URCA documentation

Pages for the Unified Resource for Cryptographic Arrays project.

Motivation

When talking about simmetric ciphers, it could be handy having a resource that efficiently encrypt and decrypt batch of texts. Moreover, if a change of sizes is needed, there are many constraints to take into account in order to have the standard implementation working correctly.

The URCA project tries to provide a standard for the above tasks. Moreover, URCA tries to provide a tool that evaluates a generalised version of the cipher keeping the reference configurations.

Note that all over the documentation, \(\mathbb{N}\) is the set of natural numbers excluding \(0\), unless otherwise specified.

First and last rounds

One of the most common scheme for a simmetric cipher is to have a number of rounds repeating the same structure. This is the most important skeleton of the cipher.

However, it is frequent to have some changes in the first round or even in the last one. When this occurs in some design, it is important to say what is and what is not a round. Think about Speck [BSS2015], PRESENT [BKL2007] and AES [DR2003].

They represent three important cases.

  1. Speck is word oriented and has not corner cases. Its implementation is suitable for any word size, unless constraints on parameters are met.

  2. PRESENT is a bit oriented cipher. Note that there is a final XOR with the key, meaning that the last round is different from the other ones. Moreover, in order to be compliant with the reference implementation, some constraints on the parameters should be met (e.g. the key must be 5/4 or 8/4 of the text).

  3. AES has a declared different last round. It lacks of the mixcolumn operation.

Numbering rounds

Numbering rounds is another task that should be accomplished carefully and consistently. Think again about Speck. The key scheduling can be inserted starting from the second round, but this could be misleading since the state of the cipher is not preserved as per round. The suggestion from this project is to consider ciphers as follows.:

| ---- (registers == state) ----------------------------------- |
| ---- text[0] (plaintext) ---- | ---- key[0] (master-key) ---- |
         |                                                         |
         | (schedule)                                              |
         |                                                         |
         V                                                         |
| ---- text[1] ---------------- | ---- key[0] ----------------- |  | round[0]
                                        |                          |
                                        | (key-schedule)           |
                                        |                          |
                                        V                          V
| ---- text[1] ---------------- | ---- key[1] ----------------- |

Note that the schedule and the key-schedule can be swapped.

Moreover, the proposed standard from this project is to use 0 as index for the plaintext and the master-key and considering both the starting round and the number of rounds for encryption/decryption.

API

Bibliography