ARMv7 32-bit Linux Pseudo Random String Generator

Overview

prand is a minimal user space program written in ARMv7 (32-bit ARM) assembly for Linux. It generates a secure 32 character pseudo random string sampled from a custom set of alphanumeric and symbol characters. This is done purely by invoking Linux system calls, without any C runtime or libraries, making the program extremely small and self contained.

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Features

• Directly uses the Linux getrandom system call (syscall number 384 on ARMv7) to gather 32 bytes of cryptographically secure random data.
• Transforms each raw random byte into a readable character by taking its remainder modulo the size of a predefined character set and using that as an index into the set.
• Prints the generated string followed by a newline to standard output.
• Implements its own program entry (_start) with no reliance on libc or startup files.
• Statically linked and designed to run on bare ARMv7 Linux systems or emulators.

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Character Set

The random string is composed from this character set of length 78:

ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789():;!+-$#@/*={}

This set includes uppercase, lowercase, digits, and a selection of common symbols ensuring a broad range of output characters.

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Low Level Implementation Details

Syscall Interface

The program directly interfaces with Linux syscalls following the ARM 32-bit EABI convention:

• r7 holds the syscall number.
• Arguments are passed in r0, r1, r2, etc.
• svc #0 triggers the syscall.

For the key operations, syscalls used:

• getrandom (384):
  Reads 32 random bytes into a buffer.

  • r0: pointer to buffer
  • r1: number of bytes to read
  • r2: flags (0)

• write (4):
  Writes the generated string to stdout.

  • r0: file descriptor (1 for stdout)
  • r1: pointer to output buffer
  • r2: buffer length (33 bytes including null terminator)

• exit (1):
  Cleanly exits the program with status code in r0.

Memory Layout

• .data section stores the constant character set.
• .bss section reserves uninitialized memory:
  • randbuf: 32 bytes (random bytes buffer)
  • outbuf: 33 bytes (output string buffer, including null terminator)

Control Flow

1. Call getrandom syscall to fill randbuf with 32 random bytes.
2. Verify if the syscall returned exactly 32 bytes, else exit with error.
3. For each byte in randbuf:
   • Calculate the modulo against the character set size (78) by repeated subtraction.
   • Use the remainder as index to lookup a character in the character set.
   • Store the selected character sequentially in outbuf.
4. Append a null terminator byte to outbuf.
5. Use the write syscall to output the full string to stdout.
6. Exit cleanly with success status.

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Assembly Code Structure

• _start: program entry point.
• Simple loops written with explicit labels (convert_loop, mod_loop) to process each byte.
• Error handling jumps to error_exit label, which exits with status 1.
• Uses ARM instructions like ldr, mov, cmp, b, blt, sub, ldrb, strb.

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Building the Project

You can build the project using the provided Makefile:

git clone https://github.com/gaidardzhiev/getprand
cd getprand
make

Ensure you use a assembler and linker for ARMv7 (32-bit ARM).

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Usage

Run the resulting executable on an ARMv7 Linux system or emulator. The program will print a single 32 character pseudo random string to stdout.

./getprand

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Limitations and Notes

• The modulo operation on random bytes uses repeated subtraction, which is simple but not the most efficient; this is acceptable given the small buffer size.
• There is no error output (no stderr) beyond process exit codes.
• This program assumes the ARM 32 bit Linux syscall convention and wont run on other Linux architectures without modification.

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License

This project is provided under the GPL3 License.

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