Difference Between Cookie and Checksum
Cookie:
The purpose of the cookie is to login to the monitor from an ALIVE command. We detail how this can happend as follows. The opening exchange from the monitor looks like this: 
   COMMENT: Monitor Version 2.2
   REQUIRE: IDENT
   WAITING:
A possible response is 
   IDENT franco_1
The Monitor then sends 
 
   REQUIRE: PASSWORD 
   WAITING:
A possible response is 
   PASSWORD 0/G^f)_W9-r'Td.pD=D::0o+YfL-d-m>ia\*7i;@'7
The monitor sends back something like this: 
   RESULT: PASSWORD 8QQVEJGSJWQ7IGQ0EUB
   REQUIRE: HOST_PORT
   WAITING:
The value 8QQVEJGSJWQ7IGQ0EUB is called the monitor password or the cookie. It is needed to log in to the monitor when an ALIVE is received. For example, the monitor sends 
   REQUIRE: ALIVE
   WAITING:
and the response of the player (either the active client or passive server - either one may alive itself) is 
   ALIVE 8QQVEJGSJWQ7IGQ0EUB

Checksum:
The purpose of the checksum is to authenticate the monitor. The message group 

   PARTICIPANT_PASSWORD_CHECKSUM:  1ead131dd863da2ef558580d29080af8effa6d7
   RESULT: IDENT
   REQUIRE: ALIVE
   WAITING:
contains the checksum. It is the user's responsibility to use the checksum to authenticate the monitor before proceeding with the ALIVE. The checksum is a SHA-1 digest of the participant's password, represented in hexadecimal. If the checksum matches the hash of the password, the player can be assured that whatever it is talking to knows its password (well, maybe the password was stolen - but hopefully not). You might use code such as the following to make this check (I have not checked this but it has the important characteristics such as BigInteger(1,md.digest()), and big.toString(16), and equals instead of ==: 
   public boolean Verify(String passwd, String chksum) 
      try {
         MessageDigest md = MessageDigest.getInstance("SHA"); 
         md.update(passwd.toUpperCase().getBytes());
         BigInteger big = new BigInteger(1,md.digest());
         if ((big.toString(16).equals(chksum.trim()))) return true;
      } catch (NoSuchAlgorithmException e) { }
      return false;
   }