CCNx makes heavy use of cryptography. Here we present a brief discussion of algorithm agility in CCNx.

Signature Algorithms

Whenever possible CCNx leaves the choice of signature algorithms (digest and public key) to use up to individual publishers and consumers, and specifies the algorithm used for a particular data item as part of the encoding of that item (for example, see SignatureGeneration). This is both an attempt to accommodate the needs of different users and to allow for the advancement in cryptographic algorithm design over time.

That said, CCNx nodes are free to drop content whose signatures they cannot verify, and so by selecting an exotic signature algorithm, a producer risks the non-propagation of their content. Eventually, like many network standards, there will likely be a list of generally-supported signature algorithms associated with CCNx that cautious producers may prefer to use.

Encryption Algorithms

Content encryption is opaque to the operation of CCNx. Content producers and consumers are free to use any encryption algorithm that they can agree on, specified in any fashion that works for them.

For ease of use and maximal efficiency, the CCNx core libraries provide basic encryption functionality; these currently support a limited algorithm set (AES in CTR mode in Java, no encryption support in C). Eventually the built-in encryption support will expand to allow for a wide range of potential encryption algorithms, with algorithm choice up to the producer, mindful of its intended consumers. Producers and consumers are always free to ignore this functionality, and apply encryption at the application layer.

Built-in Digest Algorithm

There are a small number of places in the core CCNx protocol where cryptographic digests are used. These are:

  • the calculation of PublisherPublicKeyDigests from publisher public keys, both specified in ContentObjects and used as selectors in Links and Interests

  • the calculation of ContentObject digests as the implicit last component of content names in Interest matching (see InterestMessage and DigestComponent)

  • the calculation of ContentObject digests as an Exclude specifier in an Interest, to avoid receiving that ContentObject again (this is not yet completely implemented)

To simplify operation of the lowest levels of CCNx, we choose to implement all three of these using a single, fixed digest algorithm tied to the top-level version of the CCNx protocol. While we understand this algorithm choice will evolve, we expect it to evolve slowly. For the immediate future, this algorithm will be SHA-256; we expect it to shift to SHA-3 when that standard is finalized.

This does leave the protocol open to the risk that legacy systems running old software supporting only outdated versions of the CCNx protocol may be vulnerable in the face of a catastrophic break of the algorithm used in those versions. Given the fact that such breaks tend not to happen overnight, and backwards-compatible evolution of the CCNx protocol is relatively simple, we see this as an acceptable risk given the implementation complexity and equivalent risks introduced by allowing this set of operations to specify their algorithm choice on the fly. Most of these examples, calculate their data on the fly, and so different digest algorithms can be used to query over the same set of (previously signed) ContentObjects over time.