Chaos Digest Vendredi 11 Juin 1993 Volume 1 : Numero 52
ISSN 1244-4901
Editeur: Jean-Bernard Condat (
[email protected])
Archiviste: Yves-Marie Crabbe
Co-Redacteurs: Arnaud Bigare, Stephane Briere
TABLE DES MATIERES, #1.52 (11 Juin 1993)
File 1--40H VMag Number 6 Volume 2 Issue 2 #00B-00C (reprint)
File 2--Fortress: services de securite europeens a la loupe (rapport)
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----------------------------------------------------------------------
Date: Tue May 11 09:24:40 PDT 1993
From:
[email protected] (American_Eagle_Publication_Inc. )
Subject: File 1--40H VMag Number 6 Volume 2 Issue 2 #00B-00C (reprint)
40Hex Number 6 Volume 2 Issue 2 File 00B
++++++++++++++++++++++++++++++
SCAN STRINGS, HOW THEY WORK,
AND HOW TO AVOID THEM
++++++++++++++++++++++++++++++
By Dark Angel
++++++++++++++++++++++++++++++
Scan strings are the scourge of the virus author and the friend of anti-
virus wanna-bes. The virus author must find encryption techniques which
can successfully evade easy detection. This article will show you several
such techniques.
Scan strings, as you are well aware, are a collection of bytes which an
anti-viral product uses to identify a virus. The important thing to keep
in mind is that these scan strings represent actual code and can NEVER
contain code which could occur in a "normal" program. The trick is to use
this to your advantage.
When a scanner checks a file for a virus, it searches for the scan string
which could be located ANYWHERE IN THE FILE. The scanner doesn't care
where it is. Thus, a file which consists solely of the scan string and
nothing else would be detected as infected by a virus. A scanner is
basically an overblown "hex searcher" looking for 1000 signatures.
Interesting, but there's not much you can do to exploit this. The only
thing you can do is to write code so generic that it could be located in
any program (by chance). Try creating a file with the following debug
script and scanning it. This demonstrates the fact that the scan string
may be located at any position in the file.
---------------------------------------------------------------------------
n marauder.com
e 0100 E8 00 00 5E 81 EE 0E 01 E8 05 00 E9
rcx
000C
w
q
---------------------------------------------------------------------------
Although scanners normally search for decryption/encryption routines, in
Marauder's case, SCAN looks for the "setup" portion of the code, i.e.
setting up BP (to the "delta offset"), calling the decryption routine, and
finally jumping to program code.
What you CAN do is to either minimise the scannable code or to have the
code constantly mutate into something different. The reasons are readily
apparent.
The simplest technique is having multiple encryption engines. A virus
utilising this technique has a database of encryption/decryption engines
and uses a random one each time it infects. For example, there could be
various forms of XOR encryption or perhaps another form of mathematical
encryption. The trick is to simply replace the code for the encryption
routine each time with the new encryption routine.
Mark Washburn used this in his V2PX series of virii. In it, he used six
different encryption/decryption algorithms, and some mutations are
impossible to detect with a mere scan string. More on those later.
Recently, there has been talk of the so-called MTE, or mutating engine,
from Bulgaria (where else?). It utilises the multiple encryption engine
technique. Pogue Mahone used the MTE and it took McAfee several days to
find a scan string. Vesselin Bontchev, the McAfee-wanna-be of Bulgaria,
marvelled the engineering of this engine. It is distributed as an OBJ file
designed to be able to be linked into any virus. Supposedly, SCANV89 will
be able to detect any virus using the encryption engine, so it is worthless
except for those who have an academic interest in such matters (such as
virus authors).
However, there is a serious limitation to the multiple encryption
technique, namely that scan strings may still be found. However, scan
strings must be isolated for each different encryption mechanism. An
additional benefit is the possibility that the antivirus software
developers will miss some of the encryption mechanisms so not all the
strains of the virus will be caught by the scanner.
Now we get to a much better (and sort of obvious) method: minimising scan
code length. There are several viable techniques which may be used, but I
shall discuss but three of them.
