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January 8, 2015 / Jim Fenton

Level of Assurance alternatives: A modest proposal

There has recently been a good deal of attention to the concept of Level of Assurance (LOA), and the unsuitability of a one-dimensional, four-level classification of authentication quality for many current requirements including NSTIC and many non-government uses that had adopted the LOA model. On top of that, President Obama issued an Executive Order last October that further motivates change by requiring two-factor authentication for all US Government transactions with consumers that release their personal information.

I have been thinking about ways to improve (or replace) Levels of Assurance for a few years now. Among the comments I made about NIST Special Publication 800-63 during its 2011 revision to SP-800-63-1 was that LOA didn’t support the NSTIC model of separate identity (authentication) and attribute providers very well, since LOA encompasses aspects of both. That would have required major surgery to SP 800-63, and would make its responsiveness to OMB M-04-04, the requirements document from the White House, less clear. So a paragraph was inserted in the introduction:

Current government systems do not separate functions related to identity proofing in registration from credential issuance. In some applications, credentials (used in authentication) and attribute information (established through identity proofing) could be provided by different parties. While a simpler model is used in this document, it does not preclude agencies from separating these functions.

Briefly, the four LOA levels are:

  • LOA 1: Some assurance that this is the same Claimant who participated in previous transactions
  • LOA 2: Single factor network authentication
  • LOA 3: Multi-factor remote network authentication
  • LOA 4: Strong multi-factor cryptographic authentication

Here are some of the problems that I see with the current LOA structure:

  • By combining authentication and identity proofing, strong pseudonymous authentication is not recognized. Pseudonymous transactions are limited to LOA 1 or 2.
  • LOA 2 is sometimes pseudonymous and sometimes not (requiring identity proofing). It should only mean one thing.
  • LOA 2 and 3 identity proofing requirements are different, but not substantially so.

To avoid confusion, we should avoid the use of the term “Assurance” in the new structure.


I propose that we have two dimensions to the new structure, which I will call authentication strength and attribute reliability. Authentication strength refers to the technical strength of the authentication process itself, for example, whether it is single or multi-factor. Attribute reliability refers to the quality of the identity proofing process, as well as the strength of the binding of the identifying attributes to the authentication.

I am proposing that we have three levels each for strength and reliability, as follows:

Authentication Strength

  • Level 1: Some confidence in the authentication (typically username/password)
  • Level 2: High confidence (Two or more factors used together)
  • Level 3: Very high confidence (Two or more factors including a hardware token)

Note that account reset and recovery must be done at a commensurate level of strength with the original authentication.

Attribute Reliability

  • Level 1: Self-asserted
  • Level 2: Reliable attribute (Remotely identity proofed)
  • Level 3: Very reliable (In-person identity proofed or derived from identity-proofed assertions)

Detailed requirements for the strength and reliability levels will of course need to be worked out.

Why not more levels? One has to ask whether additional levels would be actionable on the part of relying parties. As it is, one could consider there to be are theoretically nine strength x reliability levels, although not all of them may be useful. High strength with low reliability might be used in cases where strong pseudonymous authentication is required, but I struggle to find a use for high reliability with low strength.

These strength/reliability levels map into the existing assurance levels as follows:

Level of Assurance Strength Reliability
1 1 1
2/Pseudonymous 1 1
2/Non-pseudonymous 1 2
3 2 2
4 3 3


What about other dimensions?

Various other dimensions have been proposed in discussions about LOA alternatives. For example, in a message to the IETF Vectors of Trust mailing list, Justin Richer offered a strawman that included assertion presentation and operational management. Assertion presentation (leakiness of the federation protocol) is more related to the choice of network protocols, and is therefore more directly visible to the relying party, as compared with strength and reliability that need to be explicitly asserted. Operational management is a characteristic that would impact the accreditation (through a trust framework or otherwise) of identity and attribute providers, and therefore generally outside the scope of a given transaction. Assertion presentation and operational management might, however, place upper bounds on the levels of strength and reliability that a relying party is able to accept from a given provider.

Additional dimensions have a major impact on the complexity of the framework, probably multiplying the number of possible combinations by a factor of 3 or 4 per dimension.


Much of the inspiration and many ideas for this framework come from discussion on the IETF Vectors of Trust mailing list.


Leave a Comment
  1. Lauren / Jun 15 2017 7:58 am

    Hi Mr. Fenton,

    I am a student researcher at CEPS interested in alternative forms of measuring secure authentication. I would be curious if you have any information particularly on the use of double-authentication measures in e-commerce. Thanks.

    • Jim Fenton / Jun 16 2017 8:44 pm

      Hi Lauren,

      E-commerce is one of the areas where 2-factor authentication is infrequently used. Merchants are concerned with making it as easy as possible for people to buy things, and are willing to sustain some losses in order to do so. It’s effectively the online version of shoplifting, which merchants simply build into their pricing structure.

      Besides, the losses are “only money” (or goods), in contrast to the disclosure of sensitive personal information, such as medical records. It’s easy to mitigate the loss of money with insurance (either purchased or self-provided).

      There was considerable concern when the United States recently began using EMV (chip) cards for transactions that it would make in-person fraud harder to commit and more of it would move online. I haven’t heard the extent to which that happened, but it was primarily a question of whether the e-merchants had enough reserves set aside for a change in the likely proportion of fraudulent transactions.

      Two-factor authentication requires more effort on the part of the customer, and is therefore considered unattractive for e-commerce. In contrast, things like Amazon’s one-click purchasing are popular with retailers.

      Hope this helps!

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