openssl - In which library is the MD5() function?

Question

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

Answer 1

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