Simple public key infrastructure (SPKI, pronouncedspoo-key) was an attempt to overcome the complexity of traditionalX.509public key infrastructure. It was specified in twoInternet Engineering Task Force (IETF)Request for Comments (RFC) specifications – RFC 2692[1] andRFC 2693[2] – from the IETFSPKI working group. These two RFCs never passed the "experimental" maturity level of the IETF'sRFC status. The SPKI specification defined an authorization certificate format, providing for the delineation of privileges, rights or other such attributes (calledauthorizations) and binding them to a public key. In 1996, SPKI was merged withSimple Distributed Security Infrastructure[3] (SDSI, pronouncedsudsy) byRon Rivest andButler Lampson.
The original SPKI had identified principals only aspublic keys but allowed binding authorizations to those keys and delegation of authorization from one key to another. The encoding used was attribute:value pairing, similar toRFC 822 headers.
The original SDSI bound local names (of individuals or groups) to public keys (or other names), but carried authorization only inAccess Control Lists (ACLs) and did not allow for delegation of subsets of a principal's authorization. The encoding used was standardS-expression. Sample RSA public key in SPKI in "advanced transport format" (for actual transport the structure would beBase64-encoded):
(public-key (rsa-pkcs1-md5 (e#03#) (n |ANHCG85jXFGmicr3MGPj53FYYSY1aWAue6PKnpFErHhKMJa4HrK4WSKTO YTTlapRznnELD2D7lWd3Q8PD0lyi1NJpNzMkxQVHrrAnIQoczeOZuiz/yY VDzJ1DdiImixyb/Jyme3D0UiUXhd6VGAz0x0cgrKefKnmjy410Kro3uW1|)))
The combined SPKI/SDSI allows the naming of principals, creation of named groups of principals and the delegation of rights or other attributes from one principal to another. It includes a language for expression of authorization – a language that includes a definition of "intersection" of authorizations. It also includes the notion ofthreshold subject – a construct granting authorizations (or delegations) only whenK ofN of the listed subjects concur (in a request for access or a delegation of rights). SPKI/SDSI uses S-expression encoding, but specifies a binary form that is extremely easy to parse – an LR(0) grammar – calledCanonical S-expressions.
SPKI/SDSI does not define a role for a commercialcertificate authority (CA). In fact, one premise behind SPKI is that a commercial CA serves no useful purpose.[4]As a result of that, SPKI/SDSI is deployed primarily in closed solutions and in demonstration projects of academic interest. Another side-effect of this design element is that it is difficult to monetize SPKI/SDSI by itself.[citation needed] It can be a component of some other product, but there is no business case for developing SPKI/SDSI tools and services except as part of some other product.
The most prominent general deployments of SPKI/SDSI are E-speak, a middleware product fromHP that used SPKI/SDSI for access control of web methods, andUPnP Security, that uses an XML dialect of SPKI/SDSI[citation needed] for access control of web methods, delegation of rights among network participants, etc.