ofdgo/ofdgo_sign.go

// Copyright 2025-2026 肖其顿 (XIAO QI DUN)
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package ofdgo

import (
	"bytes"
	"encoding/asn1"
	"encoding/binary"
	"encoding/xml"
	"path"
	"strings"
)

const (
	// SignTypeSeal 签章类型: seal
	SignTypeSeal = "seal"
	// SignTypeSign 签章类型: sign
	SignTypeSign = "sign"
)

// OFDSignatures 签名列表列表
type OFDSignatures struct {
	XMLName   xml.Name       `xml:"Signatures"`
	MaxSignId string         `xml:"MaxSignId"`
	List      []OFDSignature `xml:"Signature"`
}

// OFDSignature 签名列表引用
type OFDSignature struct {
	ID      string `xml:"ID,attr"`
	BaseLoc string `xml:"BaseLoc,attr"`
}

// SignatureFile 签名文件内容描述
type SignatureFile struct {
	XMLName     xml.Name `xml:"Signature"`
	SignedValue string   `xml:"SignedValue"`
	SignedInfo  struct {
		Seal struct {
			BaseLoc string `xml:"BaseLoc"`
		} `xml:"Seal"`
		StampAnnot []struct {
			ID       string `xml:"ID,attr"`
			PageRef  string `xml:"PageRef,attr"`
			Boundary string `xml:"Boundary,attr"`
		} `xml:"StampAnnot"`
	} `xml:"SignedInfo"`
}

// parseSignatures 解析签名文件
// 入参: doc 文档结构
// 返回: error 错误信息
func (r *Reader) parseSignatures(doc *Document) error {
	if doc.Signatures == "" {
		return nil
	}
	f, err := r.openFile(doc.Signatures)
	if err != nil {
		return err
	}
	defer f.Close()
	var signatures OFDSignatures
	if err := xml.NewDecoder(f).Decode(&signatures); err != nil {
		return err
	}
	for _, sigRef := range signatures.List {
		func(sigRef OFDSignature) {
			sigPath := path.Join(path.Dir(doc.Signatures), sigRef.BaseLoc)
			sf, err := r.openFile(sigPath)
			if err != nil {
				return
			}
			defer sf.Close()
			var sigFile SignatureFile
			if err := xml.NewDecoder(sf).Decode(&sigFile); err != nil {
				return
			}
			var sealType string
			var sealData []byte
			if sigFile.SignedInfo.Seal.BaseLoc != "" {
				sealPath := path.Join(path.Dir(sigPath), sigFile.SignedInfo.Seal.BaseLoc)
				if data, err := r.ResData(sealPath); err == nil {
					sealType, sealData = extractSeal(data)
				}
			}
			if len(sealData) == 0 && sigFile.SignedValue != "" {
				signedValuePath := path.Join(path.Dir(sigPath), sigFile.SignedValue)
				if data, err := r.ResData(signedValuePath); err == nil {
					sealType, sealData = extractSeal(data)
				}
			}
			for _, annot := range sigFile.SignedInfo.StampAnnot {
				pageID := annot.PageRef
				bbox, _ := ParseBox(annot.Boundary)
				r.addStamp(pageID, bbox, sealType, sealData)
			}
		}(sigRef)
	}
	return nil
}

