Unlike full disk encryption, filesystem-level encryption does not typically encrypt filesystem metadata, such as the directory structure, file names, modification timestamps or sizes. Full disk encryption and Trusted Platform Module.

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An Emergency Recovery Information (ERI) file provides an alternative for recovery if a challenge response mechanism is unfeasible due to the cost of helpdesk operatives for small companies or implementation challenges. Challenge/response password recovery mechanism.

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An Emergency Recovery Information (ERI) file provides an alternative for recovery if a challenge response mechanism is unfeasible due to the cost of helpdesk operatives for small companies or implementation challenges.

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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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In general, a hashing function may map several different keys to the same index. Therefore, each slot of a hash table is associated with (implicitly or explicitly) a set of records, rather than a single record. For this reason, each slot of a hash table is often called a bucket, and hash values are also called bucket indices.

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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 some applications, the input data may contain features that are irrelevant for comparison purposes. When looking up a personal name, for instance, it may be desirable to ignore the distinction between upper and lower case letters. For such data, one must use a hash function that is compatible with the data equivalence criterion being used: that is, any two inputs that are considered equivalent must yield the same hash value.

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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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For example, text in any natural language has highly non-uniform distributions of characters, and character pairs, very characteristic of the language. For such data, it is prudent to use a hash function that depends on all characters of the string — and depends on each character in a different way.

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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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The detailed operation of a cipher is controlled both by the algorithm and in each instance by a key. This is a secret parameter (ideally known only to the communicants) for a specific message exchange context. Keys are important, as ciphers without variable keys are trivially breakable and therefore less than useful for most purposes.

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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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The potential effects of quantum computing are already being considered by some cryptographic system designers; the announced imminence of small implementations of these machines may be making the need for this preemptive caution less than merely speculative.

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Note that if neither Alice nor Bob wishes the bank to know that they performed a transaction with each other, then it is hard for the bank to find out. However, in order to ensure this is the case many people need to be making transactions at the same time. Otherwise the bank can figure it out by the timing of the transactions, using traffic analysis.

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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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In modern cryptosystems designs, both asymmetric (public key) and symmetric algorithms are used to take advantage of the virtues of both. Asymmetric algorithms are used to distribute symmetric-keys at the start of a session. 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.

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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 DES can be said to have "jump started" the nonmilitary study and development of encryption algorithms. In the 1970s there were very few cryptographers, except for those in military or intelligence organizations, and little academic study of cryptography. There are now many active academic cryptologists, mathematics departments with strong programs in cryptography, and commercial information security companies and consultants.

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Various minor cryptanalytic properties are known, and three theoretical attacks are possible which, while having a theoretical complexity less than a brute force attack, require an unrealistic amount of known or chosen plaintext to carry out, and are not a concern in practice.

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