Reversing the report encryption algorithm for the flexlm license manager.
Explains the encryption algorithm used by vendor daemons for reporting.
flexlm
flexlm
21-oct-1999
by Nolan Blender
Courtesy of Reverser's page of reverse engineering
slightly edited
by reverser+
fra_00xx
980926
Nolan Blender
0100
FL
PC
Lately, many of my more industrious and investigative contributors have taken it upon themselves to supply documents which purport to explain and/or illuminate the most secret workings of the FlexLM schemes.
You may be able to read a very interesting example of team-reversing (about FlexLM) here

Nolan Blender writes in the following essay:
"In many ways, reversing code is a mental exercise of examining what is happening in the program, and asking "If I were writing this, what would I be trying to accomplish?"
It is indeed, and this very good essay will delight many readers.
There is a crack, a crack in everything That's how the light gets in
Rating
( )Beginner (X)Intermediate ( )Advanced ( )Expert

This document describes the reversing of the FlexLM reporting algorithm on the HP platform. Mostly this was done as an exercise in reverse engineering, and to determine exactly how the encryption of the reporting logfiles was done in Flexlm.
Reversing the report encryption algorithm for the flexlm license manager.
Explains the encryption algorithm used by vendor daemons for reporting.
Written by Nolan Blender


Introduction
This essay describes in some depth how the reversing of the Flexlm reporting log encryption algorithms was done. The reversing of one of the smaller modules is discussed in depth, as well as some of the techniques used to decode sections of the more difficult to understand programs. Methods to replicate the functionality of the reversed modules is discussed as well. The ability to decode the Flexlm logfiles isn't a generally useful thing, but the methods described can be used for reversing other programs.

Tools required
DDE for HP-UX
ANSI C compiler.

Target's URL/FTP
ftp.globes.com
Get the version 6.1 Flexlm SDK for HP, and use Pilgrim's or my techniques to unpack.

Program History
Flexlm generates logfiles which can be interpreted by special tools provided by Globetrotter, most notably Samreport, Samsuite and by the repapi product. The file is encrypted, supposedly to prevent tampering, but most likely to force end users to purchase their expensive license reporting tool rather than exporting the raw data into a spreadsheet and generating the reports there. The logfile is somewhat tamper resistant, and it is possible to determine if lines have been deleted from the log file. The capability to decode what is in the file directly isn't too useful since the Globetrotter reporting tools have been cracked, but it is satisfying to know what is going into the log.

Essay
Preliminaries.

Although this paper is mostly about reversing the algorithm that is used for encryption, I am including some assembly code sections as well. There are some fundamental differences between how the Intel and PA-RISC processors work, and how the calling sequences are implemented.

Firstly, the byte order is different on the machines. PA-RISC (and SPARC and MIPS) are big endian implementations. The most significant byte is stored at the starting address for these machines. On Intel and VAX machines, the least significant byte is stored at the starting address. This is important if you are debugging because when , say, using Softice to debug on Intel machines, on memory operations you will see that 0x31323334 is equivalent to "4321" where it is "1234" on the big endian machines.

The subroutine calling sequence is significantly different as well. On the HP-UX implementation on PA-RISC, registers are used to pass arguments where possible. The calling sequence on Windows NT intel pushes the arguments onto the stack.

Outline of the process.
1) A daemon was built using the Flexlm Software Development Kit that had a complete symbol table and debug information.

2) A license file for that daemon was created that created a reportlog.

3) The debugger was run on the daemon, and the relevant calls to the logging routines determined. This was done by tracing the vendor daemon while a checkout was done.

4) The relevant object file was extracted from the Globetrotter supplied library, and a program that emulated the calls to the relevant routines was created. The output of the program was checked against the logfile to make sure that the same behaviour was exhibited by the test program as the actual daemon.

5) The program was analysed and a program that emulated the behaviour of the test program was written.

Step 1.
This process was described in an earlier essay, but the essential part is to change the makefile so that everything is compiled with the -g option, and the rules which strip the resulting executable are removed. It is possible to debug a completely stripped executable, but life is difficult enough as it is, so any advantages which can be gained should be used.

Step 2. A license file that contains options for creating a logfile was generated. Here is the relevant line from that file:

VENDOR blenderd blenderd blenderd.opts

blenderd.opts contains

REPORTLOG +myreport

All this is documented in the Flexlm end user manual, and there is nothing difficult or undocumented with this step.

