indexOf method similar to Strings and Arrays in Javascript to find the index of the ByteRange, the code for that could look something like this: const byteRangeStart = pdf.indexOf("/ByteRange", byteRangePos)
Once the PDF buffer has been passed in as an argument we can use the. Note: The final code for this article can be found here const pdf = fs.readFileSync("./test.pdf") We need it to be a buffer so that the file can be streamed meaning we don’t wait for the whole PDF to be loaded in before we start reading from it. We can use the file system module in Node.js to pass the PDF file into the code as a Buffer. Note: Please keep in mind that there may be multiple ByteRanges in a PDF, this could be because a document has been revised with a new signature so it would make sense to pick the latest ByteRange to have the most up-to-date signature.
The third number tells us where the last byte of the signature is, and the fourth tells us how many bytes are left from the end of the signature to the end of the PDF, so in the image below the fourth value in the ByteRange is 240, meaning if we add the third (960) and fourth values together, we will get the last byte in the pdf which is 1200.Īs you can see in the above image the ByteRange is. The first number which should always be 0 shows the start of the PDF and the second number shows the start of the signature. Extracting the digital signatureĪ digitally signed PDF contains an array called ByteRange which contains 4 numbers. We will discuss later how this can be used to verify the signature but for now, let’s focus on extraction. The digital embedded signature not only contains the public key and other information about the signer but also the certificate which was used to sign the PDF. Luckily programmatically finding a signature in a PDF is very simple. To follow along please make sure you have access to a signed PDF and a Hexadecimal editor, I use the Hex VS Code plugin.
#Adobe pdf signature validation how to#
In this article, we’re going to talk about how to extract a digital signature from a PDF using Javascript.
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