How do I use Bouncy Castle to read an App Store In App Purchase receipt? (PKCS7) - ios

Question

I have a receipt in PKCS7 that I obtained from my iOS app. Apple says this is a PKCS7 structure, and within that, is information regarding past recurring purchases.
I have the raw receipt here, encoded in Base64.
I've sent this payload, with my secret key, to Apple and got this response. Based on WWDC videos, and documentation, I believe I should be able to read this receipt directly, and without sending it to apple.
I'm guessing that PEMReader in BC is the correct starting point parse it, however I'm not sure how to actually use it. I've scanned the BC source code for the strings "PKCS", and looked at unit tests, however all I ever see are casts from PEMReader into another format.
using (var stream1 = new MemoryStream(receipt.Data))
using (var stream2 = new StreamReader(stream1))
{
var pp = new PemReader(stream2);
pp.ReadObject();
}
Question
How do I use Bouncy Castle to verify a raw receipt payload generated from Apple Store?
Note to self: I intend to use this to inspect the actual binary to see if ApplicationUsername is included in the receipt, yet for some reason isn't returned in the JSON result when posting the server. (Bug on Apple's side?)

Answer 1

I've made this using Java 7 and BouncyCastle 1.56.
For the code below, consider that pemString is the PEM string you provided. But I had to make some modifications:
format (break lines for every 64 characters) - I've made a small program to do that
include BEGIN and END headers
So my PEM looks like:
-----BEGIN PKCS7-----
MIIv5gYJKoZIhvcNAQcCoIIv1zCCL9MCAQExCzAJBgUrDgMCGgUAMIIfhwYJKoZI
hvcNAQcBoIIfeASCH3Qxgh9wMAoCAQgCAQEEAhYAMAoCARQCAQEEAgwAMAsCAQEC
AQEEAwIBADALAgELAgEBBAMCAQAwCwIBDwIBAQQDAgEAMAsCARACAQEEAwIBADAL
....
gdTu2uzkTyT+vcBlaLHK1ZpjKozsBds7ys6Q4EFp7OLxtJTj7saEDYXCNQtXBjwl
UfSGvQkXeIbsaqSPvOVIE83K3ki5i64gccA=
-----END PKCS7-----
For the code below, I followed the definition in Apple's doc:
ReceiptAttribute ::= SEQUENCE {
type INTEGER,
version INTEGER,
value OCTET STRING
}
Payload ::= SET OF ReceiptAttribute
Code:
import org.bouncycastle.asn1.ASN1InputStream;
import org.bouncycastle.asn1.ASN1Object;
import org.bouncycastle.asn1.DEROctetString;
import org.bouncycastle.asn1.DLSequence;
import org.bouncycastle.asn1.DLSet;
import org.bouncycastle.cms.CMSSignedData;
import org.bouncycastle.util.io.pem.PemObject;
import org.bouncycastle.util.io.pem.PemReader;
String pemString = // PEM String as described above
PemReader reader = new PemReader(new StringReader(pemString));
PemObject pemObject = reader.readPemObject();
reader.close();
CMSSignedData s = new CMSSignedData(pemObject.getContent());
byte[] content = (byte[]) s.getSignedContent().getContent();
ASN1InputStream in = new ASN1InputStream(content);
// Payload: a SET of ReceiptAttribute
DLSet set = (DLSet) DLSet.fromByteArray(in.readObject().getEncoded());
int size = set.size();
for (int i = 0; i < size; i++) {
// ReceiptAttribute is a SEQUENCE
DLSequence seq = (DLSequence) set.getObjectAt(i);
// value is the third element of the sequence
DEROctetString oct = (DEROctetString) seq.getObjectAt(2);
ASN1Object obj = readObject(oct.getOctets()); // *** see comments below ***
}
in.close();
// readObject method
public ASN1Object readObject(byte[] b) throws IOException {
ASN1InputStream in = null;
try {
in = new ASN1InputStream(b);
return in.readObject();
} catch (Exception e) {
// if error occurs, just return the octet string
return new DEROctetString(b);
} finally {
in.close();
}
}
Variable obj will be the content of the ReceiptAttribute, and it can vary a lot - I've seen DERIA5String, DERUTF8String, ASN1Integer and many others. As I don't know all possible values of this field, I think it's up to you to check each value.