PDF 安全指南——加密與隱私
PDF 支援多種安全功能——密碼保護、加密、權限限制和數位簽章。每種功能的作用都不同，理解這些差異有助於您針對自身情況選擇適當的保護措施。
兩種 PDF 密碼類型
PDFs have two separate password slots:
**User password (Document Open password):** Required to open the document. Anyone without this password sees a blank page or an error in their PDF viewer.
**Owner password (Permissions password):** Controls what an authorised reader can do — printing, copying text, editing. This is weaker than it sounds (see below).
You can set both on the same document, one, or neither. Most 'password protect PDF' tools set the User password by default.
PDF 加密的實際作用
When you set a User password, the PDF is encrypted (typically AES-256 for modern PDFs). The file contents are mathematically scrambled and cannot be read without the correct password. This is robust — brute-forcing a strong AES-256 password is not practical with current hardware.
Caution: PDF encryption protects the file at rest. Once a recipient opens the PDF with the correct password, they have access to the decrypted content on their device. PDF encryption does not prevent an authorised reader from screenshotting or printing.
權限限制「無法」防止的事項
Owner password (permissions) restrictions — 'no printing', 'no copying', 'no editing' — are enforced only by well-behaved PDF viewers. Any software that does not respect these flags can ignore them. This includes free tools, scripts, and even some popular PDF readers.
Conclusion: Use permissions restrictions to signal intent ('this PDF should not be edited'), but do not rely on them to technically prevent a determined person from copying or printing content.
浮水印與密碼保護的比較
A watermark does not prevent copying or editing — it is a visible deterrent and a way to identify leaked copies. Use watermarks when you want to:
- Attribute a document to a specific recipient (e.g. 'Confidential: John Smith') - Mark draft versions clearly - Protect brand assets shared for review
Watermarks and password protection can be combined: encrypt the file AND add a watermark for layered protection.
共用敏感 PDF 的最佳做法
1. **Set a strong User password** (12+ characters, mixed case, numbers, symbols) 2. **Share the password through a separate channel** — never in the same email as the PDF 3. **Set an expiry if possible** — some document management systems support link-based access with expiry 4. **Flatten forms before sharing** — if the PDF contains editable fields, flatten them to prevent modification 5. **Remove metadata** — PDF metadata can contain author name, creation software, and revision history
常見問題
- PDF 密碼保護有多安全？
- 如果您使用良好的密碼，它會非常安全。現代 PDF 採用 AES-256 加密，在任何實際的時間範圍內，都無法透過暴力破解法破解包含 12 個以上字元的隨機密碼。不過，弱密碼（字典單字、短字串）可以透過現成的工具破解。
- 受密碼保護的 PDF 可以在沒有密碼的情況下解鎖嗎？
- 針對使用者密碼（加密），基本上不行——使用 AES-256 與強密碼是無法解開的。針對擁有者密碼（僅限權限），可以——許多工具可以在不需要擁有者密碼的情況下移除權限限制，因為檔案內容本身並未加密。
- WhiskFile 會儲存我的密碼嗎？
- 不會。WhiskFile 在記憶體中處理您的 PDF，套用密碼保護，然後傳回加密檔案。密碼不會被儲存、記錄，也不會用於加密檔案以外的任何用途。檔案會在 1 小時內刪除。
- 為了符合 GDPR 合規性，我應該使用 PDF 密碼保護嗎？
- PDF 密碼保護可以是資料保護方法的一個層面，但若僅靠它本身並不足以符合 GDPR 合規性。靜態與傳輸中的檔案加密、存取控制以及資料保留政策，都是合規工作流程的一部分。