With the recent proliferation of virus encryption "engines," I was
  inspired to write my own.  In a few short weeks, I was able to construct one
  such routine which can hold its own.  A polymorphic encryption routine is
  nothing more than a complex code generator.  Writing such a routine, while
  not incredibly difficult, requires careful planning and perhaps more than a
  few false starts.

       The utility of true polymorphism is, by now, an accepted fact.
  Scanning for the majority of viruses is a trivial task, involving merely the
  identification of a specific pattern of bytes in executable files.  This
  approach is quick and may be used to detect nearly all known viruses.
  However, polymorphism throws a monkey wrench into the works.  polymorphic
  viruses encode each copy of the virus with a different decryption routine.
  Since (theoretically) no bytes remain constant in each generated decryption
  routine, virus detectors cannot rely on a simple pattern match to locate
  these viruses.  Instead, they are forced to use an algorithmic appproach
  susceptible to "false positives," misleading reports of the existence of the
  virus where it is not truly present.  Creating a reliable algorithm to
  detect the polymorphic routine takes far more effort than isolating a usable
  scan string.  Additionally, if a virus detector fails to find even one
  instance of the virus, then that single instance will remain undetected and
  spawn many more generations of the virus.  Survival, of course, is the
  ultimate goal of the virus.

       Before attempting to write a polymorphic routine, it is necessary to
  obtain a manual detailing the 80x86 instruction set.  Without bit-level
  manipulation of the opcodes, any polymorphic routine will be of limited
  scope.  The nice rigid structure of the 80x86 instruction set will be
  readily apparent after a simple perusal of the opcodes.  Exploitation of
  this structured instruction set allows for the compact code generation
  routines which lie at the heart of every significant polymorphic routine.

       After examining the structure of the opcodes, the basic organisation of
  the polymorphic routine should be laid out.  Here, an understanding of the
  basics behind such routines is required.  The traditional approach treats
  the decryption routine as a simple executable string, such as
  "BB1301B900022E8137123483C302E2F6."  A true (advanced) polymorphic routine,
  by contrast, views the decryption routine as a conceptual algorithm, such
  as, "Set up a 'pointer' register, that is, the register whose contents hold
  a pointer to the memory to be decrypted.  Set up a counter register.  Use
  the pointer register to decrypt one byte.  Update the pointer register.
  Decrement the count register, looping if it is not zero."  Two routines
  which fit this algorithm follow:

  Sample Encryption 1
  ------ ---------- -
       mov  bx,offset startencrypt   ; here, bx is the 'pointer' register
       mov  cx,viruslength / 2       ; and cx holds the # of iterations
  decrypt_loop:
       xor  word ptr [bx],12h        ; decrypt one word at a time
       inc  bx                       ; update the pointer register to
       inc  bx                       ; point to the next word
       loop decrypt_loop             ; and continue the decryption
  startencrypt:

  Sample Encryption 2
  ------ ---------- -
  start:
       mov  bx,viruslength           ; now bx holds the decryption length
       mov  bp,offset start          ; bp is the 'pointer' register
  decrypt_loop:
       add  byte ptr [bp+0Ch],33h    ; bp+0Ch -> memory location to be
                                     ; decrypted at each iteration
       inc  bp                       ; update the pointer register
       dec  bx                       ; and the count register
       jnz  decrypt_loop             ; loop if still more to decrypt

       The number of possibilities is essentially infinite.  Naturally,
  treating the decryption as an algorithm rather than as an executable string
  greatly increases the flexibility in creating the actual routine.  Various
  portions of the decryption algorithm may be tinkered with, allowing for
  further variations.  Using the example above, one possible variation is to
  swap the order of the setup of the registers, i.e.

       mov  cx,viruslength
       mov  bx,offset startencrypt

  in lieu of

       mov  bx,offset startencrypt
       mov  cx,viruslength

       It is up to the individual to decide upon the specific variations which
  should be included in the polymorphic routine.  Depending upon the nature of
  the variations and the structure of the polymorphic routine, each increase
  in power may be accompanied with only a minimal sacrifice in code length.
  The goal is for the routine to be capable of generating the greatest number
  of variations in the least amount of code.  It is therefore desirable to
  write the polymorphic routine in a manner such that additional variations
  may be easily accommodated.  Modularity is helpful in this respect, as the
  modest overhead is rapidly offset by substantial space savings.

