Error message when calling Api Gateway with signature

Question

Here's the error I'm getting....

The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.

The Canonical String for this request should have been
'GET
/devcam/securehello

host:*.execute-api.us-west-2.amazonaws.com
x-amz-date:20161223T170922Z

host;x-amz-date
e3b0c44098098s098df098s...8f0s98df098sd09f8s0d98f095'

The String-to-Sign should have been
'AWS4-HMAC-SHA256
20161223T170922Z
20161223/us-west-2/execute-api/aws4_request
828177fca39making23409...9840928304980238402983282d'

The strange thing is I compare this to my Canonical String and String to Sign and they are identical... What else do I need to check?

I am attempting to call this with temporary aws lambda credentials. I'm not exactly sure how I'm supposed to use the Session token, but right now it's being added as a header (I've tried including it in the signed headers and not including it--neither work) "X-Amz-Security-Token".

Edit:

Here's the JSON Serialized HttpMessageRequest object from .NET core that I'm using to send the request.

{
  "Version": {
    "Major": 1,
    "Minor": 1,
    "Build": -1,
    "Revision": -1,
    "MajorRevision": -1,
    "MinorRevision": -1
  },
  "Content": null,
  "Method": {
    "Method": "GET"
  },
  "RequestUri": "https:\/\/somerandomapigatewaystring.execute-api.us-west-2.amazonaws.com\/devcam\/securehello",
  "Headers": [
    {
      "Key": "x-amz-security-token",
      "Value": [
        "FQoDYXdzEKr\/\/\/\/\/\/\/\/\/\/wEaDPuFLZoHsomefmorefakevaluesin5here0+36yIwO1kh8sdkeuf98eek3kj4934898erjthisisafakevaluehahahahahahahahaz8FE7jKvENO1IdshzqjDFWhpnfLESAS4B404dBW\/VAkH1jJDvpiFylP5qiZk6q+S\/eJHDdLUoVAXDqkh0w2wkFdXNpsMsfr8eLdzXP8jT\/ZDA+OxG2qzxd1UaotDl\/\/NSzWjNyOYGX3pEoNk9LtefKBGKbgTvH5lrguydfN8JFb4mGjtP8wRY7tjHHq8jtLQhJMRiub3eGnR\/vxwNQk8EJ5usJxB1UF2Kl0p5LqSohViivqfXCBQ=="
      ]
    },
    {
      "Key": "host",
      "Value": [
        "somerandomapigatewaystring.execute-api.us-west-2.amazonaws.com"
      ]
    },
    {
      "Key": "x-amz-date",
      "Value": [
        "20161223T200041Z"
      ]
    },
    {
      "Key": "Authorization",
      "Value": [
        "AWS4-HMAC-SHA256 Credential=********************\/20161223\/us-west-2\/execute-api\/aws4_request, SignedHeaders=host;x-amz-date, Signature=3e0d3***********************************************************"
      ]
    }
  ],
  "Properties": {

  }
}

Here's my code--I apologize for the mess.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
using Amazon.CloudFormation;
using Amazon.IdentityManagement;
using Amazon.IdentityManagement.Model;
using Amazon.Lambda.Core;
using Amazon.Runtime.Internal;
using Amazon.Runtime.Internal.Auth;
using Amazon.SecurityToken;
using Amazon.SecurityToken.Model;
using bifrost;
using Newtonsoft.Json;

namespace Bifrost
{
    public class Handler
    {
        private static HttpClient httpClient = new HttpClient(); //yes we want this to be static https://aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/
        private static string Service = "execute-api";
//        private static string Service = "apigateway";
        private static string Algorithm = "AWS4-HMAC-SHA256";
        public const string Iso8601DateTimeFormat = "yyyyMMddTHHmmssZ";
        public const string Iso8601DateFormat = "yyyyMMdd";

        public Response Hello(Request request, ILambdaContext context)
        {
            try
            {

                var now = DateTime.Now;
                var signedHeaders = new string[]
                {
//                    "content-type",
                    "host", //http://docs.aws.amazon.com/general/latest/gr/rande.html#apigateway_region
                    "x-amz-date",
//                    "x-amz-security-token",
//                    "Authorization",
//                    "Credential"
                };

