Today I went to my internship and we talked about encryption. First I made a substitution cipher, which is when you substitute every letter of the alphabet with another for example P could be substituted in for A and Y could be substituted in for T so the word AT would be encrypted as PY.  Here is a picture of the code (white) and the actual program in run in the command line (black):

We also talked about decryption. People look at letter frequencies, letter combinations (ex. qu, th, and st), and permutation in order to crack the encrypted message. The encryption above is vulnerable to things such as repeat messages, chosen plaintext attack, chosen ciphertext attack, and English frequency analysis.
In plaintext attack you chose the text you want them to encrypt (leak info on purpose). In ciphertext attack you give them the plaintext and they give you back ciphertext for that message (man in the middle).