may be interpreted by a computer to perform the selected operations in their given sequence and produce a desired result. Also see: software and hardware. Operating Mode With respect to block ciphers, a way to handle messages which are larger than the defined block size. Usually this means one of the four block cipher "applications" defined for use with DES: ECB or Electronic CodeBook; CBC or Cipher Block Chaining; CFB or Ciphertext FeedBack; and OFB or Output FeedBack. It can be argued that block cipher operating modes are stream "meta- ciphers" in which the streamed transformation is of full block cipher width, instead of the usual stream cipher bit- or byte-width transformations. Opponent A term used by some cryptographers to refer to the opposing cryptanalyst or opposing team. Sometimes used in preference to "the enemy." OR A Boolean logic function which is also nonlinear under mod 2 addition. Order In mathematics, typically the number of elements in a structure, or the number of steps required to traverse a cyclic structure. Ordinal In statistics, measurements which are ordered from smallest to largest. Also see: nominal, and interval. Orthogonal At right angles; on an independent dimension. Two structures which each express an independent dimension. Orthogonal Latin Squares Two Latin squares of order n, which, when superimposed, form each of the n 2 possible ordered pairs of n symbols exactly once. At most, n-1 Latin squares may be mutually orthogonal. 3 1 2 0 0 3 2 1 30 13 22 01 0 2 1 3 2 1 0 3 = 02 21 10 33 1 3 0 2 1 2 3 0 11 32 03 20 2 0 3 1 3 0 1 2 23 00 31 12 Also see Balanced Block Mixing. OTP One Time Pad. Overall Diffusion That property of an ideal block cipher in which a change of even a single message or plaintext bit will change every ciphertext bit with probability 0.5. In practice, a good block cipher will approach this ideal. This means that about half of the output bits should change for any possible change to the input block. Overall diffusion means that the ciphertext will appear to change at random even between related message blocks, thus hiding message relationships which might be used to attack the cipher. Overall diffusion can be measured statistically in a realized cipher and used to differentiate between better and worse designs. Overall diffusion does not, by itself, define a good cipher, but it is required in a good block cipher. Also see diffusion, avalanche, strict avalanche criterion and complete. Padding In classical cryptography, random data added to the start and end of messages so as to conceal the length of the message, and the position where coding actually starts. In more conventional computing, some additional data needed to fill-out a fixed-size data structure. This meaning also exists in cryptography, where the last block of a fixed-size block cipher often must be padded to fill the block. Password A key, in the form of a word. Also "pass phrase," for multiple-word keys. See: user authentication. Patent The legal right, formally granted by a government, to exclude others from making, selling or using the particular invention described in the patent deed. (The term "selling" is generally understood to cover free distribution.) Note that a patent is not the right to make the invention, if it is covered by other unexpired patents. A patent constitutes the open publication of an invention, in return for a limited-term monopoly on its use. A patent is said to protect the application of an idea (as opposed to the idea itself), and is distinct from copyright, which protects the expression of an idea. The concept behind patenting is to establish intellectual property in a way somewhat related to a mining claim or real estate. An inventor of a machine or process can file a claim on the innovation, provided that it is not previously published, and that someone else does not already have such a claim. Actual patents normally do not claim an overall machine, but just the newly-innovative part, and wherever that part is used, it must be licensed from the inventor. It is common for an inventor to refine earlier work patented by someone else, but if the earlier patent has not expired, the resulting patent often cannot be practiced without a license from the earlier patent holder. Someone who comes up with a patentable invention and wishes to give up their rights can simply publish a full description of the invention. Simple publication should prevent an application from anyone who has not already established legal proof that they previously came up with the same invention. In the U.S., publication also apparently sets a 1-year clock running for an application to be filed by anyone who does have such proof. But coming up with an invention does not take away someone else's rights if they came up with the same thing first, they may have a year to file, and their case might take several years to prosecute and issue. In the U.S., a patent is a non-renewable grant, previously lasting 17 years from issue date, now lasting 20 years from application date. Both an application fee and an issue fee are required, as are periodic "maintenance" fees throughout the life of the patent. There are four main requirements: 1. Statutory Class (35 USC 101): The invention must be either: o a process, o a machine, o a manufacture, o a composition of materials, or o a new use for one of the above. 2. Utility (35 USC 101): The invention must be of some use. 3. Novelty (35 USC 102): The invention must have some aspect which is different from all previous inventions and public knowledge. A U.S. patent is not available if before the invention date the invention was: o Publicly known or used the United States of America, or o Described in a printed publication (e.g., available at a public library) anywhere (35 USC 102(a)). A U.S. patent is not available if more than a year before the application date the invention was: o In public use or on sale in the United States of America, or o Described in a printed publication (e.g., available at a public library) anywhere (35 USC 102(b)). 4. Unobviousness (35 USC 103): The invention must have not been obvious to someone of ordinary skill in the field of the invention at the time of the invention. Unobviousness has various general arguments, such as: o Unexpected Results, o Unappreciated Advantage. o Solution of Long-Felt and Unsolved Need, and o Contrarian Invention (contrary to teachings of the prior art), among many others. When the same invention is claimed by different inventors, deciding who has "priority" to be awarded the patent can require legally provable dates for both "conception" and "reduction to practice": Conception can be proven by disclosure to others, preferably in documents which can be signed and dated as having been read and understood. The readers can then testify as to exactly what was known and when it was known. Reduction to Practice may be the patent application itself, or requires others either to watch the invention operate or to make it operate on behalf of the inventor. These events also should be carefully recorded in written documents with signatures and dates. "In determining priority of invention, there shall be considered not only the respective dates of conception and reduction to practice of the invention, but also the reasonable diligence of one who was first to conceive and last to reduce to practice . . ." (35 USC 102(g)). Also see: prior art and our claims tutorial. In practice, a patent is rarely the intrusive prohibitive right that it may at first appear to be, because patents are really about money and respect. Ideally, a patent rewards the inventor for doing research and development, and then disclosing an invention to the public; it is also a legal recognition of a contribution to society. If someone infringes a patent in a way which affects sales, or which implies that the inventor cannot do anything about it, the patent holder can be expected to show some interest. But when little or no money is involved, a patent can be infringed repeatedly with little or no response, and typically this will have no effect on future legal action. This simple introduction cannot begin to describe the complexity involved in filing and prosecuting a patent application. Your author does not recommend going it alone, unless one is willing to put far more time into learning about it and doing it than one could possibly imagine. Patent Infringement Patent infringement occurs when someone makes, sells, or uses a patented invention without license from the patent holder. Normally the offender will be contacted, and there may be a settlement and proper licensing, or the offender may be able to design around the patent, or offender may simply stop infringing. Should none of these things occur, the appropriate eventual response is a patent infringement lawsuit in federal court. Perfect Secrecy The unbreakable strength delivered by a cipher in which all possible ciphertexts may be key-selected with equal probability given any possible plaintext. This means that no ciphertext can imply any particular plaintext . Actual patents normally do not claim an overall machine, but just the newly-innovative part, and wherever that part is used, it must be licensed from the inventor. It is common for an inventor to. legal right, formally granted by a government, to exclude others from making, selling or using the particular invention described in the patent deed. (The term "selling" is generally understood. the four block cipher "applications" defined for use with DES: ECB or Electronic CodeBook; CBC or Cipher Block Chaining; CFB or Ciphertext FeedBack; and OFB or Output FeedBack.