Private Key Algorithms

The private key or symmetric algorithm is the quickest and simplest encryption algorithm in widespread use today. The term symmetric refers to the fact that the same key is used for encryption and decryption (using a encryption notation this means that K = K-1), hence only one key is required in order to encrypt and decrypt data.

We use the following terms when talking about encryption -Plaintext - the initial unencrypted data or the encrypted data after it has been decrypted.

Ciphertext - the plaintext after is has been encrypted and is no longer readable.
Key - the value applied to the plaintext and encryption algorithm in order to achieve encryption. Conversely the value applied to the ciphertext and decryption algorithm in order to achieve decryption.

Encryption - the process of converting the plaintext to ciphertext.
Decryption - the process of converting the ciphertext back to plaintext.

The different algorithms in use have various means of achieving the encryption and decryption but they all have the same general goals -
Secure Encryption - It should not be practical to decipher the ciphertext without the key.
Key secrecy - It should not be feasible to deduce the key from the ciphertext.

The key point here is the word “practical” in practise it is almost always mathematically possible to decipher the ciphertext without a key. The aim is to make it so difficult (usually in terms of required computing power) that it is not practical to achieve this decryption - within a week, year, decade or whatever timescale is required. This is typically achieved by varying the key size, longer keys take longer to break, but require more computing power to encrypt and decrypt with in the first place.

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