# Hash collision – Wikipedia

John Smith and Sandra Dee share the same hash value of 02, causing a hash collision. In calculator skill, a hash collision or clash [ citation needed ] is when two pieces of data in a hashish table plowshare the same hash value. The hashish value in this case is derived from a hash routine which takes a datum input and returns a fix duration of bits. [ 1 ] Although hash algorithms have been created with the purpose of being collision immune, they can still sometimes map different data to the lapp hash ( by virtue of the cubbyhole principle ). malicious users can take advantage of this to mimic, access, or interpolate data. [ 2 ] due to the potential negative applications of hash collisions in data management and calculator security ( in particular, cryptanalytic hash functions ), collision avoidance has become an authoritative topic in calculator security.

## background

Hash collisions can be ineluctable depending on the number of objects in a typeset and whether or not the moment string they are mapped to is hanker enough in length. When there is a located of n objects, if n is greater than | R |, which in this case R is the range of the hashish value, the probability that there will be a hash collision is 1, meaning it is guaranteed to occur. [ 3 ] Another reason hash collisions are probable at some point in time stems from the mind of the birthday paradox in mathematics. This problem looks at the probability of a jell of two randomly chosen people having the same birthday out of n total of people. [ 4 ] This estimate has led to what has been called the birthday attack. The precede of this attack is that it is unmanageable to find a birthday that specifically matches your birthday or a specific birthday, but the probability of finding a set of any two people with matching birthdays increases the probability greatly. Bad actors can use this set about to make it simpler for them to find hash values that collide with any other hash prize – rather than searching for a specific value. [ 5 ] The shock of collisions depends on the application. When hash functions and fingerprints are used to identify exchangeable data, such as homologous DNA sequences or alike audio files, the functions are designed so as to maximize the probability of collision between distinct but exchangeable data, using techniques like locality-sensitive hash. [ 6 ] Checksums, on the early pass, are designed to minimize the probability of collisions between similar inputs, without regard for collisions between very unlike inputs. [ 7 ] Instances where bad actors attempt to create or find hashish collisions are known as collision attacks. [ 8 ] In commit, security-related applications use cryptanalytic hash algorithm, which are designed to be long enough for random matches to be unlikely, debauched enough that they can be used anywhere, and safe enough that it would be highly hard to find collisions. [ 7 ]

## probability of occurrence

Hash collisions can occur by luck and can be intentionally created for many hashish algorithm. The probability of a hash collision therefore depends on the size of the algorithm, the distribution of hash values, and whether or not it is both mathematically known and computationally feasible to create particular collisions. Take into account the succeed hashish algorithms – CRC-32, MD5, and SHA-1. These are common hash algorithm with varying levels of collision risk. [ 9 ]

CRC-32 poses the highest risk for hash collisions. This hash officiate is generally not recommended for use. If a hub were to contain 77163 hashish values, the gamble of a hashish collision occurring is 50 %, which is extremely high compared to other methods. [ 10 ]

### MD5

MD5 is the most normally used and when compared to the early two hash functions, it represents the middle ground in terms of hash collision risk. In ordain to get a 50 % luck of a hash collision occurring, there would have to be over 5.06 billion records in the hub [ 10 ]
SHA-1 offers the lowest risk for hashish collisions. For a SHA-1 function to have a 50 % chance of a hash collision occurring, there would have to be 1.42 ten 10²⁴ records in the hub. note, the number of records mentioned in these examples would have to be in the same hub. [ 10 ] Having a hub with a smaller number of records could decrease the probability of a hash collision in all of these hashish functions, although there will constantly be a minor risk present, which is inevitable, unless used collision resolution techniques .

## Collision Resolution

Since hashish collisions are inevitable, hash tables have mechanisms of dealing with them, known as collision resolutions. Two of the most common strategies are open addressing and separate chain. The cache-conscious collision resolution is another scheme that has been discussed in the by for string hash tables .
John Smith and Sandra Dee are both being directed to the like cell. open Addressing will cause the hash postpone to redirect Sandra Dee to another cell.

Cells in the hashish table are assigned one of three states in this method – occupied, empty, or deleted. If a hash collision occurs, the board will be probed to move the record to an alternate cellular telephone that is stated as evacuate. There are different types of probing that take place when a hash collision happens and this method is implemented. Some types of probing are linear Probing, Double Hashing, and quadratic Probing. [ 11 ] Open Addressing is besides known as closed Hashing. [ 12 ]

### separate chain

This scheme allows more than one record to be ‘chained ‘ to the cells of a hash table. If two records are being directed to the same cell, both would go into that cellular telephone as a linked tilt. This efficiently prevents a hash collision from occurring since records with the same hashish values can go into the lapp cell, but it has its disadvantages. Keeping track of then many lists is unmanageable and can cause whatever instrument that is being used to become identical slow. [ 11 ] Separate Chaining is besides known as open Hashing .

### Cache-Conscious Collision Resolution

Although much less used than the previous two, Askitis et alabama has proposed the hoard -conscious collision solution method acting in 2005, although this method might have been improved since then. [ 13 ] It is a alike idea to the discriminate chain methods, although it does not technically involve the chain lists. In this lawsuit, alternatively of chain lists, the hash values are represented in a adjacent tilt of items. This is better suited for string hash tables and the use for numeral values is inactive unknown. [ 11 ]