Problem - Kevin Tafuro

}
static void spc_ssock_write( int fd, unsigned char *msg, size_t mlen) {
ssize_t w;
while (mlen) {
if ((w = write(fd, msg, mlen)) == -1) {
switch (errno) {
case EINTR:
break;
default:
abort();
}
} else {
mlen -= w;
msg += w;
}
}
}
Let’s look at the rest of the client side of the connection, before we turn our attention
to the server side. When the client wishes to terminate the connection politely, it
will send an empty message but pass 0xff as the status byte. It must still send the
proper nonce and encrypt a zero-length message (which CWC will quite happily do).
That can be done with code very similar to the code shown previously, so we won’t
waste space by duplicating the code.
Now let’s look at what happens when the client receives a message. The status byte
should be 0x00. The nonce we get from the server should be unchanged from the one
we just sent, except that the first byte should be SPC_SERVER_DISTINGUISHER. If the
nonce is invalid, we’ll just fail by aborting, though you could instead discard the
message if you choose to do so (doing so is a bit problematic, though, because you
then have to resync the connection somehow).
Next, we’ll read the length value, dynamically allocating a buffer that’s big enough to
hold the ciphertext. This code can never allocate more than 232–1 bytes of memory.
In practice, you should probably have a maximum message length and check to
make sure the length field doesn’t exceed that. Such a test can keep an attacker from
launching a denial of service attack in which she has you allocate enough memory to
slow down your machine.
Finally, we’ll call cwc_decrypt_message( ) and see if the MAC validates. If it does,
we’ll return the message. Otherwise, we will abort.
static void spc_ssock_read(int, unsigned char *, size_t);
static void spc_get_status_and_nonce(int, unsigned char *, unsigned char *);
static unsigned char *spc_finish_decryption(spc_ssock_t *, unsigned char,
unsigned char *, size_t *);
unsigned char *spc_client_read(spc_ssock_t *ctx, size_t *len, size_t *end) {
unsigned char status;
unsigned char nonce[SPC_CWC_NONCE_LEN];
Building an Authenticated Secure Channel Without SSL | 497
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