error while using pysnmp with the option privProtocol=usm3DESEDEPrivProtocol - python-2.7

Question

I want to get information from a snmp device which is configured with auth protocol MD5 and priv protocol 3DES.
The configuration statement is
snmp-server user testuser testgroup v3 auth md5 authmd5pwd priv 3des privpwddes
The pysnmp code to access the device is
from pysnmp.hlapi import *
def get(mib):
errorIndication, errorStatus, errorIndex, varBinds = next(
getCmd(SnmpEngine(),
UsmUserData('nmsuser', 'authmd5pwd', 'privpwddes',
authProtocol=usmHMACMD5AuthProtocol,
privProtocol=usm3DESEDEPrivProtocol),
UdpTransportTarget(('10.96.158.251', 161)),
ContextData(),
ObjectType(ObjectIdentity(mib)))
)
if errorIndication:
print(errorIndication)
elif errorStatus:
print('%s at %s' % (errorStatus.prettyPrint(),
errorIndex and varBinds[int(errorIndex) - 1][0] or '?'))
else:
for varBind in varBinds:
print(varBind[1])
if __name__ == "__main__":
get('.1.3.6.1.2.1.1.5.0')
When I execute this I get
Traceback (most recent call last):
File "pytest.py", line 24, in
get('.1.3.6.1.2.1.1.5.0')
File "pytest.py", line 11, in get
ObjectType(ObjectIdentity(mib)))
File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/hlapi/asyncore/sync/cmdgen.py", line 113, in getCmd
File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/carrier/asyncore/dispatch.py", line 50, in runDispatcher
pysnmp.error.PySnmpError: poll error: Traceback (most recent call last):
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/carrier/asyncore/dispatch.py", line 46, in runDispatcher
use_poll=True, map=self.__sockMap, count=1)
; File "/usr/lib/python2.7/asyncore.py", line 220, in loop
poll_fun(timeout, map)
; File "/usr/lib/python2.7/asyncore.py", line 201, in poll2
readwrite(obj, flags)
; File "/usr/lib/python2.7/asyncore.py", line 123, in readwrite
obj.handle_error()
; File "/usr/lib/python2.7/asyncore.py", line 108, in readwrite
obj.handle_read_event()
; File "/usr/lib/python2.7/asyncore.py", line 449, in handle_read_event
self.handle_read()
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/carrier/asyncore/dgram/base.py", line 163, in handle_read
self._cbFun(self, transportAddress, incomingMessage)
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/carrier/base.py", line 70, in _cbFun
self, transportDomain, transportAddress, incomingMessage
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/entity/engine.py", line 152, in __receiveMessageCbFun
self, transportDomain, transportAddress, wholeMsg
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/rfc3412.py", line 344, in receiveMessage
statusInformation
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/rfc3412.py", line 533, in __expireRequest
cachedParams['cbCtx'])
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/entity/rfc3413/cmdgen.py", line 104, in processResponsePdu
(origSendRequestHandle, cbFun, cbCtx))
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/rfc3412.py", line 153, in sendPdu
pduVersion, PDU, expectResponse, sendPduHandle
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/mpmod/rfc3412.py", line 240, in prepareOutgoingMessage
securityEngineId, securityName, securityLevel, scopedPDU
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/secmod/rfc3414/service.py", line 525, in generateRequestMsg
None)
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/secmod/rfc3414/service.py", line 395, in __generateRequestOrResponseMsg
(snmpEngineBoots, snmpEngineTime, None), dataToEncrypt
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/secmod/eso/priv/des3.py", line 117, in encryptData
encryptKey, snmpEngineBoots
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/secmod/eso/priv/des3.py", line 77, in __getEncryptionKey
preIV = privKey[24:32]
; File "/usr/local/lib/python2.7/dist-packages/pyasn1/type/univ.py", line 996, in __getitem__
return self.clone(self._value[i])
; File "/usr/local/lib/python2.7/dist-packages/pysnmp-4.4.3-py2.7.egg/pysnmp/proto/rfc1902.py", line 202, in clone
return univ.OctetString.clone(self, *args, **kwargs).setFixedLength(self.getFixedLength())
; File "/usr/local/lib/python2.7/dist-packages/pyasn1/type/base.py", line 349, in clone
return self.__class__(value, **initilaizers)
; File "/usr/local/lib/python2.7/dist-packages/pyasn1/type/univ.py", line 819, in __init__
base.AbstractSimpleAsn1Item.__init__(self, value, **kwargs)
; File "/usr/local/lib/python2.7/dist-packages/pyasn1/type/base.py", line 246, in __init__
raise exType('%s at %s' % (exValue, self.__class__.__name__))
;ValueConstraintError: , > failed at: ValueConstraintError(" failed at: ValueConstraintError('',)",) at OctetString

Answer 1

Seems to be a bug in pysnmp 4.4.3 when using 3DES with short-hash HMAC such as MD5.
You can either use some other AUTH algorithm which produces at least 32-octet long hash or pull fixed pysnmp (4.4.4) from its master branch.
Your script seems to work with the simulator once you change username/passphrase.