A useful topic to understand when trying to decipher the parts of Kryptos. I won’t lie and say I wrote this. I will however cite a noted free online encyclopedia.
Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related, often identical, cryptographic keys for both decryption and encryption.
The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transform to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link.
Other terms for symmetric-key encryption are secret-key, single-key, shared-key, one-key, and private-key encryption. Use of the last term conflicts with the term private key in public-key cryptography.
One disadvantage of symmetric-key algorithms is the requirement of a shared secret key, with one copy at each end. In order to ensure secure communications between everyone in a population of n people a total of n(n − 1)/2 keys are needed, which is the total number of possible communication channels. To limit the impact of a potential discovery by a cryptographic adversary, they should be changed regularly and kept secure during distribution and in service. The process of selecting, distributing and storing keys is known as key management, and is difficult to achieve reliably and securely.
Symmetric ciphers have historically been susceptible to known-plaintext attacks, chosen plaintext attacks, differential cryptanalysis and linear cryptanalysis. Careful construction of the functions for each round can greatly reduce the chances of a successful attack.
So what has this got to do with Kryptos?
Kryptos implies the need for symmetric-key algorithms with the keyed Vigenere ciphers of K1 and K2. A K3 method has been found that uses a route transposition followed by a keyed-columnar transposition which could indicate another use of keywords. I initially figured a key was a key and you had to use the same one to encrypt and decrypt. I was wrong and am working to understand the difference and communicate it with others.
The other method of key use is asymmetric-key algorithms.