A research available through the Academy. "By developing Cryptography enemy spies have it 20% harder."
In essence, this means that it will take your enemy 20% more silver coins invested into an espionage action to successfully spy on your polis.
1.) 2500 Wood
2.) 3000 Stone
3.) 5100 Silver
4.) 6 Research Points
5.) Academy level of 19 or Greater
Example You have 9,000 silver coins in your cave.
An enemy invests 10,000 silver coins into an espionage attempt.
Since you have Cryptography, the enemy's espionage attempt is now 20% weaker.
10000 * 20% = 2000
10000 - 2000 = 8000
Since you have more than 8000 silver coins in your cave, your enemy failed in his/her espionage attempt. Without Cryptography, this would have been a successful espionage attempt.