How to decrypt a file with GPGme

The main problem with GPGme is its (lack of) documentation.
After looking aroung for a while, I found that there is an
obscure (i.e., undocumented) way to generate a PDF manual.
The examples contained there, however, are far from being
tutorial material. What that manual needs badly are
examples for the most used operations: decrypt and encrypt.

I used Google to find the following code which seems to be
what I am looking for (sample code to decrypt a file).
Unfortunately the posting was in German and the code in C++,
two languages that I don't know that well.

Anyway, if somebody out there could translate the code below
to C (_specially_ the callback that handles the passphrase)
I for one, would be most grateful.

TIA,

-Ramon F Herrera

---------------------------------------------------------------

#include <iostream>
#include <iomanip>

#include "gpgme.h"

// Callback function to retrieve the passphrase
const char *myPass(void *hook, const char *desc, void **r_hd)
{
char *sNull = "NULL";
const char *p;
if( desc ) p = desc;
else p = sNull;

std::cout << "myPass(" << p << ")" << std::endl;
static const char *passPhrase = "secret";

if( desc ) p = passPhrase;
else p = NULL;

return p;
}

int main(int argc, char *argv[])
{
GpgmeCtx ctx;
GpgmeData ciphertext, plaintext;
GpgmeRecipients rset;

gpgme_new (&ctx);
gpgme_set_armor (ctx, 1);
gpgme_set_passphrase_cb (ctx, myPass, NULL);

char *plain = "Hallo Welt";

gpgme_data_new_from_mem (&plaintext, plain, strlen(plain), 1 );
std::cout << "gpgme_data_new_from_mem(plaintext) ok" << std::endl;

GpgmeError err = gpgme_data_new ( &ciphertext );
if( err != GPGME_No_Error )
{
std::cout << "gpgme_data_new error " << err << std::endl;
exit(0);
}
std::cout << "gpgme_data_new(cipher) ok" << std::endl;

err = gpgme_recipients_new (&rset);
if( err != GPGME_No_Error )
{
std::cout << "gpgme_recipients_new error " << err << std::endl;
exit(0);
}
std::cout << "gpgme_recipients_new() ok" << std::endl;

err = gpgme_recipients_add_name (rset, "neo42@gmx.de");
if( err != GPGME_No_Error )
{
std::cout << "gpgme_recipients_add_name error " << err << std::endl;
exit(0);
}
std::cout << "gpgme_recipients_add_name() ok" << std::endl;

err = gpgme_op_encrypt (ctx, rset, plaintext, ciphertext );
if( err != GPGME_No_Error )
{
std::cout << "gpgme_op_encrypt error " << err << std::endl;
exit(0);
}
std::cout << "gpgme_op_encrypt() ok" << std::endl;

char buf[4096];
size_t nread;
err = gpgme_data_read( ciphertext, buf, sizeof(buf), &nread );
if( err != GPGME_No_Error )
{
std::cout << "gpgme_data_read error " << err << std::endl;
exit(0);
}
std::cout << "gpgme_data_read() ok " << nread << " bytes" << std::endl;

buf[nread] = 0;
std::cout << buf;

// free all used resources

gpgme_data_release (plaintext);
gpgme_data_release (ciphertext);
gpgme_recipients_release (rset);

//
// Decrypt
//
err = gpgme_data_new_from_mem (&ciphertext, buf, nread, 1 );
if( err != GPGME_No_Error )
{
std::cout << "gpgme_data_new_from_mem(buf) error " << err << std::endl;
exit(0);
}
std::cout << "gpgme_data_new_from_mem(ciphertext) ok" << std::endl;

gpgme_data_new (&plaintext);
err = gpgme_op_decrypt (ctx, ciphertext, plaintext);
if( err != GPGME_No_Error )
{
std::cout << "gpgme_op_decrypt error " << err << std::endl;
exit(0);
}
std::cout << "gpgme_op_decrypt() ok" << std::endl;

gpgme_data_release(ciphertext);

err = gpgme_data_read( plaintext, buf, sizeof(buf), &nread );
if( err != GPGME_No_Error )
{
std::cout << "gpgme_data_read error " << err << std::endl;
exit(0);

I have a vague knowledge of c++. Hope this helps.

1. [std::]cout is the method of printing to standard output in c++. You
need to replace it with printfs, or the like. Notice that it understands
different data types intuitively, but printf doesn't:

cout << "hello" << 2;
printf("hello%d", 2);

The std:: is just a way of referencing which cout is being used. In this
case, the standard one. << is the operator which does the actual work, as
well as separates parts, and std::endl is the endline, replaceable with
"\n".

2. I personally have never used const in c, but I guess it compiles... It
just makes variables immutable (i.e., you can't change them) in c++.

3. Other than that, myPass appears to be coded in normal C.

4. Looking at the variables, and the way they are initialized by calls to
functions, I suspect that they are merely C structures, and not c++
objects. But don't take my word on this, go ahead and check the header
files. (though, if you want to be technical about it, structs are classes
in C++ with default to public access permissions...)

Jon.

-- * Does the walker choose the path, or does the path choose the walker?
(fr. Sabriel) * --

1) unpack the src and then do info -f doc/gpgme.info
2) unpack the src and look at the examples in tests/gpg, for example
t-decrypt.c (looks pretty simple to me, completely c, including the
t-support.h)

Of course, I'm only glancing at it.

Jon.

-- * Does the walker choose the path, or does the path choose the walker?
(fr. Sabriel) * --