The one mentioned before which Mark Washburn used in V2P6 was interesting.
He first filled the space to be filled in with the encryption mechanism
with dummy one byte op-codes such as CLC, STC, etc. As you can see, the
flag manipulation op-codes were exploited. Next, he randomly placed the
parts of his encryption mechanism in parts of this buffer, i.e. the gaps
between the "real" instructions were filled in with random dummy op-codes.
In this manner, no generic scan string could be located for this encryption
mechanism of this virus. However, the disadvantage of this method is the
sheer size of the code necessary to perform the encryption.
A second method is much simpler than this and possibly just as effective.
To minimise scan code length, all you have to do is change certain bytes at
various intervals. The best way to do this can be explained with the
following code fragment:
mov si, 1234h ;Starting location of encryption
mov cx, 1234h ;Virus size / 2 + variable number
loop_thing:
xor word ptr cs:[si], 1234h ;Decrypt the value
add si, 2
loop loop_thing
In this code fragment, all the values which can be changed are set to 1234h
for the sake of clarity. Upon infection, all you have to do is to set
these variable values to whatever is appropriate for the file. For
example, mov bx, 1234h would have to be changed to have the encryption
start at the wherever the virus would be loaded into memory (huh?). Ponder
this for a few moments and all shall become clear. To substitute new
values into the code, all you have to do is something akin to:
mov [bp+scratch+1], cx
Where scratch is an instruction. The exact value to add to scratch depends
on the coding of the op-code. Some op-codes take their argument as the
second byte, others take the third. Regardless, it will take some
tinkering before it is perfect. In the above case, the "permanent" code is
limited to under five or six bytes. Additionally, these five or six bytes
could theoretically occur in ANY PROGRAM WHATSOEVER, so it would not be
prudent for scanners to search for these strings. However, scanners often
use scan strings with wild-card-ish scan string characters, so it is still
possible for a scan string to be found.
The important thing to keep in mind when using this method is that it is
best for the virus to use separate encryption and decryption engines. In
this manner, shorter decryption routines may be found and thus shorter scan
strings will be needed. In any case, using separate encryption and
decryption engines increases the size of the code by at most 50 bytes.
The last method detailed is theft of decryption engines. Several shareware
products utilise decryption engines in their programs to prevent simple
"cracks" of their products. This is, of course, not a deterrent to any
programmer worth his salt, but it is useful for virus authors. If you
combine the method above with this technique, the scan string would
identify the product as being infected with the virus, which is a) bad PR
for the company and b) unsuitable for use as a scan string. This technique
requires virtually no effort, as the decryption engine is already written
for you by some unsuspecting PD programmer.
All the methods described are viable scan string avoidance techniques
suitable for use in any virus. After a few practice tries, scan string
avoidance should become second nature and will help tremendously in
prolonging the effective life of your virus in the wild.
+++++
40Hex Number 6 Volume 2 Issue 2 File 00C
++++++++++++++++++++++++
Virus Contest!
'The Spammies(tm)'
++++++++++++++++++++++++
Deadline: July 4th, 1992
This is the first PHALCON/SKISM virus contest. As a matter of fact, this
is the first contest of its kind. We believe that it will motivate you to
produce more original code, rather than more hacks. Winners may have already
won $10,000,000, as well as the prestige of winning the first ever 'Spammie'
awards.
Rules and Regulations:
1) All submissions must be original source code. (no hacks)
2) Only one submission is allowed per programmer, plus one group project.
3) All viruses must be recieved by us before July 4th, 1992.
4) Viruses must be accompanied by a complete entry form. (see below)
5) The original, compilable, commented source MUST be included, along with an
installer program, or a dropper, in the case of boot block viruses.
6) Entries must include a location where the author may be contacted, such as
an email address or a BBS.
7) Personnel or persons related to personnel of PHALCON/SKISM are not
eligable.