Step 3.
The executable was run, and calls to ls_append_repfile and ls_lf were called with information, then the calls to the file system write were called with data somewhere in those routines. These calls were carefully examined and the interesting routines determined. The tactic of writing an intercepting routine that passes arguments through and logs them was used to get typical input into ls_lf.

Step 4.
The relevant object was then extracted from liblmgr_s.a. This object is ls_filter.o - this contains the encrypting code that converts the plaintext into the form that is written to the file. The next step was to build an executable that would reproduce the function in question. The unix nm command was used to figure out the missing globals or functions, then the vendor daemon operation was examined to extract the relevant values from the executable.

Makefile contains

CFLAGS= -Ae -g

all: democrack

democrack.o: democrack.c
cc $(CFLAGS) democrack.c -c

democrack: democrack.o lsfilter.o
cc $(CFLAGS) democrack.o lsfilter.o -o democrack
Program initially contains

#include<stdio.h>
main(int argc, char *argv[])
{
}



% make
        cc -Ae -g democrack.c -c
        cc -Ae -g   democrack.o lsfilter.o  -o
democrack
/usr/ccs/bin/ld: Unsatisfied symbols:
   ls_replog_ascii (data)
   ls_log_repfile_client (data)
   ls_replog_clear (data)
   ls_log_repfile (data)
   ls_client_send_str (code)
*** Error exit code 1
At this point, we know that there are a whole bunch of variables and subroutines messing. My tactic here is to start debugging the license server, and to declare the missing variables as initialized data space.

The working vendor daemon must now be examined, and the process of determining what variables are getting passed in. The value of the missing variables can be determined too.

Since we know the argument list that is passed to the vendor daemons by using ps -ef, we call the vendor daemon directly. DDE allows you to attach to an existing process, however it's much simpler to call the program directly.

% /opt/langtools/bin/dde ./blenderd -T myhost 6.1 3 -c test.dat

Once dde has started, we want to put a breakpoint at ls_lf, the routine which writes to the logfile.

dde> break ls_lf
Breakpoint(s) set.

Then go is typed to run the program.

At this point, I should note that it may not be possible to set a breakpoint at some locations because the scope of the procedure is not visible to main. You can get the address to break on by using the nm command. nm -x gives values which are suitable for dde. The syntax is break `va(address) where address is the hex address of the item of interest.

After the program hits the breakpoint, we look at the call in the preceeding stack frame (by opening the stack view, then clicking on the frame of interest, in this case, ls_append_repfile+0c8. Since we don't have the source, we must look at the assembly code at this point:
0000EA68  ls_append_repfile+00ac  STB         %r0,R'590(%r1)
0000EA6C  ls_append_repfile+00b0  ADDIL       L'0,%r27
0000EA70  ls_append_repfile+00b4  STW         %r0,R'3c(%r1)
0000EA74  ls_append_repfile+00b8  ADDIL       L'2800,%r27
0000EA78  ls_append_repfile+00bc  STW         %r0,R'528(%r1)
0000EA7C  ls_append_repfile+00c0  LDW         -420(%r30),%r26
0000EA80  ls_append_repfile+00c4  LDIL        L'6800,%r31
0000EA84  ls_append_repfile+00c8  BLE         R'3e8(%sr4,%r31)
0000EA88  ls_append_repfile+00cc  COPY        %r31,%r2
The program counter is at EA84 at this frame. I looked at the preceeding frames, and I saw that only r26 was set, and that r25 was not set.
Since the arguments under HPUX are passed with registers, I conclude that ls_lf takes one argument. So far so good. But what is that argument?
At this point, we are at the entry of ls_lf so the register should still contain the first argument.
dde> regs
r0: 00000000  00000000  0000EA8F
r3 : 7B03AE50  00000007  7B03A51C  7B03A53C
r7 : 7B03A650  7B03A650  00000020  0000001F
r11: 00000000  00000000  08000084  08000084
r15: 00000000  00000001  4001CAF0  00000008
r19: 00000040  00000040  00000108  00004000
r23: 00000141  008C0A00  00000000  7B03F118
r27: 4000A610  00000040  00000008  7B03F550
r31: 0000EA8F
pc: 0000EA8C  ipsw: 0006000F  sar: 00000018
So r26 contains 7b03f118 - a pointer to some sort of automatic variable on the stack. Let's check it out:
dde> dump -from 7b03f118 -to 7b03f128
7B03F118: 2320464C  45586C6D  20526570  6F727420
7B03F128: 4C6F672C
That sure looks like character data to me!
dde> dump -from 7b03f118 -to 7b03f128 -char
7B03F118: "# FL"  "EXlm"  " Rep"  "ort "
7B03F128: "Log,"
After checking a few more runs, it was clear that the input was a null terminated string. So now we can start to fill in the blanks in our "main" program.
#include<stdio.h>
int ls_replog_ascii[6];
int ls_log_repfile_client;
int ls_replog_clear[6];
FILE *ls_log_repfile;
extern char *cksumbuf;  /* declared in lsfilter.o */
extern long buflen;
extern char *curra;

