Such software encrypts data stored on a computer's mass storage and transparently decrypts the information when an authorized user requests it: no special action by the user (except supplying a password or passphrase at the beginning of a session) is required. Some also provide plausible deniability with deniable encryption techniques.

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Examples of software that can be used for file encryption are special-purpose software (e.g., GNU Privacy Guard and PGP), file archivers, and even some text editors (e.g., emacs or vi). This section lists features commonly found in disk encryption software, though not all functions may be provided in any given system. Some disk encryption systems offer two levels of Plausible Deniability, which might be useful in case a user is required to reveal the password of an encrypted volume.

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In the event the hidden volume user is forced to reveal their password, he can divulge the password to the outer volume (not disclosing the fact that he actually has a hidden volume within), and sensitive data within the hidden volume are not compromised; provided certain additional precautions are taken in overwriting the free areas of the "host" disk.

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This is particularly important for sensitive data like credit card numbers. The purpose of encryption is to prevent third parties from recovering the original information. Once a hidden volume has been created inside another volume, the user will store important-looking information (but which the user does not actually mind revealing) on the outer volume, whereas sensitive information is stored within the hidden volume.

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Encrypted volumes contain "header" (or "CDB") data, which may be backed up. Overwriting these data will destroy the volume, so the ability to back them up is useful. Restoring the backup copy of these data may reset the volume's password to what it was when the backup was taken.

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Most Full Disk Encryption solutions utilize Pre-Boot Authentication by loading a small, highly secure operating system which is strictly locked down and hashed versus system variables to check for the integrity of the Pre-Boot kernel. Some implementations such as BitLocker Drive Encryption can make use of hardware such as a Trusted Platform Module to ensure the integrity of the boot environment.

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In these applications, the set of all inputs is some sort of metric space, and the hashing function can be interpreted as a partition of that space into a grid of cells. The table is often an array with two or more indices (called a grid file, grid index, bucket grid, and similar names), and the hash function returns an index tuple. This special case of hashing is known as geometric hashing or the grid method.

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A perfect function for a given set of n keys, suitable for use in a hash table, can be found in time proportional to n, can be represented in less than 3*n bits, and can be evaluated in a constant number of operations. There are generators that will emit optimized executable code to evaluate a perfect hash for a given input set.

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An early example, from Herodotus, concealed a message - a tattoo on a slave's shaved head - under the regrown hair.[2] More modern examples of steganography include the use of invisible ink, microdots, and digital watermarks to conceal information. The earliest forms of secret writing required little more than local pen and paper analogs, as most people could not read.

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Nonetheless, good modern ciphers have stayed ahead of cryptanalysis; it is typically the case that use of a quality cipher is very efficient (i.e., fast and requiring few resources), while breaking it requires an effort many orders of magnitude larger than before, making cryptanalysis so inefficient and impractical as to be effectively impossible.

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Cryptography is, also, a branch of engineering, but an unusual one as it deals with active, intelligent, and malevolent opposition (see cryptographic engineering and security engineering); most other kinds of engineering need deal only with neutral natural forces. There is also active research examining the relationship between cryptographic problems and quantum physics (see quantum cryptography and quantum computing).

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It was named after Julius Caesar who is reported to have used it, with a shift of 3, to communicate with his generals during his military campaigns, just like EXCESS-3 code in boolean algebra. Some of the existing work-around designs work, more or less, but are not fully satisfactory.

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In the case of Octopus Card in Hong Kong, deposits work similarly to banks'. After Octopus Card Limited receives money for deposit from users, the money is deposited into banks, which is similar to debit-card-issuing banks redepositing money at central banks.

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Keys are important, as ciphers without variable keys are trivially breakable and therefore less than useful for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks.

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Once a symmetric key is known to all parties of the session, faster symmetric-key algorithms using that key can be used to encrypt the remainder of the session. This simplifies the key distribution problem, because asymmetric keys only have to be distributed authentically, whereas symmetric keys need to be distributed in an authentic and confidential manner.

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The resultant ciphertext replaces P1 and P2. The ciphertext is then encrypted again with the new subkeys, and P3 and P4 are replaced by the new ciphertext. This continues, replacing the entire P-array and all the S-box entries. In all, the Blowfish encryption algorithm will run 521 times to generate all the subkeys - about 4KB of data is processed.

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This prevents its use in certain applications, but is not a problem in others. In one application, it is actually a benefit: the password-hashing method used in OpenBSD uses an algorithm derived from Blowfish that makes use of the slow key schedule; the idea is that the extra computational effort required gives protection against dictionary attacks. See key strengthening.

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The belief that Blowfish is the fastest algorithm is widespread, although currently it's not true on mainstream PC hardware. It remains the case on some embedded CPUs (e.g. Atom, ARM[7]). Encryption speed is more likely to be a bottleneck on these slower systems, so Blowfish is certainly useful there.

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It is based on a symmetric-key algorithm that uses a 56-bit key. The algorithm was initially controversial with classified design elements, a relatively short key length, and suspicions about a National Security Agency (NSA) backdoor. DES consequently came under intense academic scrutiny which motivated the modern understanding of block ciphers and their cryptanalysis.

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DES is now considered to be insecure for many applications. This is chiefly due to the 56-bit key size being too small; in January, 1999, distributed.net and the Electronic Frontier Foundation collaborated to publicly break a DES key in 22 hours and 15 minutes (see chronology).

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