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.

It depends on what type of water related potential energy you are tapping into to generate the power. There are essentially four types:

* Tidal power where you use the tides to fill abasindururing high tide and drain the water through a turbine during ebb and low tdes. * Wave power where you use the oscillation of the waves to activate pumps or turbine to store water at an elevation or generate power directly * Hydropower generated by water diverted from a natural reservoir where it originally fell over a cliff and now powers your turbines * Essentially the same as the above but the water is contained in a reservoir you construct by damming a river or stream then use the head to power your generators. All of these systems pollute and degrade the environment in some small measure by the construction of their infrastructure (roads, power lines, control buildings etc)

Only the creation of reservoirs has an ongoing emission, that is methane from the anaerobic decomposition of vegetation at the bottom of the reservoir.

Answer: Data encryption is the process of scrambling stored or transmitted information so that it is unintelligible until it is unscrambled by the intended recipient. Historically, data encryption has been used primarily to protect diplomatic and military secrets from foreign governments. It is also now used increasingly by the financial industry to protect money transfers, by merchants to protect credit-card information in electronic commerce, and by corporations to secure sensitive communications of proprietary information. Encryption systems are often grouped into families. Common families include symmetric systems (e.g. AES) and asymmetric systems (e.g. RSA), or may be grouped according to the central algorithm used (e.g. elliptic curve cryptography). As each of these is of a different level of cryptographic complexity, it is usual to have different key sizes for the same level of security, depending upon the algorithm used. For example, the security available with a 1024-bit key using asymmetric RSA is considered approximately equal in security to an 80-bit key in a symmetric algorithm (Source: RSA Security). The actual degree of security achieved over time varies, as more computational power and more powerful mathematical analytic methods become available. For this reason cryptologists tend to look at indicators that an algorithm or key length shows signs of potential vulnerability, to move to longer key sizes or more difficult algorithms. For example as of May 2007, a 1039 bit integer was factored, with the special number field sieve using 400 computers over 11 months. The factored number was of a special form; the special number field sieve cannot be used on RSA keys. The computation is roughly equivalent to breaking a 700 bit RSA key. However, this might be an advanced warning that 1024 bit RSA used in secure online commerce should be deprecated, since they may become breakable in the near future. Cryptography professor Arjen Lenstra observed that "Last time, it took nine years for us to generalize from a special to a non-special, hard-to-factor number" and when asked whether 1024-bit RSA keys are dead, said: "The answer to that question is an unqualified yes 64 bit encryption indicates that the size of the key used to encrypt the messageis 64 bits. The 64-bit encryption standard was used in some early Internet and wireless communication encryption algorithms such as DES and WEP. Unfortunately, 64-bit encryption has proven too easy to decipher or crack in practice. Now, 128-bit encryption (in 3DES or TDES) have replaced the 64-bit encryption keys (DES).