Security officers should be aware that many legacy systems cannot be upgraded to full PIV capability because the interoperability and PKI requirements inherent in a PIV system exceed the computing power and memory capabilities of the legacy system. It may possible, however, to utilize the existing cabling and door hardware when installing a new secure access system.

Most legacy systems (proximity, magnetic stripe and bar code) are based on 26-bit card numbers, of which 8 bits are reserved for the local facility code and 16 bits for the card ID number. One low cost solution that has been proposed is to "migrate" legacy systems simply by replacing the existing access readers with PIV readers.

Table 1
CAC CUID Number	16-bit Hash
4090000C22B9C8111C16	07190
409000029840000121C16	07190

This can be risky. First, PIV is a cross-agency solution that does not provide for local facility codes. The result is that the security provided by the site code data element will be lost when PIV credentials are used in a legacy system. This leaves only the 16-bit card number to match.

Second, for a PIV to be "matched" in a 16-bit system, the credential ID number (FASC-N or CHUID) must be truncated (hashed down) to 16 bits. Any hashing down process, regardless of the "algorithm" used, causes a loss of data and therefore a loss of uniqueness. Two credentials can now appear identical to the system. This is termed an "identifier collision" and the probability of this happening is surprisingly large: in a 16 bit system there is a 50% probability of the first collision after just 309 PIV credentials are enrolled.

Table 1 shows the result of two distinctly different Card Unique Identifiers (CUIDs) from two issued CAC cards being hashed down from 80 bits to 16 bits. The result is a collision with both showing access identifier codes of 07190.

When a collision happens, it happens at both "ends" of the access system. First, at enrollment, a hashed down number that collides cannot be enrolled because the number is already enrolled in the system. Second, the colliding credential will be permitted access just as the enrolled credential is permitted access because the system thinks the credential is enrolled and authorized.

A number of vendors provide access readers that perform some form of hashing operation. This approach to "upgrade" an access system can save money, but if every component in the system cannot support a match of the complete identifier there is significantly reduced security. NIST 800-116 recommends against this solution.

NIST Recommendation: "The presence of a unique identifier allows a PIV card to be uniquely identified by a relying system such as a PACS. If the unique identifier is ever truncated, compressed, hashed, or modified, information could be lost. If information is lost from the unique identifier before it is compared against Access Control List entries, multiple cards may generate the same reduced identifier. This is called an identifier collision. A collision means that multiple PIV Cards will appear to belong to the same person, and will all be granted the same access privileges." "The PIV card mitigates the risk of collision by defining a unique FASC-N identifier for the purposes of physical access control decisions. To prevent collisions, all access control decisions should be made by comparing the 14 digit FASC-N Identifier." --NIST SP-800-116, Paragraph 4.1 Identifier Collisions

Some reader manufacturers provide "multi-technology" readers that are capable of reading both legacy and PIV credentials. In this scenario any legacy token can collide with any hashed- down PIV number and conversely, any hashed-down PIV number can collide with any legacy card. NIST 800-116 also recommends against this solution.

NIST Statement: "Deployed proximity and magnetic stripe authentication are not PIV authentication mechanisms and when used in conjunction with PIV authentication mechanisms there is a strong potential for "collisions." ...Moreover, proximity and magnetic stripe cards can be easily cloned." --NIST SP-800-116, Paragraph 7.1.1 Deployed Proximity or Magnetic Stripe Authentication

Collisions are actually considered a form of "attack" by crypto-analysts and the probability of this type of attack succeeding is simply too high when the security of federal facilities, information and personnel is considered.

HSPD-12 is not about replacing access readers to read a particular set of data. The Directive is about securing America’s assets. Every agency’s strategy should be to achieve an end-state for physical access control that delivers the assurance needed when that agency is "under attack" from terrorism, cyber-attack or natural disaster (all hazards).