5.1.2 Encryption

Encryption—one of the central operations in cryptography—is the principal mechanism used to protect information as it traverses a communication channel. It converts the original message, or plaintext, into an encoded sequence, or ciphertext, that appears as a random stream of symbols. Only an authorized recipient possessing the correct key can reconstruct the original information through decryption.

Encryption supports several of the security services described earlier:

Confidentiality, by protecting information from interception and disclosure.
Authentication, by confirming that only authorized transmitters can generate valid ciphertext.
Integrity, by assuring that any modification to the ciphertext during transmission will be detected during decryption.

The basic structure of a secure communication system is as follows. Data pass through an encryption stage before entering the transmission channel and are recovered by a corresponding decryption stage at the receiver. Both transformations are governed by a shared secret key, the management of which is typically the responsibility of a trusted third party.