Add documentation of PKCS12_newpass()

=pod

=head1 NAME

PKCS12_newpass - change the password of a PKCS12 structure

=head1 SYNOPSIS

 #include <openssl/pkcs12.h>

 int PKCS12_newpass(PKCS12 *p12, char *oldpass, char *newpass);

=head1 DESCRIPTION

PKCS12_newpass() changes the password of a PKCS12 structure.

B<p12> is a pointer to a PKCS12 structure. B<oldpass> is the existing password

and B<newpass> is the new password.

=head1 RETURN VALUES

PKCS12_newpass() returns 1 on success or 0 on failure. Applications can

retrieve the most recent error from PKCS12_newpass() with ERR_get_error().

=head1 EXAMPLE

This example loads a PKCS#12 file, changes its password and writes out

the result to a new file.

 #include <stdio.h>

 #include <stdlib.h>

 #include <openssl/pem.h>

 #include <openssl/err.h>

 #include <openssl/pkcs12.h>

 int main(int argc, char **argv)

 {

    FILE *fp;

    PKCS12 *p12;

    if (argc != 5) {

        fprintf(stderr, "Usage: pkread p12file password newpass opfile\n");

        return 1;

    }

    if ((fp = fopen(argv[1], "rb")) == NULL) {

        fprintf(stderr, "Error opening file %s\n", argv[1]);

        return 1;

    }

    p12 = d2i_PKCS12_fp(fp, NULL);

    fclose(fp);

    if (p12 == NULL) {

        fprintf(stderr, "Error reading PKCS#12 file\n");

        ERR_print_errors_fp(stderr);

        return 1;

    }

    if (PKCS12_newpass(p12, argv[2], argv[3]) == 0) {

        fprintf(stderr, "Error changing password\n");

        ERR_print_errors_fp(stderr);

        PKCS12_free(p12);

        return 1;

    }

    if ((fp = fopen(argv[4], "wb")) == NULL) {

        fprintf(stderr, "Error opening file %s\n", argv[4]);

        PKCS12_free(p12);

        return 1;

    }

    i2d_PKCS12_fp(fp, p12);

    PKCS12_free(p12);

    fclose(fp);

    return 0;

 }

=head1 NOTES

If the PKCS#12 structure does not have a password, then you must use the empty

string "" for B<oldpass>. Using NULL for B<oldpass> will result in a

PKCS12_newpass() failure.

If the wrong password is used for B<oldpass> then the function will fail,

with a MAC verification error. In rare cases the PKCS12 structure does not

contain a MAC: in this case it will usually fail with a decryption padding

error.

=head1 BUGS

The password format is a NULL terminated ASCII string which is converted to

Unicode form internally. As a result some passwords cannot be supplied to

this function.

=head1 SEE ALSO

L<PKCS12_create(3)>, L<ERR_get_error(3)>

=cut