The request signature we calculated does not match the signature you provided.

[castrors]

Describe the bug
I'm using AWS Appsync and then, when I try to make a request with the same parameters via http, it goes with success.
But when I try with graphql-flutter, I'm getting this error:

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
'POST
/graphql

accept:application/json
content-type:application/graphql; charset=utf-8
host:www.host.com
x-amz-date:0000000000000
accept;content-type;host;x-amz-date
AAAAA'

The String-to-Sign should have been
'AWS4-HMAC-SHA256
0000000000000
1111111/eu-west-1/appsync/aws4_request
BBBBB'

To Reproduce
This is my creation of the link using graphql-flutter:

import 'package:amazon_cognito_identity_dart_2/sig_v4.dart';

    final awsSigV4Client = AwsSigV4Client(
        credentials.accessKeyId, credentials.secretAccessKey, endpoint,
        serviceName: 'appsync',
        sessionToken: credentials.sessionToken,
        region: 'eu-west-1');

    final signedRequest = SigV4Request(awsSigV4Client,
        method: 'POST',
        path: '/graphql',
        headers: Map<String, String>.from(
            {'Content-Type': 'application/graphql; charset=utf-8'}),
        body: Map<String, String>.from({'query': query}));

    final CustomAuthLink authLink =
        CustomAuthLink(getHeaders: () => signedRequest.headers);

    return authLink.concat(httpLink);

Steps to reproduce the behavior:

1. Just consume the AuthLink in the flutter app.

Expected behavior
This goes well when I'm using http.Request:

import 'package:amazon_cognito_identity_dart_2/sig_v4.dart';

    final awsSigV4Client = AwsSigV4Client(
        credentials.accessKeyId, credentials.secretAccessKey, endpointStaging,
        serviceName: 'appsync',
        sessionToken: credentials.sessionToken,
        region: 'eu-west-1');

    final signedRequest = SigV4Request(awsSigV4Client,
        method: 'POST',
        path: '/graphql',
        headers: Map<String, String>.from(
            {'Content-Type': 'application/graphql; charset=utf-8'}),
        body:
            Map<String, String>.from({'query': query}));

    http.Response response;
    try {
      response = await http.post(signedRequest.url,
          headers: signedRequest.headers, body: signedRequest.body);
    } catch (e) {
      print(e);
    }
    print(response.body);

Desktop (please complete the following information):

• OS: macOS Catalina
• Version 10.15.4

Smartphone (please complete the following information):

• Device: android-x86 emulator
• OS: Android
• Version 10 (api 29)

[micimize]

Are you sure the only thing SigV4Request changes are the headers? Try

      response = await http.post(endpoint,
          headers: signedRequest.headers, body: {'query': query});

Other than that, try using content-type (lowercased), since the default is lowercased: https://github.com/zino-app/graphql-flutter/blob/eec7d2c5e8d753f7d2d62b2964cb61730caeae42/packages/graphql/lib/src/link/http/fallback_http_config.dart#L14

You can try something like #88 to debug as well

[2coo]

@castrors Did you fix that? I have faced the problem too like yours

[micimize]

This is outdated / likely solved by us using gql_links in 4.0.0-beta.1 (#648). Please test there and have a look at appsync support section and related issues https://github.com/zino-app/graphql-flutter/tree/beta/packages/graphql#aws-appsync-support