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HMAC SHA256 hash calculation in C #

I need to calculate HMAC using the SHA256 hash function. I have a secret key encoded in base64 format. There is also an online tool that correctly calculates HMAC (verified). http://www.freeformatter.com/hmac-generator.html I wrote the following code snippet:

var signatureHashHexExpected = "559bd871bfd21ab76ad44513ed5d65774f9954d3232ab68dab1806163f806447";
var signature = "123456:some-string:2016-04-12T12:44:16Z";
var key = "AgQGCAoMDhASFAIEBggKDA4QEhQCBAYICgwOEBIUAgQ=";

var shaKeyBytes = Convert.FromBase64String(key);
using (var shaAlgorithm = new System.Security.Cryptography.HMACSHA256(shaKeyBytes))
{
    var signatureBytes = System.Text.Encoding.UTF8.GetBytes(signature);
    var signatureHashBytes = shaAlgorithm.ComputeHash(signatureBytes);
    var signatureHashHex = string.Concat(Array.ConvertAll(signatureHashBytes, b => b.ToString("X2"))).ToLower();

    System.Diagnostics.Debug.Assert(signatureHashHex == signatureHashHexExpected);
}

PROBLEM: My code does not generate the correct HMAC. I checked the various steps using various online tools and alternative C # implementations. Only the conversion from base64 is not confirmed. What am I missing?

UPDATE: The calculated HashHex signature in my code is "a40e0477a02de1d134a5c55e4befa55d6fca8e29e0aa0a0d8acf7a4370208efc"

ANSWER: The problem was caused by misleading documentation that the key is provided in Base64 format. See Accepted Answer:

var shaKeyBytes = System.Text.Encoding.UTF8.GetBytes(key);
+4
2

, , , , Base64 .

. , :

var shaKeyBytes = System.Text.Encoding.UTF8.GetBytes("AgQGCAoMDhASFAIEBggKDA4QEhQCBAYICgwOEBIUAgQ=");


559bd871bfd21ab76ad44513ed5d65774f9954d3232ab68dab1806163f806447

(, , )

+5

HMAC: . . , . -, . UTF8 ASCIIEncoding.

UTF8:

var signature = "123456:some-string:2016-04-12T12:44:16Z";
var key = "AgQGCAoMDhASFAIEBggKDA4QEhQCBAYICgwOEBIUAgQ=";


var signatureBytes = System.Text.Encoding.UTF8.GetBytes(signature);
var shaKeyBytes = System.Text.Encoding.UTF8.GetBytes(key);

ASCIIEncoding:

var signature = "123456:some-string:2016-04-12T12:44:16Z";
var key = "AgQGCAoMDhASFAIEBggKDA4QEhQCBAYICgwOEBIUAgQ=";

var signatureBytes = System.Text.ASCIIEncoding.GetBytes(signature);
var shaKeyBytes = System.Text.ASCIIEncoding.GetBytes(key);

.

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