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hash, hmac, RSA, and X.509 with an mbed TLS back end

Project description

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python-mbedtls is a free cryptographic library for Python that uses mbed TLS for back end.

mbed TLS (formerly known as PolarSSL) makes it trivially easy for developers to include cryptographic and SSL/TLS capabilities in their (embedded) products, facilitating this functionality with a minimal coding footprint.

python-mbedtls API follows the recommendations from PEP 452: API for Cryptographic Hash Functions v2.0 and PEP 272 API for Block Encryption Algorithms v1.0 and can therefore be used as a drop-in replacements to PyCrypto or Python’s hashlib and hmac

License

python-mbedtls is licensed under the MIT License (see LICENSE.txt). This enables the use of python-mbedtls in both open source and closed source projects. The MIT License is compatible with both GPL and Apache 2.0 license under which mbed TLS is distributed.

Installation

The bindings are tested with Python 2.7, 3.4, 3.5, and 3.6.

mbedtls is available on Debian. Install with:

# apt-get install libmbedtls-dev
# apt-get install libpython-dev   # for Python 2, or
# apt-get install libpython3-dev  # for Python 3

and pyton-mbedtls:

$ python -m pip install python-mbedtls

Message digest with mbedtls.hash

The mbedtls.hash module provides MD5, SHA-1, SHA-2, and RIPEMD-160 secure hashes and message digests. The API follows the recommendations from PEP 452 so that it can be used as a drop-in replacement to e.g. hashlib or PyCrypto.

Here are the examples from hashlib ported to python-mbedtls:

>>> from mbedtls import hash as hashlib
>>> m = hashlib.md5()
>>> m.update(b"Nobody inspects")
>>> m.update(b" the spammish repetition")
>>> m.digest()
b'\xbbd\x9c\x83\xdd\x1e\xa5\xc9\xd9\xde\xc9\xa1\x8d\xf0\xff\xe9'
>>> m.digest_size
16
>>> m.block_size
64

More condensed:

>>> hashlib.sha224(b"Nobody inspects the spammish repetition").hexdigest()
'a4337bc45a8fc544c03f52dc550cd6e1e87021bc896588bd79e901e2'

Using new():

>>> h = hashlib.new('ripemd160')
>>> h.update(b"Nobody inspects the spammish repetition")
>>> h.hexdigest()
'cc4a5ce1b3df48aec5d22d1f16b894a0b894eccc'

HMAC algorithm with mbedtls.hmac

The mbedtls.hmac module computes HMAC. The API follows the recommendations from PEP 452 as well.

Example:

>>> from mbedtls import hmac
>>> m = hmac.new(b"This is my secret key", digestmod="md5")
>>> m.update(b"Nobody inspects")
>>> m.update(b" the spammish repetition")
>>> m.digest()
b'\x9d-/rj\\\x98\x80\xb1rG\x87\x0f\xe9\xe4\xeb'

Warning:

The message is cleared after calculation of the digest. Only call mbedtls.hmac.Hmac.digest() or mbedtls.hmac.Hmac.hexdigest() once per message.

Symmetric cipher with mbedtls.cipher

The mbedtls.cipher module provides symmetric encryption. The API follows the recommendations from PEP 272 so that it can be used as a drop-in replacement to e.g. PyCrypto.

mbedtls provides the following algorithms:

  • Aes encryption/decryption (128, 192, and 256 bits) in ECB, CBC, CFB128, CTR, GCM, or CCM mode;
  • Arc4 encryption/decryption;
  • Blowfish encryption/decryption in ECB, CBC, CFB64, or CTR mode;
  • Camellia encryption/decryption (128, 192, and 256 bits) in ECB, CBC, CFB128, CTR, GCM, or CCM mode;
  • DES encryption/decryption in ECB, or CBC mode;
Notes:
  • Tagging and padding are not wrapped.
  • The counter in CTR mode cannot be explicitly provided.

Example:

>>> from mbedtls import cipher
>>> c = cipher.AES.new(b"My 16-bytes key.", cipher.MODE_CBC, b"CBC needs an IV.")
>>> enc = c.encrypt(b"This is a super-secret message!")
>>> enc
b'*`k6\x98\x97=[\xdf\x7f\x88\x96\xf5\t\x19J7\x93\xb5\xe0~\t\x9e\x968m\xcd\x9c3\x04o\xe6'
>>> c.decrypt(enc)
b'This is a super-secret message!'