main(int argc, char *argv[])
{
    ls_lf("# FLEXlm Report Log, 8 September, 1999,
\"blenderd\" on
\"myhost\"\n"
);
    ls_lf("5 2 2\n");
    ls_lf("7 1 78261234\n");
    ls_lf("23 1\n");
    ls_lf("7 2 hp700_u9\n");
    ls_lf("27 2\n");
    ls_lf("7 3 /usr/tmp/lockblenderd\n");
}
int ls_client_send_str()
{
    printf ("ls_client_send_str called.\n");
}
Attempting to compile and run this program gets a segmentation fault. Some of these variables need to be initialized. The next step is to run DDE against our program. The program will fault, and we can then look at the assembly listing to see what made it fault.

Break at: \\democrack\main\12
dde> go
Intercepted event type 'signal SIGSEGV'.
Signal received by process 11165: (SIGSEGV).
Stopped at: ls_lf+0254 (000038B8)

Assembly shows us:

000038A0  ls_lf+023c  LDW             -1044(%r30),%r31
000038A4  ls_lf+0240  LDO             R'1(%r31),%r19
000038A8  ls_lf+0244  STW             %r19,-1044(%r30)
000038AC  ls_lf+0248  ADDIL           L'5800,%r27
000038B0  ls_lf+024c  LDO             R'2f8(%r1),%r20
000038B4  ls_lf+0250  LDW             -1044(%r30),%r21
000038B8  ls_lf+0254  LDWX,S          %r21(%r20),%r22
000038BC  ls_lf+0258  COMIBF,=        -1,%r22,ls_lf+021c

regs are

r0: 00000000  40006B10  0000386B
r3 : 7B03A000  00000001  7B03A4F0  7B03A4F8
r7 : 7B03A608  7B03A608  00000020  0000001F
r11: 00000000  00000000  08000084  08000084
r15: 00000000  00000001  4001CA20  00000002
r19: 0000007E  40006E08  0000007E  00000000
r23: 00000000  00000001  4000117A  00000000
r27: 40001310  00000001  00000001  7B03AAC8
r31: 0000007D
pc: 000038B8  ipsw: 0006000F  sar: 00000008

Looking at r20 we see

dde> dump -from 40006e08
40006E08: 00000000
So it sure looks like an attempt to reference through a bad pointer. In this case we're lucky - this is one of the variables which we're looking for. To get the values, we need to look at the vendor daemon again though.
% nm -x democrack | grep 6e08
ls_replog_ascii     |0x40006e08|extern|data   |$BSS$
0.1u 0.0s 0:00 11%
The offset was at ls_lf+254, so now we must hunt down that offset in the vendor daemon.
In our daemon directory:


% nm -x blenderd | grep ls_lf
ls_lf               |0x00005db0|extern|code   |$CODE$
So we can figure out the equivalent offset in that routine in the vendor daemon. It's the base address+offset.
dde> print 0x5db0+0x254 -hex
0x0000000000006004
We start up the vendor daemon again, and set a breakpoint at that offset.
% /opt/langtools/bin/dde ./blenderd -T myhost 6.1 3 -c test.dat

Once dde has started

dde> break `va(6004)
Breakpoint(s) set.
dde> go
Break at: ls_lf+0254 (00006004)
We look at the assembly code again, and we can see that we're in the same spot in ls_lf, just that everything should be OK at this point.
dde> regs
r0: 00000000  40005E10  00005FB7
r3 : 7B03AE50  00000007  7B03A51C  7B03A53C
r7 : 7B03A650  7B03A650  00000020  0000001F
r11: 00000000  00000000  08000084  08000084
r15: 00000000  00000001  4001CAF0  00000008
r19: 00000001  40006320  00000001  00000006
r23: 00000000  00000001  400010C2  00000000
r27: 4000A610  00000001  00000001  7B03F9D0
r31: 00000000
pc: 00006004  ipsw: 0004000F  sar: 0000001B

