Twofish, Advanced Encryption Standard, Blowfish, Serpent, CAST 5, RC4, TDES, and IDEA are all examples of symmetric encryption methods. In short, any algorithm that uses the same or similar cryptographic keys for encryption and decryption is a symmetric encryption method. Symmetric encryption methods are also divided into two different categories, stream ciphers and block ciphers. These two encryption methods are much like the difference between an interpreter and a compiler in programming. Stream encrypts individual bytes one at a time, while Block encrypts volumes as a whole.

Bruce Schneier has written:

'Schneier on security' -- subject(s): Computer networks, Computer security, Prevention, Privacy, Right of, Right of Privacy, Security measures, Terrorism

'The Twofish Encryption Algorithm'

'Applied Cryptography' -- subject(s): Security measures, Telecommunication, Computer security, Cryptography

'Liars and outliers' -- subject(s): Interpersonal relations, Sociology, Social stability, Social aspects, Trust, Social interaction, Truthfulness and falsehood

'Protect Your Macintosh'

'Beyond fear' -- subject(s): Terrorism, Prevention, War on Terrorism, 2001-, War on Terrorism, 2001-2009

If you're talking about symmetric key encryption (the kind where you just use one key for encryption and decryption), then arguably, the best encryption algorithm you can use is the Rijndael algorithm, better known now as AES (advanced encryption standard). It is the encryption standard used by the U.S. government for classified information. It is fast, requires little memory, and the only potential attacks against it are highly theoretical.

Rijndael beat out Twofish and Serpent in the AES standard contest, but those other two algorithms will provide more than enough security as well. In the end, it doesn't really matter, since most successful attacks are made simply by finding out your key through brute force, espionage or extortion, rather than pure data analysis.

Humans are almost always the weakest point when it comes to security, and it doesn't matter what algorithm you use if someone can guess your password.

Well because it's the newest. DES has been around since 1973 but wasn't formally a federal standard until 1976. It had to recertify every 5 years, the probem with this was....No company's came forth to go against the standard. So 30 years go by and tons of people have found ways to decipher DES. Weak keys are a major problem, and there are only a certain amount of keys allowed due to it's bit size. 2^56 keys available. It's susceptable to brute force attacks. 3DES goes through 3 rounds of DES, but it still has the same weaknesses. After 30 years of the standard, NIST called for a new standard the AES, it went with a contest design. There were 5 finalist. MARS by IBM, RC6 by RSA, Rijndael by John Daemen and Vincent Rigmen, Serpent by Ross Anderson, Eli Biham, and Lars Knudsen, and Twofish by Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, and Niels Fergson. The winner was Rigndael which was chosen in 2000.

The competition called for a block cipher using symmetric Key cryptography. It also had to support key sizes of 128, 192 and 256 bits. For big and small devices. It standardizes at 128 bits blocks.

the size of the key depends on the number of rounds of encrpytion.

Wikipedia it to find out more about how the new standard works. Since it has 3 different sizes, brute force attack with current systems would take a really long time. But as technology grows so will the need for an even stronger crypto system. 3AES may be the next standard :)