It's fairly straight-forward to use. This isn't tested, but should work:

# load OpenSSL.crypto
from OpenSSL import crypto

# open it, using password. Supply/read your own from stdin.
p12 = crypto.load_pkcs12(open("/path/to/cert.p12", 'rb').read(), passwd)

# get various properties of said file.
# note these are PyOpenSSL objects, not strings although you
# can convert them to PEM-encoded strings.
p12.get_certificate()     # (signed) certificate object
p12.get_privatekey()      # private key.
p12.get_ca_certificates() # ca chain.

For more examples, have a look through the unit test code of pyopenssl. Pretty much every way you might want to use the library is there

See also here or without adverts here.

For people not using openssl who have this problem (missing sign method) there is another solution.

Check the import section on the top of your code, you need to have this import :

from Crypto.Signature import PKCS1_v1_5

and not the Crypto.Cipher implementation:

from Crypto.Cipher import PKCS1_v1_5

This class doesn't have the sign method: https://www.dlitz.net/software/pycrypto/api/2.6/Crypto.Cipher.PKCS1_v1_5.PKCS115_Cipher-class.html

The Crypto.Signature PKCS1_v1_5 class has the sign method :https://www.dlitz.net/software/pycrypto/api/2.6/Crypto.Signature.PKCS1_v1_5.PKCS115_SigScheme-class.html

Use cryptography.hazmat.primitives.serialization.pkcs12.load_key_and_certificates.

Example:

Copyfrom cryptography.hazmat.primitives.serialization import pkcs12

with open("cert.pfx", "rb") as f:
    private_key, certificate, additional_certificates = pkcs12.load_key_and_certificates(f.read(), b"1234")
print(certificate.not_valid_after)

Slightly modified pkcs12 creation code with key attrib is given below.

private static void createPKCS12File(String alias, PrivateKey key,
X509Certificate cert, char[] password, OutputStream pfxOut)
throws Exception
{
    PrivateKeyInfo pki = PrivateKeyInfo.getInstance(key.getEncoded());
    X509KeyUsage usage = new X509KeyUsage(X509KeyUsage.digitalSignature);
    DERSet usageSet = new DERSet(usage);
    DERSequence attrib = new DERSequence(new ASN1Encodable[]
        {new ASN1ObjectIdentifier("2.5.29.15"), usageSet});
    pki = new PrivateKeyInfo(pki.getPrivateKeyAlgorithm(),
        pki.parsePrivateKey(), new DLSet(attrib));

    OutputEncryptor encOut = new JcePKCSPBEOutputEncryptorBuilder(
        NISTObjectIdentifiers.id_aes256_CBC).setProvider("BC").build(
        password);
    PKCS12SafeBagBuilder certBuilder = new JcaPKCS12SafeBagBuilder(
        cert);
    certBuilder.addBagAttribute(
        PKCS12SafeBag.friendlyNameAttribute, new DERBMPString(alias));

    JcaX509ExtensionUtils extUtils = new JcaX509ExtensionUtils();
    SubjectKeyIdentifier pubKeyId = extUtils.createSubjectKeyIdentifier(
        cert.getPublicKey());
    certBuilder.addBagAttribute(PKCS12SafeBag.localKeyIdAttribute,
        pubKeyId);
    PKCS12SafeBagBuilder keyBagBuilder = new PKCS12SafeBagBuilder(pki,
        encOut);
    keyBagBuilder.addBagAttribute(PKCS12SafeBag.friendlyNameAttribute,
        new DERBMPString(alias));
    keyBagBuilder.addBagAttribute(PKCS12SafeBag.localKeyIdAttribute,
        pubKeyId);
    keyBagBuilder.addBagAttribute(new ASN1ObjectIdentifier(
        "1.3.6.1.4.1.311.17.1"),
        new DERBMPString("Microsoft Software Key Storage Provider"));
    PKCS12PfxPduBuilder builder = new PKCS12PfxPduBuilder();
    builder.addData(keyBagBuilder.build());
    builder.addEncryptedData(new JcePKCSPBEOutputEncryptorBuilder(
        PKCSObjectIdentifiers.pbeWithSHAAnd128BitRC2_CBC).
        setProvider("BC").build(password),
        new PKCS12SafeBag[]{certBuilder.build()});
    PKCS12PfxPdu pfx = builder.build(new JcePKCS12MacCalculatorBuilder(
        NISTObjectIdentifiers.id_sha256), password);
    pfxOut.write(pfx.getEncoded(ASN1Encoding.DL));
    pfxOut.flush();
}