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AWS S3 Raw Authentication written in C#

aloneguid profile image Ivan G ・4 min read

AWS SDK for C# is actually a very good one and you should use it if you can. Unfortunately in my case it had just too many dependencies and due to attempt to cover all the AWS services (which it should absolutely do!) it's a very bloated, very hard to read codebase. I needed to perform a single AWS request to S3, very specific REST one, and I don't want to bring all these packages.

AWS documentation on this topic is excellent, and after many many days of work I could write a working request authenticator in C#. Pasting it here:

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Security.Cryptography;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Web;

namespace Storage.IO.Files.Impl.Amazon
{
   /// <summary>
   /// https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-auth-using-authorization-header.html
   /// 
   /// explore python version: https://github.com/tedder/requests-aws4auth
   /// </summary>
   class S3AuthHandler : DelegatingHandler
   {
      private readonly string _accessKeyId;
      private readonly string _secretAccessKey;
      private readonly string _region;
      private readonly string _service;
      private static readonly string EmptySha256 = new byte[0].SHA256().ToHexString();

      public S3AuthHandler(string accessKeyId, string secretAccessKey, string region, string service = "s3") : base(new HttpClientHandler())
      {
         _accessKeyId = accessKeyId;
         _secretAccessKey = secretAccessKey;
         _region = region;
         _service = service;
      }

      protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
      {
         await SignAsync(request);

         return await base.SendAsync(request, cancellationToken);
      }

      protected async Task<string> SignAsync(HttpRequestMessage request, DateTimeOffset? signDate = null)
      {
         // a very helpful article on S3 auth: https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html

         DateTimeOffset dateToUse = signDate ?? DateTimeOffset.UtcNow;
         string nowDate = dateToUse.ToString("yyyyMMdd");
         string amzNowDate = GetAmzDate(dateToUse);

         request.Headers.Add("x-amz-date", amzNowDate);

         // 1. Create a canonical request

         /*
          * <HTTPMethod>\n
          * <CanonicalURI>\n
          * <CanonicalQueryString>\n
          * <CanonicalHeaders>\n
          * <SignedHeaders>\n
          * <HashedPayload>
          */

         string payloadHash = await AddPayloadHashHeader(request);

         string canonicalRequest = request.Method + "\n" +
            GetCanonicalUri(request) + "\n" +  // CanonicalURI
            GetCanonicalQueryString(request) + "\n" +
            GetCanonicalHeaders(request, out string signedHeaders) + "\n" +   // ends up with two newlines which is expected
            signedHeaders + "\n" +
            payloadHash;


         // 2. Create a string to sign

         // step by step instructions: https://docs.aws.amazon.com/general/latest/gr/sigv4-create-string-to-sign.html

         /*
          * StringToSign =
          *    Algorithm + \n +
          *    RequestDateTime + \n +
          *    CredentialScope + \n +
          *    HashedCanonicalRequest
          */

         string stringToSign = "AWS4-HMAC-SHA256\n" +
               amzNowDate + "\n" +
               nowDate + "/" + _region + "/s3/aws4_request\n" +
               canonicalRequest.SHA256();


         // 3. Calculate Signature

         /*
          * DateKey              = HMAC-SHA256("AWS4"+"<SecretAccessKey>", "<YYYYMMDD>")
          * DateRegionKey        = HMAC-SHA256(<DateKey>, "<aws-region>")
          * DateRegionServiceKey = HMAC-SHA256(<DateRegionKey>, "<aws-service>")
          * SigningKey           = HMAC-SHA256(<DateRegionServiceKey>, "aws4_request")
          */

         byte[] kSecret = Encoding.UTF8.GetBytes(("AWS4" + _secretAccessKey).ToCharArray());
         byte[] kDate = HmacSha256(nowDate, kSecret);
         byte[] kRegion = HmacSha256(_region, kDate);
         byte[] kService = HmacSha256(_service, kRegion);
         byte[] kSigning = HmacSha256("aws4_request", kService);