8) The source must compile without error under Tasm or Masm (please specify
what assembler and version you used, along with the necessary command line
switches). If we cannot compile your virus, it will be disqualified.
9) All entries recieve a free subscription to 40hex. (hehehehe)
10) The entry must be uploaded privately to the sysop, stating that it is a
contest entry.
11) The viruses must not be detectable by the current version (as of July 4th)
of any known virus scanner.
12) Viruses will be judged by our 'panel of experts' in three catagories.
6.1) Stealth
6.2) Size
6.3) Reproductivity
6.4) Performance
For example, Red Cross is an example of a 'high performance' virus.
It was entertaining and well done.
*** Entry Form
Handle ________________________
Group Afiliation ______________
Virus Name ____________________
Size ____bytes (if you need more spaces, go away)
Type ___ File Infector ___ Boot block
Infection method ___ Direct Action ___ Memory Resident ___ Directory chain
___ Other (please describe it in detail)
Encryption routine ___ None (bah) ___ Xor loop
___ Other (please describe it in detail)
Describe what makes your infection routine unique.
_____________________________________________________________________________
_____________________________________________________________________________
Describe what makes your encryption routine unique.
_____________________________________________________________________________
_____________________________________________________________________________
Describe what means your virus uses, other than encryption, to keep itself
hidden.
_____________________________________________________________________________
_____________________________________________________________________________
What is the largest possible scan string for this virus? __bytes
What else sets this virus apart from other viruses?
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
------------------------------
Date: Fri Jun 11 15:02:53 BST 1993
From: internet!baltimore.ie!susan (Susan Barry )
Subject: File 2--Fortress: services de securite europeens a la loupe (rapport)
IMPLEMENTING AND PROVING SECURITY
SERVICES FOR THE RARE/COSINE COMMUNITY
Susan Barry, Patricia McQuillan, Michael Purser (Baltimore Technologies)
Jonathan Moffett (University of York)
Summary
+-------
Secure electronic-mail (PEM) and remote access services, using asymmetric key
cryptology have been developed and proved between several European countries.
The paper reviews the project; high-lighting its shortcomings, since they are
most likely to be of interest to others.
1. Introduction
+---------------
The COSINE Sub-Project P8 (now named "FORTRESS") provides secure electronic
mail (Privacy Enhanced Mail: PEM) and remote access services over wide area
networks using asymmetric key cryptology. Its main features have been
described in a previous paper [1]. The design phase lasted from November
1991 until April 1992, and the implementation terminated in October 1992 with
the installation of systems in six different countries. From then until March
1993 evaluation and proving of FORTRESS took place. This paper is largely
concerned with this latter aspect of the project. However it is necessary
to review the security services provided if the evaluation and proving are
to be meaningful. This is done in Section 2. Sections 3 and 4 are critiques
of the user-related and technical aspects of the services. Section 5
describes the proving process, and Section 6 contains brief conclusions.
2. Security Service Overview
+----------------------------
The principal service provided is secure messaging based on PEM [2]. Messages
may be guaranteed as uncorrupted and authentic by attaching the sender's
digital signature to them; and may be made confidential by means of encryption
Public key cryptology (RSA) [3], is used for the signatures, and for
encrypting the symmetric keys which in turn are used to encrypt (DES) [4] the
message text. The use of public key cryptology implies the existence of a
Certification Authority (CA) which issues certificates guaranteeing the
genuineness of users' public keys. (This is done by having the CA sign the
user's identity and public key as a unit.) In FORTRESS the CA, operated in
Dublin by Baltimore Technologies, generates as well as certifies users' secret
and public keys. New users are "recommended" by existing privileged users,
called Security Administrators (SAs); and this is an on-line operation between
the SA's system and the CA over the network.This remote access to the CA is
made secure by means of a two-way X.509 strong authentication procedure [5],
in which both parties are reciprocally and securely identified, and during
which a symmetric key is established for stream-encrypting the subsequent
dialogue using BSA [6].
