Improve performance of ClaimsIdentity

Background and Motivation

.NET 4.5 introduced ClaimsIdentity, ClaimsPrincipal, and Claim as a standard way to represent the claims in a SecurityToken. Since then there are performance and consistency improvements that can be made.

ASP.NET Core uses Microsoft.IdentityModel.* 7.x packages. In this version, when IdentityModel reads an incoming token, it parses it into a JsonWebToken instance and the claims are stored in a dictionary. The JsonWebToken.Claims collection is lazily created by iterating through that dictionary and creating and adding a new Claim instance for each claim value. When ClaimsIdentity is requested and IdentityModel creates one, it iterates through a collection of JsonWebToken.Claims, creates a new Claim, and adds it to an instance of ClaimsIdentity to be returned. When ClaimsIdentity methods like HasClaim and FindFirst are called, they operate on the IEnumerable<Claim> in that ClaimsIdentity instance. The full Claim collection is created even if only one claim is needed. Methods like HasClaim have to iterate through the whole Claim collection.

Proposed API

IdentityModel introduces a new type, SecurityTokenClaimsIdentity, which will derive from ClaimsIdentity. When ASP.NET uses IdentityModel an instance of this new type will be returned. SecurityTokenClaimsIdentity holds a backing instance of JsonWebToken (it's of more general type SecurityToken, but currently only JsonWebToken supported). Operations like HasClaim and FindFirst will now look in the claims dictionary. Claim collection will be generated only when all claims are requested and only once.

This PR shows initial in-progress proposal. The initial performance results show ~65% reduction in latency and ~40% reduction in allocations for FindFirst and HasClaim.

This change does mean that the ClaimsIdentity is now case-sensitive. IdentityModel 8 already introduced CaseSensitiveClaimsIdentity and uses it by default. The goal is for the behavior of SecurityTokenClaimsIdentity operations to match the behavior of ClaimsIdentity (except the case-sensitivity).

Once SecurityTokenClaimsIdentity is generally available, ASP.NET can upgrade the IdentityModel package and use that type internally. IdentityModel will provide a flag to enable or disable the use of this new type by default.

ASP.NET can also expose a flag, likeUseSecurityTokenClaimsIdentity in an options class like JwtBearerOptions, OpenIdConnectOptions.

Usage Examples

ASP.NET can still use ClaimsIdentity as a return type in methods, only the implementation and case sensitive behavior will change as stated above. The users can use the new ASP.NET version without any changes in their code.

Alternative Designs

ASP.NET can make this new ClaimsIdentity behavior opt-in and expose an options flag to enable it.

Risks

The new ClaimsIdentity type will be case-sensitive, which is a breaking change.
JsonWebToken doesn't support adding and removing claims; ClaimsIdentity does. Also for these operations, ClaimsIdentity does a comparison by Claim reference while JsonWebToken compares by claim type and claim value. Work will need to be done to make sure SecurityTokenClaimsIdentity behavior is on par with ClaimsIdentity in these cases.

dotnet / aspnetcore