Signature Version 4 (SigV4) is the AWS authentication protocol used to sign API requests made to AWS services. It provides a secure method to verify the identity of the requester and ensure data integrity during transmission.
When you make requests to AWS APIs, SigV4 creates a cryptographic signat…Signature Version 4 (SigV4) is the AWS authentication protocol used to sign API requests made to AWS services. It provides a secure method to verify the identity of the requester and ensure data integrity during transmission.
When you make requests to AWS APIs, SigV4 creates a cryptographic signature using your AWS credentials (Access Key ID and Secret Access Key). This signature is included in the request header or query string, allowing AWS to authenticate and authorize your request.
The signing process involves four main steps:
1. **Create a Canonical Request**: This involves formatting your HTTP request into a standardized structure, including the HTTP method, URI, query string, headers, signed headers list, and a hash of the request payload.
2. **Create a String to Sign**: This combines the algorithm identifier (AWS4-HMAC-SHA256), request timestamp, credential scope (date/region/service/aws4_request), and the hash of the canonical request.
3. **Calculate the Signature**: Using your Secret Access Key, you derive a signing key through a series of HMAC-SHA256 operations. This derived key is then used to sign the string created in step 2.
4. **Add Signature to Request**: The final signature is added to the Authorization header or as query parameters for pre-signed URLs.
Key benefits of SigV4 include:
- **Request integrity**: Any modification to the request invalidates the signature
- **Identity verification**: Confirms the requester possesses valid AWS credentials
- **Replay protection**: Timestamps prevent reuse of old requests
- **Region and service scoping**: Signatures are bound to specific regions and services
For developers, the AWS SDKs handle SigV4 signing automatically. However, understanding SigV4 is essential when working with custom HTTP clients, debugging authentication issues, or creating pre-signed URLs for S3 object access. Pre-signed URLs allow temporary access to private resources by embedding the signature in the URL parameters.
Signature Version 4 Signing - Complete Guide for AWS Developer Associate Exam
What is Signature Version 4 (SigV4)?
Signature Version 4 is the AWS signing protocol used to authenticate API requests made to AWS services. It is a security mechanism that ensures requests are legitimate, have not been tampered with, and come from an authorized source.
Why is SigV4 Important?
• Authentication: Proves the identity of the requester using AWS access keys • Integrity: Ensures the request has not been modified in transit • Non-repudiation: The sender cannot deny having sent the request • Protection against replay attacks: Timestamps prevent old requests from being reused • Required by most AWS services: Nearly all AWS APIs require SigV4 authentication
How Does SigV4 Work?
The signing process involves four main steps:
Step 1: Create a Canonical Request • HTTP method (GET, POST, etc.) • URI path • Query string parameters • Headers (including host and x-amz-date) • Signed headers list • Hashed payload (SHA256 hash of the request body)
Step 2: Create a String to Sign • Algorithm identifier (AWS4-HMAC-SHA256) • Request date/time • Credential scope (date/region/service/aws4_request) • Hash of the canonical request
Step 3: Calculate the Signature • Derive a signing key using your secret access key • Apply HMAC-SHA256 iteratively with date, region, service, and signing constant • Sign the string to sign with the derived key
Step 4: Add Signature to Request • Include in Authorization header, or • Add as query string parameters (for presigned URLs)
Key Components of SigV4
• Access Key ID: Identifies who is making the request • Secret Access Key: Used to derive the signing key (never sent over the wire) • Credential Scope: Format is YYYYMMDD/region/service/aws4_request • Timestamp: Must be within 5 minutes of AWS server time • Signed Headers: List of headers included in signature calculation
Common Use Cases
• Making REST API calls to AWS services • Creating presigned URLs for S3 objects • Authenticating WebSocket connections to API Gateway • Cross-region API requests • IoT device authentication
Presigned URLs
SigV4 enables presigned URLs that allow temporary access to private resources: • Default expiration: 15 minutes for S3 console, up to 12 hours for CLI • Maximum expiration: 7 days when using IAM user credentials • Include signature in query string parameters
Exam Tips: Answering Questions on Signature Version 4 Signing
• Remember the four steps: Canonical Request → String to Sign → Calculate Signature → Add to Request • Know that SigV4 is the current standard: SigV2 is deprecated for most services • Understand credential scope format: date/region/service/aws4_request • Time synchronization matters: Requests fail if timestamp differs by more than 5 minutes • Secret key is never transmitted: Only the access key ID is included in requests • Regional endpoints: Signature is tied to a specific region and service • SDK handles signing: AWS SDKs implement SigV4 automatically • Presigned URL limits: Know the maximum duration (7 days for IAM users, 12 hours for STS credentials) • S3 special case: Supports both path-style and virtual-hosted-style URLs with SigV4 • Payload hashing: For streaming uploads, use STREAMING-AWS4-HMAC-SHA256-PAYLOAD
Common Exam Scenarios
• Troubleshooting authentication failures (check timestamps, region, credentials) • Choosing between SigV4 in header vs query string • Understanding when to use presigned URLs vs IAM policies • Recognizing that cross-region requests need correct regional signing