Using Signed URLs and Tokens for Secure Data Downloads

June 29, 2025

Using Signed URLs and Tokens for Secure Data Downloads

Overview

When delivering files or data over the internet, it’s important to ensure only authorized users can access them. Two common methods to protect downloads are Signed URLs and Tokens. These techniques help prevent unauthorized access, link sharing, or scraping of your data.

1. What Are Signed URLs?

A Signed URL is a link that includes an embedded signature or token that grants temporary access to a file or resource.

Key Features:

Time-limited access

Tied to specific users or permissions

Can include IP restrictions or usage limits

Example Use Case:

A file download link that expires 15 minutes after it's generated, only usable by the intended recipient.

How It Works:

User authenticates.

Server generates a URL with a secure signature, expiration time, and optional user/IP restrictions.

The URL is shared with the user.

When the user accesses the link, the server checks the signature and conditions before allowing the download.

plaintext

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https://example.com/download/file.pdf?expires=1719800400&signature=abcd1234

2. What Are Tokens?

A Token is a piece of data (like a JWT or opaque string) that proves a user’s authorization to access a resource.

Types:

Access Tokens (e.g., OAuth2, JWTs)

Refresh Tokens (to renew access)

Download Tokens (one-time use for specific files)

How It Works:

User logs in or is verified.

The server issues a token.

The client uses the token in a request header or query string to download the file.

The server validates the token before allowing access.

http

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GET /download/file.pdf

Authorization: Bearer eyJhbGciOi...

3. Signed URLs vs Tokens

Feature Signed URL Token-based Download

Access Method Link with embedded parameters Request with token in header/query

Expires Automatically Yes, via URL expiration time Yes, if token has TTL

Granularity Per-resource, per-user, per-time Per-session or per-user

Ease of Use Easy for user to click a link Requires client handling for token use

Security Can be shared, unless locked to IP More secure if token is short-lived

4. Best Practices

Use HTTPS to prevent man-in-the-middle attacks.

Set short expiration times for signed URLs.

Use HMAC or asymmetric encryption to sign URLs.

For tokens:

Keep them short-lived.

Store them securely on the client.

Use scopes or claims to limit what the token can access.

Consider revocation mechanisms for both (e.g., blacklist or allowlist).

5. Tools and Libraries

AWS S3 Signed URLs (boto3.generate_presigned_url)

Google Cloud Storage Signed URLs

Azure Blob SAS Tokens

JWT Libraries (e.g., jsonwebtoken in Node.js, pyjwt in Python)

OAuth2 Frameworks

6. Conclusion

Using signed URLs and tokens effectively allows for secure, time-bound, and user-specific access to downloadable content. Depending on your application’s needs, you might use one or a combination of both to ensure that your resources are protected from unauthorized access.

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