AWS DVA-C02 Drill: Signature Version 4 - Completing Signed Requests

Jeff’s Note

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Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.

For AWS DVA-C02 candidates, the confusion often lies in how and where to attach the AWS Signature Version 4 in HTTP requests. In production, this is about knowing exactly which HTTP headers or query parameters AWS services accept for authentication, so your API calls are properly authorized without errors. Let’s drill down.

The Certification Drill (Simulated Question)

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Scenario

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You are developing a payment processing application for FinApp Solutions, which integrates tightly with multiple AWS services through signed HTTP requests. You have successfully constructed the canonical request, generated the string to sign, and computed the signing key and signature using Signature Version 4. Now you need to complete the signed HTTP request to call AWS APIs securely.

The Requirement:

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Identify the valid ways to attach the computed signature to the HTTP request to ensure AWS service authorization. Choose two.

The Options

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• A) Add the signature to an HTTP header named Authorization.
• B) Add the signature to a session cookie.
• C) Add the signature to an HTTP header named Authentication.
• D) Add the signature to a query string parameter named X-Amz-Signature.
• E) Add the signature to an HTTP header named WWW-Authenticate.

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Correct Answer

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A and D.

Quick Insight: The Developer Imperative

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For DVA-C02 candidates, it is critical to understand that the AWS Signature Version 4 supports signing requests through either a specific Authorization HTTP header or through query string parameters (also called presigned URLs). Confusing these with other HTTP headers or cookies will cause authentication failures.

Content Locked: The Expert Analysis

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You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?

The Expert’s Analysis

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Correct Answer

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Options A and D

The Winning Logic

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The AWS Signature Version 4 signing process supports two primary ways to transmit the signature:

• Authorization header (Option A): This is the canonical way to pass the signature in AWS API requests. The header looks like:
  Authorization: AWS4-HMAC-SHA256 Credential=..., SignedHeaders=..., Signature=...
  This is the most common method for signed requests, especially for programmatic API calls.

• Query string parameter (Option D): This approach is used when generating presigned URLs, which allow access to AWS resources without requiring the client to sign the request. The signature is appended as the X-Amz-Signature query parameter along with other required signing information.

AWS services do not support passing signatures in generic headers like Authentication or WWW-Authenticate (Options C and E) or in browser session cookies (Option B). These are either unrelated HTTP authentication headers or mechanisms outside the Signature Version 4 process.

The Trap (Distractor Analysis):

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• Why not B (Session cookie)?
  AWS Signature Version 4 authentication doesn’t leverage cookies for authorization. Signed requests require explicit signature in headers or query params.

• Why not C (Authentication header)?
  The Authentication header is not recognized by AWS. The correct header is specifically named Authorization.

• Why not E (WWW-Authenticate header)?
  This header is used by servers to challenge clients for credentials during an authentication handshake, not for including signed request credentials.

The Technical Blueprint

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# Example of adding Signature Version 4 to AWS API call with Authorization Header
import boto3
from botocore.auth import SigV4Auth
from botocore.awsrequest import AWSRequest
from botocore.session import get_session

session = get_session()
credentials = session.get_credentials()
region = "us-east-1"
service = "execute-api"

request = AWSRequest(method="GET", url="https://api.example.com/resource", data=None)
SigV4Auth(credentials, service, region).add_auth(request)
headers = dict(request.headers)

print(headers["Authorization"])

# Example CLI command to generate a presigned URL (query string method)
aws s3 presign s3://mybucket/myobject --expires-in 3600
# The output URL contains X-Amz-Signature as a query parameter

The Comparative Analysis

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Real-World Application (Practitioner Insight)

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Exam Rule

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For the exam, always pick Authorization header or X-Amz-Signature query parameter when you see Signature Version 4.

Real World

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In API Gateway Lambda integrations, using the Authorization header is ideal for direct AWS SDK calls, while presigned URLs (query params) are handy for temporary access without exposing credentials.

(CTA) Stop Guessing, Start Mastering

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Disclaimer

This is a study note based on simulated scenarios for the AWS DVA-C02 exam.