@jsreport/node-signpdf/src/helpers/extractSignature.js

Simple signing of PDFs in node.

import SignPdfError from '../SignPdfError';

const getSubstringIndex = (str, substring, n) => {
    let times = 0; let
        index = null;

    while (times < n && index !== -1) {
        index = str.indexOf(substring, index + 1);
        times += 1;
    }

    return index;
};
/**
 * Basic implementation of signature extraction.
 *
 * Really basic. Would work in the simplest of cases where there is only one signature
 * in a document and ByteRange is only used once in it.
 *
 * @param {Buffer} pdf
 * @returns {Object} {ByteRange: Number[], signature: Buffer, signedData: Buffer}
 */
const extractSignature = (pdf, signatureCount = 1) => {
    if (!(pdf instanceof Buffer)) {
        throw new SignPdfError(
            'PDF expected as Buffer.',
            SignPdfError.TYPE_INPUT,
        );
    }

    // const byteRangePos = pdf.indexOf('/ByteRange [');
    const byteRangePos = getSubstringIndex(pdf, '/ByteRange [', signatureCount);
    if (byteRangePos === -1) {
        throw new SignPdfError(
            'Failed to locate ByteRange.',
            SignPdfError.TYPE_PARSE,
        );
    }

    const byteRangeEnd = pdf.indexOf(']', byteRangePos);
    if (byteRangeEnd === -1) {
        throw new SignPdfError(
            'Failed to locate the end of the ByteRange.',
            SignPdfError.TYPE_PARSE,
        );
    }

    const byteRange = pdf.slice(byteRangePos, byteRangeEnd + 1).toString();
    const matches = (/\/ByteRange \[(\d+) +(\d+) +(\d+) +(\d+) *\]/).exec(byteRange);
    if (matches === null) {
        throw new SignPdfError(
            'Failed to parse the ByteRange.',
            SignPdfError.TYPE_PARSE,
        );
    }

    const ByteRange = matches.slice(1).map(Number);
    const signedData = Buffer.concat([
        pdf.slice(ByteRange[0], ByteRange[0] + ByteRange[1]),
        pdf.slice(ByteRange[2], ByteRange[2] + ByteRange[3]),
    ]);

    const signatureHex = pdf.slice(ByteRange[0] + ByteRange[1] + 1, ByteRange[2])
        .toString('binary')
        .replace(/(?:00|>)+$/, '');

    const signature = Buffer.from(signatureHex, 'hex').toString('binary');

    return {
        ByteRange: matches.slice(1, 5).map(Number),
        signature,
        signedData,
    };
};

export default extractSignature;