3. Symmetric Encryption Key

The word 'symmetric' is used because the same key is used for both encryption and decryption.

The symmetric key must be kept secret by legitimate systems using it.

A symmetric digital key must be long enough that it becomes infeasable for a normal sized computer to crack the encrypted message in a reasonable amount of time.

The strength of the key is determined by its bit length. For example a 128 bit symmetric key would take a maximum of $2^{128}$ brute force attempts to guess it. At the moment 128 bits is a good balance between privacy for day-to-day use and the processing overhead needed to do the encrypt and decrypt.

An example of a digital symmetric key algorithm is AES-256 (short for Advanced Encryption Standard - 256 bits), and is the standard algorithm used by national governments and commercial organisations for really sensitive information.

Book cipher

You have probably seen this used in wartime spy movies. Agent A and Agent B agree to use a certain book, the huge 'War and Peace' book maybe. Then the message is created by referencing a letter or word in various pages within the book.

For example the code might be 434. Meaning use the 34th letter on page 4.

The whole scheme relies on no-one knowing which book is being used.