This section introduces the basic concepts and underlying principles of cryptography. Unfortunately, a profound mathematical knowledge is required to understand section 4.2 up to section 4.3 . RSA is a public key cryptographic algorithm. This means that everybody on the planet can encrypt a message to Alice with Alice's public key, but only Alice will be capable of decrypting the message, by using its secret key. It is important to note that there are 2 different keys that form a pair, one is public and is used to encrypt a message and the private key is kept secret, so only the receiver can decrypt the message.
Every person has a unique keypair.
DES is a private key algorithm. If Bob and Alice want to exchange encrypted information, they both need to possess one private key, which is kept secretly between them. This key is needed for both encryption and decryption of the message. Digital Signatures can be used to uniquely sign an electronic document. Similar to the RSA public key algorithm, but this time using the private key, Alice will compute a number representing the signature of the document, which depends on the document content as well as Alice's private key.