Quick script for converting PEM-format RSA keys to a format usable by .NET's RSA.FromXmlString

csp2pem.rb

#!/usr/bin/ruby
require 'openssl'

TYPES = {
  6 => {magic: 'RSA1', private_key: false},
  7 => {magic: 'RSA2', private_key: true}
}

data = ARGV[0] ? File.binread(ARGV[0]) : $stdin.binmode.read
type, version, algo, magic, bits, rest = data.unpack('ccx2l<a4l<a*')

if match = TYPES[type]
  raise 'Wrong magic' if magic != match[:magic]
  private_key = match[:private_key]
else
  raise 'Unsupported type'
end
raise 'Unsupported version' if version != 2
raise 'Unsupported algorithm' if ((algo >> 9) & 15) != 2

fields = [32, bits]
fields.push(*[bits / 2] * 5, bits) if private_key

key = OpenSSL::PKey::RSA.new
key.e, key.n, key.p, key.q, key.dmp1, key.dmq1, key.iqmp, key.d = fields.map do |bits|
  size = (bits + 7) / 8
  raise 'Unexpected end of data reached' if rest.size < size
  OpenSSL::BN.new(rest.slice!(0, size).reverse!, 2)
end
raise 'Unexpected leftover data' unless rest.empty?

puts key.to_pem

pem2xml.rb

#!/usr/bin/ruby
require 'nokogiri'
require 'openssl'
require 'byebug'

# https://ruby-doc.org/stdlib-3.1.0/libdoc/openssl/rdoc/OpenSSL/PKey/RSA.html
# https://ruby-doc.org/stdlib-3.1.0/libdoc/openssl/rdoc/OpenSSL/PKey.html
# Go the other way: https://groups.google.com/g/mailing.openssl.users/c/8hAgyv4SiFY

def base64(bn)
  [bn.to_s(2)].pack('m0')
end

# key = OpenSSL::PKey::RSA.new(File.read($stdin))
# key = OpenSSL::PKey::RSA.new(File.read('../../.ssh/cqm.pem'))
key = OpenSSL::PKey::RSA.new(2048)

byebug

key = key.public_key

key = OpenSSL::PKey::RSA.new($stdin)
builder = Nokogiri::XML::Builder.new do |xml|
  xml.RSAKeyValue do
    xml.Modulus base64(key.n)
    xml.Exponent base64(key.e)
    if key.private?
      xml.D base64(key.d)
      xml.P base64(key.p)
      xml.Q base64(key.q)
      xml.DP base64(key.dmp1)
      xml.DQ base64(key.dmq1)
      xml.InverseQ base64(key.iqmp)
    end
  end
end
puts builder.to_xml