5 Abstract key types and Hardware security modules

In several cases storing the long term cryptographic keys in a hard disk or even in memory poses a significant risk. Once the system they are stored is compromised the keys must be replaced as the secrecy of future sessions is no longer guaranteed. Moreover, past sessions that were not protected by a perfect forward secrecy offering ciphersuite are also to be assumed compromised.

If such threats need to be addressed, then it may be wise storing the keys in a security module such as a smart card, an HSM or the TPM chip. Those modules ensure the protection of the cryptographic keys by only allowing operations on them and preventing their extraction. The purpose of the abstract key API is to provide an API that will allow the handle of keys in memory and files, as well as keys stored in such modules.

In GnuTLS the approach is to handle all keys transparently by the high level API, e.g., the API that loads a key or certificate from a file. The high-level API will accept URIs in addition to files that specify keys on an HSM or in TPM, and a callback function will be used to obtain any required keys. The URI format is defined in [PKCS11URI].

More information on the API is provided in the next sections. Examples of a URI of a certificate stored in an HSM, as well as a key stored in the TPM chip are shown below. To discover the URIs of the objects the p11tool (see p11tool Invocation).

pkcs11:token=Nikos;serial=307521161601031;model=PKCS%2315; \
manufacturer=EnterSafe;object=test1;type=cert