C crypto hash
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FORUM FOREX INDONESIA KASKUS
A key feature of these schemes is their asymmetry: the work must be moderately hard but feasible on the requester side but easy to check for the service provider. One popular system — used in Bitcoin mining and Hashcash — uses partial hash inversions to prove that work was done, to unlock a mining reward in Bitcoin, and as a good-will token to send an e-mail in Hashcash.
The sender is required to find a message whose hash value begins with a number of zero bits. The average work that the sender needs to perform in order to find a valid message is exponential in the number of zero bits required in the hash value, while the recipient can verify the validity of the message by executing a single hash function. For instance, in Hashcash, a sender is asked to generate a header whose bit SHA-1 hash value has the first 20 bits as zeros. The sender will, on average, have to try times to find a valid header.
File or data identifier[ edit ] A message digest can also serve as a means of reliably identifying a file; several source code management systems, including Git , Mercurial and Monotone , use the sha1sum of various types of content file content, directory trees, ancestry information, etc.
Hashes are used to identify files on peer-to-peer filesharing networks. For example, in an ed2k link , an MD4 -variant hash is combined with the file size, providing sufficient information for locating file sources, downloading the file, and verifying its contents.
Magnet links are another example. Such file hashes are often the top hash of a hash list or a hash tree which allows for additional benefits. One of the main applications of a hash function is to allow the fast look-up of data in a hash table. Being hash functions of a particular kind, cryptographic hash functions lend themselves well to this application too.
However, compared with standard hash functions, cryptographic hash functions tend to be much more expensive computationally. For this reason, they tend to be used in contexts where it is necessary for users to protect themselves against the possibility of forgery the creation of data with the same digest as the expected data by potentially malicious participants. Hash functions based on block ciphers[ edit ] There are several methods to use a block cipher to build a cryptographic hash function, specifically a one-way compression function.
The methods resemble the block cipher modes of operation usually used for encryption. Many well-known hash functions, including MD4 , MD5 , SHA-1 and SHA-2 , are built from block-cipher-like components designed for the purpose, with feedback to ensure that the resulting function is not invertible. SHA-3 finalists included functions with block-cipher-like components e. A standard block cipher such as AES can be used in place of these custom block ciphers; that might be useful when an embedded system needs to implement both encryption and hashing with minimal code size or hardware area.
However, that approach can have costs in efficiency and security. The ciphers in hash functions are built for hashing: they use large keys and blocks, can efficiently change keys every block, and have been designed and vetted for resistance to related-key attacks.
General-purpose ciphers tend to have different design goals. In particular, AES has key and block sizes that make it nontrivial to use to generate long hash values; AES encryption becomes less efficient when the key changes each block; and related-key attacks make it potentially less secure for use in a hash function than for encryption. This can be achieved by breaking the input up into a series of equally sized blocks, and operating on them in sequence using a one-way compression function.
The compression function can either be specially designed for hashing or be built from a block cipher. The last block processed should also be unambiguously length padded ; this is crucial to the security of this construction. This design causes many inherent flaws, including length-extension , multicollisions, [9] long message attacks, [10] generate-and-paste attacks,[ citation needed ] and also cannot be parallelized.
For these other primitives to be cryptographically secure, care must be taken to build them correctly. Message authentication codes MACs also called keyed hash functions are often built from hash functions. Just as block ciphers can be used to build hash functions, hash functions can be used to build block ciphers.
Luby-Rackoff constructions using hash functions can be provably secure if the underlying hash function is secure. Pseudorandom number generators PRNGs can be built using hash functions. This is done by combining a secret random seed with a counter and hashing it. Often this is done by first building a cryptographically secure pseudorandom number generator and then using its stream of random bytes as keystream. SEAL is a stream cipher that uses SHA-1 to generate internal tables, which are then used in a keystream generator more or less unrelated to the hash algorithm.
Concatenation[ edit ] Concatenating outputs from multiple hash functions provide collision resistance as good as the strongest of the algorithms included in the concatenated result. The additional work needed to find the SHA-1 collision beyond the exponential birthday search requires only polynomial time. A more extensive list can be found on the page containing a comparison of cryptographic hash functions.
Collisions against MD5 can be calculated within seconds which makes the algorithm unsuitable for most use cases where a cryptographic hash is required. MD5 produces a digest of bits 16 bytes. Government's Capstone project.
You only need one class per hash algorithm Indeed, as you mentioned in your own answer, the Sha1 class seems superfluous, since it doesn't store any state. However, instead of creating static functions inside a Sha1 namespace, you could make those functions member functions of the class that holds the actual state.
So you would have to add functions to create a HMAC to each namespace that implements a hash algorithm. Allow hashes to be updated The code you wrote only performs one-shot conversions of some input to a hash. However, it is not uncommon for programs to not have all the data they want to create a hash for in a single, contiguous memory region. You could add an explicit finish function or call this implicitly when accessing the digest result.