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Cryptographic hash function
In cryptography, a cryptographic hash function is a hash function with certain additional security properties to make it suitable for use as a primitive in various information security applications, such as authentication and message integrity. A hash function takes a long string (or message) of any length as input and produces a fixed length string as output, sometimes termed a message digest or a digital fingerprint.
Broadly speaking, a cryptographic hash function should behave as much as possible like a random function while still being deterministic and efficiently computable.
A cryptographic hash function is considered insecure if either of the following is computationally feasible:
- finding a (previously unseen) message that matches a given digest
- finding "collisions", wherein two different messages have the same message digest.
An attacker who can do either of these things might, for example, use them to substitute an unauthorized message for an authorized one.
It should not even be feasible to find two messages whose digests are substantially similar; nor should an attacker learn anything useful about a message given only its digest.
Checksums and Cyclic redundancy checks (CRCs) are quite distinct from cryptographic hash functions, and are used for different applications. If used for security, they are vulnerable to attack; for example, a CRC was used for message integrity in the WEP encryption standard, but an attack was readily discovered which exploited the linearity of the checksum specified.
A message authentication code or MAC takes a message and a secret key and generates a "MAC tag", such that it is difficult for an attacker to generate a valid (message, tag) pair that doesn't match one they've already seen; they are used to prevent attackers forging messages, among other uses. Though it is sometimes referred to as a "keyed hash function", a MAC serves a very different purpose and has very different security properties than a cryptographic hash function; for example, it is not considered a flaw if it is easy for someone who knows the MAC key to generate two messages that have the same MAC. Hash functions can be used to create MAC functions; see for example HMAC.
There is no formal definition which captures all of the properties considered desirable for a cryptographic hash function. These properties below are generally considered prerequisites:
- Preimage resistant (See one way for a related but slightly different property): given h it should be hard to find m such that h = hash(m).
- Second preimage resistant: given an input m1, it should be hard to find another input, m2 (not equal to m1) such that hash(m1) = hash(m2).
- Collision-resistant: it should be hard to find two different messages m1 and m2 such that hash(m1) = hash(m2). Because of the birthday paradox this means the hash function must have a larger image than is required for preimage-resistance.
A hash function meeting these criteria may still have undesirable properties. For instance, most popular hash functions are vulnerable to length-extension attacks: given h(m) and len(m) but not m, by choosing a suitable m' an attacker can calculate h(m || m'), where || denotes concatenation. This property can be used to break naive authentication schemes based on hash functions. The HMAC construction works around these problems.
An ideal hash function would be maximally "boring": it would have no interesting properties such as length extension, and the only interesting way it would differ from a random function would be in that it was deterministic and efficiently computable. This criterion is of course deeply resistant to formal expression; the closest thing to formal expression is the random oracle model, which is an idealization no real hash function can satisfy.
Applications of hash functions
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. Bob would like to try it himself, but would yet like to be sure that Alice is not bluffing. Therefore, Alice writes down her solution, appends a random nonce, computes its hash and tells Bob the hash value (whilst keeping the solution secret). This way, when Bob comes up with the solution himself a few days later, Alice can verify his solution but still be able to prove that she had the solution earlier.
In actual practice, Alice and Bob will often be computer programs, and the secret would be something less easily spoofed than a claimed puzzle solution. The above application is called a commitment scheme. Another important application of secure hashes is verification of message integrity. Determination of whether or not any changes have been made to a message (or a file), for example, can be accomplished by comparing message digests calculated before, and after, transmission (or any other event) (see Tripwire, a system using this property as a defense against malware and malfeasance). A message digest can also serve as a means of reliably identifying a file. A related application is password verification. Passwords are usually not stored in cleartext, for obvious reasons, but instead in digest form. To authenticate a user, the password presented by the user is hashed and compared with the stored hash.
For both security and performance reasons, most digital signature algorithms specify that only the digest of the message be "signed", not the entire message. Hash functions can also be used in the generation of pseudorandom bits.
SHA-1, MD5, and RIPEMD-160 are among the most commonly-used message digest algorithms as of 2005. In August 2004, researchers found weaknesses in a number of hash functions, including MD5, SHA-0 and RIPEMD. This has called into question the long-term security of later algorithms which are derived from these hash functions — in particular, SHA-1 (a strengthened version of SHA-0), RIPEMD-128, and RIPEMD-160 (both strengthened versions of RIPEMD). Neither SHA-0 nor RIPEMD are widely used since they were replaced by their strengthened versions. In February 2005, an attack on SHA-1 was reported, finding collisions in about 269 hashing operations, rather than the 280 expected for a 160-bit hash function.
Hashes are used to identify files on peer-to-peer filesharing networks. For example, in a ed2k link the hash is combined with the filename and the file size, providing sufficient information for locating file sources, downloading the file and verifying its contents.
Merkle-Damgård hash functions
A hash function must be able to process an arbitrary-length message into a fixed-length output. This can be achieved by breaking the input up into a series of equal-size blocks, and operating on them in sequence using a compression function. The last block processed will also contain the length of the entire message; this is crucial to the security of this construction, which is known as the Merkle-Damgård structure. Most widely-used hash functions, including SHA-1 and MD5, take this form.
In the diagram, the compression function is denoted by f, and transforms a fixed-length input to an output of the same size. The algorithm starts with an initial value, the initialisation vector (IV). For each message block, the compression (or compacting) function f takes the result so far, combines it with the block, and produces an intermediate result. Bits representing the length of the entire message are appended to the message and padded suitably as part of the last block. The value after the last block is taken to be the hash value for the entire message.
The popularity of this construction is due to the fact, proven by Merkle and Damgård, that if the compression function f is collision-resistant, then so is the hash function constructed using it. Unfortunately, this construction also has several undesirable properties:
- length-extension attacks are always possible
- second-preimage attacks against long messages are always much more efficient than brute force
List of cryptographic hash functions
Some of the following algorithms are known to be insecure; consult the relevant articles for more information on the status of the algorithm.
- The SHA hash functions are a series of functions developed by the NSA: SHA, also known as SHA-0, SHA-1 and four flavours of a function known as SHA-2.
- Avalanche effect
- Message authentication code
- Keyed-hash message authentication code
- CRYPTREC and NESSIE (projects which recommend hash functions)
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