Add Postal support

Thanks to @tiltec for researching, implementing, testing and documenting it.

tests/utils_postal.py

@@ -0,0 +1,58 @@
+from base64 import b64encode
+
+from django.test import override_settings
+
+from tests.utils import ClientWithCsrfChecks
+
+HAS_CRYPTOGRAPHY = True
+try:
+    from cryptography.hazmat.primitives.asymmetric import rsa
+    from cryptography.hazmat.primitives import serialization
+    from cryptography.hazmat.primitives import hashes
+    from cryptography.hazmat.primitives.asymmetric import padding
+except ImportError:
+    HAS_CRYPTOGRAPHY = False
+
+
+def make_key():
+    """Generate RSA public key with short key size, for testing only"""
+    private_key = rsa.generate_private_key(
+        public_exponent=65537,
+        key_size=512,
+    )
+    return private_key
+
+
+def derive_public_webhook_key(private_key):
+    """Derive public """
+    public_key = private_key.public_key()
+    public_bytes = public_key.public_bytes(
+        encoding=serialization.Encoding.PEM,
+        format=serialization.PublicFormat.SubjectPublicKeyInfo
+    )
+    public_bytes = b'\n'.join(public_bytes.splitlines()[1:-1])
+    return public_bytes.decode('utf-8')
+
+
+def sign(private_key, message):
+    """Sign message with private key"""
+    signature = private_key.sign(message, padding.PKCS1v15(), hashes.SHA1())
+    return signature
+
+
+class _ClientWithPostalSignature(ClientWithCsrfChecks):
+    private_key = None
+
+    def set_private_key(self, private_key):
+        self.private_key = private_key
+
+    def post(self, *args, **kwargs):
+        signature = b64encode(sign(self.private_key, kwargs['data'].encode('utf-8')))
+        kwargs.setdefault('HTTP_X_POSTAL_SIGNATURE', signature)
+
+        webhook_key = derive_public_webhook_key(self.private_key)
+        with override_settings(ANYMAIL={'POSTAL_WEBHOOK_KEY': webhook_key}):
+            return super().post(*args, **kwargs)
+
+
+ClientWithPostalSignature = _ClientWithPostalSignature if HAS_CRYPTOGRAPHY else None