C :: Mcrypt Lib And String Encoding

I've got an application here which connects over the internet to a webserver and sends some json strings. This is all working already.

Now I want to encode one string via mcrypt (because it seemed the easiest library of all) AES and send it over to the other server where it should get decrypted again and checked for validity.

I'll be using this sample code as "starting base". i've found it on the internet:

Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/*
* MCrypt API available online:

[Code] ....

The code works as it is, it encodes my string into a ciphered text and displays the text via the display function to stdout.

From previous projects I know I usually used Base64 for transporting strings via json, but in this example the string is encoded into "%d" - so decimals.. It works too, and I dont care if I send a base64 encoded string or these decimals but how would I calculate back the %d encoded string ? So how would a "undisplay()" class look alike ?

Or maybe there is an easier way to transport the string and re-decode it ?

I'd expect something like:

base64string = base64encode(ciphertext);
...
send base64string to host2
...
at host2:
ciphertext = base64decode(base64string)
...
and then mcrypt_decode that ciphertext...