Problem - Kevin Tafuro

xl
Pointer into which the length of the application-specific data is placed.
On success, AX_srv_exchange( ) will return a connected socket descriptor in blocking
mode that you can then use to talk to the client. On failure, an XXLexception will be
raised. The value of the exception will be either AX_CAUTH_ERR if we believe the client
refused our credentials or AX_SAUTH_ERR if we refused the client’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.
With a valid socket descriptor in hand, you can now use the exchanged key to set up
a secure channel, as discussed in Recipe 9.12. When you are finished communicating,
you may simply close the socket descriptor.
Note that whether or not the exchange with the client succeeds, AX_srv_exchange( )
will free the AC_CLIENT object passed into it. If the exchange fails, the socket descriptor
will be closed, and the client will have to reconnect in order to attempt another
exchange.
The client
The client side is a bit less work. We first connect to the server with the following
function:
AX *AX_connect(char *addr, unsigned short port, char *uname, size_t ulen,
char *extra, size_t elen, size_t protocol);
This function has the following arguments:
addr
IP address (or DNS name) of the server as a NULL-terminated string.
port
Port to which we should connect on the remote machine.
uname
Username.
ulen
Length of the username in bytes.
extra
Application-specific data discussed above.
elen
Length of the application-specific data in bytes.
protocol
Indication of the protocol you’re using to connect. As of this writing, the only
valid values are SAX_PROTOCOL_v1 and PAX_PROTOCOL_v1.
Performing Password-Based Authentication and Key Exchange | 427
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