Exact, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.IO;
using System.Security.Cryptography;
using System.Text;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
string apiId = "1lie8ficql9h5";
string apiSecret = "j6hriaKY2iZi+Y2uo9JJldmO1Bq79XB8d1v2uHzAK0Zvy972mIs8ThsJSQeDlZJz+HzmLD6Q1MUZb5X1Zf9MzQ==";

//build the string to sign
//note the order of the entries is important.
//The http headers must be in alphabetical order by key name
string httpMethod = "POST";
string httpUrl = "/services/api/sts/session";
StringBuilder stringToSign = new StringBuilder();
stringToSign.Append(httpMethod).Append("\n")
                    .AppendFormat("x-csod-api-key:{0}", apiId).Append("\n")
    .AppendFormat("x-csod-date:{0}", ("2015-09-08T11:27:32.000")).Append("\n")
    .Append(httpUrl);

//Generate the signature
string signature = string.Empty;
            
/*byte[] secretkeyBytes = (apiSecret);*/
            
byte[] secretkeyBytes = Convert.FromBase64String(apiSecret);
            
byte[] inputBytes = Encoding.UTF8.GetBytes(stringToSign.ToString());
using (var hmac = new HMACSHA512(secretkeyBytes))
{
    byte[] hashValue = hmac.ComputeHash(inputBytes);
    signature = System.Convert.ToBase64String(hashValue);
}

/*
* signature produced:
* 3x5ETGSoqJa4vLl8gOFzdhxReOS0k8Nk2CpKVFN2A60ItF8wfP2tr+GUY2mELXjL90B57B5imLIrzou3ZQMfqQ==
*/
            Console.WriteLine(signature);
            Console.WriteLine(stringToSign);
        }
    }
}

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Compilation time: 0,14 sec, absolute running time: 0,09 sec, cpu time: 0,08 sec, average memory usage: 13 Mb, average nr of threads: 2