         // final signature
         byte[] signatureRaw = HmacSha256(stringToSign, kSigning);
         string signature = signatureRaw.ToHexString();

         string auth = $"Credential={_accessKeyId}/{nowDate}/{_region}/s3/aws4_request,SignedHeaders={signedHeaders},Signature={signature}";
         request.Headers.Authorization = new AuthenticationHeaderValue("AWS4-HMAC-SHA256", auth);

         return signature;
      }

      private static string GetAmzDate(DateTimeOffset date)
      {
         return date.ToString("yyyyMMddTHHmmssZ");
      }

      private string GetCanonicalUri(HttpRequestMessage request)
      {
         string path = request.RequestUri.AbsolutePath.TrimStart('/');

         return "/" + path.UrlEncode();
      }

      private string GetCanonicalQueryString(HttpRequestMessage request)
      {
         /**
          * CanonicalQueryString specifies the URI-encoded query string parameters. You URI-encode name and values individually. You must also sort the parameters in the canonical query string alphabetically by key name. The sorting occurs after encoding.
          */


         NameValueCollection values = HttpUtility.ParseQueryString(request.RequestUri.Query);
         var sb = new StringBuilder();

         foreach(string key in values.AllKeys.OrderBy(k => k))
         {
            if(sb.Length > 0)
            {
               sb.Append('&');
            }

            string value = HttpUtility.UrlEncode(values[key]);

            if(key == null)
            {
               sb
                  .Append(value)
                  .Append("=");
            }
            else
            {
               sb
                  .Append(HttpUtility.UrlEncode(key.ToLower()))
                  .Append("=")
                  .Append(value);

            }
         }

         return sb.ToString();
      }

      private string GetCanonicalHeaders(HttpRequestMessage request, out string signedHeaders)
      {
         // List of request headers with their values.
         // Individual header name and value pairs are separated by the newline character ("\n").
         // Header names must be in lowercase. You must sort the header names alphabetically to construct the string.

         // Note that I add some headers manually, but preserve sorting order in the actual code.

         var headers = from kvp in request.Headers
                       where kvp.Key.StartsWith("x-amz-", StringComparison.OrdinalIgnoreCase)
                       orderby kvp.Key
                       select new { Key = kvp.Key.ToLowerInvariant(), kvp.Value };

         var sb = new StringBuilder();
         var signedHeadersList = new List<string>();

         // The CanonicalHeaders list must include the following:
         // - HTTP host header.
         // - If the Content-Type header is present in the request, you must add it to the CanonicalHeaders list.
         // - Any x-amz-* headers that you plan to include in your request must also be added. For example, if you are using temporary security credentials, you need to include x-amz-security-token in your request. You must add this header in the list of CanonicalHeaders.

         if(request.Headers.Contains("date"))
         {
            sb.Append("date:").Append(request.Headers.GetValues("date").First()).Append("\n");
            signedHeadersList.Add("date");
         }

         sb.Append("host:").Append(request.RequestUri.Host).Append("\n");
         signedHeadersList.Add("host");

         string contentType = request.Content?.Headers.ContentType?.ToString();
         if(contentType != null)
         {
            sb.Append("content-type:").Append(contentType).Append("\n");
            signedHeadersList.Add("content-type");
         }

         if(request.Headers.Contains("range"))
         {
            sb.Append("range:").Append(request.Headers.GetValues("range").First()).Append("\n");
            signedHeadersList.Add("range");
         }

         // Create the string in the right format; this is what makes the headers "canonicalized" --
         //   it means put in a standard format. http://en.wikipedia.org/wiki/Canonicalization
         foreach(var kvp in headers)
         {
            sb.Append(kvp.Key).Append(":");
            signedHeadersList.Add(kvp.Key);

            foreach(string hv in kvp.Value)
            {
               sb.Append(hv);
            }

            sb.Append("\n");
         }

         signedHeaders = string.Join(";", signedHeadersList);

         return sb.ToString();
      }

      /// <summary>
      /// Hex(SHA256Hash(<payload>))
      /// </summary>
      /// <param name="request"></param>
      /// <returns></returns>
      private async Task<string> AddPayloadHashHeader(HttpRequestMessage request)
      {
         string hash;

         if(request.Content != null)
         {
            byte[] content = await request.Content.ReadAsByteArrayAsync();
            hash = content.SHA256().ToHexString();
         }
         else
         {
            hash = EmptySha256;
         }

         request.Headers.Add("x-amz-content-sha256", hash);

         return hash;
      }

      private static byte[] HmacSha256(string data, byte[] key)
      {
         var alg = KeyedHashAlgorithm.Create("HmacSHA256");
         alg.Key = key;
         return alg.ComputeHash(Encoding.UTF8.GetBytes(data));
      }
   }
}

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P.S. Originally published on my blog.

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