                //X-Amz-Security-Token

                HttpRequestMessage httpRequest = new HttpRequestMessage(HttpMethod.Get, "https://somerandomapigatewaystring.execute-api.us-west-2.amazonaws.com/devcam/securehello");
                httpRequest.Headers.Add("x-amz-security-token", EnvironmentVariables.SessionToken);

//                httpRequest.Headers.Add("host", "apigateway.us-west-2.amazonaws.com");
                httpRequest.Headers.Add("host", "somerandomapigatewaystring.execute-api.us-west-2.amazonaws.com");
                httpRequest.Headers.Add("x-amz-date", now.ToString(Iso8601DateTimeFormat));
//                httpRequest.Content.Headers.ContentType = MediaTypeHeaderValue.Parse("application/json");

//                httpRequest.Headers.Add("X-Amz-Security-Token", EnvironmentVariables.SessionToken);

                string credentialScope = String.Format("{0}/{1}/{2}/aws4_request", now.ToString(Iso8601DateFormat),
                    EnvironmentVariables.Region, Service);
                string credentialString = EnvironmentVariables.AccessKeyId + "/" + credentialScope;

                string signedHeadersString = string.Join(";",
                    signedHeaders.Select(x => x.Trim().ToLowerInvariant()).OrderBy(x => x));

//                AwsV4SignatureCalculator signatureCalculator = new AwsV4SignatureCalculator(EnvironmentVariables.SecretKey, Service, "us-west-2");
//                string signature = signatureCalculator.CalculateSignature(httpRequest, signedHeaders, now);

                string signature = getSignature(EnvironmentVariables.AccessKeyId, Service, "us-west-2", httpRequest, signedHeaders, now);


                // task 4:  add signing info to request
                string authorization = CreateAuthorizationGET(signature, signedHeadersString, credentialScope);


                Console.WriteLine("authorization:");
                Console.WriteLine(authorization);

//                The format of value 'AWS4-HMAC-SHA256 Credential=/20161222/us-west-2/execute-api/aws4_request, SignedHeaders=host;x-amz-date;x-amz-security-token, Signature=.

                //                httpRequest.Headers.Add("Authorization", authorization);
                httpRequest.Headers.TryAddWithoutValidation("Authorization", authorization);
//                httpRequest.Headers.TryAddWithoutValidation("Authorization", authorization + ", X-Amz-Security-Token=" + EnvironmentVariables.SessionToken);


//                httpRequest.Version = Version./
                var response = httpClient.SendAsync(httpRequest);

                    return new Response
                    {
                        Body =
                            "Go Serverless v1.0! Your function executed successfully!\r\n" +
                            JsonConvert.SerializeObject(request)
                            + "\r\n\r\n" + JsonConvert.SerializeObject(context) + "\r\n\r\n" +
                            JsonConvert.SerializeObject(Environment.GetEnvironmentVariables()) + "\r\n\r\n" +
                            JsonConvert.SerializeObject(httpRequest) + "\r\n\r\n" +
                            JsonConvert.SerializeObject(response.Result) + "\r\n\r\n" + 
//                            "",
                            response.Result.Content.ReadAsStringAsync().Result,
                        StatusCode = 200,
                        Headers = new Dictionary<string, string>()
                    };
            }
            catch (Exception e)
            {

                return new Response
                {
                    StatusCode = 200,
                    Body = e.Message + "\r\n\r\n" + e.StackTrace + 
                    "\r\n\r\n" + e.InnerException.StackTrace,
                    Headers = new Dictionary<string, string>()
                };
            }
        }

        private string getSignature(string accessKey, string service, string region, HttpRequestMessage httpRequest, string[] signedHeaders, DateTime now)
        {

//            httpRequest.Content = new StringContent("", Encoding.UTF8,
//                        "application/json");

            var signedHeadersDict = createSignedHeadersDictionary(signedHeaders, httpRequest);

            string hashedPayload = AWSAuthenticationHelper.HashAndHexEncodeString("");
            string canonicalHeaders = string.Join("\n", signedHeadersDict.Select(x => x.Key.ToLowerInvariant() + ":" + x.Value.Trim())) + "\n";