Looking at that location, it appears as though this is
a pointer to an array of ints.

dde> dump -from 40006320 -to 40006340
40006320: 00000006  00000007  00000019  00000025
40006330: 00000026  FFFFFFFF  0A000000  0A000000
40006340: 0A000000

After tracing the program a bit, we find that it uses -1 as a terminator,
thus we can initialize the array and quit at the first -1.

    ls_replog_ascii[0] = 0x00000006;
    ls_replog_ascii[1] = 0x00000007;
    ls_replog_ascii[2] = 0x00000019;
    ls_replog_ascii[3] = 0x00000025;
    ls_replog_ascii[4] = 0x00000026;
    ls_replog_ascii[5] = 0xffffffff;

After a few hours of digging around looking at variables, the following
program can be had:

#include<stdio.h>
int ls_replog_ascii[6];
int ls_log_repfile_client;
int ls_replog_clear[6];
FILE *ls_log_repfile;
extern char *cksumbuf;  /* declared in lsfilter.o */
extern long buflen;
extern char *curra;

main(int argc, char *argv[])
{
    char instr[1024];
    char outstr[1024];
    char instr1[1024];
    int num;
    int i;
    unsigned long testval;
    unsigned long *fooval;
    FILE *fp;
    char ibuf[1024];

    ls_log_repfile = stdout;
    ls_replog_ascii[0] = 0x00000006;
    ls_replog_ascii[1] = 0x00000007;
    ls_replog_ascii[2] = 0x00000019;
    ls_replog_ascii[3] = 0x00000025;
    ls_replog_ascii[4] = 0x00000026;
    ls_replog_ascii[5] = 0xffffffff;
    ls_replog_clear[0] = 0x00000005;
    ls_replog_clear[1] = 0xffffffff;
    ls_replog_clear[2] = 0x000001f3;
    ls_replog_clear[3] = 0x00000000;
    ls_replog_clear[4] = 0x00000000;
    ls_replog_clear[5] = 0x00000000;
    ls_log_repfile_client = 0;

    curra = (char *) malloc(1024 * sizeof(char));
    cksumbuf = (char *) malloc(1024 * sizeof(char));
    buflen = 0;
    strcpy(curra, "");

    ls_lf("# FLEXlm Report Log, 8 September, 1999,
\"blenderd\" on
\"myhost\"\n"
);
    ls_lf("5 2 2\n");
    ls_lf("7 1 78261234\n");
    ls_lf("23 1\n");
    ls_lf("7 2 hp700_u9\n");
    ls_lf("27 2\n");
    ls_lf("7 3 /usr/tmp/lockblenderd\n");

    ls_flush_replog(stdout);
    fflush(stdout);
}
int ls_client_send_str()
{
    printf ("ls_client_send_str called.\n");
    return(0);
}
Now we are ready to start reversing the actual code. The procedure that I use to reverse engineer programs is to build a program that duplicates the behaviour of the target routines.
The procedure is to step through the target routines one instruction at a time, at the same time building a program which generates the same data.
By examining the code which is produced, an idea of what the original programmer was trying to accomplish can be extracted. In many ways, reversing code is a mental exercise of examining what is happening in the program, and asking "If I were writing this, what would I be trying to accomplish?"

If you watch the data being fed into ls_lf (via the code interception trick) you can see how the programmers of this algorithm save space. Since there is a lot of information associated with an event (checkout, checkin, etc) that is common, a set of values associated with a user or machine or hosttype or whatever is assigned a code, and the individual events are a combination of these codes. It is a clever idea, and will prevent the log files from growing at an excessive rate.
However, I digress from the task at hand.

The first complete routine that is of value to examine is incr_currc. This routine is called many times, and is very straightforward. What is is trying to accomplish?

As it turns out, in general, the first character of each line indicates what kind of data the line contains. The sequence Copyright_1988-1996_Globetrotter_Software_Inc_ will be repeated over and over in the first character position, If there is a change of encryption mode, the sequence will skip 3 positions instead of one.

