PGP(7)

DESCRIPTION
This document describes the process of integrating PGP with third party applications.

STRATEGIES
There are two basic possible strategies for PGP integration: source level (calling the PGP libraries directly) and executable level (using system(2) or some other method to call the PGP executables). This document will not detail the library method at this time. When PGP 5.5 is released, all support will be migrated to the new Source Development Kit (SDK) from the existing APIs. Developers are encouraged to either use the executable integration method detailed here or wait for the SDK to be released.

EXECUTABLE INTEGRATION
The most straightforward means of integrating PGP support into your application is to open input and output pipes to the PGP executables. For a very thorough example of PGP integration, see the "plugins" directory of the PGP 5.0 source distribution, which has a patch for the mutt email client. Most of the ideas in this text are a result of experiences modifying Michael Elkins' PGP integration code from mutt. The basic strategy is to use the PGP executables to perform encryption, decryption and verification. While it is possible to take a much more complex approach (for example, Mr. Elkins' code actually parses the output of pgpk(1) in order to present users with a list of keys), the more complex efforts are outside the scope of this document. All examples are in "C", and assume the following variables:
char *recipients; /*A space-delimited list of message recipients*/
char *buf;        /*The message to be operated on*/
char *passphrase; /*The user's passphrase*/
All allocation is assumed already handled by the application. The reader is assumed to be familiar with concepts of C programming and to have read and understood the pgp.cfg(5) man page. Also be aware that the following code assumes all buffers are big enough and isn't always perfect about handling errors gracefully. This should be considered a starting point, not a complete system.

COMMON ROUTINE
The following routine uses fork(2) and pipe(2) to open both read and write file handles to a child process. It is used to invoke PGP in these examples. You may use it effectively without understanding how it works.
/*Runs cmd, and hooks in to the process' stdin, and out to its stdout.
 *Inspiration stolen from Michael Elkins; see mutt for how to do this
 *properly with error checking, etc.
 */
int run_pgp(char *cmd, FILE **in, FILE **out)
{
  int pin[2], pout[2], child_pid;
  *in = *out = NULL;
  pipe(pin);
  pipe(pout);
  if(!(child_pid = fork()))
  {
    /*We're the child.*/
    close(pin[1]);
    dup2(pin[0], 0);
    close(pin[0]);
    close(pout[0]);
    dup2(pout[1], 1);
    close(pout[1]);
    execl("/bin/sh", "sh", "-c", cmd, NULL);
    _exit(127);
  }
  /*Only get here if we're the parent.*/
  close(pout[1]);
  *out = fdopen(pout[0], "r");
  close(pin[0]);
  *in = fdopen(pin[1], "w");
  return(child_pid);
}

ENCRYPTION
The easiest case is to simply pass your buffer to PGP. The most minimal form of this would be to write your data out, and have PGP encrypt it. Unfortunately, this would mean that the cleartext document would be placed on the hard drive, which is undesirable. However, we can use PGP's stream mode (the -f option) to get PGP to accept the plaintext on stdin and place the encrypted text on stdout:
/*Takes the buffer you wish to encrypt and the recipients; writes the
 *result into encrypt_buffer.
 */
void encrypt_buffer(char *buf, char *recipients)
{
  FILE *pgpin, *pgpout;
  char sys_buf[256], tmpbuf[1024] = "\0";
  sprintf(sys_buf, "pgpe -at -r %s -f +batchmode=1", recipients);
  run_pgp(sys_buf, &pgpin, &pgpout); /*Execute PGP*/
  if(pgpin && pgpout)
  {
    /*Output buffer to PGP:*/
    fwrite(buf, sizeof(char), strlen(buf), pgpin);
    fclose(pgpin);
    /*Now, read the result back from PGP:*/
    *buf = '\0';
    fgets(tmpbuf, sizeof(tmpbuf), pgpout);
    while(!feof(pgpout))
    {
       strcat(buf, tmpbuf);
       fgets(tmpbuf, sizeof(tmpbuf), pgpout);
    }
    fclose(pgpout);
  }
}
Additional functionality would include using the -s flag to sign the message if the user so requested. Implementation is quite similar to the pgps(1) integration, shown next. If you find that PGP is mysteriously failing during this step, it may be because you are encrypting to keys that are not completely valid. By default, PGP will fail during batch mode when this is encountered. The solution is to place NoBatchInvalidKeys = off in your pgp.cfg file. It is requested that you not pass this option on the command line, as some users will prefer not to encrypt to invalid keys.

