How do I calculate the equivalent to SQL Server (hashbytes('SHA1',[ColumnName])) in C#?

Question

In my database I have a computed column that contains a SHA1 hash of a column called URLString which holds URLs (e.g. \"http://xxxx.com/index.html\").

I often need t

Answer 1

The below code is equivalent to SQL Server's hashbytes('sha1')

using (SHA1Managed sha1 = new SHA1Managed()) {
    var hash = sha1.ComputeHash(Encoding.Unicode.GetBytes(input));
    var sb = new StringBuilder(hash.Length * 2);

    foreach (byte b in hash) {
        // can be "x2" if you want lowercase
        sb.Append(b.ToString("X2"));
    }

    string output = sb.ToString();
}

Answer 2

You're likely getting bitten by character encoding differences:

http://weblogs.sqlteam.com/mladenp/archive/2009/04/28/Comparing-SQL-Server-HASHBYTES-function-and-.Net-hashing.aspx

You could try getting the bytes via Encoding.ASCII.GetBytes(url) or Encoding.Unicode.GetBytes(url) and see which one your db is using.

Answer 3

Below are two methods that do hashing of string and of bytes. The HashBytes method returns Base64 of the resulting bytes but you can return just the bytes if you prefer them

public static string HashString(string cleartext)
{
    byte[] clearBytes = Encoding.UTF8.GetBytes(cleartext);
    return HashBytes(clearBytes);
}  

public static string HashBytes(byte[] clearBytes)
{
    SHA1 hasher = SHA1.Create();
    byte[] hashBytes =   hasher.ComputeHash(clearBytes);
    string hash = System.Convert.ToBase64String(hashBytes);
    hasher.Clear();
    return hash;
}