import <CommonCrypto/CommonCrypto.h>//头文件

-(NSString *)MD5ForLower32Bate:(NSString *)str{

//要进行UTF8的转码
const char* input = [str UTF8String];
unsigned char result[CC_MD5_DIGEST_LENGTH];
CC_MD5(input, (CC_LONG)strlen(input), result);

NSMutableString *digest = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
for (NSInteger i = 0; i < CC_MD5_DIGEST_LENGTH; i++) {
    [digest appendFormat:@"%02x", result[i]];
}

return digest;

}