1. PUBLIC KEY CRYPTOGRAPHY A brief overview By Andy Brodie

2. PUBLIC/PRIVATE KEYS A normal lock, for example on a door, has one key. If you have a key, you can lock and unlock the door. This is a useful way of securing content (whatever is behind the door). Asymmetric keys mean that the lock has two keys. One key can only lock the door (a public key) One key can only unlock the door (a private key) Encryption and decryption is a metaphor for locking and unlocking the door to get at whatever is behind the door. This is really, really powerful.

3. BENEFITS OF ASYMMETRIC KEYS If you give away the public key (that encrypts) then anyone can protect content. No-one else can decrypt and read the content unless they have the private key. But, there’s more… it works the other way around too!

4. BUT HOW DO THEY WORK? Private keys must be unguessable. It must be impossible to derive the private key from the public key. Keys are calculated from very, very large prime numbers. Key “strength” is measure in bits, i.e. the number of bits the key consists of. E.g. an 8-bit key has a range of 256 potential values (2 8 ). This is not very secure as it would be trivial to manufacture all 256 keys and try them against the lock (a brute force attack) A 2048 bit key has a range of 3.23x10 616. 3.23 with 616 zeros after the end. Even the Milliard Gargantubrain could only manage ~1x10 57 in a millisecond!

5. MATHS

6. REAL MATHS!

7. ENCRYPTING AND DECRYPTING

8. END This work is licensed under the Creative Commons Attribution 4.0 International License.Creative Commons Attribution 4.0 International License