pdf-signature-reader-sl/helpers/extractSignature.js

Verify digital signatures in PDF documents with content validation and enhanced support for Spanish certificates (FNMT, DNIe, corporate)

const { Buffer } = require('../packages/buffer');

const VerifyPDFError = require('../VerifyPDFError');
const { getSignatureMeta, preparePDF } = require('./general');

const DEFAULT_BYTE_RANGE_PLACEHOLDER = '**********';

const getByteRange = (pdfBuffer) => {
  const byteRangeStrings = pdfBuffer.toString().match(/\/ByteRange\s*\[{1}\s*(?:(?:\d*|\/\*{10})\s+){3}(?:\d+|\/\*{10}){1}\s*\]{1}/g);
  if (!byteRangeStrings) {
    throw new VerifyPDFError(
      'Failed to locate ByteRange.',
      VerifyPDFError.TYPE_PARSE,
    );
  }

  const byteRangePlaceholder = byteRangeStrings.find((s) => s.includes(`/${DEFAULT_BYTE_RANGE_PLACEHOLDER}`));
  const strByteRanges = byteRangeStrings.map((brs) => brs.match(/[^[\s]*(?:\d|\/\*{10})/g));

  const byteRanges = strByteRanges.map((n) => n.map(Number));

  return {
    byteRangePlaceholder,
    byteRanges,
  };
};

const extractSignature = (pdf) => {
  const pdfBuffer = preparePDF(pdf);

  const { byteRanges } = getByteRange(pdfBuffer);
  // const lastIndex = byteRanges.length - 1;
  // const endOfByteRange = byteRanges[lastIndex][2] + byteRanges[lastIndex][3];

  // if (pdfBuffer.length > endOfByteRange) {
  //   throw new VerifyPDFError(
  //     'Failed byte range verification.',
  //     VerifyPDFError.VERIFY_BYTE_RANGE,
  //   );
  // }

  const signatureStr = [];
  const signedData = [];
  byteRanges.forEach((byteRange) => {
    signedData.push(Buffer.concat([
      pdfBuffer.slice(byteRange[0], byteRange[0] + byteRange[1]),
      pdfBuffer.slice(byteRange[2], byteRange[2] + byteRange[3]),
    ]));

    const signatureHex = pdfBuffer.slice(byteRange[0] + byteRange[1] + 1, byteRange[2]).toString('latin1');
    signatureStr.push(Buffer.from(signatureHex, 'hex').toString('latin1'));
  });

  const signatureMeta = signedData.map((sd) => getSignatureMeta(sd))

  return {
    byteRanges,
    signatureStr,
    signedData,
    signatureMeta,
  };
};

module.exports = extractSignature;