Async method to determine whether or not a PDF has any encryption applied to it.

pdf-encryption-check.ts

export const checkPDFEncryption = (file: Blob): Promise<boolean> => {
  const allTrailerOccurrences = (source: string, search: string) => {
    const trailers = [];

    // TODO: Reverse the search as trailers should appear at the end of the file according to the spec.
    for (let i = 0; i < source.length; ++i) {
      if (source.substring(i, i + search.length) === search) {
        trailers.push(i);
      }
    }
    return trailers;
  };

  return new Promise((resolve, reject) => {
    const fileReader = new FileReader();

    fileReader.onload = (result) => {
      const fileData = result.target?.result;
      let isEncrypted = false;

      if (fileData instanceof ArrayBuffer) {
        let fileAsString = '';

        // First we need to convert the entire PDF file from ArrayBuffer to human-readable text.
        if ('TextDecoder' in window) {
          // Decode as UTF-8
          const dataView = new DataView(fileData);
          const decoder = new TextDecoder('utf-8');
          fileAsString = decoder.decode(dataView);
        } else {
          // Fallback method
          const dataArray = new Uint8Array(fileData);
          for (const byte of dataArray) {
            // Convert decimal to UTF-8 character (e.g. 87 -> W)
            fileAsString += String.fromCharCode(byte);
          }
        }

        //  # According to the official PDF spec (Page 55):
        //
        //  Encryption-related information is stored in a document's _encryption dictionary_, which
        //  is the value of the *Encrypt* entry in the document's *trailer* dictionary.
        //  The absence of this entry from the trailer dictionary means that a conforming reader
        //  should consider the given document to be not encrypted.
        //  See page 55: https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf
        //
        //  Example of a trailer dictionary:
        //    trailer
        //        << /Size 8
        //        << /Root 1 0 R
        //        << /Encrypt 8 0 R

        // This will find all trailers in a given document (in case there are multiple).
        // Reference: https://resources.infosecinstitute.com/topic/pdf-file-format-basic-structure/
        const allTrailers = allTrailerOccurrences(fileAsString, 'trailer');

        // Then we need to go through each trailer and look for the "Encrypt" keyword
        allTrailers.forEach((trailer) => {
          const encryptKey = fileAsString.indexOf('Encrypt', trailer);
          // By looking for indexes higher than 0, we only find values like '/Encrypt' (after the `/` prefix).
          if (encryptKey > 0) {
            isEncrypted = true;
          }
        });

        resolve(isEncrypted);
      } else {
        reject(`FileReader result was not an ArrayBuffer: ${fileReader.result}`);
      }
    };

    fileReader.onerror = () => reject;
    fileReader.readAsArrayBuffer(file);
  });
};