Generate JWT token signed with RSA key in python

Question

I am trying to convert this java code for generating JWT token in python.

String privateKeyContent = privateKey
    .replaceAll(Definitions.ApiGeneral.LINE_BREAKER, "")
    .replace(Definitions.AuthProperty.PRIVATE_KEY_START, "")
    .replace(Definitions.AuthProperty.PRIVATE_KEY_END, "");

PKCS8EncodedKeySpec keySpecPKCS8 = new PKCS8EncodedKeySpec(Base64.getDecoder().decode(privateKeyContent));

KeyFactory kf = KeyFactory.getInstance(Definitions.AuthProperty.RSA_KEY_FACTORY);
PrivateKey privKey = kf.generatePrivate(keySpecPKCS8);

String jwtAudUrl = System.getenv(Definitions.IamProperty.IAM_URL_KEY) + System.getenv(Definitions.IamProperty.JWT_AUD_URI_KEY);
String jwtToken = Jwts.builder()
    .setAudience(jwtAudUrl)
    .setSubject(serviceId)
    .setIssuer(serviceId)
    .setExpiration(new Date(new Date().getTime() + TimeUnit.MINUTES.toMillis(Definitions.AuthProperty.JWT_TOKEN_EXPIRY_IN_MINUTES)))
    .signWith(privKey)
    .compact();

Python:

import jwt

serviceID = "abc"
secret = '-----BEGIN RSA PRIVATE KEY-----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-----END RSA PRIVATE KEY-----'

due_date = datetime.now() + timedelta(minutes=10)
header = {"alg": "RS256"}
expiry = int(due_date.timestamp())
payload = {"iss": serviceID, "sub": serviceID, "exp": expiry, "aud": iam_url + "/oauth2/access_token"}

priv_rsakey = serialization.load_pem_private_key(secret.encode('utf8'), password=None, backend=default_backend())
token=jwt.encode(payload, priv_rsakey, algorithm='RS256')

However, I keep on getting this error :

ValueError: ('Could not deserialize key data. The data may be in an incorrect format, it may be encrypted with an unsupported algorithm, or it may be an unsupported key type (e.g. EC curves with explicit parameters).', [_OpenSSLErrorWithText(code=503841036, lib=60, reason=524556, reason_text=b'error:1E08010C:DECODER routines::unsupported')])

Can someone please help me with this ?

Answer 1

The issue is precisely identified in the error message: Your private key is incorrectly formatted. A PEM encoded key consists of the Base64 encoded body, which contains a line break after every 64 characters, and a header and footer on separate lines. Your key is missing the line breaks.

load_pem_private_key() expects at least header and footer on separate lines, but is tolerant about line breaks in the body, i.e. they are optional. So you have to pass your key e.g. like this:

secret = '-----BEGIN RSA PRIVATE KEY-----\nMIIEogIBAAKCAQEAhstYtRbkgQkFwlVr8QjSCQqqRTDMKWHdIGRYBpXcQmvKfagId9nBA2Ygh7cOrT9g8MhxYo8U1jYmPQpv6gf3LgO/J0qspLdaAhZP6LusA/HHJBR7kjTXBsLcsEDyd8S0UioBYP3DLvtWhGIR2f4o7SH1TlE96tldV6FZKGO2NHsJrJwTd+ym0AeZe0b7QZLe43LBCTLdqk05U34jrknJliSAEbGqYg4h6nrJsKBC/0pmiQ9ptD1N/Kl4bqffMWIbZq2bPP6jFrmBLe+7yTeVMKltVbJZys4nHhyYngBtbAxynXeB2tpE8If7cK75fj42MlFgquEiEZZVSzNNmrmPOwIDAQABAoH/B18Xes/Fr0jPB9GkFYpl8hijNyV0BM9VSHA0YCfR49ABQt3tmKBP7d+n58QbCV5t7r0Hdlxcx1ouvSfU9vd4jQunaH6s8lUUlwihVhjtT0npmg+EsnoxSC1f5EOo/uPC+LtTV/qIsgkMsjCqyUEc+9rfj2jh+fXpJOGt/od1b2k2xs84MsXmSF/As7GYRdw+FLbkN64R/SGmv3NLtQdg5uvBKLuKvtQWIJBuPqgKOsJWaCVO0XaoUDQeav/nTfP0ntmF0QH9JtXYzBldhGq2FPVQRUaCuJ4YPEpXlD3FptQBlX/Wu7wXbdwDz3qyXbGqSkiaR3gN+QR+IgjG3oQxAoGBAML5M4Dg0S/yobHuCRcvpngraXGBWnCTaD0mMh/cosV0HBNdZbglwVpG8Agz9BEYIrDM777HFuB+lWUvGNMR45rObPcDqn7Qj8Uwnj775Bg0Sx09MCBds6IGce/92uPUsHPx4pj8dzCj1s3cfUKO+EaUE33bnFXcMCGh/0x0sIVXAoGBALD8H4Aph2OPQK9OOHb3ULDvrmMPrpe3xv5iSzfLt7xBIP2G0Wl+q0hzEtdEOUZGQemOtyuV5t9Jwwfh9uH9jAhk9OEvoNg3F3mQl6hjmHPd1xUNvRTqwnV+VvQnw7Aeq2TZMcAKwbXf3+p430wMsxR6m0Y7rGi0FIxWbILp0RK9AoGBAIfiPBXnGYOsOysRtb42FHQN9WgI+eoZof10IFz6XWr12Bda8Wicz5vGcsWUx9YeFxdXTQOOJ5CASEiDwW5hOlqK4YBqSqolWv3YO4Gz9i00TOFs4py8EVSr3z6ekq5UbkHwY7exxLPei/dfYuE/WSN/UfJWWyev1M+r4oz7iobzAoGAKe8S56LvWT+P6/l0l3txuvqPLxmAHKKGm69ecxHprskfr/JJm91PaBMb27VmfKgY5eXSsJkL4svvUebQQCt7CmIhQ1mtmo0zGrKPvG4cqRde5rYintogyQXuRFtHmmsp4PM1PnNOAnHQ9BU/kx1PMQL712A8MXK5i6bOfxY3W2ECgYEAp9cw3NeMSy/WcXyZoyNFUdEQiHTJGPBtELjHWRnjMU1454EkWYYPiYXUnElfxP6InxHgMNbGVsd1BUEhXOdSUS5bO/WBOOPqSh6MIGfKFuMN/9SI/Y/UZKltCD1CboTvDfmkD+opkLM6YZpW9CRT7Szn7ivdFr5KqGQZUZOwn3k=\n-----END RSA PRIVATE KEY-----'

or

secret = '''-----BEGIN RSA PRIVATE KEY-----
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
-----END RSA PRIVATE KEY-----'''

With this change the code works (after adding the missing iam_url and the missing import statements).

---

Note that PKCS8EncodedKeySpec in the Java code expects a DER encoded private key in PKCS#8 format, while in the Python code a PEM encoded private key in PKCS#1 format is applied.

A DER encoded key results from a PEM encoded key by removing header, footer and all line breaks, and Base64 decoding the rest. The Cryptography library supports the import of a DER encoded private key with load_der_private_key():

import base64
secret = base64.b64decode('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') 
priv_rsakey = serialization.load_der_private_key(secret, password=None, backend=default_backend())

load_pem_private_key() and load_der_private_key() support both PKCS#8 and PKCS#1 format.