Crash course on cryptography
Using cryptography, Alice (the sender) can scramble (encrypt) all messages intended for Bob (the recipient) so that no one but Bob can read them. Usually Alice and Bob agree on a common, secret key that they use to encrypt and decrypt messages. This type of cryptography is known as secret key cryptography, shared-key cryptography or symmetric cryptography.
On the internet, a more common form of cryptography is public key cryptography, public-private cryptography or asymmetric cryptography. Using this type of cryptography, Alice and Bob can simply obtain each other's public key and use it to encrypt messages for one another. Only with the corresponding private key can those messages be decrypted.
Public key cryptography also allows the creation of digital signatures. This allows Bob to verify that a message sent by Alice was really sent by her and was not modified by anyone else. For this to work, Bob also needs Alice's so-called digital certificate.