Double SHA256 giving me an incorrect value with this code VB.net

Question

I'm trying to build a bitcoin miner. I have generated the message successfully, but I can't get the double SHA256 correctly. The value I'm getting is incorrect. What is wrong with this code?

 Private Sub Button9_Click(sender As Object, e As EventArgs) Handles Button9.Click

    Dim message_combined as string = "02000000671D0E2FF45DD1E927A51219D1CA1065C93B0C4E8840290A00000000000000002CD900FC3513260DF5BD2EABFD456CD2B3D2BACE30CC078215A907C045F4992E3E5F4A5E747B1B1843F740C0"


    Dim sha256 As SHA256 = SHA256Managed.Create()
    Dim bytes As Byte() = Encoding.UTF8.GetBytes(message_combined)
    Dim hash As Byte() = sha256.ComputeHash(bytes)
    Dim stringBuilder1 As New StringBuilder()

    For i As Integer = 0 To hash.Length - 1
        stringBuilder1.Append(hash(i).ToString("X2"))
    Next

    '  Dim sha256 As SHA256 = SHA256Managed.Create()
    Dim sha256_2 As SHA256 = SHA256Managed.Create()
    Dim bytes2 As Byte() = Encoding.UTF8.GetBytes(stringBuilder1.ToString())
    Dim hash2 As Byte() = sha256_2.ComputeHash(bytes2)
    'bytes2 = Encoding.UTF8.GetBytes(bytes)
    'hash2 = sha256_2.ComputeHash(bytes2)
    Dim stringBuilder2 As New StringBuilder()

    For i As Integer = 0 To hash2.Length - 1
        stringBuilder2.Append(hash2(i).ToString("X2"))
    Next


    final_hash = stringBuilder2.ToString()

    TextBox9.Text = final_hash
End Sub

After double hashing, the end hash should be: 920F0D3EF835E723DBDA8AE0E1EA0705FAF53AC784A8116639BCB215F3BC9D8B

Answer 1

You seem to be interpreting the hex encoded data as a UTF-8 string. Instead, you should be decoding it as a byte dump.

To illustrate, decoding 12 as a UTF-8 string will give you:

0x3132

while decoding it as a hex dump will give you:

0x12

In bitcoin, all hashing it done on the byte values, which are often printed as hex strings for convenience - you must decode them back into a byte array as hex strings, not as actual strings.