Problem - Kevin Tafuro

communicate asynchronously (that is, without taking turns), there is the possibility
for a race condition in which the SPC_CIPHERQ states on each side of the communication
get out of sync, which will needlessly cause decryption operations to fail.
If you need to perform asynchronous communication with an infrastructure like this,
you could use two SPC_CIPHERQ instances, one where the client encrypts messages for
the server to decrypt, and another where the server encrypts messages for the client
to decrypt.
The choice you need to make is whether each SPC_CIPHERQ object should be keyed
separately or should share the same key. Sharing the same key is possible, as long as
you ensure that the same {key, nonce} pair is never reused. The way to do this is to
manage two sets of nonces that can never collide. Generally, you do this by setting
the high bit of the nonce buffer to 1 in one context and 0 in another context.
Here’s a function that takes an existing context that has been set up, but not otherwise
used, and turns it into two contexts with the same key:
void spc_cipherq_async_setup(SPC_CIPHERQ *q1, SPC_CIPHERQ *q2) {
memcpy(q2, q1, sizeof(SPC_CIPHERQ));
q1->nonce[0] &= 0x7f; /* The upper bit of q1's nonce is always 0. */
q2->nonce[0] |= 0x80; /* The upper bit of q2's nonce is always 1. */
}
We show a similar trick in which we use only one abstraction in Recipe 9.12.
See Also
Recipes 4.11, 5.5, 5.10, 9.12, 11.2
5.17 Performing Block Cipher Setup (for CBC,
CFB, OFB, and ECB Modes) in OpenSSL
Problem
You need to set up a cipher so that you can perform encryption and/or decryption
operations in CBC, CFB, OFB, or ECB mode.
Solution
Here are the steps you need to perform for cipher setup in OpenSSL, using their
high-level API:
1. Make sure your code includes openssl/evp.h and links to libcrypto (-lcrypto).
2. Decide which algorithm and mode you want to use, looking up the mode in
Table 5-6 to determine which function instantiates an OpenSSLobject repre-
Performing Block Cipher Setup (for CBC, CFB, OFB, and ECB Modes) in OpenSSL | 221
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