SIGNING
Conceptually, signing is quite similar, but with a twist - the user must specify a passphrase! The simple way to handle this is to allow PGP to ask the user for the passphrase. However, if you wish to hide PGP's functionality a little bit more, you may wish to ask for the passphrase yourself, prior to signing, and pass it through. There are a number of ways to get the passphrase to PGP, but the best is probably using the environment variable PGPPASSFD. This allows you to specify a file descriptor on which you will pass the passphrase as the first input. Commonly, this is set to 0, stdin:
/*Signs a buffer.  The output is placed in buf.*/
void sign_buffer(char *buf, char *passphrase)
{
  FILE *pgpin, *pgpout;
  char tmpbuf[1024] = " ";
  setenv("PGPPASSFD", "0", 1);
  run_pgp("pgps -at -f +batchmode=1", &pgpin, &pgpout);
  if(pgpin && pgpout)
  {
    fprintf(pgpin, "%s\n", passphrase); /*Send the passphrase in, first*/
    memset(passphrase, '\0', strlen(passphrase)); /*Burn the passphrase*/
    fwrite(buf, sizeof(char), strlen(buf), pgpin);
    fclose(pgpin);
    *buf = '\0';
    fgets(tmpbuf, sizeof(tmpbuf), pgpout);
    while(!feof(pgpout))
    {
      strcat(buf, tmpbuf);
      fgets(tmpbuf, sizeof(tmpbuf), pgpout);
    }
    wait(NULL);
    fclose(pgpout);
  }
  unsetenv("PGPPASSFD");
}
Note the use of memset(3) to clear the passphrase immediately after use. Not clearing the passphrase can result in numerous security issues.

DECRYPTION/VERIFICATION
Decryption is almost identical to signing (the same passphrase requirements apply). The options passed in are slightly different, however:
/*Verifies a PGP buffer.  Note that, if the buffer is only signed, the
 *passphrase may be unnecessary - a complete program should probably
 *check for the "BEGIN PGP SIGNED MESSAGE" tag before prompting the
 *user for a passphrase.  The output is placed in buf, as well.
 */
void verify_buffer(char *buf, char *passphrase)
{
  FILE *pgpin, *pgpout;
  char tmpbuf[1024] = " ";
  setenv("PGPPASSFD", "0", 1);
  run_pgp("pgpv -f +batchmode=1 +OutputInformationFD=1",
          &pgpin, &pgpout);
  if(pgpin && pgpout)
  {
    fprintf(pgpin, "%s\n", passphrase); /*Send the passphrase in, first*/
    memset(passphrase, '\0', strlen(passphrase)); /*Burn the passphrase*/
    fprintf(pgpin, "%s", buf);
    fclose(pgpin);
    *buf = '\0';
    fgets(tmpbuf, sizeof(tmpbuf), pgpout);
    while(!feof(pgpout))
    {
      strcat(buf, tmpbuf);
      fgets(tmpbuf, sizeof(tmpbuf), pgpout);
    }
    wait(NULL);
    fclose(pgpout);
  }
  unsetenv("PGPPASSFD");
}
The +OutputInformationFD option is used to have PGP output information about the message (in this case, who signed it, if anyone) on the same stream as the decrypted or verified data.

CONFIGURATION OPTIONS
Application integrators are encourages to read the pgp.cfg(5) documentation, which details how to pass configuration options on the command line, including the public and private keyrings your application wishes to use.

SEE ALSO
pgp(1), pgpe(1), pgpv(1), pgps(1), pgpk(1), http://www.pgp.com (US versions) and http://www.pgpi.com (International versions)