Verifying Auth0 JWT throws invalid algorigthm

If you are using only a secret key then using RS256 won't work, as it's based on a private/public key pair. Using only a secret key usually indicates H256. In my answer I assume that what you call MYSECRET is just the content of certificate.pem.

Anyways, I would assume your string has to contain

-----BEGIN RSA PRIVATE KEY-----

and

-----END RSA PRIVATE KEY-----

or PUBLIC instead of PRIVATE.

You can see this in source. The lines mentioned in your error message contains:

if (!~options.algorithms.indexOf(header.alg)) {
  return done(new JsonWebTokenError('invalid algorithm'));
}

and options.algorithms is defined as

if (!options.algorithms) {
  options.algorithms = ~secretOrPublicKey.toString().indexOf('BEGIN CERTIFICATE') ||
                       ~secretOrPublicKey.toString().indexOf('BEGIN PUBLIC KEY') ?
                        [ 'RS256','RS384','RS512','ES256','ES384','ES512' ] :
                       ~secretOrPublicKey.toString().indexOf('BEGIN RSA PUBLIC KEY') ?
                        [ 'RS256','RS384','RS512' ] :
                        [ 'HS256','HS384','HS512' ];

}

If you don't have the RSA things at the start and end it will look for the following algorithms: 'HS256','HS384','HS512'.

I haven't used RS256 with JWT before, but I have used it with ssh, and I know that it's very sensitive to having the header. The string has to be in the exactly correct format.

You need to specify the allowed algorithms as an Array of Strings, instead of an algorithm String.

jwt.verify(token, MYSECRET, { algorithms: ['RS256'] });