Reading public and private key from stored files in node js

Question

I wanted to encrypt and decrypt a message in node using public and private keys stored in my system. I was using the following java code to read the file and use the keys.

Java Code:

byte[]  keyBytes = Files.readAllBytes(new File(publicKeyFileName).toPath());
X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
KeyFactory kf = KeyFactory.getInstance("RSA");
publicKey=kf.generatePublic(spec);

I am able to use the above java method without any issues to read the public key from file. However, I want to achieve similar functionality in node. I have tried using crypto for achieving the same but it gives me error while passing the key to publicEncrypt method.

Node:

var encryptStringWithRsaPublicKey = function(toEncrypt, relativeOrAbsolutePathToPublicKey) {
    var absolutePath = path.resolve(relativeOrAbsolutePathToPublicKey);
    var publicKey = fs.read(absolutepath, "utf-8");
    console.log(publicKey);
    var buffer = Buffer.from(toEncrypt);
    var encrypted = crypto.publicEncrypt(publicKey, buffer);
    return encrypted.toString("base64");
};

Error

internal/crypto/cipher.js:43
    return method(toBuf(key), buffer, padding, passphrase);
           ^

Error: error:0906D06C:PEM routines:PEM_read_bio:no start line

Please help. Thanks

user_8166854 · Answer

Your problem is located in the file format you are actually using with Java. You probably save the private and the public in encoded ("byte array") to a file and rebuild the keys e.g. with X509EncodedKeySpec.

This format is not compatible to Node.JS and you have 3 ways to solve it:

a) you write the keys in Java with re neccessary format for usage in Node.JS

b) you write a converter in Node.JS to get the correct format

c) you convert the files with a tool like OPENSSL.

Here I show you the "c-way" as you are handling just one keypair and probably don't need a programatically solution.

Let's say you have two files with the private key ("rsa_privatekey_2048.der") and the public key ("rsa_publickey_2048.der").

In OPENSSL you are using the command line with

openssl rsa -inform der -in rsa_privatekey_2048.der -outform pem -out rsa_privatekey_2048.pem
openssl rsa -inform der -pubin -in rsa_publickey_2048.der -outform pem -RSAPublicKey_out -out rsa_publickey_2048.pem

to convert the files to their PEM-encoded formats.

Below you can find the two sample files I created.

rsa_privatekey_2048.pem:

rsa_publickey_2048.pem:

user_596325 · Answer

There's potentially a few issues with your code or the encryption key you're using:

You're using fs.read incorrectly as Node is asynchronous and it needs a callback function to properly read the file.
The encryption key you're using is formatted incorrectly for crypto.publicEncrypt. You must have the proper RSA headers.

I modified your code to use fs.readFile properly instead in the standard Node callback form, and here's an example encryption key in the correct RSA format to use:

var path = require('path');
var crypto = require('crypto');
var fs = require('fs');

var encryptStringWithRsaPublicKey = function(toEncrypt, relativeOrAbsolutePathToPublicKey, callback) {
    var absolutePath = path.resolve(relativeOrAbsolutePathToPublicKey);
    fs.readFile(absolutePath, 'utf-8', (err, publicKey) => {
        // The value of `publicKey` is in the callback, not the return value
        console.log(publicKey);
        var buffer = Buffer.from(toEncrypt);
        var encrypted = crypto.publicEncrypt(publicKey, buffer);
        if (err) {
            callback(err);
        } else {
            callback(null, encrypted.toString("base64"));
        }
    });
};

encryptStringWithRsaPublicKey('hello world', 'test.pub', (err, encrypted) => {
    // If you're using a callback in a function,
    // the original function must have a callback as well
    console.log(encrypted);
});

Example encryption key at test.pub (must have the RSA headers as shown below):

As of 2020, there are also other ways of making the code cleaner, such as with using the Promises version of the fs module and async / await, though I wanted to keep this answer as simple as possible for now.

Reading public and private key from stored files in node js

2 Answers