Exercise: XOR Encryption — Build and Use a Library¶

Tools: GCC, Make, ar, file, ldd, nm

Goal¶

Build a static library and a shared library from the same source file, link a program against each, and follow a symbol from undefined to resolved. This is a guided walkthrough: there is no code to write, only commands to run and output to read. Everything you do here, you will do on your own in 03-stream-ciphers.

Background¶

xor_encrypt() (in xor.c / xor.h) XORs every byte of a buffer with the bytes of a repeating key. XOR is its own inverse, so the same call with the same key also decrypts. main.c encrypts its <text> argument, prints the result in hexadecimal (it can hold any byte value, 00 included), then decrypts it again.

A static library (.a) is an archive of object files; the linker copies what it needs out of it into the executable. A shared library (.so) stays a separate file; the executable only records its name, and the loader maps it in each time the program starts.

To keep the two apart, the libraries have different names: libxorstatic.a and libxordyn.so.

Your Task¶

Run every command, and read every output before moving on.

Compile each source file to an object file, and look at the symbols in each:
```
gcc -Wall -c -o main.o main.c
gcc -Wall -c -o xor.o xor.c
file main.o xor.o
nm main.o
nm xor.o
```
U means undefined: used here, defined somewhere else. T means defined here, in the code (text) section. Find xor_encrypt in both outputs.
Try to make an executable from main.o alone, with gcc -o xor main.o, and read the error. Someone has to provide xor_encrypt.

Build the static library and look inside it:

ar rcs libxorstatic.a xor.o
ar t libxorstatic.a
file libxorstatic.a
nm libxorstatic.a

Link main.o against it, in three attempts:
```
gcc -o xor-static-lib main.o -lxorstatic
gcc -o xor-static-lib -L. -lxorstatic main.o
gcc -o xor-static-lib main.o -L. -lxorstatic
```
-lxorstatic asks for a file named libxorstatic.a (or libxorstatic.so): the linker adds the lib prefix and the extension. -L. adds the current directory to the places the linker searches; without it, only the system directories are searched. The linker reads its arguments left to right and takes from a library only the symbols that are undefined so far. Explain why only the third attempt works, then run ./xor-static-lib key "Hello, World!".

Check where xor_encrypt ended up, and what the executable depends on:

nm xor-static-lib | grep xor_encrypt
file xor-static-lib
ldd xor-static-lib

Build the shared library — position-independent code (-fPIC), linked with -shared — and look at it:

gcc -Wall -fPIC -c -o xor_pic.o xor.c
gcc -shared -o libxordyn.so xor_pic.o
file libxordyn.so
nm -D libxordyn.so

Link against it, again with -L. and with -l at the end, and run the result:
```
gcc -o xor-dyn main.o -L. -lxordyn
./xor-dyn key "Hello, World!"
ldd xor-dyn
```
Read the error message, and the not found in the ldd output. -L. was for the linker, at build time; it told the loader nothing. LD_LIBRARY_PATH is the environment variable that adds directories to the loader's search, at run time:
```
LD_LIBRARY_PATH=. ldd xor-dyn
LD_LIBRARY_PATH=. ./xor-dyn key "Hello, World!"
```

Check where xor_encrypt is now, and watch the loader resolve it:

nm xor-dyn | grep xor_encrypt
LD_DEBUG=bindings LD_LIBRARY_PATH=. ./xor-dyn key hi 2>&1 | grep xor_encrypt

Read the Makefile, match each rule to a command you typed, then rebuild everything with it:
```
make clean
make
```

Check Your Work¶

Both executables print the same encrypted bytes for the same key and text, and both decrypt back to the text.
For each of xor.o, libxorstatic.a, libxordyn.so, xor-static-lib and xor-dyn, you can say what file reports and why.
You can say which of the two executables ldd shows depending on libxordyn.so, and why the other one does not need it.
xor_encrypt is U in main.o and in xor-dyn, but T in xor-static-lib. Explain to the teaching assistant who resolves it in each case, and when: at link time, or at load time.
You can explain why -l goes after the object files, why -L. is needed, and why LD_LIBRARY_PATH is needed for xor-dyn but not for xor-static-lib.