The previous chapter described specific X.400 services that counter 
threats to the MHS.  Although the services employ a broad scope of
security mechanisms, there are some limitations to the 1988 X.400 
security architecture.  These limitations pertain to the token, the message
store, and some services provided by the MTS that access the content of 
messages. 
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One security limitation is that data in the token is encrypted before it
is signed.  This is considered a bad practice, because the 
recipient can only authenticate the encrypted data, not the plaintext data.  
In a worst case scenario, a malicious party can intercept a message, and 
create a new message keeping the encrypted content of the original message, 
but generating a new message token.  The new token would be signed by the 
malicious party.  Under these circumstances, the message recipient could be 
fooled into believing that the malicious party originated the message.  It 
should be noted that depending on how the security services are implemented, 
this scenario can be avoided.
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A second limitation pertains to the MS (Message Store).  An MS can accept
the delivery of messages on behalf of a UA.  If a message originator
requests {\em proof of delivery} for a message whose content is encrypted, 
and the message is delivered to an MS, the MS would require access to the 
encryption key to provide the service (the proof must be computed using 
the plaintext content).  This would involve providing the MS with the 
recipient's private asymmetric key, or the symmetric key shared between 
the originator and the recipient.  Neither case is desirable from a 
security standpoint.  This is one scenario where a recipient (i.e., the
MS) might ignore the originator's request for {\em proof of delivery}.
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A final limitation pertains to MTS services which access the message
content.  An MTA may perform conversion on incoming messages,
such as converting telex data to IA5 data.  Any type of conversion 
invalidates integrity checks.  Also, if the content is encrypted, the
conversion cannot be performed without the MTA first decrypting the 
content.  To decrypt the content, the MTA would require access to the 
encryption key, which is not desirable from a security standpoint. 
