Validate receipt using Apple Certificate data robotmedia#198

Adds a method to provide Apple's certificate data as an alternative to the certificate url.
This can be useful when Apple's certificate is embedded in the source code (base64) instead of being in the app's resources. It's specially useful in Mac Apps, where anyone could replace the certificate in the bundle.

This change does not affect setAppleRootCertificateURL. The receipt can still be validated with the url of the certificate, but if the certificate data exists, it takes precedence over the url.

RMStore/Optional/RMAppReceipt.h

@@ -102,6 +102,12 @@ __attribute__((availability(ios,introduced=7.0)))
  */
 + (void)setAppleRootCertificateURL:(NSURL*)url;
 
+/**
+ Sets the data of the Apple Root certificate that will be used to verifiy the signature of the bundle receipt. This takes precedence over AppleRootCertificateURL if both are set. If none is provided, AppleRootCertificateURL will be used. If no certificate is available, no signature verification will be performed. 
+ @param data containing the Apple Root certificate.
+ */
++ (void)setAppleRootCertificateData:(NSData*)data;
+
 @end
 
 /** Represents an in-app purchase in the app receipt.

RMStore/Optional/RMAppReceipt.m

@@ -103,6 +103,8 @@ static int RMASN1ReadInteger(const uint8_t **pp, long omax)
 
 static NSURL *_appleRootCertificateURL = nil;
 
+static NSData *_appleRootCertificateData = nil;
+
 @implementation RMAppReceipt
 
 - (instancetype)initWithASN1Data:(NSData*)asn1Data
@@ -215,6 +217,11 @@ + (void)setAppleRootCertificateURL:(NSURL*)url
     _appleRootCertificateURL = url;
 }
 
++ (void)setAppleRootCertificateData:(NSData*)data
+{
+  _appleRootCertificateData = data;
+}
+
 #pragma mark - Utils
 
 + (NSData*)dataFromPCKS7Path:(NSString*)path
@@ -229,8 +236,13 @@ + (NSData*)dataFromPCKS7Path:(NSString*)path
     if (!p7) return nil;
 
     NSData *data;
-    NSURL *certificateURL = _appleRootCertificateURL ? : [[NSBundle mainBundle] URLForResource:@"AppleIncRootCertificate" withExtension:@"cer"];
-    NSData *certificateData = [NSData dataWithContentsOfURL:certificateURL];
+    NSData *certificateData = _appleRootCertificateData;
+
+    if( certificateData.length == 0 ) {
+        NSURL *certificateURL = _appleRootCertificateURL ? : [[NSBundle mainBundle] URLForResource:@"AppleIncRootCertificate" withExtension:@"cer"];
+        certificateData = [NSData dataWithContentsOfURL:certificateURL];
+    }
+
     if (!certificateData || [self verifyPCKS7:p7 withCertificateData:certificateData])
     {
         struct pkcs7_st *contents = p7->d.sign->contents;