A blockchain is a distributed ledger with growing lists of records (blocks) that are securely linked together via cryptographic hashes.Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data (generally represented as a Merkle tree, where data nodes are represented by leaves). Since each block contains information about the previous block, they effectively form a chain (compare linked list data structure), with each additional block linking to the ones before it. Consequently, blockchain transactions are irreversible in that, once they are recorded, the data in any given block cannot be altered retroactively without altering all subsequent blocks.
A cryptographic hash function (CHF) is a hash algorithm (a map of an arbitrary binary string to a binary string with a fixed size of n bits that has special properties desirable for a cryptographic application
-The probability of a particular n-bit output result (hash value) for a random input string ("message") (as for any good hash), so the hash value can be used as a representative of the message;-Finding an input string that matches a given hash value (a pre-image) is unfeasible, assuming all input strings are equally likely. The resistance to such search is quantified as security strength, a cryptographic hash with n bits of hash value is expected to have a preimage resistance strength of n bits, unless the space of possible input values is significantly smaller than 2^(n) (a practical example can be found in § Attacks on hashed passwords);-A second preimage resistance strength, with the same expectations, refers to a similar problem of finding a second message that matches the given hash value when one message is already known;-Finding any pair of different messages that yield the same hash value (a collision) is also unfeasible, a cryptographic hash is expected to have a collision resistance strength of n/2 bits (lower due to the birthday paradox).
Cryptographic hash functions have many information-security applications, notably in digital signatures, message authentication codes (MACs), and other forms of authentication. They can also be used as ordinary hash functions, to index data in hash tables, for fingerprinting, to detect duplicate data or uniquely identify files, and as checksums to detect accidental data corruption. Indeed, in information-security contexts, cryptographic hash values are sometimes called (digital) fingerprints, checksums, or just hash values, even though all these terms stand for more general functions with rather different properties and purposes.Non-cryptographic hash functions are used in hash tables and to detect accidental errors, their construction frequently provides no resistance to a deliberate attack. For example, a denial-of-service attack on hash tables is possible if the collisions are easy to find, like in the case of linear cyclic redundancy check (CRC) functions.
Password verificationFile or data identifierContent-addressable storageVerifying the integrity of msg and filesHash functions we buy\sell as cloud resource:- sha-1 - sha-2 - sha-3 - sha-256 - keccak - scrypt - blake