Problem - Kevin Tafuro

This call will throw an XXLexception if there’s a socket error. Otherwise, it will
return an object dynamically allocated with malloc( ) that contains the key exchange
state.
If the user is expected to know the salt (i.e., if the server will not send it over the network),
you must enter it at this time, with the following function:
void AX_set_salt(AX *p, char *salt, size_t saltlen);
AX_set_salt( ) expects the binary encoding that the server-side API produced. It is
your responsibility to make sure the user can enter this value. Note that this function
copies a reference to the salt and does not copy the actual value, so do not modify
the memory associated with your salt until the AX context is deallocated (which
happens as a side effect of the key exchange process; see the following discussion).
Note that, the first time you make the user type in the salt on a particular client
machine, you should save the salt to disk. We strongly recommend encrypting the
salt with the user’s supplied password, using an authenticated encryption mode and
the key derivation function from Recipe 4.10.
Once the client knows the salt, it can initiate key exchange using the following function:
int AX_exchange(AX *p, char *pw, size_t pwlen, size_t keylen, char *key);
This function has the following arguments:
p
Pointer to the context object that represents the connection to the server.
pw
Password, treated as a binary string (i.e., not NULL-terminated).
pwlen
Length of the associated password in bytes.
keylen
Key length the client desires in the exchange. The server must be prepared to
serve up keys of this length; otherwise, the exchange will fail.
key
Buffer into which the key will be placed if authentication and exchange are successful.
On success, AX_exchange( ) will return a connected socket descriptor in blocking
mode that you can then use to talk to the server. On failure, an XXLexception will
be raised. The value of the exception will be either AX_CAUTH_ERR if we believe the
server refused our credentials or AX_SAUTH_ERR if we refused the server’s credentials.
In both cases, it is possible that an attacker’s tampering with the data stream caused
the error. On the other hand, it could be that the two parties could not agree on the
protocol version or key size.
428 | Chapter 8: Authentication and Key Exchange
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