CSR submitted to IIS CA fails due to ASN1 value

I have generated a private key / CSR from pyOpenSSL - code snippet below:

Key:

key = crypto.PKey()
key.generate_key(type, bits)
if os.path.exists(_keyfile):
    print "Certificate file exists, aborting."
    print " ", _keyfile
    sys.exit(1)
else:
    f = open(_keyfile, "w")
    f.write(crypto.dump_privatekey(crypto.FILETYPE_PEM, key))
    f.close()
return key

CSR:

req = crypto.X509Req()
# Return an X509Name object representing the subject of the certificate.
req.get_subject().countryName = country
req.get_subject().stateOrProvinceName = state
req.get_subject().localityName = location
req.get_subject().organizationName = organisation
req.get_subject().organizationalUnitName = organisational_unit
req.get_subject().CN = nodename
# Add in extensions
#base_constraints = ([
#    crypto.X509Extension("keyUsage", False, "Digital Signature, Non Repudiation, Key Encipherment"),
#    crypto.X509Extension("basicConstraints", False, "CA:FALSE"),
#])
#x509_extensions = ([])
x509_extensions = []
# If there are SAN entries, append the base_constraints to include them.
if ss:
    san_constraint = crypto.X509Extension("subjectAltName", False, ss)
    x509_extensions.append(san_constraint)
req.add_extensions(x509_extensions)
# Set the public key of the certificate to pkey.
req.set_pubkey(key)
# Sign the certificate, using the key pkey and the message digest algorithm identified by the string digest.
req.sign(key, "sha1")
# Dump the certificate request req into a buffer string encoded with the type type.
if os.path.exists(_csrfile):
    print "Certificate file exists, aborting."
    print " ", _csrfile
    sys.exit(1)
else:
    f = open(_csrfile, "w")
    f.write(crypto.dump_certificate_request(crypto.FILETYPE_PEM, req))
    f.close()

The error that I get back from the IIS CA is:

ASN1 bad tag value met. 0x8009310b (ASN: 267)

According to Microsoft this is caused by:

This behavior occurs when certificate request is stored in a file in Unicode encoding. Microsoft Certificate Services do not support Unicode-encoded files request files. Only ANSI encoding is supported.

I know that if I generate a CSR from openssl on the command line it is accepted and issued by the IIS CA RESTful webservice without error.

I want to know if there is some way I can generate 'ANSI' encoded files from pyOpenSSL - I am not sure if it is the keyfile or the CSR that is signed with the keyfile that is causing the issues.

I have solved it with the help of this stackoverflow question thanks to @yodatg.

The problem occurs due to a bug in pyOpenSSL that has been fixed.

By issuing:

openssl asn1parse -in certificates/cert.csr

I could see the ASN1 value:

8:d=2  hl=2 l=   1 prim: INTEGER           :01

In a working CSR it looks like this:

8:d=2  hl=2 l=   1 prim: INTEGER           :00

I then changed my code to include a set_version call on the req object prior to signing:

#set version - IIS CA required this
req.set_version(0)

# Set the public key of the certificate to pkey.
req.set_pubkey(priv_key)

This is now resolved.