SSCC - Keys

For more info on cryptography keys, this wikipedia page is a good start:

Now my short summary

Q: What are keys?
A: The things that lock and unlock our doors so we can keep our home safe and yet let us in.

Q: So, what are keys regarding computer data?
A: Things that lock and unlock our data so we can keep our data safe and yet let us use it.

All the cryptography keys (or simply keys from now on) can be divided in two categories:
  • Symmetric keys
  • Asymmetric keys
If the same key can be used for locking and unlocking the data then we are talking about symmetric key.

On the image above persons A and B both have the same symmetric key K and can share a message securely between each other, as the unauthorized  person C does not have the key and can only see scrambled data.
Usually when we talk about keys one immediately thinks of encryption, but we want use this word yet as encryption might refer to several different types of data scrambling :). Since data scrambling does not sound too much profesional, let us use the term bits manipulation.
When it comes to bits manipulation algorithms that use symmetric keys we are talking about ciphering.

Ciphering is one of the earliest types of protecting data and it dates from long before computers were invented. In some cases, where less security is required cypher algorithms can be very simple. One such example is the XOR logic operator:
100101101 XOR 110011001 = 010110100
010110100 XOR 110011001 = 100101101

Now imagine that your secure zone consists of a group of more than two people. Let say a group of police officers communicating over some computer network. One of the many in the group is the chief and gives orders to the others, while the others must only receive and not be able to send orders. Obviously this restriction can not be done by using a symmetric key, as in order for the group to receive the orders they will need to share the same key, which will enable them to also send orders to the rest of the group. But if we had a key pair instead of only one key, and we use one key from the pair to lock the message, while the other to unlock it, then the officer can have the first key and hence would be able to send orders, while the group can share the second key and would be only able to receive orders.

If we use separate keys for locking and unlocking the data, then we are talking about asymmetric keys.
The key used by the chief and not shared by the others in the group is called a private key, while the shared one is called a public key.

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