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

//            httpRequest.Headers.Add("X-Amz-SignedHeaders", signedHeadersString);


            string canonicalRequest = AWSAuthenticationHelper.CreateCanconicalRequestGET(hashedPayload, signedHeadersString, canonicalHeaders,
                httpRequest.RequestUri.AbsolutePath,
                httpRequest.RequestUri.Query);

            Console.WriteLine("Canonical String:");
            Console.WriteLine(canonicalRequest);

            string hashedCanonicalRequest = AWSAuthenticationHelper.HashAndHexEncodeString(canonicalRequest);

            string dateStamp = now.ToString(Iso8601DateFormat);
            string dateTimeStamp = now.ToString(Iso8601DateTimeFormat);

//            httpRequest.Headers.Add("X-Amz-Date", now.ToString(Iso8601DateTimeFormat));

            string credentialScope = String.Format("{0}/{1}/{2}/aws4_request", dateStamp, region, service);

//            httpRequest.Headers.Add("X-Amz-Credential", credentialScope);

            string stringToSign = CreateStringToSign(hashedCanonicalRequest, dateTimeStamp, credentialScope);

            Console.WriteLine("stringToSign:");
            Console.WriteLine(stringToSign);

            // task 3:  calculate signature
            string signature = AWSAuthenticationHelper.CreateSignature(stringToSign, dateStamp, region, service, accessKey);
            return signature;
        }

        private string CreateAuthorizationGET(string signature, string signedHeaders, string credentialScope)
        {
            return String.Format("{0} Credential={1}/{2}, SignedHeaders={3}, Signature={4}", Algorithm, EnvironmentVariables.AccessKeyId, credentialScope, signedHeaders, signature);
        }

        private string CreateStringToSign(string hashedRequest, string requestDate, string credentialScope)
        {
            string stringToSign = string.Format("{0}\n{1}\n{2}\n{3}", Algorithm, requestDate, credentialScope, hashedRequest);
            return stringToSign;
        }

        private SortedDictionary<string, string> createSignedHeadersDictionary(string[] signedHeaders, HttpRequestMessage httpRequest)
        {
            SortedDictionary<string, string> ret = new SortedDictionary<string, string>();
            List<string> signedHeadersList = signedHeaders.Select(x => x.ToLowerInvariant().Trim()).OrderBy(x => x).ToList();

            foreach (KeyValuePair<string, IEnumerable<string>> header in httpRequest.Headers)
            {
                if (signedHeadersList.Contains(header.Key.ToLowerInvariant().Trim()))
                {
                    ret.Add(header.Key, String.Join(",", header.Value)); // TODO:  is tostring safe here?
                }
            }

            if (httpRequest.Content != null && httpRequest.Content.Headers != null)
            {
                foreach (var contentHeader in httpRequest.Content.Headers)
                {
                    if (signedHeaders.Contains(contentHeader.Key.ToLowerInvariant().Trim()))
                    {
                        ret.Add(contentHeader.Key, string.Join(",", contentHeader.Value));
                    }
                }
            }
//            else if (signedHeadersList.Contains("content-type"))
//            {
//                throw new Exception("content-type header set but no Content found");
//            }

            return ret;
        }

        public Response SecureHello(Request request, ILambdaContext context)
        {
            return new Response
            {
                Body = "securely called!\r\n" + JsonConvert.SerializeObject(request)
                       + "\r\n\r\n" + JsonConvert.SerializeObject(context) + "\r\n\r\n" +
                       JsonConvert.SerializeObject(Environment.GetEnvironmentVariables()),
                StatusCode = 200,
                Headers = new Dictionary<string, string>()
            };
        }
    }


    public class Response : BaseResponse
    {
    }

    public class Request : BaseRequest
    {
    }
}

And more code...

