Amazon Lex authorization with Flutter

Question

I want write Flutter app which work with Amazon Lex REST API.Amazon has specific way for authenticating I used SigV4 package to make required headers

  Sigv4Client client = Sigv4Client(
  keyId: kAccessKey,
  accessKey: kSecretKey,
  region: "us-east-1",
  serviceName: "lex",
);

final request = client.request(
  "https://runtime.lex.us-east-1.amazonaws.com/bot/myBotName/alias/BETA/user/myUserId/text",
  method: 'POST',
  body: jsonEncode({'inputText': 'hi'}),
);

var response=post(request.url, headers: request.headers, body: request.body);
print(response.body);

but i get this message in print :

{"message":"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."}

I get valid response on postman with same data the only part is different in postman and this package is "X-Amz-Content-Sha256" value and of course Signature value (it change every time) . "X-Amz-Content-Sha256" postman value:

beaead3198f7da1e70d03ab969765e0821b24fc913697e929e726aeaebf0eba3

"X-Amz-Content-Sha256" my code value:

ee9ef87bd5a357cff93b1d83d1e8a1b47fb3fa2e94251711c6a30250119e6369

I tried to write function to calculate authenticating string but it was so complex for me .

Solution

I changed the package and use amazon-cognito-identity-dart-2

and send the request like this :

AwsSigV4Client client=AwsSigV4Client(
      kAccessKey,
      kSecretKey,
      'https://runtime.lex.us-east-1.amazonaws.com',
      region: 'us-east-1',
      serviceName: 'lex',
    );

final signedRequest = new SigV4Request(
      client,
      method: 'POST',
      path: '/bot/MyBotName/alias/BETA/user/MyUser/text',
      headers: new Map<String, String>.from({
        'Content-Type': 'application/json; charset=utf-8',
        'ACCEPT': 'application/json',
      }),
      body: new Map<String, dynamic>.from({"inputText": "hi"}),
    );

var response = await http.post(
      signedRequest.url,
      headers: signedRequest.headers,
      body: signedRequest.body,
    );

it so important to add

'Content-Type': 'application/json; charset=utf-8',

in your headers .