Here is the assembly code: 0000438C incr_currc ADDIL L'0,%r27 ;load the base reg into r1 00004390 incr_currc+0004 LDW R'4(%r1),%r22 ;grab the value at base+4 00004394 incr_currc+0008 COMIBF,=,N 0,%r22,incr_currc+0020 ;not null, go to 20 otherwise 00004398 incr_currc+000c ADDIL L'fffff800,%r27 ; 0000439C incr_currc+0010 LDW R'758(%r1),%r31 ;set the pointer to a string 000043A0 incr_currc+0014 ADDIL L'0,%r27 000043A4 incr_currc+0018 B incr_currc+005c ;and bail out, but first 000043A8 incr_currc+001c STW %r31,R'4(%r1) ;save the value at base+4 000043AC incr_currc+0020 ADDIL L'0,%r27 ;Come here if not null 000043B0 incr_currc+0024 LDW R'4(%r1),%r19 ;Increment the pointer 000043B4 incr_currc+0028 LDO R'1(%r19),%r20 000043B8 incr_currc+002c ADDIL L'0,%r27 000043BC incr_currc+0030 STW %r20,R'4(%r1) 000043C0 incr_currc+0034 ADDIL L'0,%r27 000043C4 incr_currc+0038 LDW R'4(%r1),%r21 000043C8 incr_currc+003c ADDIL L'fffff800,%r27 000043CC incr_currc+0040 LDW R'758(%r1),%r22 000043D0 incr_currc+0044 LDO R'2e(%r22),%r1 000043D4 incr_currc+0048 COMBT,<<,N %r21,%r1,incr_currc+005c ;If it's more than 0x2e past start 000043D8 incr_currc+004c ADDIL L'fffff800,%r27 000043DC incr_currc+0050 LDW R'758(%r1),%r31; set the pointer back to the start 000043E0 incr_currc+0054 ADDIL L'0,%r27 000043E4 incr_currc+0058 STW %r31,R'4(%r1) 000043E8 incr_currc+005c BV %r0(%r2); go back from whence we came 000043EC incr_currc+0060 NOP Because of the pipelined behaviour of the PA-RISC processor, instructions are only sort of executed sequentially. The ,N modifier means DON'T execute the following instruction. The branch instruction at incr_currc+0018 and the store to memory instruction at at incr_currc+001c are both executed during the branch, but they only take one operation. If you'll notice, all the instructions are the same size.

We can dig out the value at fffff800+758 by examining r31 at incr_currc+10. The value is 40001191, so now
dde> dump -from 40001191 -to 400011c1 -char
40001191: "Copy"  "righ"  "t_19"  "88-1"
400011A1: "996_"  "Glob"  "etro"  "tter"
400011B1: "_Sof"  "twar"  "e_In"  "c_\0\377"
400011C1: "\377\377\377\377"
Which gives us the string that it's stepping through.

chptr is set to this string, and it goes over and over the same pattern.
char
*cstring="Copyright_1988-1996_Globetrotter_Software_Inc_";


int incr_currc(void) {
    if (chptr == 0) { /* incr_currc+8 */
        chptr = cstring;
    }
    else {
        chptr++;
        if (chptr >= cstring+46) {
            chptr = cstring;
        }
    }
    return(0);
}
Now after understanding a little bit about how the incr_currc works, we can turn our attention to the encryption algorithm itself. There are a couple of things which seem relatively straightforward in C, but become somewhat warped and more difficult to understand after compilation.

The designers of the this processor implemented a shift and add instruction. A lot of the time, programmers are multiplying by some integer constant, and rather than chew up a lot of cycles doing a multiply, the shift/add instructions can be combined for faster execution. For example, a multiplication by 5 could be expressed as a multiplication by 4 and an addition.
So in the case of 3 * 5, it could be expressed as 3+(3*4). The processor could implement this by left shifting 3 2 bits, then adding 3. So 0x00000011<<2 is 0x00001100. add 0x00000011 and you get 0x00001111, or 15 (0x0e) which is 3*5. I explain this so that if the software designers use multiplication, you can figure out what is going on.

Some of the subroutines, such as remI (remainder) are used to implement modulo (%) operations. It is better to examine the output of these routines as they are used to implement standard operations, and are twisted hand tuned assembler.
Look at this if you want to see the ingenuity of the designers of this machine. If you see $$ calls, they are usuall implementations of standard operations.