using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace bifrost
{
    public class AWSAuthenticationHelper
    {

        public static string HashAndHexEncodeString(string str)
        {
            return HexEncode(GenerateHash(ToBytes(str)));
        }

        public static string CreateSignature(string stringToSign, string dateStamp, string region, string serviceName, string awsKey)
        {
            byte[] key = GenerateKey(dateStamp, region, serviceName, awsKey);
            return HexEncode(HmacSha256(stringToSign, key));
        }

        public static string CreateCanconicalRequestPOST(string hashedPayload, string signedHeaders, string canonicalHeaders, string canonicalURI)
        {
            string canonicalRequest = "POST" + "\n" + canonicalURI + "\n" + "" + "\n" + canonicalHeaders + "\n" +
                                      signedHeaders + "\n" + hashedPayload;

            return canonicalRequest;
        }

        public static string CreateCanconicalRequestGET(string hashedPayload, string signedHeaders, string canonicalHeaders, string canonicalURI, string canonicalQueryString)
        {
            string canonicalRequest = "GET" + "\n" +
                                      canonicalURI + "\n" +
                                      canonicalQueryString + "\n" +
                                      canonicalHeaders + "\n" +
                                      signedHeaders + "\n"
                                    + hashedPayload;

            return canonicalRequest;
        }

        private static byte[] GenerateKey(string dateStamp, string region, string serviceName, string awsSecretKey)
        {
            byte[] kDate = HmacSha256(dateStamp, ToBytes("AWS4" + awsSecretKey));
            byte[] kRegion = HmacSha256(region, kDate);
            byte[] kService = HmacSha256(serviceName, kRegion);
            return HmacSha256("aws4_request", kService);
        }

        private static string HexEncode(byte[] bytes)
        {
            return BitConverter.ToString(bytes).Replace("-", string.Empty).ToLowerInvariant();
        }

        private static byte[] ToBytes(string str)
        {
            return Encoding.UTF8.GetBytes(str.ToCharArray());
        }

        private static byte[] GenerateHash(byte[] bytes)
        {
            return SHA256.Create().ComputeHash(bytes);
        }

        private static byte[] HmacSha256(String data, byte[] key)
        {
            return new HMACSHA256(key).ComputeHash(ToBytes(data));
        }
    }
}

Updated Canonical Request String:

GET
/devcam/securehello

host:somerandomapigatewaystring.execute-api.us-west-2.amazonaws.com
x-amz-date:20161223T200041Z

host;x-amz-date
e3b0ooooyeaaac149afbf4c89somemorefake8888fvaluesa495991oooyea855

Updated String to Sign

AWS4-HMAC-SHA256
20161223T200041Z
20161223/us-west-2/execute-api/aws4_request
01b7ad17a58c442ed893ece5acb63a650b9781e32d23cb4d4114777d224deaf7

Updated Authorization Header:

AWS4-HMAC-SHA256 Credential=ASPECIALSECRETEKEYKK/20161223/us-west-2/execute-api/aws4_request, SignedHeaders=host;x-amz-date, Signature=3e0immodifyingthesevaluesjusttobesafeoiuoiuoiuoiukjh8a30540609fe

Answer 1

Okay I've found the code to make it work.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Text.RegularExpressions;

namespace bifrost
{
    public class AwsSigningHelper
    {
        private const string Service = "execute-api";
        public static readonly StringContent EmptyContent = new StringContent("", Encoding.UTF8, "application/json");
        public const string Iso8601DateTimeFormat = "yyyyMMddTHHmmssZ";
        public const string Iso8601DateFormat = "yyyyMMdd";

        private static string GetHost(string url)
        {
            Regex reg = new Regex(@"https\:\/\/([a-z|0-9|A-Z]+\.execute-api\.[a-z|A-Z|0-9|-]+.amazonaws\.com).*");

            Match match = reg.Match(url);

