Public key / Asymmetric encryption

• Public key / Asymmetric encryption
  • Overview
  • Pair of keys
  • Examples
    • Browser example
    • Real life analogy

Overview

Public key cryptography, also known as asymmetric cryptography, is an encryption scheme that uses two mathemtically related, but not identical, keys: a public and a private key.

There are many public key cryptography algorithms, such as RSA, or ECDSA in the case of Bitcoin. Some are thought to be quantum computing-resistant; others are chosen for their speed or ease.

tip

Great non-geek explanaiton from Panayotis Vryonis here!

Pair of keys

Unlike symmetric key algorithms that rely on one single key to both enctypt and decrypt, each key performs an unique action.

If the public key is used for encryption, then the related private key is used for decryption. If the private key is used for encryption, then the related public key is used for decryption.

• Your private key. It must be kept secret, known only to you. You can use your private key to sign a message, and your public key (known to others) can verify the signature.
• Your public key can be publicized on the Internet, and someone can use it to encrypt a message that only you can decrypt (using your private key).

Examples

Browser example

1. A client sends its public key to the server and requests for some data.
2. The server encrypts the data using client's public key and sends the encrypted data.
3. Client receives this data and decrypts it.
4. Vice versa.

When you access a website over https, you're using the website's B to encrypt your outgoing data so that only the website may read it.

Real life analogy

1. Alice wants something valuable shipped to her.
   1. It arrives securely (i.e., hasn't been opened or tampered with).
   2. It's not a forgery (i.e., the sender is who she's expecting it to be).
2. She provides the sender, called Bob, with a high-security box.
3. She provides Bob with a padlock without a key (i.e., Alice's public key), which she kept for herself (i.e., Alice's private key).
   1. Bob can put items in the box then put the padlock onto it.
   2. Once the padlock snaps shut, the box cannot be opened unless you have Alice's private key.
4. Bob also puts a padlock onto the box (i.e., Bob's private key) and uses a key Bob has published to the world (i.e., Bob's public key).
   1. If you have one of Bob's keys, you know a box came from him.
   2. For the sake of the example, imagine a world where padlocks cannot be forged even if you know the key.
5. Bob then sends the box to Alice.
6. In order for Alice to open the box, she needs two keys:
   1. Her private key that opens her own padlock.
   2. Bob's well-known key.
7. If Bob's key doesn't open the second padlock, then Alice knows that this is not the box she was expecting from Bob, it's a forgery.

This bidirectional guarantee around identity is known as mutual authentication.