敏感文件REST传输、第三方服务器存储及SFTP取回全流程安全方案问询

Alright, let's tackle this end-to-end security challenge step by step—your constraints (no pre-transmission encryption, reliance on third-party storage) are tricky, but there are solid ways to lock down every stage of the workflow.

First, let's ground this in your specific setup: files go over SSL REST to a third-party server, sit there for 7 days, users pull via SFTP then delete. Since you can't encrypt before the file hits the server, we need to double down on every other layer.

1. Lock Down the Initial REST Upload #

Even though you're using SSL, don't take that for granted:

• Force TLS 1.3 only: Ditch older TLS versions (1.0, 1.1, even 1.2 if possible) on both your sending client and the third-party server. TLS 1.3 is faster and eliminates a host of legacy vulnerabilities like BEAST or CRIME.
• Strict certificate validation: Make sure your client doesn't skip hostname checks or accept self-signed certificates. Verify the third-party's cert chain against a trusted root store—this stops man-in-the-middle attackers from intercepting the file mid-upload.
• Mutual TLS (mTLS) if feasible: If the third-party supports it, require your client to present a unique client certificate for authentication. This adds a hard barrier beyond basic auth, ensuring only your authorized systems can send files to their server.

2. Secure the File While It's Stored #

Since you can't encrypt before transmission, server-side at-rest encryption is non-negotiable:

• AES-256 encryption at rest: Work with the third-party to enable this—make sure the encryption keys are managed in a dedicated KMS (Key Management Service), not stored alongside the files. The third-party should not have access to the decryption keys unless absolutely necessary (and even then, with audit trails).
• Least-privilege access controls: Restrict file permissions so only the specific SFTP user accounts that need to download each file have read access. No shared accounts, no broad group permissions—each user gets exactly what they need, nothing more.
• Comprehensive audit logging: Demand that the third-party logs every single file action: upload timestamps, download attempts (success/failure), user IDs, source IPs, and deletion events. Store these logs separately from the files so you can investigate any suspicious activity later.

3. Harden the SFTP Download Process #

The final leg to the user's device needs just as much attention:

• TLS 1.3 for SFTP control channels: Modern SFTP uses SSH, but make sure the SSH connection is wrapped in TLS 1.3 (or use SSH with only the strongest cipher suites—no outdated ones like 3DES or AES-CBC).
• SSH key-based auth only: Ban password logins entirely. Each user should have a unique SSH key pair, with the private key stored securely on their device (encrypted keychain, hardware security key like YubiKey if available). Never share private keys.
• Immediate local encryption post-download: As soon as the user saves the file to their device, they should encrypt it with a strong passphrase or enable device-level encryption (BitLocker for Windows, FileVault for macOS) if they need to retain it. Since the server deletes the file after download, this protects the file on the user's end from theft or unauthorized access.

4. Extra Safeguards to Close Gaps #

• Enforce strict auto-deletion: Confirm with the third-party that files are permanently deleted after exactly one week—not moved to a recoverable trash folder. Get this in writing as part of your agreement.
• Audit the third-party regularly: Ask for their SOC 2 compliance reports, penetration test results, and security policies. Make sure they're adhering to the same security standards you would apply to your own systems.
• Educate users on safe handling: Remind users to delete the file from their device as soon as they're done with it, and never share the downloaded file via unsecure channels (email, unencrypted cloud storage).

内容的提问来源于Stack Exchange，提问作者Zachary