做一个类别吧，以后用来很方便。代码如下：

.h

#import <Foundation/Foundation.h>

@interface NSString (Additions)

- (NSString *) MD5;

@end

.m

#import "NSString+Additions.h"
#import <sys/xattr.h>
#import <CommonCrypto/CommonDigest.h>

@implementation NSString (Additions)

- (NSString *) MD5 {
    // Create pointer to the string as UTF8
    const char* ptr = [self UTF8String];
    unsigned char md5Buffer[CC_MD5_DIGEST_LENGTH];
    
    // Create 16 byte MD5 hash value, store in buffer
    CC_MD5(ptr, (CC_LONG)strlen(ptr), md5Buffer);
    
    // Convert MD5 value in the buffer to NSString of hex values
    NSMutableString* output = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
    for (int i = 0; i < CC_MD5_DIGEST_LENGTH; i++) {
        [output appendFormat:@"%02x",md5Buffer[i]];
    }
    
    return output;
}
@end