OSX Receipt validation in Swift, part 4: Accessing a C union from Swift

Receipts are stored in a so called PKCS7 container. Accessing the components in this container using openSSL will hit a snag: the PKCS7 structure is defined using a "union". Unfortunately, Swift 1.2 does not allow direct access to C union elements. So it will be necessary to define a few C-functions to access this part of the container.

We find the definition of the PKCS7 structure in "openssl/pkcs7.h" as follows:

typedef struct pkcs7_st {

    /*

     * The following is non NULL if it contains ASN1 encoding of this

     * structure

     */

    unsigned char *asn1;

    long length;

# define PKCS7_S_HEADER  0

# define PKCS7_S_BODY    1

# define PKCS7_S_TAIL    2

    int state;                  /* used during processing */

    int detached;

    ASN1_OBJECT *type;

    /* content as defined by the type */

    /*

     * all encryption/message digests are applied to the 'contents', leaving

     * out the 'type' field.

     */

    union {

        char *ptr;

        /* NID_pkcs7_data */

        ASN1_OCTET_STRING *data;

        /* NID_pkcs7_signed */

        PKCS7_SIGNED *sign;

        /* NID_pkcs7_enveloped */

        PKCS7_ENVELOPE *enveloped;

        /* NID_pkcs7_signedAndEnveloped */

        PKCS7_SIGN_ENVELOPE *signed_and_enveloped;

        /* NID_pkcs7_digest */

        PKCS7_DIGEST *digest;

        /* NID_pkcs7_encrypted */

        PKCS7_ENCRYPT *encrypted;

        /* Anything else */

        ASN1_TYPE *other;

    } d;

} PKCS7;

In Swift we can access the normal elements as follows:

if OBJ_obj2nid(receiptPKCS7.memory.type) != NID_pkcs7_signed {...}

But for the union part this does not work.

Instead we can define accessor functions in C and made these visible to Swift in the MyGreatApp-Bridging-Header.h file.

The header file (pkcs7_union_accessors.h) for these accessors is:

#ifndef __MyGreatApp__pkcs7_union_accessors__

#define __MyGreatApp__pkcs7_union_accessors__

#include <openssl/pkcs7.h>

char *pkcs7_d_char(PKCS7 *ptr);

ASN1_OCTET_STRING *pkcs7_d_data(PKCS7 *ptr);

PKCS7_SIGNED *pkcs7_d_sign(PKCS7 *ptr);

PKCS7_ENVELOPE *pkcs7_d_enveloped(PKCS7 *ptr);

PKCS7_SIGN_ENVELOPE *pkcs7_d_signed_and_enveloped(PKCS7 *ptr);

PKCS7_DIGEST *pkcs7_d_digest(PKCS7 *ptr);

PKCS7_ENCRYPT *pkcs7_d_encrypted(PKCS7 *ptr);

ASN1_TYPE *pkcs7_d_other(PKCS7 *ptr);

#endif /* defined(__MyGreatApp__pkcs7_union_accessors__) */

The corresponding implementation file (pkcs7_union_accessors.c) is as follows:

#include "pkcs7_union_accessors.h"

inline char *pkcs7_d_char(PKCS7 *ptr) {

    return ptr->d.ptr;

}

inline ASN1_OCTET_STRING *pkcs7_d_data(PKCS7 *ptr) {

    return ptr->d.data;

}

inline PKCS7_SIGNED *pkcs7_d_sign(PKCS7 *ptr) {

    return ptr->d.sign;

}

inline PKCS7_ENVELOPE *pkcs7_d_enveloped(PKCS7 *ptr) {

    return ptr->d.enveloped;

}

inline PKCS7_SIGN_ENVELOPE *pkcs7_d_signed_and_enveloped(PKCS7 *ptr) {

    return ptr->d.signed_and_enveloped;

}

inline PKCS7_DIGEST *pkcs7_d_digest(PKCS7 *ptr) {

    return ptr->d.digest;

}

inline PKCS7_ENCRYPT *pkcs7_d_encrypted(PKCS7 *ptr) {

    return ptr->d.encrypted;

}

inline ASN1_TYPE *pkcs7_d_other(PKCS7 *ptr) {

    return ptr->d.other;

}

I am not sure if the 'inline' works across languages, but it won't hurt so I just put it in.

Using these functions we can now access the union part as follows:

if OBJ_obj2nid(pkcs7_d_sign(receiptPKCS7).memory.contents.memory.type) != NID_pkcs7_data {...}

I am pretty sure it is also possible to use some nifty type conversion, but this works for me.

Next up in the series part 5: The device GUID

Happy coding...

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