RSA public key with mbedtls.pk

The mbedtls.pk module provides the RSA cryptosystem. This includes:

  • Public-private key generation and key import/export in PEM and DER formats;
  • Asymmetric encryption and decryption;
  • Message signature and verification.

Key generation, the default size is 2048 bits:

>>> from mbedtls import pk
>>> rsa = pk.RSA()
>>> prv = rsa.generate()
>>> rsa.key_size
256

Message encryption and decryption:

>>> enc = rsa.encrypt(b"secret message")
>>> rsa.decrypt(enc)
b'secret message'

Message signature and verification:

>>> sig = rsa.sign(b"Please sign here.")
>>> rsa.verify(b"Please sign here.", sig)
True
>>> rsa.verify(b"Sorry, wrong message.", sig)
False
>>> pub = rsa.export_public_key(format="DER")
>>> other = pk.RSA.from_buffer(pub)
>>> other.verify(b"Please sign here.", sig)
True

Static and ephemeral Elliptic curve Diffie-Hellman

The mbedtls.pk module provides the ECC cryptosystem. This includes:

  • Public-private key generation and key import/export in the PEM and DER formats;
  • Asymmetric encrypt and decryption;
  • Message signature and verification;
  • Ephemeral ECDH key exchange.

get_supported_curves() returns the list of supported curves.

The API of the ECC class is the same as the API of the RSA class but ciphering (encrypt() and decrypt() is not supported by MBED TLS).

Message signature and verification—elliptic curve digital signature algorithm (ECDSA):

>>> from mbedtls import pk
>>> ecdsa = pk.ECC()
>>> prv = ecdsa.generate()
>>> sig = ecdsa.sign(b"Please sign here.")
>>> ecdsa.verify(b"Please sign here.", sig)
True
>>> ecdsa.verify(b"Sorry, wrong message.", sig)
False
>>> pub = ecdsa.export_public_key(format="DER")
>>> other = pk.ECC.from_buffer(pub)
>>> other.verify(b"Please sign here.", sig)
True

The classes ECDHServer and ECDHClient may be used for ephemeral ECDH. The key exchange is as follows:

>>> srv = pk.ECDHServer()
>>> cli = pk.ECDHClient()

The server generates the ServerKeyExchange encrypted payload and passes it to the client:

>>> ske = srv.generate()
>>> cli.import_SKE(ske)

then the client generates the ClientKeyExchange encrypted payload and passes it back to the server:

>>> cke = cli.generate()
>>> srv.import_CKE(cke)

Now, client and server may generate their shared secret:

>>> secret = srv.generate_secret()
>>> cli.generate_secret() == secret
True
>>> srv.shared_secret == cli.shared_secret
True

Diffie-Hellman-Merkle key exchange

The classes DHServer and DHClient may be used for DH Key exchange. The classes have the same API as ECDHServer and ECDHClient, respectively.

The key exchange is as follow:

>>> from mbedtls import pk
>>> srv = pk.DHServer(23, 5)
>>> cli = pk.DHClient(23, 5)

The values 23 and 5 are the prime modulus (P) and the generator (G).

The server generates the ServerKeyExchange payload:

>>> ske = srv.generate()
>>> cli.import_SKE(ske)

The payload ends with G^X mod P where X is the secret value of the server.

>>> cke = cli.generate()
>>> srv.import_CKE(cke)

cke is G^Y mod P (with Y the secret value from the client) returned as its representation in bytes so that it can be readily transported over the network.

As in ECDH, client and server may now generate their shared secret:

>>> secret = srv.generate_secret()
>>> cli.generate_secret() == secret
True
>>> srv.shared_secret == cli.shared_secret
True

X.509 Certificate writing and parsing with mbedtls.x509

The x509 module can be used to parse X.509 certificates or create and verify a certificate chain.

Here, the trusted root is a self-signed CA certificate ca0_crt signed by ca0_key:

>>> import datetime as dt
>>>
>>> from mbedtls import hash as hashlib
>>> from mbedtls import pk
>>> from mbedtls import x509
>>>
>>> now = dt.datetime.utcnow()
>>> ca0_key = pk.RSA()
>>> _ = ca0_key.generate()
>>> ca0_csr = x509.CSR.new(ca0_key, "CN=Trusted CA", hashlib.sha256())
>>> ca0_crt = x509.CRT.selfsign(
...     ca0_csr, ca0_key,
...     not_before=now, not_after=now + dt.timedelta(days=90),
...     serial_number=0x123456,
...     basic_constraints=x509.BasicConstraints(True, 1))
...

