Hashing more than 8000 bytes in SQL Server

Question

SQL Server's hashing function HASHBYTES has an input limit of 8000 bytes.

How do you hash larger strings?

Answer 1

You could write a SQL CLR function:

[Microsoft.SqlServer.Server.SqlFunction]
public static SqlBinary BigHashBytes(SqlString algorithm, SqlString data)
{
    var algo = HashAlgorithm.Create(algorithm.Value);

    var bytes = Encoding.UTF8.GetBytes(data.Value);

    return new SqlBinary(algo.ComputeHash(bytes));
}

And then it can be called in SQL like this:

--these return the same value
select HASHBYTES('md5', 'test stuff')
select dbo.BigHashBytes('md5', 'test stuff')

The BigHashBytes is only necessary if the length would be over 8k.

Answer 2

You could hash 8k (or 4k or 2k) chunks of the input and then either concatenate those hashes or hash them into a new hash value. This might get difficult though if you have to create a similar algorithm (in an external .NET app for example) to compare hashes created outside of SQL Server.

Another option: Lean on SQL Server's CLR integration and perform the hashing in a .NET assembly.

Answer 3

Like Paul's idea, one idea that comes to mind for chunking would be to store the hashed string in an XML column, with each chunk as a separate XML element.