imgtool: Add ECIES-X25519 image encryption support
Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/imgtool/keys/x25519.py b/scripts/imgtool/keys/x25519.py
new file mode 100644
index 0000000..6c6f60f
--- /dev/null
+++ b/scripts/imgtool/keys/x25519.py
@@ -0,0 +1,105 @@
+"""
+X25519 key management
+"""
+
+from cryptography.hazmat.backends import default_backend
+from cryptography.hazmat.primitives import serialization
+from cryptography.hazmat.primitives.asymmetric import x25519
+
+from .general import KeyClass
+
+
+class X25519UsageError(Exception):
+ pass
+
+
+class X25519Public(KeyClass):
+ def __init__(self, key):
+ self.key = key
+
+ def shortname(self):
+ return "x25519"
+
+ def _unsupported(self, name):
+ raise X25519UsageError("Operation {} requires private key".format(name))
+
+ def _get_public(self):
+ return self.key
+
+ def get_public_bytes(self):
+ # The key is embedded into MBUboot in "SubjectPublicKeyInfo" format
+ return self._get_public().public_bytes(
+ encoding=serialization.Encoding.DER,
+ format=serialization.PublicFormat.SubjectPublicKeyInfo)
+
+ def get_private_bytes(self, minimal):
+ self._unsupported('get_private_bytes')
+
+ def export_private(self, path, passwd=None):
+ self._unsupported('export_private')
+
+ def export_public(self, path):
+ """Write the public key to the given file."""
+ pem = self._get_public().public_bytes(
+ encoding=serialization.Encoding.PEM,
+ format=serialization.PublicFormat.SubjectPublicKeyInfo)
+ with open(path, 'wb') as f:
+ f.write(pem)
+
+ def sig_type(self):
+ return "X25519"
+
+ def sig_tlv(self):
+ return "X25519"
+
+ def sig_len(self):
+ return 32
+
+
+class X25519(X25519Public):
+ """
+ Wrapper around an X25519 private key.
+ """
+
+ def __init__(self, key):
+ """key should be an instance of EllipticCurvePrivateKey"""
+ self.key = key
+
+ @staticmethod
+ def generate():
+ pk = x25519.X25519PrivateKey.generate()
+ return X25519(pk)
+
+ def _get_public(self):
+ return self.key.public_key()
+
+ def get_private_bytes(self, minimal):
+ raise X25519UsageError("Operation not supported with {} keys".format(
+ self.shortname()))
+
+ def export_private(self, path, passwd=None):
+ """
+ Write the private key to the given file, protecting it with the
+ optional password.
+ """
+ if passwd is None:
+ enc = serialization.NoEncryption()
+ else:
+ enc = serialization.BestAvailableEncryption(passwd)
+ pem = self.key.private_bytes(
+ encoding=serialization.Encoding.PEM,
+ format=serialization.PrivateFormat.PKCS8,
+ encryption_algorithm=enc)
+ with open(path, 'wb') as f:
+ f.write(pem)
+
+ def sign_digest(self, digest):
+ """Return the actual signature"""
+ return self.key.sign(data=digest)
+
+ def verify_digest(self, signature, digest):
+ """Verify that signature is valid for given digest"""
+ k = self.key
+ if isinstance(self.key, x25519.X25519PrivateKey):
+ k = self.key.public_key()
+ return k.verify(signature=signature, data=digest)