PDF Contract Redactor
Redact sensitive values from scanned PDF contracts while preserving field names.
What It Does
- OCR Recognition: Uses Alibaba Cloud OCR to extract text and positions from scanned PDFs
- Field-Value Matching: Finds field names (e.g., "合同金额") and their corresponding values (e.g., "45640元")
- Selective Redaction: Covers only the values with black boxes, keeping field names readable
Workflow
Step 1: PDF to Images
Convert PDF pages to high-resolution PNG images (200 DPI) for OCR.
Step 2: OCR with Alibaba Cloud
Call Alibaba Cloud OCR API to get:
- All text blocks
- Bounding box coordinates for each text block
- Confidence scores
Step 3: Match Fields to Values
For each field in the field list:
- Find the field name text block
- Look for the corresponding value in:
- Right side: Same row, to the right of field name
- Below: Next row, aligned with field name
- Record field-value pair with both bounding boxes
Step 4: Generate Redacted PDF
For each matched value:
- Convert image coordinates to PDF coordinates
- Draw black rectangle over the value area
- Keep field name area unchanged
Field List
The following fields are searched and their values are redacted:
- 法务部归档编号, 归档时间, 申请人工号, 申请人姓名, 申请人部门
- 申请人部门负责人, 所涉项目名称（如有）, 所涉项目编号（如有）
- 对方编号（如有）, 合同编号, 合同名称, 合同甲方名称, 合同乙方名称
- 合同相对方, 相对方所属行业, 相对方是否为世界500强
- 相对方是央企/国企, 相对方是否为涉密单位, 业务类别, 合同类别
- 合同类型, 合同状态, 扫描件状态, 对方是否签章, 我方是否签章
- 销售、采购标的（非一起译填）, 语种, 单价, 合同金额（元）, 币种
- 支付/收款方式, 付款/收款条件, 合同结算周期, 是否使用公司模板
- 用章主体, 印章类型, 签订时间, 合同开始时间, 合同到期时间
- 收支类型, 我方联系人姓名, 我方联系人电话, 对方联系人姓名
- 对方联系人电话, 对方邮寄地址, 归档状态, 开票名称, 开票账号
- 开票银行, 收款名称, 收款账号, 收款银行, 验收时间, 验收标准
- 合同是否自动续期, 合同续期时间, 合同特殊约定
- 协议内是否有结算单, 结算单（如有）内容是否填写
Usage
Prerequisites
- Alibaba Cloud account with OCR service enabled
- AccessKey ID and AccessKey Secret
Running the Tool
python scripts/redact_contract.py <input.pdf> <access_key_id> <access_key_secret> [output.pdf]
Example:
python scripts/redact_contract.py contract.pdf LTAIxxx xxx contract_redacted.pdf
Output
<name>_redacted.pdf: Redacted PDF with values covered<name>_fields.json: JSON file listing all matched field-value pairs
Implementation Notes
OCR API
Uses Alibaba Cloud "通用文字识别-高精度版" (RecognizeAdvanced API):
- Endpoint:
https://ocr.aliyuncs.com - Returns text content and quadrilateral coordinates
- Supports automatic rotation detection
Field-Value Matching Logic
# For a field at (fx0, fy0, fx1, fy1)
# Look for values that are:
# 1. To the right: vx0 > fx1 and |vy0 - fy0| < field_height * 2
# 2. Below: vy0 > fy1 and vx0 >= fx0 - field_width * 0.3
# Choose the closest match
Coordinate Transformation
OCR returns coordinates in image space (200 DPI).
Convert to PDF space (72 DPI) using scale factor: scale = 72 / 200 = 0.36
Dependencies
pip install pymupdf pillow requests
Error Handling
- If OCR API fails, retry with exponential backoff
- If field not found, skip silently (don't fail entire document)
- If value not found for a field, log warning and continue