Here is an example of this from the "filter" routine. The "shift and add" technique is used to combine the values.
4028  filter+0254  ADDIL           L'fffff800,%r27
402C  filter+0258  LDW             R'694(%r1),%r21
4030  filter+025c  SH2ADD          %r21,%r21,%r22
4034  filter+0260  SH2ADD          %r22,%r0,%r31
4038  filter+0264  ADDIL           L'fffff800,%r27
403C  filter+0268  LDW             R'690(%r1),%r19
4040  filter+026c  ADD             %r19,%r31,%r20
4044  filter+0270  ADDIL           L'fffff800,%r27
4048  filter+0274  LDW             R'698(%r1),%r21
404C  filter+0278  SH2ADD          %r21,%r21,%r22
4050  filter+027c  SH2ADD          %r22,%r0,%r1
4054  filter+0280  SH2ADD          %r1,%r1,%r31
4058  filter+0284  SH2ADD          %r31,%r0,%r19
405C  filter+0288  ADD             %r20,%r19,%r20
4060  filter+028c  STW             %r20,-52(%r30)
This actually translates to code similar to what follows.
/* This is globally declared. */
int sv_690[4]; /* local storage for filter() */
/* Local variable */
int fv_d;        /* offset -52 */
fv_d = sv_690[0] + sv_690[1] * 20 + sv_690[2] * 400;
Algorithm reversing.
If you examine the algorithm which is used to select the character to be printed in nb_encrypt (a version of ascii_filter) you can see that they use an index into the string pointed to by curra, which in turn is incremented each time the algorithm is run. We need an inverse of this function, but this function is not available in the report generation code that we are inspecting. The algorithm itself must be reversed. An inverse mapping array is built by mapping the index into the location pointed to by the value in initvals. This may not be too clear, so I have included the code which generates the inverse decoding array.
#include<stdio.h>
#include<string.h>

char initvals[] =
{
 0x37, 0x5a, 0x4b, 0x43, 0x71, 0x36,
 0x34, 0x29, 0x5e, 0x21, 0x79, 0x45,
 0x5b, 0x7a, 0x5f, 0x7d, 0x59, 0x2b,
 0x46, 0x66, 0x53, 0x42, 0x3a, 0x3b,
 0x76, 0x3d, 0x25, 0x74, 0x58, 0x63,
 0x38, 0x26, 0x35, 0x73, 0x40, 0x28,
 0x27, 0x47, 0x23, 0x61, 0x6d, 0x41,
 0x7c, 0x68, 0x4d, 0x44, 0x7e, 0x52,
 0x51, 0x39, 0x55, 0x67, 0x30, 0x4e,
 0x69, 0x4a, 0x3f, 0x24, 0x5d, 0x4c,
 0x56, 0x6f, 0x50, 0x60, 0x6e, 0x2d,
 0x5c, 0x72, 0x2a, 0x49, 0x75, 0x31,
 0x3e, 0x6b, 0x70, 0x7b, 0x22, 0x2f,
 0x64, 0x33, 0x54, 0x78, 0x3c, 0x4f,
 0x48, 0x2e, 0x20, 0x6c, 0x2c, 0x62,
 0x32, 0x57, 0x77, 0x65, 0x6a, 0x00
};

int main()
{
 char *p;
 char outarr[256]; /* 256 different mappings */
 int i;
 int outindex; /* index into output table */
 int numindex = 0; /* the current index into initvals
table */

 /*
  * "unused" values will have 255 for easier fault
  * detection.
  */
 for (i = 0; i < 256; i++)
 {
  outarr[i] = 0xff;
 }

 /*
  * step through the initvals array, and load
  * the location in outarr with the index into
  * the initvals array.
  */

 for (p = initvals; *p != '\0'; p++)
 {
  outindex = (int) *p & 0xff;
  outarr[outindex] = numindex;
  numindex++;
 }

 /* load the last value anyway, probably not used */
 outindex = (int) *p & 0xff;
 outarr[outindex] = numindex;

 /* dump the array to stdout */
 printf ("char * decodetable[] = {");
 for (i = 0; i < 256; i++)
 {
  if (i%8==0)
  {
   printf ("\n     ");
  }
  printf ("%02x", outarr[i] & 0xff);
  if (i != 255)
  {
   printf (", ");
  }
 }
 printf ("\n};\n");
}
Here we have the completed encrypt/decrypt (for ascii_filter). Sections which should be examined in depth are the inner for loops in the encrypt and decrypt sections.
#include<stdio.h>
#include<string.h>
/* Now internal */

char tstbuf[1024]; /* 40002978? */
char *curra; /* In other at 40006ee4 */
int curoffset; /* current offset value */

