corps.
Their role has evolved somewhat over the intervening years. As Vice Admiral Jan Tighe, Commander of the US Fleet Cyber Command/US 10th Fleet, put it on the occasion of the CSG's 81st birthday, "we execute cryptologic warfare, which encompasses signals intelligence, cyberspace operations and electronic warfare operations in order to deliver effects through sea, air, land, space and cyber domains at all levels of war."
Modern cryptology is now about far more than simply eavesdropping on the airwaves.
In Julius Caesar's day, sending a coded message was not a particularly difficult undertaking, not least because most of the barbarian enemy tribes would scarcely have been able to read, much less understand Latin syntax. The 'Caesar cipher' was a relatively unsophisticated substitution code, replacing each letter with another letter, according to Suetonius, which was three along in the alphabet. Thus 'Caesar' becomes 'Fdhvdu' (or more accurately, 'Fdhxdv' using the Roman alphabet as he would have known it) and so on.
Move forward 800 years, and the Arab polymath Al-Kindi documented - conceivably having also first invented - the use of frequency analysis to correlate the most commonly used letters in a given language with their substitutes in the code, and so determine the shift that has been used. Secret communications remained vulnerable to being deciphered by this method for the next 600 years, until the development of the polyalphabetic cipher, often credited to Leon Battista Alberti, which changes the distance of the substitution shift used for different parts of the message.
From then on, through the 19th and into the 20th century, the parallel struggle between encryption and decryption saw the deployment of increasingly complex algorithms and the invention of various mechanical devices, including the well-known Enigma machine of World War Two.
At the other end of the spectrum, the 20th century also saw examples of the use of obscure languages as the basis for securing communication with the rise of 'code talkers'. The Navajo speakers serving with the US Marines in the Pacific during the Second World War are the most famous example, but the method had also been used by Cherokee and Choctaw soldiers in the previous conflict, and reportedly re-emerged again in the 1990s amongst Welsh-speaking contingents of the British forces in Bosnia.
The big advantage of code talking is speed; in tests under combat conditions in early 1942, Marine Corps Navajo code talkers successfully encrypted, transmitted and decrypted a short message in less than 30 seconds - not the 30 minutes necessary for the coding machines of the time.
Fast though this approach is, the very shortage of speakers that enables it to work as a secret code also restricts its use to just a few exponents. For wider military applications, the need remains for a more conventional algorithmic approach.
The burgeoning sophistication of such ciphers and the increasingly arduous and repetitive tasks involved in attempting to decipher them eventually led to the development of the world's first programmable, electronic, digital computer - Colossus - by British code-breakers during the Second World War. The arrival of this computing power was quickly to prove a game-changer and ultimately set the shape of cryptography into the digital age. The global proliferation of sophisticated information technology and online connectivity has changed the way the world communicates, put high-level asymmetric key encryption in the public domain - and by implication, into the hands of terrorists too - and, from a military perspective, irrevocably altered the conduct and speed of warfare.
As an early adopter of high-tech data links, worldwide communication networks and advanced reconnaissance and surveillance systems, and a major investor in the necessary people skills to make use of them, the US Navy has for some time enjoyed virtually unrivalled informational superiority over its potential adversaries.
However, as Vice Admirals Kendall L. Card, former Deputy Chief of Naval Operations for Information Dominance and Michael S. Rogers, then the Fleet Cyber Commander, point out in their foreword to 'Navy Strategy for Achieving Information Dominance 2013-2017' that advantage is being steadily eroded. They note that a growing number of countries, transnational criminal organisations and non-state actors are acquiring and using ever-more advanced computer and network systems, and amongst them are some who are actively seeking to exploit weaknesses and challenge the US' long-standing technological edge.
It leaves the Navy dependent for everything from complex intelligence missions and drone piloting right down to the straightforward tasks of communication and navigation, on increasingly vulnerable networks. Addressing this is a rising challenge for the naval cryptologists and information specialists, and the fight-back has already begun.
The next step
Since the beginning of the decade, the US Navy has been consolidating its information-related initiatives, resources and manpower under a single umbrella, and integrating the doctrine of Information Dominance as a core naval warfighting competency. Now that looks set to go even further with the possible adoption of the electromagnetic (EM) spectrum as a separate battlespace domain in its own right - distinct from cyberspace which was itself designated a domain in 2006.
The idea has been under discussion in the Pentagon for some time, and with the Navy widely recognised as the lead service when it comes to understanding and using the EM spectrum, the concept would have particular resonance for the kind of comprehensive cryptologic warfare Vice Admiral Jan Tighe described. Having already seen its role expanded over the years from simple communications intercepts and signals intelligence to cover cyberspace operations and electronic warfare across several domains, for the Navy's cryptographic community, encompassing the whole of EM within its remit would certainly seem to be the logical next step.
In 1956, observing the fast-maturing radar, navigation and communication technologies of the early Cold War era, Soviet Admiral Sergei Gorshkov said that "the next war will be won by whichever side best exploits the electromagnetic spectrum." Sixty years later, that still looks to hold true.