HttpResponseChecksumHandler.cs

using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

namespace App.API.Handlers
{
	/// <summary>
	/// A custom handler for adding an MD5 hash of the HTTP response content to the HTTP response headers.
	/// </summary>
	public class HttpResponseChecksumHandler : DelegatingHandler
	{
		protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
		{
			return base.SendAsync(request, cancellationToken)
				.ContinueWith(task =>
				{
					var response = task.Result;
					if (response.Content != null)
					{
						response.Headers.Add("X-Checksum",
							CryptographyHelper.ComputeMd5Hash(response.Content.ReadAsStringAsync().Result));
					}

					return task.Result;
				});
		}
	}
}

using System.Security.Cryptography;
using System.Text;

namespace App.API.Helpers
{
	/// <summary>
	/// Provides utility methods for cryptographic operations.
	/// </summary>
	public static class CryptographyHelper
	{
		/// <summary>
		/// Computes an MD5 hash of the specified string.
		/// </summary>
		/// <param name="source">The string.</param>
		/// <returns>A <see cref="string"/> representing the computed hash.</returns>
		public static string ComputeMd5Hash(string source)
		{
			if (string.IsNullOrEmpty(source))
			{
				return null;
			}

			var hash = MD5.Create().ComputeHash(Encoding.ASCII.GetBytes(source));
			var sb = new StringBuilder();
			for (var i = 0; i < hash.Length; i++)
			{
				sb.Append(hash[i].ToString("X2"));
			}

			return sb.ToString();
		}
	}
}