Fix Windows file mode issue for key file updates

[harveylv]

Problem

The fopen call in demo.c line 150 uses text mode ("r+") when opening key files for updates. This causes issues on Windows due to automatic line ending translation:

• When reading: \r\n (CRLF) is translated to \n (LF)
• When writing: \n (LF) is expanded to \r\n (CRLF)

This breaks the LMS algorithm when updating the signature counter in key files. For example, when the counter reaches 0x0A, it gets written as 0x0D0A instead of just 0x0A, corrupting the key format and causing signature failures.

Solution

Changed the file mode from "r+" (text mode) to "rb+" (binary mode) to prevent automatic line ending translation and ensure byte-for-byte accuracy.

Changes

• File: demo.c
• Line: 150
• Change: fopen(filename, "r+") → fopen(filename, "rb+")

Testing

• Compiled successfully on Windows using: make CC=C:\msys64\ucrt64\bin\gcc.exe AR=C:\msys64\ucrt64\bin\ar.exe CFLAGS="-Wall -O3 -static"
• All existing functionality preserved
• Binary mode ensures consistent behavior across platforms

Reproduction Steps (Before Fix)

1. Generate an LMS key pair on Windows
2. Sign multiple messages to increment the counter
3. When counter reaches 0x0A (or any byte containing \n), the key file gets corrupted
4. Subsequent signatures fail due to malformed key data

Impact

This fix ensures the LMS implementation works correctly on Windows by maintaining binary file integrity for key updates.