"Technology changes, tactics change, but the principles never do. You need to get information from your adversary, and guess what? They are trying to do the same thing to you," Patrick Weadon, curator of the US National Cryptologic Museum, said about the evolution of spycraft half a decade ago.
During the First World War, pigeon cameras were used to explore the secrets of enemies. The birds would carry messages at times when radio communication couldn't. These pigeons would even receive medals of honour for their services.
The pigeon camera was invented in 1907 by German apothecary Julius Neubronner. He used the birds to deliver medicines.
The last century saw many such spying gadgets like tree stump bug, insectothopter (insect-size unmanned aerial vehicle), lipstick pistol, and spy shoe with a heel transmitter that sounds like fiction in the present day.
With technological advancement over the years, the craft of espionage has also evolved.
Within a few years of the first world war, Germany developed the Enigma machine to transmit coded messages, which made it difficult for the Allies to decode them during the Second World War.
Only those with the enigma machine and its secret knowledge could decode the messages.
If you have watched 2014's Hollywood movie "The Imitation Game", you know already how the Allies eventually cracked those codes and won the war.
Two years prior to the beginning of the Cold War, when the allies defeated the Axis in WWII, some Soviet school children gifted a carved wooden replica of the Great Seal of the United States to W Averell Harriman, the United States Ambassador to the Soviet Union, on his birthday.
Touched by such a beautiful present, the ambassador carefully hung the Great Seal carving on the wall of his study room. It was there from 1945 to 1952, before it was finally discovered that the Soviets put a listening device in the carving.
Such bugging and wiretaps became critical tools during the first half and early second half of the last century.
But the world of espionage changed dramatically after the Vietnam War. The eavesdropping culture via bugging and wiretapping took a new form as internet appeared to transform spycraft.
The modern era is a time of Edward Snowden and Julian Assange that CBS News' Jake Miller described as the "democratisation of espionage."
Nowadays, spying is no longer limited to the nation-state; rather the individual has become a major player in the realm of espionage.
With a computer and excellent hacking tools and knowledge, anyone can become a spy in this day and age and expose governments' secrets or wrongdoings.
"I was trained as a spy in sort of the traditional sense of the word, in that I lived and worked undercover overseas," said Former US spy agency contractor Edward Snowden, who ended up exposing a million of classified US information.
Whether his whistleblowing was good or bad is a matter of debate. But his actions show how in spycraft, an individual can emerge as equally powerful as a nation-state.
The modern spies are often expert computer hackers, who make the idea of classified encryption advice meaningless. Now hackers from countries with good internet services, sometimes even from countries where internet service is very poor, can crack any government, business and private cyber-security systems.
Do you remember the Sony Picture's hacking story of 2014?
A North Korean group, which introduced itself as the Guardians of Peace, hacked Sony Pictures. It exposed Sony's confidential documents after the film studio released "The Interview" comedy, portraying two American journalists sent to North Korea to assassinate Kim Jong Un.
There are many other cases like this. The most relatable ones are perhaps the hacking missions of subcontinental hackers – following some heated cricket matches – who hacked various private and public websites of the opponent countries.
As there are no agreed-upon international rules or norms to govern international conflict in cyberspace, and as such 'democratisation' in the field of espionage widens, cyber domain remains the most challenging part from the defence perspective.
The cyber espionages, in experts' opinions, are far more dangerous than conventional old-school spying. It could spark a large-scale cyber war at any moment if things get out of control.
You do not need a large government programme now. "You just need a dedicated group of hackers, or even one who has the skills to do some real damage overseas," said Vincent Houghton, a historian and curator at the International Spy Museum.
The major concern of cyber espionage is no matter how amazing the technology you have to fight it, spies have much better technology to infiltrate into your 'encrypted' system.
The modern time espionages may sound less fancy than the old-school stories, where the spies would engage in daring James Bond style missions, but the spies of the modern era are actually more lethal and more destructive. They have both the eavesdropping gadgets and the latest technology of scooping up big classified government data.