/*
 * This is the encryption values array
 */
char initvals[] =
{
 0x37, 0x5a, 0x4b, 0x43, 0x71, 0x36,
 0x34, 0x29, 0x5e, 0x21, 0x79, 0x45,
 0x5b, 0x7a, 0x5f, 0x7d, 0x59, 0x2b,
 0x46, 0x66, 0x53, 0x42, 0x3a, 0x3b,
 0x76, 0x3d, 0x25, 0x74, 0x58, 0x63,
 0x38, 0x26, 0x35, 0x73, 0x40, 0x28,
 0x27, 0x47, 0x23, 0x61, 0x6d, 0x41,
 0x7c, 0x68, 0x4d, 0x44, 0x7e, 0x52,
 0x51, 0x39, 0x55, 0x67, 0x30, 0x4e,
 0x69, 0x4a, 0x3f, 0x24, 0x5d, 0x4c,
 0x56, 0x6f, 0x50, 0x60, 0x6e, 0x2d,
 0x5c, 0x72, 0x2a, 0x49, 0x75, 0x31,
 0x3e, 0x6b, 0x70, 0x7b, 0x22, 0x2f,
 0x64, 0x33, 0x54, 0x78, 0x3c, 0x4f,
 0x48, 0x2e, 0x20, 0x6c, 0x2c, 0x62,
 0x32, 0x57, 0x77, 0x65, 0x6a, 0x00};

/*
 * This is the decryption values array.  Invalid values
 * are set to ff
 */
char decodetable[] = {
 0x5f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0x56, 0x09, 0x4c, 0x26, 0x39, 0x1a, 0x1f, 0x24,
 0x23, 0x07, 0x44, 0x11, 0x58, 0x41, 0x55, 0x4d,
 0x34, 0x47, 0x5a, 0x4f, 0x06, 0x20, 0x05, 0x00,
 0x1e, 0x31, 0x16, 0x17, 0x52, 0x19, 0x48, 0x38,
 0x22, 0x29, 0x15, 0x03, 0x2d, 0x0b, 0x12, 0x25,
 0x54, 0x45, 0x37, 0x02, 0x3b, 0x2c, 0x35, 0x53,
 0x3e, 0x30, 0x2f, 0x14, 0x50, 0x32, 0x3c, 0x5b,
 0x1c, 0x10, 0x01, 0x0c, 0x42, 0x3a, 0x08, 0x0e,
 0x3f, 0x27, 0x59, 0x1d, 0x4e, 0x5d, 0x13, 0x33,
 0x2b, 0x36, 0x5e, 0x49, 0x57, 0x28, 0x40, 0x3d,
 0x4a, 0x04, 0x43, 0x21, 0x1b, 0x46, 0x18, 0x5c,
 0x51, 0x0a, 0x0d, 0x4b, 0x2a, 0x0f, 0x2e, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff};

/*
 * String containing array used for marking ascii_filter or filter
 */
char
*cstring="Copyright_1988-1996_Globetrotter_Software_Inc_";

unsigned long myedata; /* in other at 400013d8 */
/*
 * pointer to current value in cstring
 */
char *chptr; /* in other at 400013dc */

main()
{
 char mystr[1024];
 char encstr[1024];
 char decstr[1024];

 nb_crypt_init();
 mystr[0] = *chptr;
 /* Load in the test string. */
 strcpy(mystr+1,"The quick brown fox jumps over the lazy dog. 1234567890\n");
 printf ("orig  : %s\n", mystr);

 /* encrypt the string */
 nb_encrypt(mystr, encstr);
 printf ("encstr: %s\n", encstr);
 nb_crypt_init();
 nb_decrypt(encstr, decstr);
 printf ("decstr: %s\n", decstr);
}

/*----------------------------------------------------------*
 *
 * nb_crypt_init()
 *
 * initialize pointers and variables.  This is a consolidation
 * of various initializations.
 *----------------------------------------------------------*/
int nb_crypt_init()
{
 myedata = 0;
 chptr = 0;
 curra = 0;
 curoffset = 0;
 if (curra == 0)
 {
  curra = tstbuf;
 }
 if (tstbuf[0] == '\0')
 {
  strcpy(tstbuf, initvals);
  strcat(tstbuf, initvals);
 }
 if (chptr == 0)
 {
  chptr = cstring;
 }
 return(0);
}
/*----------------------------------------------------------*
 *
 * nb_encrypt

