这是我使用的类别:

NSString+MD5.h

@interface NSString (MD5)

- (NSString *)MD5String;

@end

NSString+MD5.m

#import <CommonCrypto/CommonDigest.h>

@implementation NSString (MD5)

- (NSString *)MD5String {
    const char *cStr = [self UTF8String];
    unsigned char result[CC_MD5_DIGEST_LENGTH];
    CC_MD5( cStr, (CC_LONG)strlen(cStr), result );

    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

使用

NSString *myString = @"test";
NSString *md5 = [myString MD5String]; // returns NSString of the MD5 of test