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

Answer 1

In which library is the MD5() function?

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

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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.

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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