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Reverse-Engineering the First Pocket PC Trojan, Part 2

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In part 2 of their series on the Brador Trojan horse for the Pocket PC, Cyrus Peikari, Seth Fogie, Ratter/29A, and Jonathan Read take us on a detailed tour of exactly how this nasty piece of business works.
See Part 1 of this article.
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Now that we've done a macro analysis in part 1, we get to the fun part: reverse-engineering the first Pocket PC Trojan. To perform micro analysis, you should be familiar with assembly language. In addition, you have to know the specific assembly registers used by the ARM processor. We've included a few paragraphs here from Security Warrior (O'Reilly, 2004) to give you an overall review.

The Advanced RISC Microprocessor (ARM) is a low-power, 32-bit microprocessor based on the Reduced Instruction Set Computer (RISC) principles. In particular, the ARM is used in small devices that have a limited power source and low threshold for heat, such as PDAs, telecommunication devices, and other miniature devices that require a relatively high level of computing power.

There are a total of 37 registers within this processor that hold values used in the execution of code. Six of these registers are used to store status values needed to hold the results of compare and mathematical operations, among others. This leaves 31 registers to the use of the program, of which a maximum of 16 are generally available to the programmer. Of these 16, Register 15 (R15) is used to hold the Program Counter (PC), which is used by the processor to keep track of where in the program it is currently executing. R14 is also used by the processor as a subroutine link register (Lr), which is used to temporarily hold the value held by R15 when a Branch and Link (BL) instruction is executed. Finally, R13, known as the Stack Pointer (Sp), is used by the processor to hold the memory address of the stack, which is used to hold all values about to be used by the processor in its execution.

In addition to these first 16 registers, some debuggers allow the programmer to monitor the last four registers (28–31), which are used to hold conditional values. These registers are used to hold the results of arithmetic and logical operations performed by the processor (addition, subtraction, compares, and so on). Here's a list of the registers and their purposes (in descending order because the processor bits are read from high to low):

  • R31: Negative/less than

  • R30: Zero

  • R29: Carry/borrow/extend

  • R28: Overflow

For more on Windows CE reverse-engineering, as well as software reversing in general, check out Security Warrior.

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