            Console.WriteLine("host: " + match.Groups[1].Value);

            return match.Groups[1].Value;
        }

        public static HttpRequestMessage GenerateAwsSignedGetRequest(string endpointUrl, string awsRegion, string accessKeyId, string secretAccessKey, string sessionToken)
        {
            return GenerateAwsSignedRequest(HttpMethod.Get, endpointUrl, EmptyContent,
                awsRegion, accessKeyId, secretAccessKey, sessionToken);
        }

        public static HttpRequestMessage GenerateAwsSignedGetRequest(string endpointUrl, string awsRegion, string accessKeyId, string secretAccessKey)
        {
            return GenerateAwsSignedRequest(HttpMethod.Get, endpointUrl, EmptyContent,
                awsRegion, accessKeyId, secretAccessKey, null);
        }

        public static HttpRequestMessage GenerateAwsSignedPostRequest(string endpointUrl, HttpContent content, string awsRegion, string accessKeyId, string secretAccessKey, string sessionToken)
        {
            return GenerateAwsSignedRequest(HttpMethod.Post, endpointUrl, content,
                awsRegion, accessKeyId, secretAccessKey, sessionToken);
        }

        public static HttpRequestMessage GenerateAwsSignedPostRequest(string endpointUrl, HttpContent content, string awsRegion, string accessKeyId, string secretAccessKey)
        {
            return GenerateAwsSignedRequest(HttpMethod.Post, endpointUrl, content,
                awsRegion, accessKeyId, secretAccessKey, null);
        }

        public static HttpRequestMessage GenerateAwsSignedPutRequest(string endpointUrl, HttpContent content, string awsRegion, string accessKeyId, string secretAccessKey, string sessionToken)
        {
            return GenerateAwsSignedRequest(HttpMethod.Put, endpointUrl, content,
                awsRegion, accessKeyId, secretAccessKey, sessionToken);
        }

        public static HttpRequestMessage GenerateAwsSignedPutRequest(string endpointUrl, HttpContent content, string awsRegion, string accessKeyId, string secretAccessKey)
        {
            return GenerateAwsSignedRequest(HttpMethod.Put, endpointUrl, content,
                awsRegion, accessKeyId, secretAccessKey, null);
        }

        private static string CreateAuthorizationHeader(string awsAccessKey, string signature, string signedHeaders, string dateStamp, string region, string serviceName)
        {
            return String.Format("AWS4-HMAC-SHA256 Credential={0}/{1}/{2}/{3}/aws4_request, SignedHeaders={4}, Signature={5}",
                awsAccessKey, dateStamp, region, serviceName, signedHeaders, signature);
        }

        private static HttpRequestMessage GenerateAwsSignedRequest(HttpMethod method, string endpointUrl, HttpContent content, string awsRegion, string accessKeyId, string secretAccessKey, string sessionToken)
        {

            DateTime now = DateTime.UtcNow;
            HttpRequestMessage httpRequest = new HttpRequestMessage(method, endpointUrl);
            httpRequest.Headers.Add("host", GetHost(endpointUrl));
            httpRequest.Headers.Add("x-amz-date", now.ToString(Iso8601DateTimeFormat));
            httpRequest.Content = content;

            List<string> signHeadersList = new List<string>
            {
                    "content-type",
                    "host",
                    "x-amz-date",
            };

            if (!string.IsNullOrWhiteSpace(sessionToken))
            {
                signHeadersList.Add("x-amz-security-token");
                httpRequest.Headers.Add("x-amz-security-token", sessionToken);
            }

            string[] signedHeaders = signHeadersList.ToArray();

            string signedHeadersString = string.Join(";",
                signedHeaders.Select(x => x.Trim().ToLowerInvariant()).OrderBy(x => x));

            AwsV4SignatureCalculator signatureCalculator = new AwsV4SignatureCalculator(secretAccessKey, Service, awsRegion);
            string signature = signatureCalculator.CalculateSignature(httpRequest, signedHeaders, now);

            string authorization = CreateAuthorizationHeader(accessKeyId, signature,
                signedHeadersString, now.ToString(Iso8601DateFormat), awsRegion, Service);

            httpRequest.Headers.TryAddWithoutValidation("Authorization", authorization);

            return httpRequest;
        }
    }
}

And from https://gist.github.com/yvanin/0bdf68c1139ad698519e:

