try:
    from hashlib import sha1
except ImportError:
    import sha
    sha1 = sha.new
from pyasn1.type import univ
from pysnmp.proto.secmod.rfc3414.auth import base
from pysnmp.proto.secmod.rfc3414 import localkey
from pysnmp.proto import errind, error

_twelveZeros = univ.OctetString((0,)*12).asOctets()
_fortyFourZeros = (0,)*44

# 7.2.4

class HmacSha(base.AbstractAuthenticationService):
    serviceID = (1, 3, 6, 1, 6, 3, 10, 1, 1, 3)  # usmHMACSHAAuthProtocol
    __ipad = [0x36]*64
    __opad = [0x5C]*64

    def hashPassphrase(self, authKey):
        return localkey.hashPassphraseSHA(authKey)
    
    def localizeKey(self, authKey, snmpEngineID):
        return localkey.localizeKeySHA(authKey, snmpEngineID)
    
    # 7.3.1
    def authenticateOutgoingMsg(self, authKey, wholeMsg):
        # 7.3.1.1
        # Here we expect calling secmod to indicate where the digest
        # should be in the substrate. Also, it pre-sets digest placeholder
        # so we hash wholeMsg out of the box.
        # Yes, that's ugly but that's rfc...
        l = wholeMsg.find(_twelveZeros)
        if l == -1:
            raise error.ProtocolError('Cant locate digest placeholder')
        wholeHead = wholeMsg[:l]
        wholeTail = wholeMsg[l+12:]

        # 7.3.1.2a
        extendedAuthKey = authKey.asNumbers() + _fortyFourZeros

        # 7.3.1.2b -- noop

        # 7.3.1.2c
        k1 = univ.OctetString(
            map(lambda x,y: x^y, extendedAuthKey, self.__ipad)
            )

        # 7.3.1.2d -- noop

        # 7.3.1.2e
        k2 = univ.OctetString(
            map(lambda x,y: x^y, extendedAuthKey, self.__opad)
            )
        
        # 7.3.1.3
        d1 = sha1(k1.asOctets()+wholeMsg).digest()
        
        # 7.3.1.4
        d2 = sha1(k2.asOctets()+d1).digest()
        mac = d2[:12]

        # 7.3.1.5 & 6
        return wholeHead + mac + wholeTail

    # 7.3.2
    def authenticateIncomingMsg(self, authKey, authParameters, wholeMsg):        
        # 7.3.2.1 & 2
        if len(authParameters) != 12:
            raise error.StatusInformation(
                errorIndication=errind.authenticationError
                )

        # 7.3.2.3
        l = wholeMsg.find(authParameters.asOctets())
        if l == -1:
            raise error.ProtocolError('Cant locate digest in wholeMsg')
        wholeHead = wholeMsg[:l]
        wholeTail = wholeMsg[l+12:]
        authenticatedWholeMsg = wholeHead + _twelveZeros + wholeTail

        # 7.3.2.4a
        extendedAuthKey = authKey.asNumbers() + _fortyFourZeros

        # 7.3.2.4b --> noop
        
        # 7.3.2.4c
        k1 = univ.OctetString(
            map(lambda x,y: x^y, extendedAuthKey, self.__ipad)
            )

        # 7.3.2.4d --> noop

        # 7.3.2.4e
        k2 = univ.OctetString(
            map(lambda x,y: x^y, extendedAuthKey, self.__opad)
            )

        # 7.3.2.5a
        d1 = sha1(k1.asOctets()+authenticatedWholeMsg).digest()

        # 7.3.2.5b
        d2 = sha1(k2.asOctets()+d1).digest()
        
        # 7.3.2.5c
        mac = d2[:12]
         
        # 7.3.2.6
        if mac != authParameters:
            raise error.StatusInformation(
                errorIndication=errind.authenticationFailure
                )
        
        return authenticatedWholeMsg
