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Back Door
An alternate, usually secret, method to decrypt an encrypted file or message. This method is usually only known to a few people, such as the author of the program. The presence of a back door greatly reduces the reliability of the implementation of an encryption algorithm.

Back-up
A copy of a program or data file for the purposes of protecting against loss if the original data becomes unavailable.

Back-up Operation
A method of operations to complete essential tasks as identified by a risk analysis. These tasks would be employed following a disruption of the MIS and continue until the MIS is acceptably restored. See also: Disaster Recovery, Contingency Operations.

Biometric Scans
Electronic retina, iris, voice, face or fingerprint scans used to prove who you are. This electronic information helps authenticate a user alone or in conjunction with other verifying information like passwords.

Birthday Attack
A brute-force attack used to find collisions. It gets its name from the surprising result that the probability of two or more people in a group of 23 sharing the same birthday is greater than ½.

Bit
A binary digit, either 1 or 0.

Blind Signature Scheme
Allows one party to have a second party sign a message without revealing any (or very little) information about the message to the second party.

Block
A sequence of bits of fixed length; longer sequences of bits can be broken down into blocks.

Block Cipher
A symmetric cipher which encrypts a message by breaking it down into blocks and encrypting each block.

Bluejacking (Bluetooth hacking)
Bluejacking is the sending of unsolicited messages over Bluetooth to Bluetooth-enabled devices such as mobile phones, PDAs or laptop computers, sending a vCard which typically contains a message in the name field to another bluetooth enabled device via the OBEX protocol.

Bluetooth has a very limited range, usually around 10 metres on mobile phones, but laptops can reach up to 100 metres with powerful transmitters.

The name originated with a user named ajack on esato.com. Jack was in a bank, searching for other BT enabled devices. When he found a Nokia 7650, he sent the owner a message saying "Buy Ericsson". He called it bluejacking, and it stuck ever since.

Bob
The name traditionally used for the second user of cryptography in a system; Alice's friend.

Break
The result of a successful cryptanalytic attack. To destroy the advantage of a cipher in hiding information. A cipher is "broken" when the information in a message can be extracted without the key, or when the key itself can be recovered. The strength of a cipher can be considered to be the minimum effort required for a break, by any possible attack. A break is particularly significant when the work involved need not be repeated on every message.

Brute Force Attack
This attack requires trying all (or a large fraction of all) possible values till the right value is found; also called an exhaustive search.

Bug
Technical slang for "error in design or implementation." An unexpected system flaw. Debugging is a normal part of system development and interactive system design.

Byte
A collection of eight bits. Also called an "octet." A byte can represent 256 different values or symbols. The common 7-bit ASCII codes used to represent characters in computer use are generally stored in a byte; that is, one byte per character.