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Net.Http;
using System.Security.Cryptography;
using System.Text;
using Microsoft.AspNetCore.WebUtilities;
using Microsoft.Extensions.Primitives;

namespace bifrost
{
    public class AwsV4SignatureCalculator
    {
        public const string Iso8601DateTimeFormat = "yyyyMMddTHHmmssZ";
        public const string Iso8601DateFormat = "yyyyMMdd";

        private readonly string _awsSecretKey;
        private readonly string _service;
        private readonly string _region;

        public AwsV4SignatureCalculator(string awsSecretKey, string service, string region = null)
        {
            _awsSecretKey = awsSecretKey;
            _service = service;
            _region = region ?? "us-east-1";
        }

        /// <summary>
        /// Calculates request signature string using Signature Version 4.
        /// http://docs.aws.amazon.com/general/latest/gr/sigv4_signing.html
        /// </summary>
        /// <param name="request">Request</param>
        /// <param name="signedHeaders">Canonical headers that are a part of a signing process</param>
        /// <param name="requestDate">Date and time when request takes place</param>
        /// <returns>Signature</returns>
        public string CalculateSignature(HttpRequestMessage request, string[] signedHeaders, DateTime requestDate)
        {
            signedHeaders = signedHeaders.Select(x => x.Trim().ToLowerInvariant()).OrderBy(x => x).ToArray();

            var canonicalRequest = GetCanonicalRequest(request, signedHeaders);
            var stringToSign = GetStringToSign(requestDate, canonicalRequest);
            return GetSignature(requestDate, stringToSign);
        }

        // http://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html
        private static string GetCanonicalRequest(HttpRequestMessage request, string[] signedHeaders)
        {
            var canonicalRequest = new StringBuilder();
            canonicalRequest.AppendFormat("{0}\n", request.Method.Method);
            canonicalRequest.AppendFormat("{0}\n", request.RequestUri.AbsolutePath);

            canonicalRequest.AppendFormat("{0}\n", GetCanonicalQueryParameters(QueryHelpers.ParseQuery(request.RequestUri.Query)));
            canonicalRequest.AppendFormat("{0}\n", GetCanonicalHeaders(request, signedHeaders));
            canonicalRequest.AppendFormat("{0}\n", String.Join(";", signedHeaders));
            canonicalRequest.Append(GetPayloadHash(request));
            return canonicalRequest.ToString();
        }

        private static string GetCanonicalQueryParameters(Dictionary<string, StringValues> queryParameters)
        {


            StringBuilder canonicalQueryParameters = new StringBuilder();
            foreach (KeyValuePair<string, StringValues> keyValuePair in queryParameters)
            {
                canonicalQueryParameters.AppendFormat("{0}={1}&", Utils.UrlEncode(keyValuePair.Key),
                                                      Utils.UrlEncode(keyValuePair.Value));
            }

            // remove trailing '&'
            if (canonicalQueryParameters.Length > 0)
                canonicalQueryParameters.Remove(canonicalQueryParameters.Length - 1, 1);

            return canonicalQueryParameters.ToString();
        }

        private static string GetCanonicalHeaders(HttpRequestMessage request, IEnumerable<string> signedHeaders)
        {
            var headers = request.Headers.ToDictionary(x => x.Key.Trim().ToLowerInvariant(),
                                                       x => String.Join(" ", x.Value).Trim());

            if (request.Content != null)
            {
                var contentHeaders = request.Content.Headers.ToDictionary(x => x.Key.Trim().ToLowerInvariant(),
                                                                          x => String.Join(" ", x.Value).Trim());
                foreach (var contentHeader in contentHeaders)
                {
                    headers.Add(contentHeader.Key, contentHeader.Value);
                }
            }

            var sortedHeaders = new SortedDictionary<string, string>(headers);

            StringBuilder canonicalHeaders = new StringBuilder();
            foreach (var header in sortedHeaders.Where(header => signedHeaders.Contains(header.Key)))
            {
                canonicalHeaders.AppendFormat("{0}:{1}\n", header.Key, header.Value);
            }
            return canonicalHeaders.ToString();
        }

        private static string GetPayloadHash(HttpRequestMessage request)
        {
            var payload = request.Content != null ? request.Content.ReadAsStringAsync().Result : "";
            return Utils.ToHex(Utils.Hash(payload));
        }

