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There is a good way to create StringUtil.m to be reusable for every project...
StringUtil.h

#import <Foundation/Foundation.h>
#import <CommonCrypto/CommonDigest.h>

@interface StringUitil : NSObject {
}
+ (NSString *)MD5:(NSString *)str;
@end

StringUtil.m

#import "StringUitil.h"


@implementation StringUitil

+ (NSString *)MD5:(NSString *)str {
	const char *cStr = [self UTF8String];
    unsigned char result[16];
    CC_MD5( cStr, strlen(cStr), result ); // This is the md5 call
    return [NSString stringWithFormat:
			@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
			result[0], result[1], result[2], result[3], 
			result[4], result[5], result[6], result[7],
			result[8], result[9], result[10], result[11],
			result[12], result[13], result[14], result[15]
			];  
}
@end

example:
NSLog(@"%@",[StringUitl MD5:@"sinzy"]);//output '09db6026d7bdb8dc662e8daae1db19a8'

the second waty to create catalog for NSString and NSData

MyAddition.h

@interface NSString (MyAddition)
- (NSString *)md5;
@end

@interface NSData (MyAddition)
- (NSString *)md5;
@end

MyAddition.m

#import "MyAddition.h"
#import <CommonCrypto/CommonDigest.h>

@implementation NSString (MyAddition)
- (NSString *)md5 {
	const char *cStr = [str UTF8String];
	unsigned char result[16];
	CC_MD5( cStr, strlen(cStr), result ); ;//This is the md5 call
	return [NSString stringWithFormat:
			@"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
			result[0], result[1], result[2], result[3], 
			result[4], result[5], result[6], result[7],
			result[8], result[9], result[10], result[11],
			result[12], result[13], result[14], result[15]
			];
}
@end

@implementation NSData (MyAddition)
- (NSString*)md5
{
    unsigned char result[16];
    CC_MD5( self.bytes, self.length, result ); // This is the md5 call
    return [NSString stringWithFormat:
        @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
        result[0], result[1], result[2], result[3], 
        result[4], result[5], result[6], result[7],
        result[8], result[9], result[10], result[11],
        result[12], result[13], result[14], result[15]
        ];  
}
@end

example:
NSLog(@"%@", [@"sinzy" md5]);//09db6026d7bdb8dc662e8daae1db19a8

MD5 generator online by php language:
http://www.adamek.biz/md5-generator.php