hashing

What is a Hash function?

A hash function H accepts a variable-length block of data M as input and produces a fixed-size result h = H(M), referred to as a hash value or a hash code.

A “good” hash function has the property that the results of applying the function to a large set of inputs will produce outputs that are evenly distributed and apparently random.

Use Cases of Hash Functions in Computer Security

1. Password Storage:

   • Hash functions are used to securely store passwords. Instead of storing the actual password, systems store the hash of the password. When a user logs in, the system hashes the entered password and compares it with the stored hash.

2. Data Integrity Checks:

   • Hash functions help in verifying the integrity of data transferred over unreliable networks. By comparing the hash values before and after transmission, one can determine if the data has been altered.

3. Digital Signatures:

   • Hash functions are integral to digital signature schemes, which are used to verify the authenticity and integrity of a message. The hash of the message is encrypted with a private key to create a signature that can be verified with the corresponding public key.

4. Message Authentication Codes (MACs):

   • MACs use hash functions combined with a secret key to ensure both the integrity and authenticity of a message. They are crucial in environments where security is essential, such as in banking and online transactions.

5. Blockchain and Cryptocurrency:

   • In blockchain technology, hash functions are used to create a unique fingerprint for each block and to ensure the link to the previous block, thus securing the blockchain against tampering.

6. Secure Random Number Generation:

   • Hash functions are used in the generation of secure random numbers, which are crucial for cryptography, simulations, and gaming.

7. File or Data Fingerprinting:

   • Hash functions are used to generate unique identifiers or fingerprints for files or data. This can be used for quick comparisons, such as checking if a file already exists or if it has been altered.

8. Deduplication:

   • In storage systems, hash functions can identify duplicate data blocks and thus reduce the amount of storage needed by only keeping one copy of identical data blocks. election

9. Anti-Tamper Mechanisms:

   • Software and hardware can use hash functions to detect unauthorized modifications. A valid hash on the system ensures that it has not been tampered with.

Commitment

Especially in bitcoin Hashing is used to establish a commitment. Example 1 P2SH where the hash of the Script is stored in the TxOut rather than the entire script, so that the fee of having that specialized spending condition has to be paid by the receiver who needs that spending condition in first place.

Example 2 A coin in bitcoin is a UTXO which is a chain of transactions and this chain is established because of how TxIds are chained. A TxId is just the hash of the transaction and it is included in TxIn of every transaction.