Computing hash string of specific length from seed string

Question

What would be a simple way of computing an (insecure) hash string from a string seed, with the end result having a specific length and fairly low probability of collision?

For example in C#:

string seed = "PK_DELETE_THIS_TABLE";  
Console.WriteLine("Seed: {0}", seed);  
string hashedData = SimpleHash(seed, 30/2);  
Console.WriteLine("Hash: {0}", hashedData);

Result:

Seed: PK_DELETE_THIS_TABLE
Hash: CC598675A3072C8B1768DED09BAAA5

Is there a particular reason you want to roll your own hashing algorithm? — mjwills
– mjwills, Commented Jul 29, 2020 at 8:59
rule #1 about correctly doing your own implementations of hashing, encryption, datetime-handling (and maybe some more): DON'T - unless you are a total expert. use the tools that already numerously exist — Franz Gleichmann
– Franz Gleichmann, Commented Jul 29, 2020 at 9:31
The only reason is that I need a hash string of a specific length. Security is not that important and collision concerns are quite low for my use case; I need to convert a name to hash string of 30 characters. — aleksander_si
– aleksander_si, Commented Sep 14, 2020 at 19:59

aleksander_si · Accepted Answer · 2021-01-15 19:14:34Z

2

We can compute an insecure hash with fairly low probability of collision with the following algorithm:

public string SimpleHash(string seed, int halfHashLength)  
{  
    if(seed == null) throw new ArgumentNullException(nameof(seed));
    if(halfHashLength <= 0) throw new ArgumentException("Value should be larger than 0.", nameof(halfHashLength));
    
    // Use different random algo for more deterministic hash:
    // https://stackoverflow.com/questions/17094189/crossplatform-random-number-generator
    Random rnd = new Random(seed.GetHashCode());
    byte[] tmp = new byte[halfHashLength];
    rnd.NextBytes(tmp);
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < tmp.Length; i++)
    {
        sb.Append(tmp[i].ToString("X2"));
    }
    return sb.ToString();
}

Note that the the above computed hash may not be portable across runtimes and possibly not even between runtime restarts (.NET Core will return a different seed.GetHashCode() each time the runtime is restarted). Use a more deterministic hashing algorithm if this is a problem.

edited Jan 15, 2021 at 19:14

answered Jul 29, 2020 at 8:53

aleksander_si

1,4371 gold badge15 silver badges32 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

TheGeneral Over a year ago

Note GetHashCode is not part of the Specs and is dependent on its implementation with no guarantee they will be the same on other platforms and implementations ect ect ect <insert other issues here>, if that doesn't bother you (which it likely wont), then this is actually a pretty good answer

Ackdari Over a year ago

Note that the byte array conversion could be simplified to BitConverter.ToString(tmp).Replace("-","")

aleksander_si Over a year ago

@TheGeneral Updated the answer to account for your comment.

Collectives™ on Stack Overflow

Computing hash string of specific length from seed string

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related