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+# coding: utf-8
+# PyGOST -- Pure Python GOST cryptographic functions library
+# Copyright (C) 2015-2023 Sergey Matveev <stargrave@stargrave.org>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, version 3 of the License.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+"""Key agreement functions, VKO GOST R 34.10-2001/2012
+"""
+
+from pygost.gost3410 import pub_marshal
+from pygost.gost34112012256 import GOST34112012256
+from pygost.gost34112012512 import GOST34112012512
+from pygost.gost341194 import GOST341194
+from pygost.utils import bytes2long
+
+
+def ukm_unmarshal(ukm):
+    """Unmarshal UKM value
+
+    :type ukm: little-endian bytes
+    :rtype: long
+    """
+    return bytes2long(ukm[::-1])
+
+
+def kek(curve, prv, pub, ukm):
+    if not curve.contains(pub):
+        raise ValueError("pub is not on the curve")
+    key = curve.exp(prv, pub[0], pub[1])
+    key = curve.exp(curve.cofactor * ukm, key[0], key[1])
+    return pub_marshal(key)
+
+
+def kek_34102001(curve, prv, pub, ukm):
+    """Key agreement (34.10-2001, 34.11-94)
+
+    :param GOST3410Curve curve: curve to use
+    :param long prv: private key
+    :param pub: public key
+    :type pub: (long, long)
+    :param long ukm: user keying material, VKO-factor
+    :returns: Key Encryption Key (shared key)
+    :rtype: bytes, 32 bytes
+
+    Shared Key Encryption Key computation is based on
+    :rfc:`4357` VKO GOST R 34.10-2001 with little-endian
+    hash output.
+    """
+    return GOST341194(
+        kek(curve, prv, pub, ukm),
+        sbox="id-GostR3411-94-CryptoProParamSet",
+    ).digest()
+
+
+def kek_34102012256(curve, prv, pub, ukm=1):
+    """Key agreement (34.10-2012, 34.11-2012 256 bit)
+
+    :param GOST3410Curve curve: curve to use
+    :param long prv: private key
+    :param pub: public key
+    :type pub: (long, long)
+    :param long ukm: user keying material, VKO-factor
+    :returns: Key Encryption Key (shared key)
+    :rtype: bytes, 32 bytes
+
+    Shared Key Encryption Key computation is based on
+    :rfc:`7836` VKO GOST R 34.10-2012.
+    """
+    return GOST34112012256(kek(curve, prv, pub, ukm)).digest()
+
+
+def kek_34102012512(curve, prv, pub, ukm=1):
+    """Key agreement (34.10-2012, 34.11-2012 512 bit)
+
+    :param GOST3410Curve curve: curve to use
+    :param long prv: private key
+    :param pub: public key
+    :type pub: (long, long)
+    :param long ukm: user keying material, VKO-factor
+    :returns: Key Encryption Key (shared key)
+    :rtype: bytes, 32 bytes
+
+    Shared Key Encryption Key computation is based on
+    :rfc:`7836` VKO GOST R 34.10-2012.
+    """
+    return GOST34112012512(kek(curve, prv, pub, ukm)).digest()