Reference Rewriter: Error: type `System.Security.Cryptography.HMACSHA1` doesn't exist in target framework.

I am using Unity 5, Build Plateform Windows Store Apps.

This works fine in Unity 4.3, but not working in Unity 5.
I am using HMACSHA1 for making signature, it is very important for me, if this is not supported by windows store apps then any alternative/workaround.

Any help would be appreciated.

thanx.

Got the Solution from Tomas1856 from forum thread.
http://forum.unity3d.com/threads/reference-rewriter-error-type-system-security-cryptography-hmacsha1-doesnt-exist-in-target-fram.313589/#post-2038137

#if NETFX_CORE
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Security.Cryptography;
using Windows.Security.Cryptography.Core;
#endif
 
....
 
#if NETFX_CORE
    private string GetSHA256Key(byte[] secretKey, string value)
    {
        var objMacProv = MacAlgorithmProvider.OpenAlgorithm(MacAlgorithmNames.HmacSha256);
        var hash = objMacProv.CreateHash(secretKey.AsBuffer());
        hash.Append(CryptographicBuffer.ConvertStringToBinary(value, BinaryStringEncoding.Utf8));
        return CryptographicBuffer.EncodeToBase64String(hash.GetValueAndReset());
    }
#endif