What is "private key"/"symmetric"/"secret key" cryptography?

Lennart Jorelid



Short answer: A private key cryptography algorithm uses the same key for encryption and decryption. Since the key is the only data required to decrypt the ciphertext, it must be kept private/secret.

Longer answer: Encryption is the art of turning a plain text message written by a sender to a ciphertext (encrypted) message which is sent to a recipient. Modern computer-based encryption is done by an algorithm (which is generally publicly available to anyone) and a secret (private) encryption key. For good encryption algorithms, it is nearly impossible to decrypt the ciphertext (recover the original plaintext message from the ciphertext) without the correct key ("breaking the code" in security lingo). 

The processes of encryption and decryption are fairly straightforward:


Encryption; creating ciphertext from plaintext with a specific encryption algorithm and an encryption key


In private key cryptography, the decryption key in the image below is identical to the encryption key in the image above.


Decryption; recovering plaintext from cipertext using a specific encryption algorithm and a decryption key