Understanding Security APIs - CrySyS Lab

certificates they produce. It is easy for an HSM to provide protection for the CA’s
signing keys in the face of simple threats such as theft of computer equipment or
hacking of the corporate network, but they also have potential to process policy components
and genuinely assist in the operation of a Certification Authority – but to
do this the HSM’s Security API policy must take into account the threats, which are
linked to the purpose and value of the certificates which the CA produces. Studying
the underlying PKI that the certificates belong to can help build an understanding
of the threat.
PKIs are dynamic collections of keys and certificates (documents signed by keys)
which reflect a management structure or usage policy. PKIs are used to add structure
and preserve the integrity of systems with large numbers of principals, where the
structure is constantly changing. Examples include
• Logon authentication for large companies, maybe with 10000 or more employees
and offices in different countries with different IT staff running computer
systems in each country.
• Delegating and managing code signing ability to ensure that programmers
contributing to a large website such as www.microsoft.com can only upload
bona fide code.
• Transaction authorisation – establishing digital identities for people, and enforcing
signature policies to validate transactions (e.g. a stock trade must be
authorised by one trader and one manager).
Typical tasks performed in a certification authority are registration & certification,
certificate renewal, and certificate revocation.
4.3.2 Threat Model
Most of the sensitive information within a PKI has no intrinsic value: certificates
and keys are means to an end, worthless when divorced from their context. The one
exception is the root key for an API, which could be considered to have intrinsic
value developed by expending effort securely distributing it to millions of end-users.
A corporate brand name is a good analogue for a PKI root key – it can be absolutely
mission-critical. Adequate protection for the PKI root key is vital.
Lower down the hierarchy of keys and certificates, there may be no intrinsic value
to key material, but revocation and reissue could still be expensive. It may be
permissible to detect abuse rather than absolutely prevent it. The cost of extra
protection must be weighed up against perceived threat, and cost of key material
reissue. Late detection strategies are also favourable when the security is only to
achieve “due diligence” – buying products which satisfy accreditation procedures
and insurance requirements in order to shift liability.
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