Symmetric encryption uses the same secret key, or closely related secret material, for encryption and decryption.

An encryption algorithm transforms plaintext $m$ into ciphertext $c$ using key $k$:

$$c=E_k(m).$$

Decryption recovers the plaintext:

$$m=D_k(c).$$

The intended security goal is confidentiality: without the key, the ciphertext should not reveal useful information about the plaintext under the scheme's security assumptions.

Modern encryption must also handle repeated messages safely. Secure schemes incorporate a nonce, initialization value or randomized mechanism so identical plaintexts do not simply reveal identical ciphertext patterns.

Encryption alone does not necessarily detect tampering. A ciphertext can be confidential while still being malleable, which is why practical protocols usually require integrity and authenticity in addition to secrecy.

The cryptographic algorithm can be public; security should depend on protecting the key, not on hiding how the algorithm works.