Node.js ssh-rsa public key parser

pubkey.class.js

/*
 * PubKey class
 * Reads, parses and provides a usable node-rsa key object
 * from a ssh-rsa PEM-encoded public key file.
*/
class PubKey {
	constructor(path) {
		const Rsa = require('node-rsa');
		const fs = require('fs');

		let key = Buffer.from(
			fs.readFileSync(path, {encoding: 'utf8'})			// Read .pub key
				.split(' ')[1]									// Trim ssh-rsa prefix and comments
			, 'base64');										// Decode base64 body

		// Deconstruct 'key' buffer step-by-step
		// More info:
		// https://tools.ietf.org/html/rfc4253#section-6.6
		let length = key.readUInt32BE();						// Get 4 bytes length prefix from beginning of buffer...
		const format = key.subarray(4, 4 + length)				// ...and get format identifier...
			.toString('ascii');									// ...which is an ascii-encoded string

		if (format !== 'ssh-rsa') {
			throw new Error('Unsupported key format');			// (throw error if format isn't supported)
		}

		key = key.subarray(4 + length);							// Trim original key buffer for next steps

		length = key.readUInt32BE();							// Get next length prefix
		const e = key.subarray(4, 4 + length);					// Read RSA exponent (e)

		key = key.subarray(4 + length);							// Trim key buffer again

		length = key.readUInt32BE();							// Get final length prefix
		const n = key.subarray(4, 4 + length);					// Read RSA modulus (n)

		key = key.subarray(4 + length);							// Trim key buffer, should now be empty

		if (key.length > 0) {
			throw new Error('Key is too long');					// (throw if there's data left after deconstruction)
		}

		// Construct node-rsa public key object
		key = new Rsa();
		key.importKey({
			e: e.readIntBE(0, e.length),
			n
		}, 'components-public');

		return key;
	}
}

module.exports = PubKey;