How to convert .cer file to .pem in the browser

Question

The cer file I'm reading in from a file input, I can then use the fileReader to get the contents. I was expecting the file to have start and end comments but it doesn't.

How can I convert the cer to pem in the browser, without uploading the file to a server?

What I really want is something like

openssl x509 -in cert.cer -out cert.pem

but in the browser

I've also tried a few x509 packages from npm, like...

import * as x509 from '@peculiar/x509';
...
  const uploadCsr = async (fileArr: File[]): Promise<void> => {
    const file = fileArr[0];
    const reader = new FileReader();
    reader.onload = (e: any): void => {
      const cert = new x509.X509Certificate(e.target.result);
      console.log('cert', cert); 
    };
    reader.readAsBinaryString(file);
  };

but the error I get is

TypeError: Unsupported format of 'raw' argument. Must be one of DER, PEM, HEX, Base64, or Base4Url

Answer 1

I had to use the readAsArrayBuffer with @peculiar/x509 and .toString('pem');

import * as x509 from '@peculiar/x509';
...

  const checkCsr = async (fileArr: File[]): Promise<void> => {
    setServerError('');
    const file = fileArr[0];
    const reader = new FileReader();
    reader.onload = async (e: any): Promise<void> => {
      const cert = new x509.X509Certificate(e.target.result);
      const expires = Date.parse(`${cert.notAfter}`) / 1000;
      const now = Math.floor(Date.now() / 1000);
      if (now > expires) setServerError('Certificate has expired');
      const issuerArr = cert.issuer.split('=');
      if (!issuerArr[3].includes('Apple Inc'))
        setServerError('Certificate not issued by Apple Inc');
      const pem = cert.toString('pem');
      if (!serverError) setCertificatePem(pem);
    };
    reader.readAsArrayBuffer(file);
  };