// extractSeal 尝试提取印章数据
// 入参: data 签名值数据
// 返回: string 印章类型, []byte 印章数据
func extractSeal(data []byte) (string, []byte) {
	if sealType, sealData := extractImageData(data); len(sealData) > 0 {
		return sealType, sealData
	}
	var raw asn1.RawValue
	_, err := asn1.Unmarshal(data, &raw)
	if err != nil {
		return "", nil
	}
	var foundType string
	var foundData []byte
	var search func(node asn1.RawValue) bool
	search = func(node asn1.RawValue) bool {
		if node.Tag == 4 {
			if sealType, sealData := extractImageData(node.Bytes); len(sealData) > 0 {
				foundType, foundData = sealType, sealData
				return true
			}
		}
		if node.IsCompound {
			if elements, ok := asn1Children(node.Bytes); ok {
				if len(elements) >= 2 && elements[1].Tag == 4 {
					sealType := sealString(elements[0])
					if sealType == "es" {
						sealType = "png"
					}
					if imgType, sealData := extractImageData(elements[1].Bytes); len(sealData) > 0 {
						if sealType == "" {
							sealType = imgType
						}
						foundType, foundData = sealType, sealData
						return true
					}
				}
				for _, child := range elements {
					if search(child) {
						return true
					}
				}
			}
		}
		return false
	}
	if search(raw) {
		return foundType, foundData
	}
	return "", nil
}

// asn1Children 解析ASN.1复合节点子元素
// 入参: data 复合节点内容
// 返回: []asn1.RawValue 子元素列表, bool 是否成功
func asn1Children(data []byte) ([]asn1.RawValue, bool) {
	var elements []asn1.RawValue
	for len(data) > 0 {
		var child asn1.RawValue
		rest, err := asn1.Unmarshal(data, &child)
		if err != nil || len(rest) == len(data) {
			return nil, false
		}
		elements = append(elements, child)
		data = rest
	}
	return elements, true
}

// sealString 解析印章图片类型
// 入参: raw ASN.1原始值
// 返回: string 图片类型
func sealString(raw asn1.RawValue) string {
	var s string
	if _, err := asn1.Unmarshal(raw.FullBytes, &s); err != nil {
		s = string(raw.Bytes)
	}
	s = strings.ToLower(strings.TrimSpace(strings.ReplaceAll(s, "\x00", "")))
	if s == "jpg" {
		s = "jpeg"
	}
	return s
}

// extractImageData 提取图片数据
// 入参: data 原始数据
// 返回: string 图片类型, []byte 图片数据
func extractImageData(data []byte) (string, []byte) {
	if idx := bytes.Index(data, []byte("\x89PNG\r\n\x1a\n")); idx >= 0 {
		if end := pngDataEnd(data[idx:]); end > 0 {
			return "png", data[idx : idx+end]
		}
	}
	if idx := bytes.Index(data, []byte{0xff, 0xd8, 0xff}); idx >= 0 {
		if end := bytes.Index(data[idx+2:], []byte{0xff, 0xd9}); end >= 0 {
			return "jpeg", data[idx : idx+2+end+2]
		}
	}
	if idx := bytes.Index(data, []byte("PK\x03\x04")); idx >= 0 {
		return "ofd", data[idx:]
	}
	return "", nil
}

// pngDataEnd 获取PNG数据结束位置
// 入参: data PNG数据
// 返回: int 结束位置
func pngDataEnd(data []byte) int {
	if len(data) < 8 || !bytes.Equal(data[:8], []byte("\x89PNG\r\n\x1a\n")) {
		return 0
	}
	pos := 8
	for pos+12 <= len(data) {
		size := int(binary.BigEndian.Uint32(data[pos : pos+4]))
		next := pos + 8 + size + 4
		if size < 0 || next > len(data) {
			return 0
		}
		if string(data[pos+4:pos+8]) == "IEND" {
			return next
		}
		pos = next
	}
	return 0
}

// Stamp 印章信息结构
type Stamp struct {
	Box  Box
	Type string
	Data []byte
}

// addStamp 添加印章到页面
// 入参: pageID 页面ID, box 印章区域, sType 印章类型, data 印章数据
func (r *Reader) addStamp(pageID string, box Box, sType string, data []byte) {
	if r.Stamps == nil {
		r.Stamps = make(map[string][]Stamp)
	}
	r.Stamps[pageID] = append(r.Stamps[pageID], Stamp{
		Box:  box,
		Type: sType,
		Data: data,
	})
}