Symmetric Algorithms
Symmetric cryptographic algorithms are based on the sender and receiver of the message knowing and using the same secret key. The sender uses a secret key to encrypt the message, and the receiver uses the same key to decrypt it. The main problem with using the symmetric key approach is finding a way to distribute the key without anyone else obtaining it. Anyone who overhears or intercepts the key in transit can later read and modify messages encrypted or authenticated using that key, and can forge new messages. DES, 3DES, and AES are popular symmetric encryption algorithms. A detailed explanation of these algorithms is beyond the scope of this book.
Note
DES uses a 56-bit key and is not considered secure anymore; in 1999, the DES key was cracked in less than 24 hours by using an exhaustive key search. Triple DES (3DES) and AES are the recommended encryption algorithms as of this writing.