Decipher and exchange secret messages with your friends! On the battlefield, the accumulation of secret intelligence was one of the most important weapons that can be used to gain advantages. Many spies on both sides of the conflict cleverly concealed and smuggled secret information through enemy lines. This Spy Decoder was one of the ways spies could pass coded messages back and forth. If they were intercepted by the enemy, the message would be unreadable to the enemy without the key agreed upon previously by the spies.