Authenticate a cognito user using expo AuthSession API - react-native

Question

I am using this example code
I am able to get a response from authorize endpoint.
request: {"clientId": "<retracted>", "clientSecret": undefined, "codeChallenge": "t6xISsEiAwOIwQxk0Ty1JNo2Kqa53mECL9a7YahLv_A", "codeChallengeMethod": "S256", "codeVerifier": "<retracted>", "extraParams": {}, "prompt": undefined, "redirectUri": "exp://192.168.0.22:19000", "responseType": "code", "scopes": undefined, "state": "o7FeO9ANoa", "url": "https://<retracted>"//oauth2/authorize?code_challenge=t6xISsEiAwOIwQxk0Ty1JNo2Kqa53mECL9a7YahLv_A&code_challenge_method=S256&redirect_uri=exp%3A%2F%2F192.168.0.22%3A19000&client_id=<retracted>"f&response_type=code&state=o7FeO9ANoa", "usePKCE": true}
LOG response: {"authentication": null, "error": null, "errorCode": null, "params": {"code": "<retracted>"", "state": "o7FeO9ANoa"}, "type": "success", "url": "exp://192.168.0.22:19000?code=<retracted>"&state=o7FeO9ANoa"}
const exchangeFn = async (exchangeTokenReq) => {
try {
const exchangeTokenResponse = await exchangeCodeAsync(
exchangeTokenReq,
discoveryDocument
);
setAuthTokens(exchangeTokenResponse);
} catch (error) {
console.error(error);
}
};
while exchangeFn is being invoked i am getting an error "ERROR [Error: Client authentication failed (e.g., unknown client, no client authentication included, or unsupported authentication method). The authorization server MAY return an HTTP 401 (Unauthorized) status code to indicate which HTTP authentication schemes are supported. If the client attempted to authenticate via the "Authorization" request header field, the authorization server MUST respond with an HTTP 401 (Unauthorized) status code and include the "WWW-Authenticate" response header field matching the authentication scheme used by the client.]"
Here is the application flow enter image description here

Answer 1

As per Oauth 2.0 while Exchanging an authorization code grant with PKCE for tokens we need to add Authorization header.
The authorization header string is Basic Base64Encode(client_id:client_secret). The following example is an authorization header for app client djc98u3jiedmi283eu928 with client secret abcdef01234567890, using the Base64-encoded version of the string djc98u3jiedmi283eu928:abcdef01234567890
The example code does not include this. That is the issue. we have to get the App client secret from aws cognito and add it to exchangeTokenReq.
const clientId = '<your-client-id-here>';
const userPoolUrl =
'https://<your-user-pool-domain>.auth.<your-region>.amazoncognito.com';
const redirectUri = 'your-redirect-uri';
const clientSecret = 'app-client-secret';
exchangeFn({
clientId,
code: response.params.code,
redirectUri,
clientSecret,
extraParams: {
code_verifier: request.codeVerifier,
},
});