/* Convert a 20-byte hash value to a 45-byte human-readable value */
void otrl_privkey_hash_to_human(char human[45], const unsigned char hash[20])
{
    int word, byte;
    char *p = human;

    for(word=0; word<5; ++word) {
	for(byte=0; byte<4; ++byte) {
	    sprintf(p, "%02X", hash[word*4+byte]);
	    p += 2;
	}
	*(p++) = ' ';
    }
    /* Change that last ' ' to a '\0' */
    --p;
    *p = '\0';
}


char temp1;
char arg1 = &temp1;

[... arg2 wird ausgelesen ...]
otrl_privkey_hash_to_human(arg1, arg2);