How do I convert an XML RSA key to a PEM file?

For those who want the resulting PEM to be readable by BouncyCastle:

1. use XMLSec2PEM tool to get a pem file
2. convert pem to pkcs8 and back (!)

The final solution I am happy with:

1. java XMLSec2PEM my.xml > my.pem
2. edit my.pem manually a bit
3. org.bouncycastle.openssl.PEMReader.readObject() returns null :-(
4. openssl pkcs8 -topk8 -inform pem -in my.pem -outform pem -nocrypt -out my.pkcs8
5. openssl pkcs8 -inform pem -nocrypt -in my.pkcs8 -out my.pkcs8.pem
6. now my.pkcs8.pem is readable with the PEMReader

my solution in python works like this:

1. extract modulus and exponent from xml

   xml = etree.fromstring(key_bin)
   modulus = xml.find('Modulus').text
   exponent = xml.find('Exponent').text
   

2. decode them in base64 and iterate the result to save it as a character string of length 2:

   mod_b64 = b64decode(modulus.encode())
   exp_b64 = b64decode(exponent.encode())
   exp = ''.join(['{:02x}'.format(x) for x in exp_b64])
   mod = ''.join(['{:02x}'.format(x) for x in mod_b64]) 
   

3. Convert the hexadecimal string to integer and generate the rsa public key with the rsa library:

   exp_num = int(exp, 16)
   mod_num = int(mod, 16)
   rsa_key = rsa.PublicKey(mod_num, exp_num)
   

4. Finally any text can be encrypted:

   msg_cryp = rsa.encrypt(msg.encode('ascii'), rsa_key)
   msg_cryp_str = b64encode(msg_cryp).decode('ascii') 
   

I did this development to consume a web service which requires that an encrypted password be sent from a public key in XML format. In this way I managed to encrypt the password and consume the web service without problems.

I have found a Java utility that can do it.