安全なファイル共有の方法 - 機密データを守る実践ガイド

About 14 min read

Sharing files with others occurs daily in both business and personal contexts. However, incorrect sharing methods risk exposing confidential information to unintended recipients. Information leaks caused by file sharing - misdirected email attachments, misconfigured cloud storage visibility, lost unencrypted USB drives - continue unabated. This article provides practical guidance on proper cloud storage sharing settings, correct use of password-protected ZIP files, choosing end-to-end encrypted services, setting link expiration dates, and applying the principle of least privilege for access permissions.

Managing Cloud Storage Sharing Settings Correctly

Choosing the Right Access Level

When sharing via cloud storage services like Google Drive or OneDrive, you must carefully select the access level. The three typical levels are "Viewer," "Commenter," and "Editor" - always grant the minimum necessary. If the recipient only needs to read the file, choose "Viewer." If feedback is needed, choose "Commenter." Select "Editor" only when collaborative editing is required. The "Editor" permission is particularly sensitive as it enables file deletion and re-sharing with others, so restrict it to trusted parties.

The visibility scope of sharing links is also a critical setting. "Anyone with the link" means anyone who knows the URL can access the file - never use this for confidential materials. Even for internal sharing, specifying "Specific people" is safer. Always set expiration dates on shares so access automatically expires after project completion or review. Google Workspace offers a sharing expiration feature with a maximum duration of one year.

Correct Use of Password-Protected ZIP Files

Password-protected ZIP files are still widely used, but security effectiveness drops significantly without proper practices. First, the "PPAP" method of sending the ZIP password in the same email as the file is completely pointless - if the email is intercepted, both the attachment and password are compromised simultaneously. Always communicate the password through a separate channel (SMS, phone call, chat tool, etc.). Also, always select AES-256 as the encryption method. The older ZipCrypto method is vulnerable to known-plaintext attacks and can be cracked in minutes.

Leveraging End-to-End Encrypted Services

Choosing E2EE File Sharing Services

File sharing services that employ end-to-end encryption (E2EE) prevent even the service provider from viewing file contents. Tresorit, Proton Drive, and SpiderOak are representative services. Files are encrypted on the sender's device and decrypted only on the recipient's device. Since only encrypted data is stored on servers, file contents remain protected even if servers are compromised.

Key points when choosing an E2EE service include the encryption algorithm (AES-256 is standard), key management approach (zero-knowledge design), file size limits, availability of link expiration settings, and audit log provision. Some services offer basic E2EE features on free plans, but for business use, detailed permission management and activity logs typically require paid plans.

Link Expiration and Access Control

Always set expiration dates on file sharing links. Links without expiration remain permanently accessible after the sharing purpose ends, accumulating risk over time. As a general guideline, 24 hours to 7 days is appropriate for temporary review purposes, and 30 to 90 days for sharing during a project period. Services that allow download count limits let you permit only the necessary number of downloads before invalidating the link. Following access control principles - granting minimum necessary permissions for only the required duration - is fundamental to secure file sharing.

Minimizing Access and Regular Audits

Practicing the Principle of Least Privilege

The principle of least privilege in file sharing means granting each user only the minimum access rights necessary to perform their tasks. Rather than giving everyone edit permissions, grant edit access only to those who actually need to edit, and limit others to view-only. When setting permissions at the folder level, note that parent folder permissions are inherited by subfolders. Isolate highly confidential files in dedicated folders with strictly limited access to prevent unintended information leaks.

Regular audits of sharing status are also essential. Once a month, review the list of shared files and folders and revoke unnecessary shares. In Google Drive, you can list files you're sharing from "Shared items." Check whether departed or transferred employees still have access, and whether post-project shares have been left active. Cloud storage sharing settings are not "set and forget" - they require ongoing management.

To protect sensitive files during physical transport, encrypted USB drives (Amazon) offer hardware-level security that software encryption cannot match.