Why signing data with RSA private key raise RSA key format is not supported in python?

Question

I have a method as below that sign provided data:

def sign_data(self, private_key_loc, data):
        """
        param: private_key_loc Path to your private key
        param: package Data to be signed
        return: base64 encoded signature
        """
        key = open(private_key_loc, "r").read()
        print(key)
        rsakey = RSA.importKey(key) # This raise an error!!!!
        print(rsakey)
        signer = PKCS1_v1_5.new(rsakey)
        digest = SHA256.new()
        # It's being assumed the data is base64 encoded, so it's decoded before updating the digest
        digest.update(data)
        sign = signer.sign(digest)
        return b64encode(sign)

The format of the private key is as below:

<RSAKeyValue><Modulus>nEWZmLeK0zgEWysFFcpcT6lnYpzYcXFf+r43wD73ko+CjR5EmwDsmlKqKS5y3rpIHQrj+xE+yiNGlcPFNwFl3cCcHPkeI0hqaPPYxE2XeP6Wa0keegny2AWWK2Cuv61YHVz4XgxTPBX7B/19ClN4wzI5CoNc9jkx0PIXEo5iWLk=</Modulus><Exponent>AQAB</Exponent><P>0b5D6oI3mOKKIHgfCrl03vfUTo91UeJyvpZ1l6F6NzUzPDhCG3Sin5dXtxEFejPayLsgLOIMMuqobOzI9wJpBQ==</P><Q>vrxvorDFE3g1cj2kWWWZc2BS7UMf+xxnPhtBR3x3T/DyTEnkEXvGLdRBuUGWKhxUpRcBkcBfAadJjhYUqYBvJQ==</Q><DP>SKYKiDPKZh4xkcWJmwFZxdE5rGxsSoyRCgq7eGXqGy1GLdmerDveCEE3lKVErGtBoL2QC3vQleJJrFDn2wbusQ==</DP><DQ>sIoTPd8lmd6ygVnCq6fZPywRtV9i03a3rIlng3YVrN1UNv5RZUlN5g9HAeRNzA5K3j8MCc6nYQ1ojGWtnDGvLQ==</DQ><InverseQ>rQzqbkXZ9Tmno3ElcliX57Xv4AXyF6yFO1kH4LHwkc8jN5dB9XYEdBIgm43yHFIznvOdc2L9Nkl2nHbFCeAKmg==</InverseQ><D>XA7IN+XP2zPBpS9HlJmcHbWO2NHK07FVODH8R70QPP9bieRNx/4YY6TU5uOc+cZFGw7CVLHJCm0TdMBaGlgxeC/GTwMrCJigxUgaxbvDmxuSpqYqkjoZhgoZAuUct8RaZUN8zNdEsXCqRhbU61U2Ey587RyPQ7varg9hkPicVHE=</D></RSAKeyValue>

NOTE: PRIVATE KEY IS FOR TEST NOTHING VALUEABLE

It seems that it is created in XML format.

In the line RSA.importKey(key) the error is raised as below:

ValueError: RSA key format is not supported

How should I feed that private key to sign data?

I even tried to form RSA key by parsing the XML and do something as the following:

rsakey = Crypto.PublicKey.RSA.construct((root.find('Modulus').text, root.find('Exponent').text,root.find('D').text))

It gives an error like:

assert isinstance(n, long)
AssertionError

Answer 1

RSA needs PEM file format in order to sign data. As my private key was not important! I used an online site to convert XML to PEM ( DON'T DO THIS ON REAL PRIVATE KEYS :| ). It now works.

My PEM file is now as below:

-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----