If you want to encrypt an message you need encryption key, a list of numbers that tells you how the normal message is transformed into the encrypted message. Sometimes however it is the case that some part of you encryption key is known by someone you want to hide your message from, in that case there are several ways to still secure your message one of which is using a uneversal hash fuction to transform you encryption key into a shorter but more secure key. A universal hash function has the property that the probability that you get the same key out of two different encryption keys is smaller or equal that 1 over the length of the resulting key. One of the easiest examples of an universal hash function is that of a matrix with at random 1 or 0 in every entry.
Today I was watching a video from day9 where among other things he told a story about how he asked a girl out by writing an letter in old english and got an java script as answer (if you want to hear the details watch the video) and this reminded me of the time I wanted to sent a girl a valentines day card but wanted to do it anonymus. So what did I do I made a new email adress with my name encrypted in the email adress name, by using an easy encrytion scheme namely taking the first prime number to be “a” the second prime number to be “b” etc (a key like that is called a cryptographic key) and emailed her the drawing I had made and scanned.