101-crackme

A reverse-engineering exercise: analyzing a compiled C binary using objdump, understanding its checksum-based password validation, and writing a keygen that generates valid passwords.

Files

• 101-crackme — the compiled binary to crack
• 101-keygen.c — generates random valid passwords for 101-crackme

How it works

The binary checks command-line input against a hidden algorithm:

long checksum(char *s)
{
    long sum = 0;

    while (*s != '\0')
    {
        sum += *s;
        s++;
    }
    return (sum);
}

It sums the ASCII values of every character in the input and compares the result to 0xad4 (2772). If they match, it prints Tada! Congrats.

This was reverse-engineered using:

• strings to locate key string literals
• objdump -d -Mintel to disassemble main and checksum
• Tracing register usage, stack frames, and the x86-64 calling convention

Usage

Compile the keygen:

gcc -Wall -pedantic -Werror -Wextra -std=gnu89 101-keygen.c -o 101-keygen

Generate a valid password and crack the binary:

./101-crackme "$(./101-keygen)"

Expected output on success:

Tada! Congrats

Writeup

Full breakdown of the reverse-engineering process: [https://m0ng00s3-blog.hashnode.dev/solving-a-simple-crackme]

Skills Demonstrated

• x86-64 assembly analysis
• Reverse engineering with objdump and strings
• C programming (low-level memory and pointer manipulation)
• Understanding stack frames and calling conventions