## What are the properties of secure hash function?

One of the hardest concepts my students had grasping was secure cryptographic hash functions, partially because of the number theory, but also in differentiating between the three properties of a secure hash function: collision resistance, preimage resistance, and second preimage resistance.

## What hash function is secure?

Secure Hash Function (SHA) The original version is SHA-0, a 160-bit hash function, was published by the National Institute of Standards and Technology (NIST) in 1993.

**Why is hash function used?**

Hash functions are used for data integrity and often in combination with digital signatures. With a good hash function, even a 1-bit change in a message will produce a different hash (on average, half of the bits change). With digital signatures, a message is hashed and then the hash itself is signed.

**How many hash functions are there?**

The SHA-2 family consists of six hash functions with digests (hash values) that are 224, 256, 384 or 512 bits: SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, SHA-512/256.

### What are good hash functions?

Characteristics of a Good Hash Function. There are four main characteristics of a good hash function: 1) The hash value is fully determined by the data being hashed. 2) The hash function uses all the input data. 3) The hash function “uniformly” distributes the data across the entire set of possible hash values.

### What is a good hash function?

A good hash function should map the expected inputs as evenly as possible over its output range. That is, every hash value in the output range should be generated with roughly the same probability.

**Can a perfect hash function be made?**

A perfect hash function can be constructed that maps each of the keys to a distinct integer, with no collisions. These functions only work with the specific set of keys for which they were constructed. Passing an unknown key will result a false match or even crash.

**Which hash algorithm is fastest?**

SHA-1 is fastest hashing function with ~587.9 ms per 1M operations for short strings and 881.7 ms per 1M for longer strings. MD5 is 7.6% slower than SHA-1 for short strings and 1.3% for longer strings.

## How do you optimize a hash function?

In general, to optimize a hash table, you want to minimize collisions in the determination of your hash, so your buckets won’t contain more than one item and the hash-search will return immediately. Ensure there are no collisions. If there are no collisions, you are guaranteed O(1) constant look-up time.