        // http://docs.aws.amazon.com/general/latest/gr/sigv4-create-string-to-sign.html
        private string GetStringToSign(DateTime requestDate, string canonicalRequest)
        {
            var dateStamp = requestDate.ToString(Iso8601DateFormat, CultureInfo.InvariantCulture);
            var scope = string.Format("{0}/{1}/{2}/{3}", dateStamp, _region, _service, "aws4_request");

            var stringToSign = new StringBuilder();
            stringToSign.AppendFormat("AWS4-HMAC-SHA256\n{0}\n{1}\n",
                                      requestDate.ToString(Iso8601DateTimeFormat, CultureInfo.InvariantCulture),
                                      scope);
            stringToSign.Append(Utils.ToHex(Utils.Hash(canonicalRequest)));
            return stringToSign.ToString();
        }

        // http://docs.aws.amazon.com/general/latest/gr/sigv4-calculate-signature.html
        private string GetSignature(DateTime requestDate, string stringToSign)
        {
            var kSigning = GetSigningKey(requestDate);
            return Utils.ToHex(Utils.GetKeyedHash(kSigning, stringToSign));
        }

        private byte[] GetSigningKey(DateTime requestDate)
        {
            var dateStamp = requestDate.ToString(Iso8601DateFormat, CultureInfo.InvariantCulture);
            var kDate = Utils.GetKeyedHash("AWS4" + _awsSecretKey, dateStamp);
            var kRegion = Utils.GetKeyedHash(kDate, _region);
            var kService = Utils.GetKeyedHash(kRegion, _service);
            return Utils.GetKeyedHash(kService, "aws4_request");
        }

        private static class Utils
        {
            private const string ValidUrlCharacters =
                "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_.~";

            public static string UrlEncode(string data)
            {
                StringBuilder encoded = new StringBuilder();
                foreach (char symbol in Encoding.UTF8.GetBytes(data))
                {
                    if (ValidUrlCharacters.IndexOf(symbol) != -1)
                    {
                        encoded.Append(symbol);
                    }
                    else
                    {
                        encoded.Append("%").Append(string.Format(CultureInfo.InvariantCulture, "{0:X2}", (int)symbol));
                    }
                }
                return encoded.ToString();
            }

            public static byte[] Hash(string value)
            {
                using (var sha256Algorithm = SHA256.Create())
                {
                    byte[] hash = sha256Algorithm.ComputeHash(Encoding.UTF8.GetBytes(value));
                    return hash;
                }
            }

            public static byte[] GetKeyedHash(string key, string value)
            {
                return GetKeyedHash(Encoding.UTF8.GetBytes(key), value);
            }

            public static byte[] GetKeyedHash(byte[] key, string value)
            {
                KeyedHashAlgorithm mac = new HMACSHA256(key);
                mac.Initialize();
                return mac.ComputeHash(Encoding.UTF8.GetBytes(value));
            }

            public static string ToHex(byte[] data)
            {
                StringBuilder sb = new StringBuilder();
                for (int i = 0; i < data.Length; i++)
                {
                    sb.Append(data[i].ToString("x2", CultureInfo.InvariantCulture));
                }
                return sb.ToString();
            }
        }
    }
}

Example usage:

                HttpRequestMessage httpRequestPost =
                    AwsSigningHelper.GenerateAwsSignedPostRequest(someapigatewayurl,
//                        AwsSigningHelper.EmptyContent,
//                        new StringContent("{ \"test\" : \"test\" }", Encoding.UTF8, "application/json"),
                        new StringContent("text!", Encoding.UTF8, "text/plain"),
                        EnvironmentVariables.Region, EnvironmentVariables.AccessKeyId, EnvironmentVariables.SecretAccessKey,
                        EnvironmentVariables.SessionToken);

                var responsePost = httpClient.SendAsync(httpRequestPost).Result;

Answer 2

You should be passing the session token in header or query param http://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp_use-resources.html, but you don't need to include it in the string to sign.

Have you tried testing this is something like postman? (https://www.getpostman.com/) Its possible, you are sending a few extra headers. It'll be helpful to debug if you can share the relevant code section.