对NSString类型进行MD5加密

用给时间进行加密做例子：

NSDate *  senddate=[NSDate date];

NSDateFormatter  *dateformatter=[[NSDateFormatter alloc] init];

[dateformatter setDateFormat:@"YYYY-MM-dd"];

NSString *  locationString=[dateformatter stringFromDate:senddate];

NSString *str = [locationString stringByAppendingString:@"tt.xmdj123.com"];

NSString *string = [self MD5With:str];

MD5加密方法：

-(NSMutableString *)MD5With:(NSString *)str{

const char *cStr = [str UTF8String];

unsigned char digest[CC_MD5_DIGEST_LENGTH];

CC_MD5( cStr, str.length, digest );

NSMutableString *result = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];

for(int i = 0; i < CC_MD5_DIGEST_LENGTH; i++)

[result appendFormat:@"%02x", digest[i]];

return result;

}