import random
from pyasn1.type import univ
from pysnmp.proto.secmod.rfc3414.priv import base
from pysnmp.proto.secmod.rfc3414.auth import hmacmd5, hmacsha
from pysnmp.proto.secmod.rfc3414 import localkey
from pysnmp.proto import errind, error

try:
    from Crypto.Cipher import AES
except ImportError:
    AES = None

random.seed()

# RFC3826

#

class Aes(base.AbstractEncryptionService):
    serviceID = (1, 3, 6, 1, 6, 3, 10, 1, 2, 4) # usmAesCfb128Protocol
    keySize = 16
    _localInt = random.randrange(0, 0xffffffffffffffff)
    # 3.1.2.1
    def __getEncryptionKey(self, privKey, snmpEngineBoots, snmpEngineTime):
        salt = [self._localInt>>56&0xff,
                self._localInt>>48&0xff,
                self._localInt>>40&0xff,
                self._localInt>>32&0xff,
                self._localInt>>24&0xff,
                self._localInt>>16&0xff,
                self._localInt>>8&0xff,
                self._localInt&0xff]

        if self._localInt == 0xffffffffffffffff:
            self._localInt = 0
        else:
            self._localInt += 1

        return self.__getDecryptionKey(privKey, snmpEngineBoots, snmpEngineTime, salt) + (univ.OctetString(salt).asOctets(),)

    def __getDecryptionKey(self, privKey, snmpEngineBoots,
                           snmpEngineTime, salt):
        snmpEngineBoots, snmpEngineTime, salt = (
            int(snmpEngineBoots), int(snmpEngineTime), salt
        )

        iv = [snmpEngineBoots>>24&0xff,
              snmpEngineBoots>>16&0xff,
              snmpEngineBoots>>8&0xff,
              snmpEngineBoots&0xff,
              snmpEngineTime>>24&0xff,
              snmpEngineTime>>16&0xff,
              snmpEngineTime>>8&0xff,
              snmpEngineTime&0xff] + salt

        return privKey[:self.keySize].asOctets(), univ.OctetString(iv).asOctets()

    def hashPassphrase(self, authProtocol, privKey):
        if authProtocol == hmacmd5.HmacMd5.serviceID:
            return localkey.hashPassphraseMD5(privKey)
        elif authProtocol == hmacsha.HmacSha.serviceID:
            return localkey.hashPassphraseSHA(privKey)
        else:
            raise error.ProtocolError(
                'Unknown auth protocol %s' % (authProtocol,)
            )

    def localizeKey(self, authProtocol, privKey, snmpEngineID):
        if authProtocol == hmacmd5.HmacMd5.serviceID:
            localPrivKey = localkey.localizeKeyMD5(privKey, snmpEngineID)
        elif authProtocol == hmacsha.HmacSha.serviceID:
            localPrivKey = localkey.localizeKeySHA(privKey, snmpEngineID)
        else:
            raise error.ProtocolError(
                'Unknown auth protocol %s' % (authProtocol,)
            )
        return localPrivKey[:16]

    # 3.2.4.1
    def encryptData(self, encryptKey, privParameters, dataToEncrypt):
        if AES is None:
            raise error.StatusInformation(
                errorIndication=errind.encryptionError
            )

        snmpEngineBoots, snmpEngineTime, salt = privParameters

        # 3.3.1.1
        aesKey, iv, salt = self.__getEncryptionKey(
            encryptKey, snmpEngineBoots, snmpEngineTime
        )

        # 3.3.1.3
        aesObj = AES.new(aesKey, AES.MODE_CFB, iv, segment_size=128)

        # PyCrypto seems to require padding
        dataToEncrypt = dataToEncrypt + univ.OctetString((0,) * (16-len(dataToEncrypt)%16)).asOctets()

        ciphertext = aesObj.encrypt(dataToEncrypt)

        # 3.3.1.4
        return univ.OctetString(ciphertext), univ.OctetString(salt)

    # 3.2.4.2
    def decryptData(self, decryptKey, privParameters, encryptedData):
        if AES is None:
            raise error.StatusInformation(
                errorIndication=errind.decryptionError
            )

        snmpEngineBoots, snmpEngineTime, salt = privParameters

        # 3.3.2.1
        if len(salt) != 8:
            raise error.StatusInformation(
                errorIndication=errind.decryptionError
            )

        # 3.3.2.3
        aesKey, iv = self.__getDecryptionKey(
            decryptKey, snmpEngineBoots, snmpEngineTime, salt
        )

        aesObj = AES.new(aesKey, AES.MODE_CFB, iv, segment_size=128)

        # PyCrypto seems to require padding
        encryptedData = encryptedData + univ.OctetString((0,) * (16-len(encryptedData)%16)).asOctets()

        # 3.3.2.4-6
        return aesObj.decrypt(encryptedData.asOctets())
