In which library is the MD5() function?

md5, openssl

Solution

In which library is the MD5() function?

The OpenSSL library. Link to `libcrypto`. See md5(3).

Is MD5 really implemented in libc and not in some libssl library?

Well, its not in Ubuntu's `libc`:

$ nm -D  /lib/x86_64-linux-gnu/libc.so.6 | grep -i md5
$

And it is in OpenSSL's `libcrypto`:

$ nm -D /usr/lib/x86_64-linux-gnu/libcrypto.so | grep MD5
0000000000066840 T MD5
0000000000066640 T MD5_Final
0000000000066790 T MD5_Init
0000000000066630 T MD5_Transform
0000000000066420 T MD5_Update

The `T` means the symbol (`MD5`) is defined in the TEXT section and its exported. A `t` means the symbol is defined in the TEXT section, but its not exported so you cannot link against it (think GCC's `visibility=private` or a static declaration).

If you get a `U`, then that means the symbol is required but undefined and a library will have to provide it.

#include <openssl/md5.h>

...
MD5(a, b, c, d);

`MD5(a, b, c, d);` is not OpenSSL's MD5. OpenSSL's MD5 has three parameters, not four.

objdump gives me the info about the linked library

`ldd` might give you different results. Its what I use to check dependencies (and I don't use `objdump`):

$ cat t.c
#include <openssl/md5.h>

int main(int argc, char* argv[])
{
    const char password[] = "password";
    char hash[MD5_DIGEST_LENGTH];    
    MD5(password, sizeof(password), hash);

    return 0;
}

$ gcc t.c -o t.exe -lcrypto
$ ldd t.exe 
    linux-vdso.so.1 =>  (0x00007fff435ff000)
    libcrypto.so.1.0.0 => /usr/lib/x86_64-linux-gnu/libcrypto.so.1.0.0 (0x00007fbaff01b000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fbafec90000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fbafea8b000)
    libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007fbafe874000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fbaff429000)

And `t.exe`'s unresolved symbol:

$ nm t.exe | grep MD5
    U MD5@@OPENSSL_1.0.0

Problem

As I'm writing code to install on a target machine, I was wondering about the dependencies and noticed that there were no openssl library needed. I wondered because I know I am using OpenSSL: ``` #include <openssl/md5.h> ... MD5(a, b, c); ... ``` To my surprise, it seems that we only get linked against libc. Is MD5 really implemented in libc and not in some libssl library? objdump gives me the info about the linked library: ``` Dynamic Section: NEEDED libQtCore.so.4 NEEDED libstdc++.so.6 NEEDED libgcc_s.so.1 NEEDED libc.so.6 SONAME libcontent.so ``` As suggested by noloader I tried with ldd and still fail to see a library that would make sense for MD5. libcontent.so is directly using MD5()... ``` ldd ../BUILD/snapwebsites/plugins/content/libcontent.so linux-vdso.so.1 => (0x00007fff4f3ff000) libQtCore.so.4 => /usr/lib/x86_64-linux-gnu/libQtCore.so.4 (0x00007ff37ad0f000) libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007ff37aa0c000) libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007ff37a7f5000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007ff37a42c000) libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007ff37a20f000) libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007ff379ff7000) libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007ff379df3000) libglib-2.0.so.0 => /lib/x86_64-linux-gnu/libglib-2.0.so.0 (0x00007ff379af7000) librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007ff3798ee000) libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007ff3795e9000) /lib64/ld-linux-x86-64.so.2 (0x00007ff37b5e5000) libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007ff3793a9000) ``` Also, just to make sure, I tried nm on that content library and I can see the MD5 entry: ``` w _ITM_registerTMCloneTable 00000000003c9468 d __JCR_END__ 00000000003c9468 d __JCR_LIST__ w _Jv_RegisterClasses U MD5 <---- it's here... U memcmp@@GLIBC_2.2.5 w pthread_cancel U pthread_mutex_destroy@@GLIBC_2.2.5 ```

Original source