Is there a way to normalize json data from NSJSONSerialization? - ios

Question

It seems that since XCode 6.1, the iPhone 5S, iPhone 6 and iPhone 6+ simulators (all 64-bit) all return data from the following system method differently (keys are ordered differently) than their 32-bit simulator counterparts (e.g. iPhone 5 simulator)
+ (NSData *)dataWithJSONObject:(id)obj options:(NSJSONWritingOptions)opt error:(NSError **)error;
This difference of key ordering caused a problem for me since we calculate the SHA1 of that JSON data (converted to NSString*) and use it for a validation test. Since the ordering changed, the SHA1 changed and the validation fails.
Simplified sample code (non-ARC) to get the SHA1 is below:
NSData* jsonData = [NSJSONSerialization dataWithJSONObject:dict
options:0
error:&error];
NSString * json = [[[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding] autorelease];
NSString * sha1 = [MyUtils computeSHA1:json];
+(NSString*) computeSHA1:(NSString*)input
{
const char *cstr = [input cStringUsingEncoding:NSUTF8StringEncoding];
NSData *data = [NSData dataWithBytes:cstr length:input.length];
NSNumber* dataLen = [NSNumber numberWithUnsignedInteger:data.length];
uint8_t digest[CC_SHA1_DIGEST_LENGTH];
CC_SHA1(data.bytes, dataLen.unsignedIntValue, digest);
NSMutableString* output = [NSMutableString stringWithCapacity:CC_SHA1_DIGEST_LENGTH * 2];
for(int i = 0; i < CC_SHA1_DIGEST_LENGTH; i++)
[output appendFormat:#"%02x", digest[i]];
return output;
}
Apparently, this key ordering difference doesn't happen on the actual devices (previous behavior was preserved).
I also tried with the NSJSONWritingPrettyPrinted option but the JSON ordering is still inconsistent between simulators.
So, the question is: Does anyone have a recommendation on how to normalize such JSON data so as to not be susceptible to key ordering changes? Alternately, is there any way to get the previous (32-bit simulator) behavior?

Answer 1

Key ordering in dictionaries is not guaranteed. If you need them sorted, put them into an array and sort them.

Answer 2

The code below (non-ARC) worked for me to better canonicalize JSON output. The code assumes the class methods below are all in a class called MyUtils.
Simply pass the the NSDictionary to serialize into "canonicalized JSON" to canonicalJSONRepresentationWithDictionary:
The returned NSString* then contains serialized JSON that has the keys ordered lexicographically/alphabetically in a non-human readable format.
+(NSString *) canonicalJSONRepresentationWithDictionary:(NSDictionary *)dict
{
NSMutableString* json = [NSMutableString string];
[json appendString:#"{"];
NSArray* keys = [[dict allKeys] sortedArrayUsingComparator:^NSComparisonResult(NSString* a, NSString* b) {
return [a compare:b];
}];
for (int i = 0; i < keys.count; i++) {
NSString* key = keys[i];
[json appendFormat:#"\"%#\":", key];
if ([dict[key] isKindOfClass:[NSString class]]) {
[json appendFormat:#"\"%#\"", [MyUtils canonicalJSONRepresentationWithString:dict[key]]];
} else if ([dict[key] isKindOfClass:[NSDictionary class]]) {
[json appendString:[MyUtils canonicalJSONRepresentationWithDictionary:dict[key]]];
} else if ([dict[key] isKindOfClass:[NSArray class]]) {
[json appendString:[MyUtils canonicalJSONRepresentationWithArray:dict[key]]];
} else {
return nil;
}
if (i < keys.count - 1) {
[json appendString:#","];
}
}
[json appendString:#"}"];
return json;
}
+(NSString *) canonicalJSONRepresentationWithArray:(NSArray *) array
{
NSMutableString* json = [NSMutableString string];
[json appendString:#"["];
for (int i = 0; i < array.count; i++) {
if ([array[i] isKindOfClass:[NSString class]]) {
[json appendFormat:#"\"%#\"", [MyUtils canonicalJSONRepresentationWithString:array[i]]];
} else if ([array[i] isKindOfClass:[NSDictionary class]]) {
[json appendString:[MyUtils canonicalJSONRepresentationWithDictionary:array[i]]];
} else if ([array[i] isKindOfClass:[NSArray class]]) {
[json appendString:[MyUtils canonicalJSONRepresentationWithArray:array[i]]];
} else {
return nil;
}
if (i < array.count - 1) {
[json appendString:#","];
}
}
[json appendString:#"]"];
return json;
}
+(NSString *) canonicalJSONRepresentationWithString:(NSString *) string;
{
NSDictionary* dict = [NSDictionary dictionaryWithObjectsAndKeys:string, #"a", nil];
NSError * error;
NSData * jsonData = nil;
NSString * json = nil;
jsonData = [NSJSONSerialization dataWithJSONObject:dict
options:0
error:&error];
if (!jsonData) {
NSLog(#"Got an error serializing json: %#", error);
return nil;
} else {
json = [[[NSString alloc] initWithData:jsonData encoding:NSUTF8StringEncoding] autorelease];
}
NSRange colonQuote = [json rangeOfString:#":\""];
NSRange lastQuote = [json rangeOfString:#"\"" options:NSBackwardsSearch];
NSRange range = NSMakeRange(colonQuote.location + 2, lastQuote.location - colonQuote.location - 2);
NSString* rc = [json substringWithRange:range];
return rc;
}