Hacker Professional Ebook part 179 pptx

It is now easy to construct large hardware or software systems which are almost unmanageably complex and never error-free. But a good design and development approach can produce systems with far fewer problems. One such approach is: 1. Decompose the system into small, testable components. 2. Construct and then actually test each of the components individually. This is both easier and harder than it looks: there are many ways to decompose a large system, and finding an effective and efficient decomposition can take both experience and trial-and-error. But many of the possible decompositions define components which are less testable or even untestable, so the testability criterion greatly reduces the search. Testing is no panacea: we cannot hope to find all possible bugs this way. But in practice we can hope to find 90 percent or more of the bugs simply by actually testing each component. (Component testing means that we are forced to think about what each component does, and about its requirements and limits. Then we have to make the realized component conform to those tests, which were based on our theoretical concepts. This will often expose problems, whether in the implementation, the tests, or the concepts.) By testing all components, when we put the system together, we can hope to avoid having to debug multiple independent problems simultaneously. Other important system design concepts include:  Build in test points and switches to facilitate run-time inspection, control, and analysis.  Use repeatable comprehensive tests at all levels, and when a component is "fixed," run those tests again.  Start with the most basic system and fewest components, make that "work" (pass appropriate system tests), then "add features" one-by- one. Try not to get too far before making the expanded system work again. Table Selection Combiner A combining mechanism in which one input selects a table or substitution alphabet, and another input selects a value from within the selected table, said value becoming the combined result. Also called a Polyalphabetic Combiner. TEMPEST Supposedly the acronym for "Transient Electromagnetic Pulse Emanation Surveillance Technology." Originally, the potential insecurity due to the electromagnetic radiation which inherently occurs when a current flow changes in a conductor. Thus, pulses from digital circuitry might be picked up by a receiver, and the plaintext data reconstructed. The general concept can be extended to the idea that plaintext data pulses may escape on power lines, or as a faint background signal to encrypted data, or in any other unexpected electronic way. Some amount of current change seems inevitable when switching occurs, and modern digital computation is based on such switching. But the amount of electromagnetic radiation emitted depends upon the amount of current switched, the length of the conductor, and the speed of the switching (that is, dI/dt, or the rate-of-change in current). In normal processing the amount of radiated energy is very small, but the value can be much larger when fast power drivers are used to send signals across cables of some length. This typically results in broadband noise which can be sensed with a shortwave receiver, a television, or an AM portable radio. Such receivers can be used to monitor attempts at improving the shielding. Ideally, equipment would be fully enclosed in an electrically unbroken conducting surface. In practice, the conductive enclosure may be sheet metal or screening, with holes for shielded cables. Shielding occurs not primarily from metal per se, but instead from the flow of electrical current in that metal. When an electromagnetic wave passes through a conductive surface, it induces a current, and that current change creates a similar but opposing electromagnetic wave which nearly cancels the original. The metallic surface must conduct in all directions to properly neutralize waves at every location and from every direction. Stock computer enclosures often have huge unshielded openings which are hidden by a plastic cover. These should be covered with metal plates or screening, making sure that good electrical contact occurs at all places around the edges. Note that assuring good electrical connections can be difficult with aluminum, which naturally forms a thin but hard and non- conductive surface oxide. It is important to actually monitor emission levels with receivers both before and after any change, and extreme success can be very difficult. We can at least make sure that the shielding is tight (that it electrically conducts to all the surrounding metal), that it is as complete as possible, and that external cables are effectively shielded. Cable shielding extends the conductive envelope around signal wires and into the envelope surrounding the equipment the wire goes to. Any electromagnetic radiation from within a shield will tend to produce an opposing current in the shield conductor which will "cancel" the original radiation. But if a cable shield is not connected at both ends, no opposing current can flow, and no electromagnetic shielding will occur, despite having a metallic "shield" around the cable. It is thus necessary to assure that each external cable has a shield, and that the shield is connected to a conductive enclosure at both ends. (Note that some equipment may have an isolating capacitor between the shield and chassis ground to minimize "ground loop" effects when the equipment at each end of the cable connects to different AC sockets.) When shielding is impossible, it can be useful to place ferrite beads or rings around cables to promote a balanced and therefore essentially non-radiating signal flow. Perhaps the most worrisome emitter on a personal computer is the display cathode ray tube (CRT). Here we have a bundle of three electron beams, serially modulated, with reasonable