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Solving buffer.readUIntBE is not a function reading a JWKS key with NodeRSA

Solving buffer.readUIntBE is not a function reading a JWKS key with NodeRSA

kleeut profile image Klee Thomas Originally published at blog.kleeut.com ・2 min read

Recently while trying to read a public key in from a JWKS endpoint I was stumped by an error when importing the public key using NodeRSA. My code looked like this:

  return rsa.importKey(
    {
      n: Buffer.from(jwksKey.n),
      e: jwksKey.e,
    },
    "components-public"
  );

The error was:

TypeError: buffer.readUIntBE is not a function
    at Object.module.exports.get32IntFromBuffer (C:\dev\jwttest\node_modules\node-rsa\src\utils.js:43:27)
    at RSAKey.module.exports.Key.RSAKey.setPublic (C:\dev\jwttest\node_modules\node-rsa\src\libs\rsa.js:172:48)
    at Object.publicImport (C:\dev\jwttest\node_modules\node-rsa\src\formats\components.js:44:17)
    at Object.detectAndImport (C:\dev\jwttest\node_modules\node-rsa\src\formats\formats.js:65:48)
    at NodeRSA.module.exports.NodeRSA.importKey (C:\dev\jwttest\node_modules\node-rsa\src\NodeRSA.js:183:22)
    at C:\dev\jwttest\dist\jwks.js:53:20
    at Generator.next (<anonymous>)
    at fulfilled (C:\dev\jwttest\dist\jwks.js:5:58)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)

I couldn't see any issue with the way I was setting up the buffer. I was able to log the contents out. Everything looked correct. When I searched for buffer.readUIntBE is not a function I found nothing useful. It left my scratching my head and saying "What the damn hell!?".

What the damn hell

Eventually I worked it out. The issue wasn't with th buffer I had so much as the one I didn't. I had been let down by own poor use of the Typescript type system. I had assumed that since the code was compiling that I was passing correct values and skipping reading the documentation. The mistake I had made was that the JWKS spec sets up the e (Exponent) parameter as a Base64urlUInt-encoded string value and the new NodeRSA().importKey({n,e}, "public-components") api takes a numberor a Buffer as the n parameter.

In this case the error I was saying that buffer.readUIntBE is not a function where buffer is a string not a Buffer

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