Michael Ash wrote:
@implementation NSDictionary (SHA1Hashing) - (NSData *)gc_sha1hash { NSMutableData *subhashes = [NSMutableData data]; for(id key in [[self allKeys] sortedArrayUsingSelector:@selector(compare:)]) { [subhashes appendData:[key gc_sha1hash]]; [subhashes appendData:[[self objectForKey:key] gc_sha1hash]]; } return [subhashes gc_sha1hash]; } @endThen define -gc_sha1hash for all the other classes you expect to encounter as well. (The prefix is just to ensure that you don't overwrite a hypothetical Apple implementation of a -sha1hash method.)
As a design, it might be more flexible to define a -gc_canonicalData method that returns an NSData containing unhashed canonical bytes, rather than -gc_sha1hash that returns a hash. This would make it easier to use different secure-hash algorithms, should that be desired. SHA1 isn't exactly state-of-the-art.
I'd also make the code that calculates the hash from a given NSDictionary or NSArray return an error (or throw an exception) if anything in the collection doesn't respond to -gc_canonicalData. Or it could silently skip those (not hash them), depending on the context.
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