In response to a successful Recommendation an SA receives, on behalf of the
new user, a Personal Secure Environment (PSE) encrypted under a DES-key,
derived from a PIN via a one-way function. This PIN is itself submitted,
encrypted, in the prior recommendation. The PSE contains the user's secret
key and other cryptographic data. It is a local matter as to how this PSE
is made available to the new user; but once this has been achieved he can
perform signing/authentication and encryption/decryption operations for PEM
and access to the CA, by entering his PIN as required to decrypt (momentarily)
his PSE. Naturally, the user's first operation on receiving a PSE will be to
change the PIN.
In addition to Recommendations, other applications at the CA are made
available to users over the secure remote access service. For example an
SA can revoke a certificate, and place it on a blacklist. Revocation would
apply if the certificate owner's secret key were compromised, or if the owner
were no longer considered to be a suitable system user. Ordinary users can
also access the blacklist on the CA, to read it or download it. They can
download the database of valid certificates. They can access news files, etc.
To enable the CA to distinguish between ordinary and privileged users (SAs),
a user's authorisation attributes are built into his certificate. (Other
information, such as an expiry date for its validity, is also built into
the certificate.) The CA uses these attributes to control Recommendations,
Revocations and other operations according to an authorisation hierarchy.
For example, only certain SAs can recommend new SAs at a new user site; SAs
in one country cannot revoke users in another; etc. However, the presence
of authorisation attributes in the certificate (a necessary concept borrowed
from architectures such as Kerberos [7], SESAME [8]) causes a FORTRESS
certificate to be non-standard; i.e. not compatible with X.509. (There are
plans to overcome this problem, maintaining FORTRESS certificates for use
with remote access, but introducing further X.509-compatible certificates
to allow interworking with other systems, e.g. the "Password" project.)
At the CA, there are additional functions which enable operators to generate
RSA keys, maintain the database and blacklist, etc. The access of operators
to the CA is controlled by smart cards.
GMD's SecuDe software was used as a basis for the development of the FORTRESS
software; in particular, it provided the PEM implementation, and the basic
functions of the CA. The associated user interfaces, and the software for
the secure remote access service, have been developed by Baltimore. The
systems run under Unix, on SUN workstations. The remote access to the CA
is over X.25 networks.
3. User Aspects - a Critique
+----------------------------
Most of the problems in FORTRESS were related to the user's view and
management of the system.
The CA and its applications, accessed over the secure remote access
service, are merely security infrastructure for PEM - or possibly some
other future applications such as secure file transfer or EDI. PEM
itself consists of two services : PEM-SCAN (process a received PEM
message) and PEM-CREATE (or generate a PEM message). These services
are essentially file-to-file processors, which perform the
cryptographic functions on the way. They are simple to use, but they
are not integrated with any mail or messaging system; principally
because too many differing mail systems, not to mention user
interfaces, exist amongst project participants. Thus a sender of a
PEM message first creates the message; then processes it with PEM-
CREATE; and finally uses his normal mail service to transmit the file
created by PEM-CREATE. This is inconvenient for users.
The security of the users' end-systems, although it is explicitly
outside the scope of the project, is however another practical
limitation on the service. Apart from the user's encrypted PSE there
is no other project-provided protection of his system. In particular,
the genuineness of the code is not checked; and there is no secure
control of local access (e.g. over a LAN) to the system. Thus a user,
accessing his PEM facility, very probably not only sends his User-ID
and Password in clear over the LAN, but also his PSE PIN. In reality,
if a user is to have confidence in the security of the PEM service, it
is necessary to install the end-system in a secure environment with
controlled access.
The decision to provide on-line access to the CA is, we believe, a
good one. But it does imply that the CA should provide fast response.
This is not always the case. Firstly, Recommendations give rise to
the generation of RSA keys, which may take many seconds - indeed more
than a minute. A solution to this can be provided by pre-generating
keys, and holding them in a cache; although there are obvious security
risks with this approach. Secondly, on-line user operations involving
the data base of extant and the blacklist of withdrawn certificates
can become quite lengthy if those lists are large. Some conventional
data processing techniques (sorting!) could have been profitably used
in the FORTRESS project.