An intermediate then issues a Certificate Singing Request (CSR) that the root CA signs:

>>> ca1_key = pk.ECC()
>>> _ = ca1_key.generate()
>>> ca1_csr = x509.CSR.new(ca1_key, "CN=Intermediate CA", hashlib.sha256())
>>>
>>> ca1_crt = ca0_crt.sign(
...     ca1_csr, ca0_key, now, now + dt.timedelta(days=90), 0x123456,
...     basic_constraints=x509.BasicConstraints(ca=True, max_path_length=3))
...

And finally, the intermediate CA signs a certificate for the End Entity on the basis of a new CSR:

>>> ee0_key = pk.ECC()
>>> _ = ee0_key.generate()
>>> ee0_csr = x509.CSR.new(ee0_key, "CN=End Entity", hashlib.sha256())
>>>
>>> ee0_crt = ca1_crt.sign(
...     ee0_csr, ca1_key, now, now + dt.timedelta(days=90), 0x987654)
...

The emitting certificate can be used to verify the next certificate in the chain:

>>> ca1_crt.verify(ee0_crt)
True
>>> ca0_crt.verify(ca1_crt)
True

Note, however, that this verification is only one step in a private key infrastructure and does not take CRLs, path length, etc. into account.

TLS client and server

The mbedtls.tls module provides TLS clients and servers. The API follows the recommendations of PEP 543. Note, however, that the Python standard SSL library does not follow the PEP so that this library may not be a drop-in replacement. Also, SSL 3 is not yet supported.

Here are some simple HTTP messages to pass from the client to the server and back.

>>> get_request = "\r\n".join((
...     "GET / HTTP/1.0",
...     "",
...     "")).encode("ascii")
...
>>> http_response = "\r\n".join((
...     "HTTP/1.0 200 OK",
...     "Content-Type: text/html",
...     "",
...     "<h2>Test Server</h2>",
...     "<p>Successful connection.</p>",
...     "")).encode("ascii")
...
>>> http_error = "\r\n".join((
...     "HTTP/1.0 400 Bad Request",
...     "",
...     ""))
...

For this example, the trust store just consists in the root certificate ca0_crt from the previous section.

>>> from mbedtls import tls
>>> trust_store = tls.TrustStore()
>>> trust_store.add(ca0_crt)

The next step is to configure the TLS contexts for server and client.

>>> srv_ctx = tls.ServerContext(tls.TLSConfiguration(
...     trust_store=trust_store,
...     certificate_chain=([ee0_crt, ca1_crt], ee0_key),
...     validate_certificates=False,
... ))
...
>>> cli_ctx = tls.ClientContext(tls.TLSConfiguration(
...     trust_store=trust_store,
...     validate_certificates=True,
... ))
...

The contexts are used to wrap TCP sockets.

>>> import socket
>>> srv = srv_ctx.wrap_socket(
...     socket.socket(socket.AF_INET, socket.SOCK_STREAM))
...
>>> from contextlib import suppress
>>> def block(callback, *args, **kwargs):
...     while True:
...         with suppress(tls.WantReadError, tls.WantWriteError):
...             return callback(*args, **kwargs)
...

The server starts in its own process in this example because accept() is blocking.

>>> def server_main_loop(sock):
...     conn, addr = sock.accept()
...     block(conn.do_handshake)
...     data = conn.recv(1024)
...     if data == get_request:
...         conn.sendall(http_response)
...     else:
...         conn.sendall(http_error)
...
>>> import multiprocessing as mp
>>> srv.bind(("localhost", 8888))
>>> srv.listen(1)
>>> runner = mp.Process(target=server_main_loop, args=(srv, ))
>>> runner.start()

Finally, a client queries the server with the get_request:

>>> cli = cli_ctx.wrap_socket(
...     socket.socket(socket.AF_INET, socket.SOCK_STREAM),
...     server_hostname=None,
... )
...
>>> cli.connect(("localhost", 8888))
>>> block(cli.do_handshake)
>>> cli.send(get_request)
18
>>> response = block(cli.recv, 1024)
>>> print(response.decode("ascii").replace("\r\n", "\n"))
HTTP/1.0 200 OK
Content-Type: text/html
<BLANKLINE>
<h2>Test Server</h2>
<p>Successful connection.</p>
<BLANKLINE>

The last step is to stop the extra process and close the sockets.

>>> cli.close()
>>> runner.join(1.0)
>>> srv.close()

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