node-signpdf/src/helpers/pdfkitAddPlaceholder.js

Simple signing of PDFs in node.

import {DEFAULT_BYTE_RANGE_PLACEHOLDER, DEFAULT_SIGNATURE_LENGTH, SUBFILTER_ADOBE_PKCS7_DETACHED} from './const';
// eslint-disable-next-line import/no-unresolved
import PDFKitReferenceMock from './pdfkitReferenceMock';
/**
 * Adds the objects that are needed for Adobe.PPKLite to read the signature.
 * Also includes a placeholder for the actual signature.
 * Returns an Object with all the added PDFReferences.
 * @param {PDFDocument} pdf
 * @param {string} reason
 * @returns {object}
 */
const pdfkitAddPlaceholder = ({
    pdf,
    pdfBuffer,
    reason,
    contactInfo = 'emailfromp1289@gmail.com',
    name = 'Name from p12',
    location = 'Location from p12',
    signatureLength = DEFAULT_SIGNATURE_LENGTH,
    byteRangePlaceholder = DEFAULT_BYTE_RANGE_PLACEHOLDER,
    subFilter = SUBFILTER_ADOBE_PKCS7_DETACHED,
}) => {
    /* eslint-disable no-underscore-dangle,no-param-reassign */
    // Generate the signature placeholder
    const signature = pdf.ref({
        Type: 'Sig',
        Filter: 'Adobe.PPKLite',
        SubFilter: subFilter,
        ByteRange: [
            0,
            byteRangePlaceholder,
            byteRangePlaceholder,
            byteRangePlaceholder,
        ],
        Contents: Buffer.from(String.fromCharCode(0).repeat(signatureLength)),
        Reason: new String(reason), // eslint-disable-line no-new-wrappers
        M: new Date(),
        ContactInfo: new String(contactInfo), // eslint-disable-line no-new-wrappers
        Name: new String(name), // eslint-disable-line no-new-wrappers
        Location: new String(location), // eslint-disable-line no-new-wrappers
    });

    // Check if pdf already contains acroform field
    const acroFormPosition = pdfBuffer.lastIndexOf('/Type /AcroForm');
    const isAcroFormExists = acroFormPosition !== -1;
    let fieldIds = [];
    let acroFormId;

    if (isAcroFormExists) {
        let acroFormStart = acroFormPosition;
        // 10 is the distance between "/Type /AcroForm" and AcroFrom ID
        const charsUntilIdEnd = 10;
        const acroFormIdEnd = acroFormPosition - charsUntilIdEnd;
        // Let's find AcroForm ID by trying to find the "\n" before the ID
        // 12 is a enough space to find the "\n"
        // (generally it's 2 or 3, but I'm giving a big space though)
        const maxAcroFormIdLength = 12;
        let foundAcroFormId = '';
        let index = charsUntilIdEnd + 1;
        for (index; index < charsUntilIdEnd + maxAcroFormIdLength; index += 1) {
            const acroFormIdString = pdfBuffer
                .slice(acroFormPosition - index, acroFormIdEnd).toString();

            if (acroFormIdString[0] === '\n') {
                break;
            }

            foundAcroFormId = acroFormIdString;
            acroFormStart = acroFormPosition - index;
        }

        const pdfSlice = pdfBuffer.slice(acroFormStart);
        const acroForm = pdfSlice.slice(0, pdfSlice.indexOf('endobj')).toString();
        acroFormId = parseInt(foundAcroFormId);

        const acroFormFields = acroForm.slice(acroForm.indexOf('/Fields [') + 9, acroForm.indexOf(']'));
        fieldIds = acroFormFields
            .split(' ')
            .filter((element, i) => i % 3 === 0)
            .map((fieldId) => new PDFKitReferenceMock(fieldId));
    }
    const signatureName = 'Signature';

    // Generate signature annotation widget
    const widget = pdf.ref({
        Type: 'Annot',
        Subtype: 'Widget',
        FT: 'Sig',
        Rect: [0, 0, 0, 0],
        V: signature,
        T: new String(signatureName + (fieldIds.length + 1)), // eslint-disable-line no-new-wrappers
        F: 4,
        P: pdf.page.dictionary, // eslint-disable-line no-underscore-dangle
    });
    pdf.page.dictionary.data.Annots = [widget];
    // Include the widget in a page
    let form;

    if (!isAcroFormExists) {
        // Create a form (with the widget) and link in the _root
        form = pdf.ref({
            Type: 'AcroForm',
            SigFlags: 3,
            Fields: [...fieldIds, widget],
        });
    } else {
        // Use existing acroform and extend the fields with newly created widgets
        form = pdf.ref({
            Type: 'AcroForm',
            SigFlags: 3,
            Fields: [...fieldIds, widget],
        }, acroFormId);
    }
    pdf._root.data.AcroForm = form;

    return {
        signature,
        form,
        widget,
    };
    /* eslint-enable no-underscore-dangle,no-param-reassign */
};

export default pdfkitAddPlaceholder;