When you enter a password at a website or make online purchases, have you ever wondered how the data is securely transmitted over the Internet? Yes, you guessed it right – the modern browsers use security protocols such as HTTPS to protect our data in transit. In this chapter, we will review one of the well-known public key cryptography methods called the RSA Cryptosystem, named after its inventors, Ron Rivest, Adi Shamir, and Leonard Adleman. In RSA, both sender and receiver (even the attacker) know the public key beforehand. The sender uses the public key to encrypt the message, and the receiver uses the private key known only to himself/herself to decrypt the message. The RSA involves a bit of math, including finding the greatest common divisor of two integers and a primality test using Fermat’s Little Theorem. Fret not, let us unpack it slowly.