Judith King/3(. 400 Security

Confidentiality

Confidentiality of the message is best covered by encryption at all levels, methods for which are de- scribed above. Also described above is the method by which X.400 provides for the labelling of the message showing the level of security and confidentiality with which it should be treated.

Conclusion

The security features defined under the 1988 version of the X.400 recommendations, whilst wide, do not fully cover all security issues on all levels,for example they do not deal with internal security horn the User Agent to the final user himself, nor do they cover the destruction of messages, although

Letter to the Editor Ross Anderson University Computer Laboratory, Pembroke Street, Cambdge CBZ 3QC, UK

In Rotraut Laun’s article ‘Asymme- tric User Identification’ (VII p 173-183), she presents a protocol with an elementary error. In it, Alice wants to check Bob’s identity, and so she sends him a random number encrypted under his public key, which he must decrypt and return.

When the encryption algorithm is RSA (which seems to be implied), this protocol works as follows. Let

Bob’s public and secret exponents be E and D respectively, and let all the arithmetic be done with respect to his public modulus. Alice chaos

!? a random number r, forms

R = r , and sends it to him. He is expected to grove his identity by calculating R , which is equal to r,

and sending it back to her.

If he does this, however, he lays himself open to fraud. Alice can abuse the protocol by sending him

X.400 can help to identify that a message has gone astray. The X.400 security features must be im- plemented as part of an overall security messaging policy, as con- formance to X.400 in itself does not provide any guarantees for se- curity.

a series ofdetective values vi, where vi = RiPI for random Ri = riE and Pi is the ith prime nsmber.Jhen once he s nds her m

% - r& , she

knows Pi and can respond cor- rectly to any challenge she can factor. Even more seriously, if he uses the same secret key for digital signatures, she can easily construct messages which are products of the primes Pi and forge his signature on these.

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