initital push

Packages/com.voidbotz.fonepayunity/Runtime/Crypto/HmacSha512Signer.cs

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+using System.Security.Cryptography;
+using System.Text;
+
+internal sealed class HmacSha512Signer
+{
+    private readonly byte[] _keyBytes;
+
+    internal HmacSha512Signer(string secretKey)
+    {
+        _keyBytes = Encoding.UTF8.GetBytes(secretKey);
+    }
+
+    internal string SignQrRequest(
+        string amount, string prn, string merchantCode,
+        string remarks1, string remarks2)
+        => Compute($"{amount},{prn},{merchantCode},{remarks1},{remarks2}");
+
+    internal string SignQrRequestWithTax(
+        string amount, string prn, string merchantCode,
+        string remarks1, string remarks2,
+        string taxAmount, string taxRefund)
+        => Compute($"{amount},{prn},{merchantCode},{remarks1},{remarks2},{taxAmount},{taxRefund}");
+
+    internal string SignStatusCheck(string prn, string merchantCode)
+        => Compute($"{prn},{merchantCode}");
+
+    internal string SignTaxRefund(
+        string fonepayTraceId, string merchantPrn,
+        string invoiceNumber, string invoiceDate,
+        string transactionAmount, string merchantCode)
+        => Compute($"{fonepayTraceId},{merchantPrn},{invoiceNumber},{invoiceDate},{transactionAmount},{merchantCode}");
+
+    private string Compute(string message)
+    {
+        using var hmac = new HMACSHA512(_keyBytes);
+        byte[] hash = hmac.ComputeHash(Encoding.UTF8.GetBytes(message));
+        return BytesToHexLower(hash);
+    }
+
+    private static string BytesToHexLower(byte[] bytes)
+    {
+        var sb = new StringBuilder(bytes.Length * 2);
+        foreach (byte b in bytes)
+            sb.Append(b.ToString("x2"));
+        return sb.ToString();
+    }
+}