*----------------------------------------------------------*/
int nb_encrypt (char *instr, char *outstr)
{

 char *asval_a;    /* offset -100 */
 char *asval_b;    /* offset -56 */
 int asval_c;    /* offset -52 */

 asval_a = instr;
 /* Determine if we were in filter mode before */
 if (myedata != 1)
 { /* ascii_filter+0x78 */
  if (myedata != 0)
  { /* ascii_filter+0x94 */
   incr_currc();
  }
  myedata = 1;
 }
 outstr[0] = chptr[0];
 /* step the firstchar pointer */
 incr_currc();
 /* Copy string to output; first char reserved for indicator */
 strcpy(outstr+1, instr);
 for (asval_b = outstr+1; *asval_b != '\0'; asval_b++)
 {
  /* Ignore low nonprintable chars */
  if (*asval_b >= ' ')
  {
   /*
    * Since the ascii range we are converting
    * starts at 32, subtract 32 to have range
    * start at 0
    */
   asval_c = (*asval_b) - 32;
   /*
    * Use the encrypting table to find a corresponding
value
    * and set the output char to that value
    */
   *asval_b = curra[asval_c];
   /*
    * move the start of the encryption table
    * to the new location ptr
    */
   curra = curra+asval_c;
    /* cycle around if  getting close to the end of
the string */
   if (curra > tstbuf+95)
   {
    curra = curra - 95;
   }

   /*
    * For keeping track of the current location
    * you can ignore this for this example.
    */
   curoffset = curoffset + asval_c;
   if (curoffset > 95)
   {
    curoffset = curoffset-95;
   }
  }
 }
 return(0);
}
/*----------------------------------------------------------*
 *
 * nb_decrypt
 *----------------------------------------------------------*/
int nb_decrypt (char *instr, char *outstr)
{

 char *asval_a;
 char *asval_b;
 int asval_c;
 int tmpval;
 int tmpval1;


 asval_a = instr;
 if (myedata != 1)
 { /* ascii_filter+0x78 */
  if (myedata != 0)
  { /* ascii_filter+0x94 */
   incr_currc();
  }
  myedata = 1;
 }
 outstr[0] = chptr[0];
 incr_currc();
 strcpy(outstr, instr+1);
 for (asval_b = outstr; *asval_b != '\0'; asval_b++)
 {
  if (*asval_b >= ' ')
  {
   /* Determine offset index */
   tmpval = *asval_b & 0xff;

   /* Decode table is basically inverse mapping of encode table */
   tmpval1 = (int) (decodetable[tmpval]&0xff) - curoffset;
   if (tmpval1 < 0)
   {
    tmpval1 += 95;
   }
   *asval_b = tmpval1 + 32;
   asval_c = tmpval1;

   curoffset = curoffset + asval_c;
   if (curoffset > 95)
   {
    curoffset = curoffset-95;
   }
  }
 }
 return(0);
}
/*----------------------------------------------------------*
 *
 * incr_currc
 *----------------------------------------------------------*/
int incr_currc(void)
{
 chptr++;
 if (chptr >= cstring+46)
 {
  chptr = cstring;
 }
 return(0);
}


One run of this program.

myhost [6]% ./a.out
orig  : CThe quick brown fox jumps over the lazy dog. 1234567890

encstr:
C(ln##v_lLaayw"*gg%yCC/r9@::4w\NN|f,,Ia@XXZT?uulyc9uwSMI.

decstr: CThe quick brown fox jumps over the lazy dog. 1234567890

0.0u 0.0s 0:00 3%


Final Notes
This particular piece of reverse engineering isn't a crack as much as it demonstrates how a particular encryption can be reverse engineered. Many keygens use some sort of trapdoor encryption where the algorithm is only meant to go one way, and the authorization code is compared against the output of the algorithm. Some methods have data hidden within the provided key, which is then used by the program. Being able to reverse encryption algorithms enables reversers to extract "secret" data from authorization codes where secret data is encoded in the provided authorization code.

I have avoided posting the full reversing of ls_filter.o because this essay is long enough as it is, and most reversers are not interested in getting usage statistics from the license daemon. It is possible with reverse engineering to build a program which can open the logfile and build an application that can extract complete information from this file.

Ob Duh
I wont even bother explaining you that you should BUY this target program if you intend to use it for a longer period than the allowed one. Should you want to STEAL this software instead, you don't need to crack its protection scheme at all: you'll find it on most Warez sites, complete and already regged, farewell, don't come back.

You are deep inside reverser's page of reverse engineering, choose your way out:


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