NSData category adding gzip functionality. Extracted from http://cocoadev.com/wiki/NSDataCategory

NSData+gzip.h

#import <Foundation/Foundation.h>

@interface NSData (gzip)

- (NSData *)gzipInflate;
- (NSData *)gzipDeflate;

@end

NSData+gzip.m

#import "NSData+gzip.h"
#include <zlib.h>

@implementation NSData (gzip)

- (NSData *)gzipInflate
{
  if ([self length] == 0) return self;
	
	unsigned full_length = [self length];
	unsigned half_length = [self length] / 2;
	
	NSMutableData *decompressed = [NSMutableData dataWithLength: full_length + half_length];
	BOOL done = NO;
	int status;
	
	z_stream strm;
	strm.next_in = (Bytef *)[self bytes];
	strm.avail_in = [self length];
	strm.total_out = 0;
	strm.zalloc = Z_NULL;
	strm.zfree = Z_NULL;
	
	if (inflateInit2(&strm, (15+32)) != Z_OK) return nil;
	while (!done)
	{
		// Make sure we have enough room and reset the lengths.
		if (strm.total_out >= [decompressed length])
			[decompressed increaseLengthBy: half_length];
		strm.next_out = [decompressed mutableBytes] + strm.total_out;
		strm.avail_out = [decompressed length] - strm.total_out;
		
		// Inflate another chunk.
		status = inflate (&strm, Z_SYNC_FLUSH);
		if (status == Z_STREAM_END) done = YES;
		else if (status != Z_OK) break;
	}
	if (inflateEnd (&strm) != Z_OK) return nil;
	
	// Set real length.
	if (done)
	{
		[decompressed setLength: strm.total_out];
		return [NSData dataWithData: decompressed];
	}
	else return nil;
}

- (NSData *)gzipDeflate
{
	if ([self length] == 0) return self;
	
	z_stream strm;
	
	strm.zalloc = Z_NULL;
	strm.zfree = Z_NULL;
	strm.opaque = Z_NULL;
	strm.total_out = 0;
	strm.next_in=(Bytef *)[self bytes];
	strm.avail_in = [self length];
	
	// Compresssion Levels:
	//   Z_NO_COMPRESSION
	//   Z_BEST_SPEED
	//   Z_BEST_COMPRESSION
	//   Z_DEFAULT_COMPRESSION
	
	if (deflateInit2(&strm, Z_DEFAULT_COMPRESSION, Z_DEFLATED, (15+16), 8, Z_DEFAULT_STRATEGY) != Z_OK) return nil;
	
	NSMutableData *compressed = [NSMutableData dataWithLength:16384];  // 16K chunks for expansion
	
	do {
		
		if (strm.total_out >= [compressed length])
			[compressed increaseLengthBy: 16384];
		
		strm.next_out = [compressed mutableBytes] + strm.total_out;
		strm.avail_out = [compressed length] - strm.total_out;
		
		deflate(&strm, Z_FINISH);
		
	} while (strm.avail_out == 0);
	
	deflateEnd(&strm);
	
	[compressed setLength: strm.total_out];
	return [NSData dataWithData:compressed];
}

@end