Alice and Bob want to communicate they have some shared information a key k in advance Alice has a plaintext message m She coverts it to a ciphertext c with c Enc k m Bob decrypts this with m Dec k c You can also do this yourself such as encrypting your own data and then decrypting it later A private key encryption algorithm scheme is defined by a message space M and three algorithms Gen Enc Dec such that Gen key generation algorithm Randomized Outputs a key k Enc encryption algorithm Takes as input a key k and message m and outputs a ciphertext c outputs a message m m Dec k c Dec decryption algorithm Takes as input a key k and ciphertext c and For all k outputed by Gen and all m in M and all c outputed by Enc k m Kerckhoff s principle Attacker knows all the details of the scheme the parties are using opposite is security through obscurity where you try to keep the algorithm secret which is bad private key setting secret key setting shared key setting symmetric key setting Consider M all strings composed of a z Gen output a uniform k in 0 25 Enc k m 1 m 2 m l output c c 1 c 2 c l Dec k c 1 c 2 c l output m m 1 m 2 m l where c i m i k 26 where m i c i k 26 This is a bad encryption scheme easy to break for example by just looking at all 26 possibilities