* `apiKey`: uma chave específica de aplicação que pode vir de:
    * Um parâmetro query.
    * Um header.
    * Um cookie.
* `http`: padrão HTTP de sistemas autenticação, incluindo:
    * `bearer`: um header de `Authorization` com valor de `Bearer` adicionado de um token. Isso é herança do OAuth2.
    * HTTP Basic authentication.
    * HTTP Digest, etc.
* `oauth2`: todas as formas do OAuth2 para lidar com segurança (chamados "fluxos").

        PERMANENT, // Operation cannot succeed
        FATAL // Connection or session must be terminated
    }

    /**
     * Error categories
     */
    public enum Category {
        AUTHENTICATION, AUTHORIZATION, NETWORK, PROTOCOL, RESOURCE, CONFIGURATION, ENCRYPTION, TIMEOUT, IO, UNKNOWN
    }

    private final int errorCode;
    private final Severity severity;
    private final Category category;

        }
      }
    };

    RequestBody requestBody = RequestBody.create(
      new File("docs/images/logo-square.png"),
      MEDIA_TYPE_PNG);

    Request request = new Request.Builder()
      .header("Authorization", "Client-ID " + IMGUR_CLIENT_ID)
      .url("https://api.imgur.com/3/image")
      .post(new ProgressRequestBody(requestBody, progressListener))
      .build();

    Response response = client.newCall(request).execute();

import org.apache.maven.wagon.Wagon;
import org.apache.maven.wagon.authentication.AuthenticationException;
import org.apache.maven.wagon.authentication.AuthenticationInfo;
import org.apache.maven.wagon.authorization.AuthorizationException;
import org.apache.maven.wagon.events.TransferListener;
import org.apache.maven.wagon.observers.ChecksumObserver;
import org.apache.maven.wagon.proxy.ProxyInfo;

	// returned early by any other middleware (but after the middleware that
	// sets the amz request id).
	httpTracerMiddleware,
	// Auth middleware verifies incoming authorization headers and routes them
	// accordingly. Client receives a HTTP error for invalid/unsupported
	// signatures.
	//
	// Validates all incoming requests to have a valid date header.
	setAuthMiddleware,

//	    modify the user-agent.
//
//	Authorization:
//
//	    Is skipped for obvious reasons
var ignoredHeaders = map[string]bool{
	"Authorization": true,
	"User-Agent":    true,
}

// Headers to ignore in streaming v4
var ignoredStreamingHeaders = map[string]bool{
	"Authorization": true,
	"Content-Type":  true,
	"Content-Md5":   true,
	"User-Agent":    true,
}

func calculateSeedSignature(r *http.Request, trailers bool) (cred auth.Credentials, signature string, region string, date time.Time, errCode APIErrorCode) {
	// Copy request.
	req := *r

	// Save authorization header.
	v4Auth := req.Header.Get(xhttp.Authorization)

	// Parse signature version '4' header.
	signV4Values, errCode := parseSignV4(v4Auth, globalSite.Region(), serviceS3)
	if errCode != ErrNone {

		// Test case - 2.
		// Test case with no sign v4 Algorithm prefix.
		// A valid authorization string should begin(prefix)
		{
			inputV4AuthStr:    "no-singv4AlgorithmPrefix",
			expectedAuthField: signValues{},
			expectedErrCode:   ErrSignatureVersionNotSupported,
		},
		// Test case - 3.
		// Test case with missing fields.
		// A valid authorization string should have 3 fields.
		{
			inputV4AuthStr:    signV4Algorithm,

    * Einem Query-Parameter.
    * Einem Header.
    * Einem Cookie.
* `http`: Standard-HTTP-Authentifizierungssysteme, einschließlich:
    * `bearer`: ein Header `Authorization` mit dem Wert `Bearer` plus einem Token. Dies wird von OAuth2 geerbt.
    * HTTP Basic Authentication.
    * HTTP Digest, usw.
* `oauth2`: Alle OAuth2-Methoden zum Umgang mit Sicherheit (genannt „Flows“).

   * due to any of:
   *
   *  * A stale connection. The request was made on a reused connection and that reused connection
   *    has since been closed by the server.
   *  * A client timeout (HTTP 408).
   *  * A authorization challenge (HTTP 401 and 407) that is satisfied by the [Authenticator].
   *  * A retryable server failure (HTTP 503 with a `Retry-After: 0` response header).
   *  * A misdirected request (HTTP 421) on a coalesced connection.