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Topic: Hash collision

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	PlanetMath: hashing
		The invariance of hashing with respect to the character of the underlying data is one of the main reasons it is such a useful technique.
		Collisions are the reason the hash function needs to spread the objects out in the hash table so they are distant from each other and “evenly” distributed.
		An abstract hash table implementation, however, will have to go to extra lengths to ensure that the tombstone is an “out-of-band” value so that no extra restrictions are put on the values of the objects which the client can store in the hash table.
	planetmath.org /encyclopedia/HashTable.html (1665 words)

	Cryptographic hash function - Wikipedia, the free encyclopedia
		A typical use of a cryptographic hash would be as follows: Alice poses to Bob a tough math problem and claims she has solved it.
		Hashes are used to identify files on peer-to-peer filesharing networks.
		Such file hashes are often the top hash of a hash list or a hash tree which allows for additional benefits.
	en.wikipedia.org /wiki/Cryptographic_hash_function (1761 words)

	Hashes and Message Digests
		Collision Resistance is a requirement on the relationship of inputs and outputs that is similar but stronger than one-way functions.
		It hashes to 128 bits, but is known to suffer from attacks on collision resistance; attacks on MD5 are known that are significantly faster than searching half the space, and more are expected to be found.
		Hashes are very useful for checking authenticity of data, but sometimes the data is so large that we don't want to recompute the hash any time some small part of it changes, since computing hashes for large amounts of data is expensive.
	www.cs.cornell.edu /Courses/cs513/2007fa/TL02.hashing.html (1433 words)

	[No title] (Site not responding. Last check: )
		Hash collisions, which cause two or more data packet addresses to transform to the same hash table entry, are handled by performing a binary tree lookup to obtain a unique forwarding address table index.
		More specifically, if the hash index identifies a non-zero entry from the first hash table which indicates no collision (the collision flag is '0', as indicated by entry 426), then a possible forwarding index can be retrieved from the first hash table as illustrated by arrow 418.
		Apparatus according to claim 1 wherein part of the hashed value is used to access the single location in the hash table and the portions of the hashed value used by the index generator are different from the part of the hashed value used to access the single location in the hash table.
	www.wipo.int /cgi-pct/guest/getbykey5?KEY=98/29986.980709&ELEMENT_SET=DECL (6148 words)

	Q94: What is a Hash Function? (Site not responding. Last check: )
		A hash function H is a transformation that takes a variable-size input m and returns a fixed-size string, which is called the hash value h (that is, h = H(m)).
		A hash function H is said to be one-way if it is hard to invert, where "hard to invert" means that given a hash value h, it is computationally infeasible to find some input x such that H(x) = h.
		Since hash functions are generally faster than digital signature algorithms, it is typical to compute the digital signature to some document by computing the signature on the document's hash value, which is small compared to the document itself.
	www.x5.net /faqs/crypto/q94.html (311 words)

	Lookup Tables
		The ideal hash function maps the actual key values uniformly onto the index type; unfortunately the ideal is not always achieved.
		For example, if a hash table is used to manage a table of identifiers in a compiler, (some) programmers are fond of names like x1, x2, x3, y1,..., z1,...
		Collisions are in fact inevitable because the key type is infinite and cannot otherwise be squeezed into a finite index range.
	www.csse.monash.edu.au /~lloyd/tildeAlgDS/Table (2864 words)

	RSA Security - 2.1.6 What is a hash function?
		A hash function H is a transformation that takes an input m and returns a fixed-size string, which is called the hash value h (that is, h = H(m)).
		Hash functions with just this property have a variety of general computational uses, but when employed in cryptography, the hash functions are usually chosen to have some additional properties.
		Since hash functions are generally faster than encryption or digital signature algorithms, it is typical to compute the digital signature or integrity check to some document by applying cryptographic processing to the document's hash value, which is small compared to the document itself.
	www.rsasecurity.com /rsalabs/node.asp?id=2176 (546 words)

	Hash Table
		With hashing =, this same element is stored in slot h(k); that is we use a hash function h to compute the slot from the key.
		When there is a collision (keys hash to the same slot), the incoming keys is stored in an overflow area and the corresponding record is appeared at the end of the linked list.
		One good approach is to derive the hash value in a way that is expected to be independent of any patterns that might exist in the data (division method).
	www.personal.kent.edu /~rmuhamma/Algorithms/MyAlgorithms/hashTable.htm (943 words)

	Storage Networking World Online - Tech Edge - What you need to know about cryptographic hashes and enterprise storage (Site not responding. Last check: )
		Cryptographic hashes are often used to verify data integrity, and recently-uncovered weaknesses in certain hash technologies could allow unauthorized users to modify or delete important records.
		Hash functions for these applications are chosen to be fast and convenient, but they're not robust against malicious attacks.
		A hash of the document is recorded in a separate location, and it is checked against the data at a later time.
	www.snwonline.com /tech_edge/what_you_need_11-01-04.asp?article_id=469 (1614 words)

