My name
is
Jon Skeet

save hash value in the database

i used this functions to compute the hash value:

public string GetSHA512(string input)
{
    byte[] data, result;
    StringBuilder hash = new StringBuilder();

    data = Encoding.UTF8.GetBytes(input);
    using (SHA512 shaM = new SHA512Managed())
    {
        result = shaM.ComputeHash(data);
    }

    for (int i = 0; i < result.Length; i++)
    {
        hash.Append(result[i].ToString());
    }

    return hash.ToString();
}

public string GetSHA256(string input)
{
    byte[] data, result;
    StringBuilder hash = new StringBuilder();

    data = Encoding.UTF8.GetBytes(input);
    using (SHA256 shaM = new SHA256Managed())
    {
        result = shaM.ComputeHash(data);
    }

    for (int i = 0; i < result.Length; i++)
    {
        hash.Append(result[i].ToString());
    }

    return hash.ToString();
}

public string GetSHA1(string input)
{
    byte[] data, result;
    StringBuilder hash = new StringBuilder();

    data = Encoding.UTF8.GetBytes(input);
    using (SHA1 shaM = new SHA1Managed())
    {
        result = shaM.ComputeHash(data);
    }

    for (int i = 0; i < result.Length; i++)
    {
        hash.Append(result[i].ToString());
    }

    return hash.ToString();
}

public string GetMD5(string input)
{
    byte[] data, result;
    StringBuilder hash = new StringBuilder();

    data = Encoding.UTF8.GetBytes(input);
    using (MD5 shaM = new MD5CryptoServiceProvider())
    {
        result = shaM.ComputeHash(data);
    }

    for (int i = 0; i < result.Length; i++)
    {
        hash.Append(result[i].ToString());
    }

    return hash.ToString();
}

but now i have few questions:

hash functions are supposed to create the fix output length for any kind of strings.(no matter my input length is 4 or 10000 the output always has a fix size) aren't they? but when my input length changes the output length changes too!! i guess my hash functions doesn't work.
if i want save the result in the database, my hash value filed type should be what?
which one of the hash functions usually used in web applications?

thank you.

Currently you're just returning the decimal representation of all the bytes, concatenated together. So { 0, 0, 0 } ends up as "000" whereas { 123, 123, 123 } ends up as "123123123". So yes, both those hashes will give the same output size for any input (SHA-1 will give 20 bytes; MD5 will give 16) but your string representations will currently vary in length.

I would recommend using either a hex representation or base64 - in particular, base64 requires rather less work:

public string GetSHA1(string input)
{
    byte[] data = Encoding.UTF8.GetBytes(input);
    using (SHA512 shaM = new SHA512Managed())
    {
        byte[] result = shaM.ComputeHash(data);
        return Convert.ToBase64String(result);
    }
}

Hex has the advantage of being a more common way of representing hashes. (Base64 is more usually used for transporting arbitrary binary data, e.g. images.) For hex, you could use:

return BitConverter.ToString(result).Replace("-", "");

(Note that I've declared local variables - you appear to be using fields for data and result, which is a bad idea - calling these methods shouldn't affect the state of the instance, IMO.)

Alternatively, you could just return a byte[] and store that directly in the database as a blob. Using base64 or hex is probably simpler though - it's easier to examine the data that way, and frankly easier to query. Strings are just simpler to handle :)

In terms of which hash you should use - I probably wouldn't use either SHA-1 or MD5 unless I had to; I'd default to SHA-256, although it depends on what you're trying to do. If this is hashing passwords for example, you probably want an HMAC of some description - or better yet, don't roll your own, and use an off-the-shelf authentication package.

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