From Cryptography to Communication Security - A Classical Introduction to Cryptography: Applications for Communications Security - page 300

Client Server

nonce C
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→
ServerHello :TLS DHE sig cipher hash

ClientHello :acceptable cipher suites ,

nonce S
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

, certificate ,

select TLS DHE sig cipher hash

))
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

ServerKeyExchange : p

, g , g x mod p , sig ( hash ( p , g , g x mod p

select p , g , pick x

ClientKeyExchange : g y mod p
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→

g xy mod p pick y pre master secret =

Figure 12.12. TLS key exchange using DHE DSS or DHE RSA.

his key and the selected authentication algorithm. The signed parameters are
sent in an additional ServerKeyExchange message which immediately
follows the ServerHello message. So the client can just take the signed
Diffie-Hellman parameters, check the signature by using the authenticated
public key from the certificate, pick his Diffie-Hellman public value g y mod
p , and send it to the server in the ClientKeyExchange message (see
Fig. 12.12).
DH anon: This is a particular case of the previous Diffie-Hellman protocols in
which the parameters are not authenticated. It does not require any certifi-
cate, but it is vulnerable to a man-in-the-middle attack. Like in the ephemeral
Diffie-Hellman algorithms, the server selects his chosen Diffie-Hellman pa-
rameters and public value ( p g x mod p ) and sends them in an additional

ServerKeyExchange message. So the client can just take the Diffie-
Hellman parameters, pick his Diffie-Hellman public value g y mod p , and
send it to the server in the ClientKeyExchange message (see Fig. 12.13).

, g ,

12.3.7 Key Derivation

A hash algorithm hash defines a pseudorandom generator P hash . Given a secret
secret and a seed seed , we define a sequence P hash ( secret , seed )by

P hash ( secret , seed )

= r 1 , r 2 , r 3 ,...

where r i =

,

a i , seed ), a i =

,

a i − 1 ), and a 0 =

HMAC hash ( secret HMAC hash ( secret

seed .

Client Server

,

nonce C
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→
ServerHello :TLS DH anon cipher hash

ClientHello :acceptable cipher suites

,

nonce S
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

select TLS DH anon cipher hash

ServerKeyExchange : p , g , g x mod p
←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− select p , g , pick x

ClientKeyExchange : g y mod p
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→

g xy mod p pick y pre master secret =

Figure 12.13. TLS key exchange using DH anon.