	PGP Corporation - Library - CTO Corner - Much ado about hash functions
		The hash is much smaller than the text and is generated in such a way that it is unlikely some other text will produce the same hash value.] The hash function itself has its complexity in how it does that.
		By signing a hash, the resultant digital signature has a small, fixed length no matter how large the signed document is. That means they can be processed quickly and are practical to use.
		With a 16-byte hash function, the odds of a collision should be 1 in 2^128; with a 20-byte hash function, 1 in 2^160.
	www.pgp.com /library/ctocorner/hashes.html (2292 words)

	What is hashing? - a definition from Whatis.com - see also: hash, hash function
		Hashing is used to index and retrieve items in a database because it is faster to find the item using the shorter hashed key than to find it using the original value.
		The digital signature is transformed with the hash function and then both the hashed value (known as a message-digest) and the signature are sent in separate transmissions to the receiver.
		The hash function is used to index the original value or key and then used later each time the data associated with the value or key is to be retrieved.
	searchsqlserver.techtarget.com /sDefinition/0,,sid87_gci212230,00.html (976 words)

	[No title]
		Hash algorithms are used by cryptographers in a variety of security protocols, for a variety of purposes, at all levels of the Internet protocol stack.
		The length of the result is called the "hash length" and is often denoted as "L"; the result of applying the hash algorithm on a particular chunk of data is called the "hash value" for that data.
		In other words, to thwart a hash collision attack in a non- repudiation protocol where a human is using a signed message as authorization, the signer needs to keep a copy of the original message they signed.
	www.faqs.org /ftp/internet-drafts/draft-hoffman-hash-attacks-04.txt (3261 words)

	An Illustrated Guide to Cryptographic Hashes
		A "hash" (also called a "digest", and informally a "checksum") is a kind of "signature" for a stream of data that represents the contents.
		If the hash algorithm is properly designed and distributes the hashes uniformly over the output space, "finding a hash collision" by random guessing is exceedingly unlikely (it's more likely that a million people will correctly guess all the California Lottery numbers every day for a billion trillion years).
		As we've mentioned several times, "collisions" play a central role in the usefulness of a cryptographic hash, mainly in the sense that the easier it is to find a collision, the less useful the hash is. Some algorithms are better than others at avoiding collisions, and this is measured by three related attributes.
	www.unixwiz.net /techtips/iguide-crypto-hashes.html (3053 words)

	Cryptography Research - Hash Collision Q&A
		The collisions against MD4, MD5, HAVAL-128, and RIPEMD were found by the Chinese researcher Xiaoyun Wang with co-authors Dengguo Feng, Xuejia Lai, and Hongbo Yu.
		There is, however, a concern that an adversary might be able to construct a valid certificate request that had a corresponding hash collision with a certificate conferring greater or different powers.
		For example, a devastating attack would be one that enabled adversaries to obtain a legitimate server certificate with a collision to one containing a wildcard for the domain name and an expiration date far in the future.
	www.cryptography.com /cnews/hash.html (1054 words)

	[No title]
		Second, the collision-resistance of a cryptographic hash function degrades over time - the cryptographic hash function that was collision-resistant when the program was first written will no longer be collision-resistant in a few years, after other cryptographers figure out how to find hash collisions quickly (hence, the term "collision-resistant" rather than "collision-free").
		It is often far simpler to use a library-provided cryptographic hash function and compare the results than to write a full hash table implementation optimized for a particular use case or keep track of changes to the data.
		Given that, historically, popular cryptographic hash functions have a useful lifetime of around 10 years, it is only prudent to plan for the eventuality of a successful attack on the hash function.
	www.linuxworld.com /cgi-bin/mailto/x_linux.cgi?pagetosend=/export/home/httpd/linuxworld/news/2007/111207-hash.html (6186 words)

	Compare-by-hash in detail
		In compare-by-hash, we assume hash collisions never occur, so we can treat the hash of a block as a unique id and compare only the hashes of blocks rather than the contents of blocks.
		From a cryptographic point of view, collision resistance means that it is difficult to find two inputs that hash to the same output.
		We assume that the inputs to the hash function are random and uniformly distributed, and the output of the hash function is also random and uniformly distributed.
	www.usenix.org /events/hotos03/tech/full_papers/henson/henson_html/node4.html (523 words)

	RSA Security - 2.4.6 What are some techniques against hash functions?
		The essential cryptographic properties of a hash function are that it is both one-way and collision-free (see Question 2.1.6).
		While collisions for the compression function of a hash function might be useful in constructing collisions for the hash function itself, this is not normally the case.
		While pseudo-collisions might be viewed as an unfortunate property of a hash function, a pseudo-collision is not equivalent to a collision - the hash function may still be considered as reasonably secure, though its use for new applications tends to be discouraged in favor of pseudo-collision-free hash functions.
	www.rsasecurity.com /rsalabs/node.asp?id=2205 (525 words)

	Database Resource
		One solution to a collision is to put hash collision entries one after another in a linked list.
		Primary keys are used to create the hash for each record, but using them is normal and in no way guarentees hash collisions will be avoided.
		If we have no way to support hash collisions if they occur (such as using hash chains) then a hash table will break if there is any hash collisions.
	db.grussell.org /mc/q47.html (183 words)