       The first step most polymorphic routines undergo is the determination
  of the precise variation which is to be encoded.  For example, a polymorphic
  routine may decide that the decryption routine is to use word-length xor
  encryption with bx as the pointer register, dx as a container for the
  encryption value, and cx as the counter register.  Once this information is
  known, the routine should be able to calculate the initial value of each
  variable.  For example, if cx is the counter register for a byte-length
  encryption, then it should hold the virus length.  To increase variability,
  the length of the encryption can be increased by a small, random amount.
  Note that some variables, in particular the pointer register, may not be
  known before encoding the rest of the routine.  This detail is discussed
  below.

       Of course, selecting the variables and registers will not in and of
  itself yield a valid decryption routine; the polymorphic routine must also
  encode the actual instructions to perform the job!  The cheesiest
  polymorphic routines encode a single "mov" instruction for the assignment of
  a value to a register.  The more complex routines encode a series of
  instructions which are functionally equivalent to the simple three byte
  "mov" statement yet far different in form.  For example,

       mov  ax, 808h

  could be replaced with

       mov  ax, 303h                 ; ax = 303h
       mov  bx, 101h                 ; bx = 101h
       add  ax, bx                   ; ax = 404h
       shl  ax, 1                    ; ax = 808h

       Recall that the registers should be encoded in a random order.  The
  counter variable, for example, should not always be the first to be encoded.
  Predictability, the bane of polymorphic routines, must be avoided at all
  costs.

       After the registers are encoded, the actual decryption loop should then
  be encoded.  The loop can perform a number of actions, the most significant
  of which should be to manipulate the memory location, i.e. the actual
  decryption instruction, and to update the pointer register, if necessary.
  Finally, the loop instruction itself should be encoded.  This can take many
  forms, including "loop," "loopnz," "jnz," etc.  Possible variations include
  altering the decryption value register and the counter register during each
  iteration.

       This is the general pattern of encoding.  By placing garbling, or "do-
  nothing," instructions between the essential pieces of code, further
  variability may be ensured.  These instructions may take many forms.  If the
  encoding routines are well-designed, the garbler can take advantage of the
  pre-existing code to generate null instructions, such as assignments to
  unused registers.

       Once the decryption routine has been written, it is necessary to
  encrypt the virus code.  The traditional approach gives the polymorphic
  routine the job of encrypting the code.  The polymorphic routine should
  therefore "remember" how the precise variation used by the decryptor and
  adjust the encryption routine in a complementary fashion.  An alternate
  approach is for the polymorphic routine to simultaneously encode both the
  encryption and decryption routines.  Although it adds overhead to the code,
  it is an extremely flexible approach that easily accommodates variations
  which may be later introduced into the polymorphic routine.

       Variable-length decryptors come at a significant trade-off; the exact
  start of the decryption cannot be known before encoding the decryptor.
  There are two approaches to working around this limitation.  The first is to
  encode the pointer register in a single instruction, i.e. mov bx,185h and to
  patch the initial value once it is known.  This is simplistic, though
  undesirable, as it decreases the variability of the routine.  An alternate
  approach is to encode the encryption instruction in the form xor word ptr
  [bx+185h], cx (as in Sample encryption 2, above) instead of xor word ptr
  [bx], cx (as in Sample encryption 1).  This increases the flexibility of the
  routine, as the initial value of the pointer register need not be any fixed
  value; correct decryption may be assured by adjusting the offset in the
  decryption instruction.  It is then possible to encode the pointer register
  with multiple instructions, increasing flexibility.  However, using either
  method alone increases the predictability of the generated code.  A better
  approach would be to incorporate both methods into a single polymorphic
  routine and randomly selecting one during each run.

       As an example of a polymorphic routine, I present DAME, Dark Angel's
  Multiple Encryptor and a simple virus which utilises it.  They appear in the
  following article.  DAME uses a variety of powerful techniques to achieve
  full polymorphism.  Additionally, it is easy to enhance; both the encoding
  routines and the garblers can be extended algorithmically with minimal
  effort.  In the next issue, I will thoroughly comment and explain the
  various parts of DAME.