#!/usr/bin/env python
# -*- mode: python -*-
#
# Check PGP signed emails
#
# This script verifies the signature on incoming mail for a couple of things
#   - That the signature is valid, recent and is not replay
#   - The signer is in the LDAP directory and is in the right group
#   - The message contains no extra text that is not signed.
#
# Options:
#  -r  Replay cache file, if unset replay checking is disabled
#  -k  Colon seperated list of keyrings to use
#  -d  LDAP search base DN
#  -l  LDAP server
#  -g  supplementary group membership
#  -p  File of Phrases that must be in the plaintext.
#  -m  Disallow PGP/MIME
#  -v  Verbose mode

# Typical Debian invokation may look like:
# sigcheck -k /usr/share/keyrings/debian-keyring.gpg:/usr/share/keyrings/debian-keyring.pgp \
#      -d ou=users,dc=debian,dc=org -l db.debian.org \
#      -m debian.org -a admin@db.debian.org \
#      -e /etc/userdir-ldap/templtes/error-reply -- test.sh

import email
import email.parser
import getopt
import os
import pwd
import sys
import time
import traceback

from userdir_gpg import *

EX_TEMPFAIL = 75
EX_PERMFAIL = 65      # EX_DATAERR

# Configuration
ReplayCacheFile = None
LDAPDn = None
LDAPServer = None
GroupMember = None
Phrases = None
AllowMIME = 1
Verbose = 0


class MessageError(Exception):
    pass


def verbmsg(msg):
    if Verbose:
        sys.stderr.write(msg + "\n")


# Match the key fingerprint against an LDAP directory
def CheckLDAP(FingerPrint):
    import ldap
    import userdir_ldap

    # Connect to the ldap server
    global ErrTyp, ErrMsg
    ErrType = EX_TEMPFAIL
    ErrMsg = "An error occurred while performing the LDAP lookup:"
    global l
    l = userdir_ldap.connectLDAP(LDAPServer)
    l.simple_bind_s("", "")

    # Search for the matching key fingerprint
    verbmsg("Processing fingerprint %s" % FingerPrint)
    Attrs = l.search_s(LDAPDn, ldap.SCOPE_ONELEVEL, "keyfingerprint=" + FingerPrint)
    if len(Attrs) == 0:
        raise MessageError("Key not found")
    if len(Attrs) != 1:
        raise MessageError("Oddly your key fingerprint is assigned to more than one account..")

    gidnumber_found = 0
    for key in Attrs[0][1].keys():
        if key == "gidNumber":
            gidnumber_found = 1

    if gidnumber_found != 1:
        raise MessageError("No gidnumber in attributes for fingerprint %s" % FingerPrint)

    # Look for the group with the gid of the user
    GAttr = l.search_s(LDAPDn, ldap.SCOPE_ONELEVEL, "(&(objectClass=debianGroup)(gidnumber=%s))" % Attrs[0][1]["gidNumber"][0], ["gid"])
    if len(GAttr) == 0:
        raise MessageError("Database inconsistency found: main group for account not found in database")

    # See if the group membership is OK
    # Only if a group was given on the commandline
    if GroupMember is not None:
        Hit = 0
        # Check primary group first
        if GAttr[0][1]["gid"][0] == GroupMember:
            Hit = 1
        else:
            # Check supplementary groups
            for x in Attrs[0][1].get("supplementaryGid", []):
                if x == GroupMember:
                    Hit = 1
        if Hit != 1:
            raise MessageError("You don't have %s group permissions." % GroupMember)


# Start of main program
# Process options
options, arguments = getopt.getopt(sys.argv[1:], "r:k:d:l:g:mp:v")
for (switch, val) in options:
    if (switch == '-r'):
        ReplayCacheFile = val
    elif (switch == '-k'):
        SetKeyrings(val.split(":"))
    elif (switch == '-d'):
        LDAPDn = val
    elif (switch == '-l'):
        LDAPServer = val
    elif (switch == '-g'):
        GroupMember = val
    elif (switch == '-m'):
        AllowMIME = 0
    elif (switch == '-v'):
        Verbose = 1
    elif (switch == '-p'):
        Phrases = val

Now = time.strftime("%a, %d %b %Y %H:%M:%S", time.gmtime(time.time()))
ErrMsg = "Indeterminate Error"
ErrType = EX_TEMPFAIL
MsgID = None
try:
    # Startup the replay cache
    ErrType = EX_TEMPFAIL
    if ReplayCacheFile is not None:
        ErrMsg = "Failed to initialize the replay cache:"
        RC = ReplayCache(ReplayCacheFile)

    # Get the email
    ErrType = EX_PERMFAIL
    ErrMsg = "Failed to understand the email or find a signature:"
    mail = email.parser.Parser().parse(sys.stdin)
    MsgID = mail["Message-ID"]

    print "Inspecting message %s" % MsgID
    verbmsg("Processing message %s" % MsgID)
    Msg = GetClearSig(mail, 1)
    if AllowMIME == 0 and Msg[1] != 0:
        raise MessageError("PGP/MIME disallowed")

    ErrMsg = "Message is not PGP signed:"
    if Msg[0].find("-----BEGIN PGP SIGNED MESSAGE-----") == -1:
        raise MessageError("No PGP signature")

    # Check the signature
    ErrMsg = "Unable to check the signature or the signature was invalid:"
    pgp = GPGCheckSig2(Msg[0])

    if not pgp.ok:
        raise UDFormatError(pgp.why)
    if pgp.text is None:
        raise UDFormatError("Null signature text")

    # Check the signature against the replay cache
    if ReplayCacheFile is not None:
        RC.process(pgp.sig_info)

    # Do LDAP stuff
    if LDAPDn is not None:
        CheckLDAP(pgp.key_fpr)

    ErrMsg = "Verifying message:"
    if Phrases is not None:
        F = open(Phrases, "r")
        while 1:
            Line = F.readline()
            if Line == "":
                break
            if pgp.text.find(Line.strip()) == -1:
                raise MessageError("Phrase '%s' was not found" % (Line.strip()))

except Exception:
    ErrMsg = "[%s] \"%s\" \"%s %s\"\n" % (Now, MsgID, ErrMsg, sys.exc_value)
    sys.stderr.write(ErrMsg)

    Trace = "==> %s: %s\n" % (sys.exc_type, sys.exc_value)
    List = traceback.extract_tb(sys.exc_traceback)
    if len(List) >= 1:
        Trace = Trace + "Python Stack Trace:\n"
        for x in List:
            Trace = Trace + "   %s %s:%u: %s\n" % (x[2], x[0], x[1], x[3])

    sys.exit(EX_PERMFAIL)

# For Main
print "Message %s passed" % MsgID
sys.exit(0)