	Software Integrity Checksum and Code Signing Vulnerability
		The cryptographic hash function MD5 was shown to be not collision resistant, by prof.
		Before the collision the two files, for which a collision is to be found, can be anything: our chosen-prefix collision finding method will always produce a collision that can be incorporated into the two files, irrespective of what data is present before the collision.
		For abusing a chosen-prefix collision on a software integrity protection or a code signing scheme, the attacker should be able to manipulate the files before they are being hashed and/or signed.
	www.win.tue.nl /hashclash/SoftIntCodeSign (2211 words)

	CIS 22: Hash Table Data Structure
		As the number of collisions increases, the distance from the array index computed by the hash function and the actual location of the element increases, increasing search time.
		For example, when the hash function produces key 79, the new node is created and the array element with index 79 now points to this node.
		When it comes to retrieve a value, the hash function is applied to the desired key and the array index is produced.
	acc6.its.brooklyn.cuny.edu /~cis22/hashing/Handling_Collisions.html (1055 words)

	[No title] (Site not responding. Last check: )
		In a first-preimage attack, you know a hash value but not the message that created it, and you want to discover that message; in the second-preimage attack, you have a message and you want to find a second message that has the same hash.
		In other words, to thwart a hash collision attack in a non-repudiation protocol where a human is using a signed message as authorization, the signer needs to keep a copy of the original message they signed.
		Thwarting hash collision attacks in automated non-repudiation protocols is potentially more difficult, because there are no humans involved to keep the signed messages or to argue about what should have happened.
	faqs.org /ftp/pub/pub/internet-drafts/draft-hoffman-hash-attacks-00.txt (3175 words)

	[No title] (Site not responding. Last check: )
		The version without collision resistance is perfectly fine for signatures on content generated by the signer; signatures with collision resistance are useful for signatures on content generated by others.
		The collision-resistant OID SHOULD be allocated as soon as a new hash algorithm is introduced; one reason is that it avoids differences in support between older and newer software; another reason is that it is good to have had protection against collisions at the time a collision attack is published.
		This initial influence on the hashing algorithm makes it impossible for an attacker to predict the context in which his data is going to be hashed.
	www.ietf.org /internet-drafts/draft-vanrein-collision-resistant-hashes-00.txt (2209 words)

	[ODE] Hash Collision space in ODE! (Site not responding. Last check: )
		As a simpler example (not hash spaces) one far field strategy is to keep 3 sorted arrays, one for each of the x y and z axes.
		Determining potential collisions now is trivial, if in a sorted list of extents, an object has some other objects extent in between its start and end extent, then that other object is potentially incident.
		In other words, it is exactly the kind of intuitive strategy that you or I might invent when faced with a problem of needing to quickly detect collisions; one more or less draws the "shadows" of the objects to a coarser resolution memory and looks for collisions.
	www.q12.org /pipermail/ode/2002-July/005422.html (541 words)

	Re: [Fwd: HMAC-MD5: to be or not to be?]
		The way the handshake hashes are used in >> SSL 3.0 requires that a collision exist in both MD5 and SHA1 for the >> same pair of message streams.
		We felt that not alternating algorithms provided at least the same level of collision intractability without the somewhat dubious mixing of algorithms that you seem to be concerned about.
		The MAC is SSL 3.0 is keyed and relies >> primarily on the secrecy of the key, not the collision intractability >> of the underlying hash function.
	lists.w3.org /Archives/Public/ietf-tls/msg00134.html (951 words)

	Nostradamus
		The cryptographic hash function MD5 that we used was shown to be not collision resistant, by prof.
		For a hash collision to be useful in realistic attack scenarios, the different messages colliding to the same hash value must be meaningful.
		In 2005 John Kelsey and Tadayoshi Kohno (see their EuroCrypt 2006 paper "Herding hash functions and the Nostradamus attack") described an attack, in which a miscreant uses hash collisions to produce one commitment for as many different messages as he wants.
	www.win.tue.nl /hashclash/Nostradamus (2980 words)

	Dynamic Hash Function / One-way and collision free (Site not responding. Last check: )
		In general a hash function is said to be "an easily implementable mapping from the set of all binary sequences to the set of binary sequences of some fixed length" [#2].
		The length of hash values is variable and depending directly on the number of data to be hashed.
		The results of the function are much shorter than conventional hash values with, for example, a fixed length of 128 bit rsp.
	www.multi-matrix.de /DYNAHASH.HTM (4076 words)

	[No title]
		To: cypherpunks@toad.com Subject: [ANNOUNCE] hash cash postage implementation From: Adam Back Date: Fri, 28 Mar 1997 16:52:26 GMT Sender: owner-cypherpunks@toad.com A partial hash collision based postage scheme I've been talking about a partial hash collision based postage scheme on the crypto lists for the last few days.
		The idea of using partial hashes is that they can be made arbitrarily expensive to compute (by choosing the desired number of bits of collision), and yet can be verified instantly.
		The exit status also tells you if the hash was ok. The database would grow indefinately as the mailer accepted new payments, so the validity period is stored in the database, and at the next use of the database after the validity period expires, the collision will be discarded.
	www.hashcash.org /papers/announce.txt (824 words)

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