The project was conceived with a single CA serving many participants
in many countries, with secure remote access to the CA over X.25.
This gives rise to many anomalies. For example, whereas participants
may have local security policies, procedures, etc.; the CA (whose
security is fundamental to all services) is not subject to those
policies. There are questions about who is responsible for handling
security incidents. There must be questions in participants' minds as
to how secure the CA really is. In reality, serious users wish to
operate their own CAs for secure internal traffic. Insofar as they
require secure external communication services; these could be
supported by the cross-certification of CAs' public keys, enabling a
user in one domain to authenticate a certificate belonging to a user
in another domain. In turn, if CAs are to be local, it might be wiser
to have them operate over a LAN-compatible infrastructure; rather than
over X.25.
In an operational environment, as opposed to FORTRESS' pilot service,
another problem affecting users is that of out-of-date certificates.
In principle, a signature to a document is valid long after the
signatory's secret key has expired; provided that the signature was
generated before the expiry. Similarly, a message may be held in
encrypted form for years; and the public key required to decrypt it
should be still available, even if withdrawn. There is a need for a
service making available out-of-date certificates to users, which has
not yet been addressed in FORTRESS.
The above problem areas:
* PEM integration with the user's preferred mail environment
* Security of users' end-systems
* On-line response delays at the CA
* In-house versus external CA
* The availability of expired certificates to users have been identified
by FORTRESS participants and the development team as important.
* Designers and implementors of future systems are advised to take note!
4. Technical Aspects - the Critique continues
+---------------------------------------------
The FORTRESS security services work well and effectively; but there is
room for improvement. In addition to aspects visible to users (see
above) there are technical problem areas visible only to system
operators, or to software specialists. Some of these are now
presented.
The initialisation of security systems usually requires manual or
semi-manual procedures for the distribution of first-time keys, PINs,
etc. Similarly, reinitialisation (e.g. when a key is compromised)
will need such procedures, which may indeed be more demanding. As an
example, if a new public key for the CA is to replace the old one,
because the old one can no longer be trusted, a major logistical
problem arises since all systems are affected. The solutions to these
initialisation problems need to be carefully specified in advance; not
left to improvisation - possibly in panic conditions.
The identification of persons and systems is always more complex than
it appears. FORTRESS' SecuDe uses serial numbers as unique
identifiers of certificates (per issuer). But users refer to a
certificate by its owner's name. The maintenance of an unambiguous
mapping between the two - given that users may have more than one
certificate, may be Revoked and re-Recommended several times, etc. -
is a potential source of problems. Additionally, how is the integrity
of this serial number within the certificate-generating software
assured? What form should the user's name take? The PEM naming
hierarchy does not recognize the existence of a supranational entity
such as COSINE (or Europe) within which countries in the X.500 name
structure can fit.
The FORTRESS CA is operated by the system developers, Baltimore
Technologies. It is a PIN-controlled application accessed from Unix.
This situation should be reversed. The CA should be the main program
(PIN-controlled), with escape to Unix barred unless the operator has
special privileges. The PIN itself is read from a chipcard; but it is
inserted into the chipcard "manually". A rigorous separation between
development and operation is required, in which developers hand over
to operators a system (such as the CA) in which the operator's
capabilities are defined and restricted. If this hand-over is
accepted the operator can thereafter be made accountable for the
system.
Users of the systems must trust the correctness of the software. But
in security systems there is ample scope for "spoof" software, which
(for example) states that a signature has been authenticated
correctly, a remote system has been identified securely, or a text has
been encrypted, when it has not. Spoof software can trick users into
entering their PINs to it, or into performing one operation
(Revocation) when another was intended (Recommendation). Really
secure software should be encrypted on file, integrity checked on
loading; backups should be integrity checked and encrypted; etc.
FORTRESS does not do this.
