Parsing Product Ids from Receipt in Swift

Parsing_productids.swift

func getProductIdFromReceipt(_ data:Data) -> String? //Swift 2: func getProductIdFromReceipt(data:NSData) -> String?
{
    //Swift 2: var p = UnsafePointer<UInt8>(data.bytes)
    //Swift 3
    var p : UnsafePointer? = (data as NSData).bytes.bindMemory(to: UInt8.self, capacity: data.count)
    
    let dataLength = data.length
        
    var type:Int32 = 0
    var tag:Int32 = 0
    var length = 0
    let end = p! + dataLength
        
    ASN1_get_object(&p, &length, &type, &tag, end - p!)
    if type != V_ASN1_SET {
        return nil
    }
    var integer: UnsafeMutablePointer<ASN1_INTEGER>
        
    while p! < end
    {
        // Expecting an attribute sequence
        ASN1_get_object(&p, &length, &type, &tag, end - p!)
        if type != V_ASN1_SEQUENCE {
            print("ASN1 error: expected an attribute sequence")
        }
            
        var attr_type = 0
            
        // The attribute is an integer
        ASN1_get_object(&p, &length, &type, &tag, end - p!)
        if type != V_ASN1_INTEGER {
            print("ASN1 error: attribute not an integer")
            return nil
        }
        integer = c2i_ASN1_INTEGER(nil, &p, length)
        attr_type = ASN1_INTEGER_get(integer)
        ASN1_INTEGER_free(integer)
            
        // The version is an integer
        ASN1_get_object(&p, &length, &type, &tag, end - p!)
        if type != V_ASN1_INTEGER {
            print("ASN1 error: version not an integer")
            return nil
        }
        integer = c2i_ASN1_INTEGER(nil, &p, length);
         ASN1_INTEGER_free(integer);
            
        // The attribute value is an octet string
        ASN1_get_object(&p, &length, &type, &tag, end - p!)
        if type != V_ASN1_OCTET_STRING {
            print("ASN1 error: value not an octet string")
            return nil
        }
            
        //For Product Id
        if attr_type == 1702
        {
            if type == V_ASN1_OCTET_STRING
            {
                ASN1_get_object(&p, &length, &type, &tag, end - p!)
                //Swift 2: let productId = NSString(bytes: p!, length: length, encoding: NSUTF8StringEncoding)
                let productId = NSString(bytes: p!, length: length, encoding: String.Encoding.utf8.rawValue)
                return productId as? String
            }
        }
            
        //Swift 2: p = p.advancedBy(length)
        p = p!.advanced(by: length)
    }
    return nil
}