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Exercise: Stream Ciphers — Four Ways to Link the Same Program

Tools: GCC, Make, ar, ldd, nm

Goal

Take one unchanged set of C source files and produce four different executables from it: dynamically linked, statically linked, linked against a shared library, and linked against a static library. Then write the Makefile rules that build them. Afterwards you will know what -L, -l, -static and LD_LIBRARY_PATH each contribute, without a walkthrough to follow.

Background

The code is already written and needs no changes:

  • caesar.c / caesar.h — the caesar() function shifts every letter by a fixed integer offset.
  • vigenere.c / vigenere.h — the vigenere() function shifts each letter by the value of the corresponding character of a repeating key.
  • main.c — parses the command line and calls one of the two.

Applying Caesar with -3 undoes Caesar with 3. Vigenere is not so obliging: this implementation only shifts forward, so decrypting needs a key whose letters are the additive inverses modulo 26 of the original key's letters. Work out that inverse key yourself — it gives you a cheap correctness check for every executable you build.

The Makefile already builds the two libraries from caesar.c and vigenere.c: the static libcipherstatic.a and the shared libcipherdyn.so. They are built exactly the way libxorstatic.a and libxordyn.so were in 02-xor-encryption.

Your Task

  1. Build the libraries with make. Read the commands it prints, and look inside both libraries with ar t, nm and nm -D.

  2. Build the program four times by hand, with gcc, one per line of the table:

    # Result Name to use
    1 Dynamically-linked executable, all sources compiled together cipher
    2 Statically-linked executable (-static), all sources compiled together cipher-static
    3 Executable from main.o, linked against libcipherdyn.so cipher-dyn
    4 Executable from main.o, linked against libcipherstatic.a cipher-static-lib

    For rows 3 and 4, use -L. and put -l<name> at the end of the command. Running cipher-dyn needs LD_LIBRARY_PATH; work out why from the error message before setting it.

  3. Add a rule for each executable to the Makefile, in the TODO 1 to TODO 4 sections, then add the executables to all (TODO 5). Each rule lists the files the executable is built from as its prerequisites, and runs the command you typed by hand.

  4. Check that the Makefile builds everything from scratch:

    make clean
make

Build & Run

Every executable takes the same arguments, whichever way it was linked:

./cipher caesar <shift> <text>
./cipher vigenere <key> <text>

Quote <text> if it contains spaces. A negative <shift> decrypts.

For example:

$ ./cipher caesar 13 solrulzforever
fbyehymsberire

$ ./cipher vigenere syscall solrulzforever
kmdtuwkxmjgvpc

Check Your Work

  • All four executables must produce byte-identical output for the same arguments. If one of them differs, you linked something you did not mean to link.
  • Encrypt a sentence and then decrypt it; you must get the original back. Non-letter characters must pass through unchanged. If running Vigenere twice with the same key does not give you the original text, that is not a bug — re-read the Background section.
  • Compare the four files with ls -l and with ldd. Predict, before running ldd, which of them depend on libc.so.6, which one also depends on libcipherdyn.so, and which one is not a dynamic executable at all. Then explain the sizes.
  • nm -u cipher-dyn should show undefined symbols that nm -u cipher-static-lib does not. Explain to the teaching assistant what "undefined" means here and who resolves those symbols, and when.
  • After make clean, a plain make builds both libraries and all four executables, and running make again rebuilds nothing.