current, switching quickly, and repeatedly tracing the exact same picture typically 60 times a second. This produces a recognizable substantial signal, and the repetition allows each display point to be compared across many different receptions, thus removing noise and increasing the effective range of the unintended communication. All things being equal, a liquid-crystal display should radiate a far smaller and also more-complex signal than a desktop CRT. Transformer A passive electrical component composed of magnetically-coupled coils of wire. When AC flows through one coil or "primary," it creates a changing magnetic field which induces power in another coil. A transformer thus isolates power or signal, and also can change the voltage-to-current ratio, for example to "step down" line voltage for low-voltage use, or to "step up" low voltages for high-voltage devices (such as tubes or plasma devices). Transistor An active semiconductor component which performs analog amplification. Originally, a bipolar version with three terminals: Emitter (e), Collector (c), and Base (b). Current flow through the base-emitter junction (I be ) is amplified by the current gain or beta (B) of the device in allowing current to flow through the collector-base junction and on through the emitter (I ce ). In a sense, a bipolar transistor consists of two back-to-back diodes: the base- collector junction (operated in reverse bias) and the base-emitter junction (operated in forward bias) which influence each other. Current through the base-emitter junction releases either electrons or "holes" which are then drawn to the collector junction by the higher potential there, thus increasing collector current. The current ratio between the base input and the collector output is amplification. Field-Effect Transistors (FET's, as in MOSFET, etc.) have an extremely high input impedence, taking essentially no input current, and may be more easily fabricated in integrated circuits than bipolars. In an FET, Drain (d) and Source (s) contacts connect to a "doped" semiconductor channel. Extremely close to that channel, but still insulated from it, is a conductive area connected to a Gate (g) contact. Voltage on the gate creates an electrostatic field which interacts with current flowing in the drain-source channel, and can act to turn that current ON or OFF, depending on channel material (P or N), doping (enhancement or depletion), and gate polarity. Sometimes the drain and source terminals are interchangeable, and sometimes the source is connected to the substrate. Instead of an insulated gate, we can also have a reverse-biased diode junction, as in a JFET. N-channel FET's generally work better than p-channel devices. JFET's can only have "depletion mode," which means that, with the gate grounded to the source, they are ON. N-channel JFET devices go OFF with a negative voltage on the gate. Normally, MOSFET devices are "enhancement mode" and are OFF with their gate grounded. N-channel MOSFET devices go ON with a positive voltage (0.5 to 5v) on the gate. Depletion mode n-channel MOSFET devices are possible, but not common. Transposition The exchange in position of two elements. The most primitive possible permutation or re-ordering of elements. Any possible permutation can be constructed from a sequence of transpositions. Trap Door A cipher design feature, presumably planned, which allows the apparent strength of the design to be easily avoided by those who know the trick. Similar to back door. Triple DES The particular block cipher which is the U.S. Data Encryption Standard or DES, performed three times, with two or three different keys. Truly Random A random value or sequence derived from a physical source. Also called really random and physically random. Trust The assumption of a particular outcome in a dependence upon someone else. Trust is the basis for communications secrecy: While secrecy can involve keeping one's own secrets, communications secrecy almost inevitably involves at least a second party. We thus necessarily "trust" that party with the secret itself, to say nothing of cryptographic keys. It makes little sense to talk about secrecy in the absence of trust. In a true security sense, it is impossible to fully trust anyone: Everyone has their weaknesses, their oversights, their own agendas. But normally "trust" involves some form of commitment by the other party to keep any secrets that occur. Normally the other party is constrained in some way, either by their own self-interest, or by contractual, legal, or other consequences of the failure of trust. The idea that there can be any realistic trust between two people who have never met, are not related, have no close friends in common, are not in the same employ, and are not contractually bound, can be a very dangerous delusion. It is important to recognize that no trust is without limit, and those limits are precisely the commitment of the other party, bolstered by the consequences of betrayal. Trust without consequences is necessarily a very weak trust. Truth Table Typically, a Boolean function expressed as the table of the value it will produce for each possible combination of input values. Type I Error In statistics, the rejection of a true null hypothesis. Type II Error In statistics, the acceptance of a false null hypothesis. Unary . secrets, communications secrecy almost inevitably involves at least a second party. We thus necessarily "trust" that party with the secret itself, to say nothing of cryptographic keys. It. normally "trust" involves some form of commitment by the other party to keep any secrets that occur. Normally the other party is constrained in some way, either by their own self-interest,. design to be easily avoided by those who know the trick. Similar to back door. Triple DES The particular block cipher which is the U.S. Data Encryption Standard or DES, performed three times,