The hidden volume can have its own separate file system and uses a different password, encryption algorithm and encryption key from that of the host volume, referred to as the "outer" volume. Which volume is actually mounted depends on the password provided; if the password to the outer volume is provided, the outer volume is mounted, and likewise for the inner volume.

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If the disk encryption hardware is integrated with the media itself the media may be designed for better integration. One example of such design would be through the use of physical sectors slightly larger than the logical sectors.

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Full disk encryption does not replace file or directory encryption in all situations. Disk encryption is sometimes used in conjunction with filesystem-level encryption with the intention of providing a more secure implementation. Since disk encryption generally uses the same key for encrypting the whole volume, all data are decryptable when the system runs.

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A hash function is any well-defined procedure or mathematical function which converts a large, possibly variable-sized amount of data into a small datum, usually a single integer that may serve as an index into an array.

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Thus, the hash function only hints at the record's location — it tells where one should start looking for it. Still, in a half-full table, a good hash function will typically narrow the search down to only one or two entries. Hash functions are an essential ingredient of the Bloom filter, a compact data structure that provides an enclosing approximation to a set of keys.

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Hash tables often contain only a small subset of the valid inputs. For instance, a club membership list may contain only a hundred or so member names, out of the very large set of all possible names. In these cases, the uniformity criterion should hold for almost all typical subsets of entries that may be found in the table, not just for the global set of all possible entries.

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In many applications, the range of hash values may be different for each run of the program, or may change along the same run (for instance, when a hash table needs to be expanded). In those situations, one needs a hash function which takes two parameters — the input data z, and the number n of allowed hash values.

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The same technique can be used to map two-letter country codes like 'us' or 'za' to country names (26²=676 table entries), 5-digit zip codes like 13083 to city names (100000 entries), etc. Invalid data values (such as the country code 'xx' or the zip code 00000) may be left undefined in the table, or mapped to some appropriate 'null' value.

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These protocols were proven secure in the so-called random oracle model (or even stronger variants), and the first protocols proven secure under standard assumptions were those of O. Goldreich and Y. Lindell (Crypto 2001) and J. Katz, R. Ostrovsky, and M. Yung (Eurocrypt 2001).

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Before the modern era, cryptography was concerned solely with message confidentiality (i.e., encryption) — conversion of messages from a comprehensible form into an incomprehensible one and back again at the other end, rendering it unreadable by interceptors or eavesdroppers without secret knowledge (namely the key needed for decryption of that message). In recent decades, the field has expanded beyond confidentiality concerns to include techniques for message integrity checking, sender/receiver identity authentication, digital signatures, interactive proofs and secure computation, among others.

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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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In recent times, IBM personnel designed the algorithm that became the Federal (ie, US) Data Encryption Standard; Whitfield Diffie and Martin Hellman published their key agreement algorithm,[9]; and the RSA algorithm was published in Martin Gardner's Scientific American column.

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In the use of off-line electronic money, the merchant does not need to interact with the bank before accepting a coin from the user. Instead he can collect multiple coins Spent by users and Deposit them later with the bank. In principle this could be done off-line, i.e. the merchant could go to the bank with his storage media to exchange e-cash for cash.

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Nevertheless the merchant is guaranteed that the user's e-coin will either be accepted by the bank, or the bank will be able to identify and punish the cheating user. In this way a user is prevented from spending the same coin twice (double-spending). Off-line e-cash schemes also need to protect against cheating merchants, i.e. merchants that want to deposit a coin twice (and then blame the user).

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The main focuses of digital cash development are 1) being able to use it through a wider range of hardware such as secured credit cards; and 2) linked bank accounts that would generally be used over an internet means, for exchange with a secure micropayment system such as in large corporations (PayPal).

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Cypherpunk, cypherpunks or cpunks are also occasionally used as a username and password on websites which require registration, especially if the user does not intend to return or does not wish to reveal information about himself. The account is left for later users. As of 2007, username "cypherpunks01" with password "cypherpunks01" seems to be one of the few of these "public use accounts" which seems to be widely available

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There are several strategies which contribute to making the e-mail so handled (more, or less) anonymous. In general, different classes of anonymous remailers differ with regard to the choices their designers/operators have made. These choices can be influenced by the legal ramifications of operating specific types of remailers

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However, if the IP source address is false, there will be no easy way to trace the originating node (and so the originating entity for the packet). In addition, all standards-based email messages contain defined fields in their headers in which the source and transmitting entities (and Internet nodes as well) are required to be included. However, since most users of email do not have very much technical expertise, the full headers are usually suppressed by mail reading software.

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Some remailers change both types of address in messages they forward, and the list of forwarding nodes in email messages as well, as the message passes through; in effect, they substitute 'fake source addresses' for the originals.

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The output from the F-function is then combined with the other half of the block, and the halves are swapped before the next round. After the final round, the halves are not swapped; this is a feature of the Feistel structure which makes encryption and decryption similar processes.

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