nyks/crypto/openssh2pem.js

"use strict";

const ASN_LONG_LEN = 0x80;
const SSH_RSA      = 'ssh-rsa';

function ASN_len(s) {
  var len = s.length;
  if(len < ASN_LONG_LEN)
    return new Buffer([len]);

  var data = len.toString(16);
  /* istanbul ignore else */
  if(data.length & 1)
    data = "0" + data;
  data = new Buffer(data, 'hex');
  return Buffer.concat([new Buffer([data.length | ASN_LONG_LEN]), data]);
}

var unpack = function(data, start) {
  var slice = data.slice(start, 4);
  var out = (slice[0] << 24) + (slice[1] << 16) + (slice[2] << 8) + slice[3];
  return [out];
};

var asnf = function(type, body) {
  return Buffer.concat([new Buffer([type]), ASN_len(body), body]);
};

module.exports = function(openssh_data) {
  if(openssh_data.substr(0, SSH_RSA.length) == SSH_RSA)
    openssh_data = openssh_data.substr(SSH_RSA.length);

  var data    = new Buffer(openssh_data, 'base64');
  var alg_len = unpack(data, 0)[0];
  var i       = 4;
  var alg     = data.slice(i, i += alg_len).toString('ascii');

  if(alg !== SSH_RSA)
    throw "Not rsa";

  var e_len = unpack(data.slice(i, i += 4))[0];
  var e     = data.slice(i, i += e_len);
  var n_len = unpack(data.slice(i, i += 4))[0];
  var n     = data.slice(i, i += n_len);
  var algid = new Buffer('06092a864886f70d0101010500', 'hex');

  algid = asnf(0x30, algid); // wrap it into sequence
  data  = asnf(0x02, n);     // numbers

  data = Buffer.concat([data, asnf(0x02, e)]);
  data = asnf(0x30, data);    // wrap it into sequence
  data = Buffer.concat([new Buffer([0]), data]);  // don't know why, but needed
  data = asnf(0x03, data);                  // wrap it into bitstring
  data = Buffer.concat([algid, data]);      // prepend algid
  data = asnf(0x30, data);                  // wrap it into sequence

  return data.toString('base64');
};