However cryptographic keys are secured (e.g. in encrypted PSEs for
FORTRESS) there comes a time when they are used. Unless this is done
within tamper-proof hardware, they are then at risk. Spoof software
(see above) can capture them. More prosaically, a user may simply be
diverted half-way through such a critical operation - and the key left
unprotected in memory. Timeouts and memory clean-up facilities are
needed to minimise such risks. FORTRESS employs timeouts, but
systematic clearing of memory after use is not practised.
In addition to the above problem areas there are several other
vulnerabilities in the FORTRESS implementation; ranging from basic
design problems (such as one PSE for the CA and all its operators) to
omissions (such as proper purging facilities, e.g. for the certificate
blacklist) due to time constraints and the limited scale of the
project. Most of these however are minor, and too particular to
FORTRESS to be worth detailing here.
5. The Proving Process
+----------------------
The points made in Sections 3 and 4 were identified in the course of
the proving phase.
The COSINE Project Management Unit, right from the start, regarded the
testing out of the effectiveness of the security services as an
integral part of the sub-project, and planned and budgeted for this
activity. Budgetary constraints meant that it was not possible to
bring in independent consultants to perform the entire proving
activity, because of the amount of time which would have been required
for familiarisation with the details of the system. Therefore the
paper analysis and computer-based attacks were undertaken by the
developers, supervised by an independent consultant.
A systematic proving process was planned, involving both developers
and users. It included :
* The regular exchange of valid and deliberately corrupted PEM messages
between participants over a 3-month period, according to a weekly
rota. These exchanges revealed much about "user-friendliness", and
the reliability of the underlying E-Mail carriers, but no occurrences
of security failures were recorded.
* A "paper" analysis of the systems implemented which culminated in a
list of some 60 potential vulnerabilities; the most important of which
have been discussed above.
* Computer-based attacks on the security mechanisms, using defective
certificates, weak keys, etc. These attacks were designed to probe
further the vulnerabilities identified in the paper analysis. In
reality, while interesting, they yielded no unforeseen results.
Conclusion
+----------
The proving activity in the project has served two useful purposes:
It has ensured that a number of weaknesses, already known to the
developers, were collected and evaluated in a methodical fashion.
The systematic computer-based attacks, which revealed no further
significant weaknesses, gave additional confidence in the robustness
of the system.
The FORTRESS systems and software have proved to be remarkably secure
and robust, given their pilot nature. The weaknesses which have been
identified are almost all in the "environment" - the physical, network
and organisational surroundings within which FORTRESS operates.
Genuine FORTRESS problems are few, and essentially easily remedied.
More generally, in retrospect it is clear that although the
international "community" approach was necessary under COSINE, the
requirement for operational security services is nearly always
internal - within an organisation. The FORTRESS products need to be
adapted to this requirement (e.g. a foolproof CA); and real user
groups (generally administrative not technical persons) need to be
prepared to receive them.
This is the intended next phase of the project.
Acknowledgements
+----------------
It is a pleasure to acknowledge the cooperation received by all
participants in the project; and the support and guidance given by the
COSINE Project Management Unit; in particular Maria Pallares.
References
+----------
1. "COSINE Sub-Project P8 : security services" Purser, Computer
Networks and ISDN Systems 25 (1992) 476-482, North Holland.
2. "Privacy Enhanced Mail" RFC 1113, 1114, 1115 Internet.
3. "A method for obtaining digital signatures and public key
cryptosystems" Rivest, Shamir and Adleman. Comm. ACM 21 (1978).
4. "Data Encryption Algorithm" ANSI X3.92.
5. "The Directory - Authentication Framework" CCITT (Blue Book)
Fascicle VIII.8 X.509 (1988).
6. "A fast stream encryptor" Purser, unpublished.
7. "The Kerberos Network Authentication Service Overview" MIT
Project Athena RFC, Draft 1, April 1989.
8. "SESAME (Secure European System for applications in a Multivendor
Environment) - An Introduction". (Bull/ICL/SNI) Tom Parker, ICL
Secure Systems.
------------------------------
End of Chaos Digest #1.52
************************************
