downgrade to plain.

Condition: after trying S256

Clients MUST NOT downgrade to "plain" after trying the "S256" method.

use S256.

Condition: if capable of using S256

If the client is capable of using "S256", it MUST use "S256".

use a PKCE challenge method that does not expose the verifier in the authorization request.

Condition: when using PKCE

When using PKCE, clients SHOULD use PKCE code challenge methods that do not expose the PKCE verifier in the authorization request.

encrypt a plain code challenge so that only the server can decrypt and extract it.

Condition: when returning the code challenge inside an authorization code to implement a stateless server

If the code challenge method is "plain" and the code challenge is to be returned inside authorization "code" to achieve a stateless server, it MUST be encrypted in such a manner that only the server can decrypt and extract it.

omit code_challenge_methods_supported rather than return an empty array.

Condition: when it would contain zero elements

Claims with zero elements MUST be omitted from the response.

support PKCE.

Authorization servers MUST support PKCE.

provide a way for clients to detect its PKCE support.

Authorization servers MUST provide a way to detect their support for PKCE.

publish code_challenge_methods_supported in its authorization server metadata with the supported PKCE challenge methods.

It is RECOMMENDED for authorization servers to publish the element code_challenge_methods_supported in their Authorization Server Metadata containing the supported PKCE challenge methods.

use a deployment-specific way to ensure or determine its PKCE support instead of publishing this metadata member.

Authorization servers MAY instead provide a deployment-specific way to ensure or determine PKCE support by the authorization server.

include code_challenge_methods_supported as a JSON array of supported PKCE code challenge methods.

OPTIONAL. JSON array containing a list of Proof Key for Code Exchange (PKCE) code challenge methods supported by this authorization server.

use PKCE to protect against authorization code misuse and injection.

Condition: when using the authorization code grant

For confidential clients, the use of PKCE is RECOMMENDED.

use the plain code challenge method.

Condition: except for compatibility with deployed implementations whose request path is already protected

"plain" SHOULD NOT be used and exists only for compatibility with deployed implementations where the request path is already protected.

use S256 or another cryptographically secure code challenge method extension.

The "S256" code challenge method or other cryptographically secure code challenge method extension SHOULD be used.

use the plain code challenge method.

Condition: unless S256 cannot be supported for a technical reason

The "plain" method SHOULD NOT be used in new implementations, unless they cannot support "S256" for some technical reason.

apply Unicode normalization to the metadata member name or method values during comparison.

Condition: when comparing JSON strings with other Unicode strings

Unicode Normalization MUST NOT be applied at any point to either the JSON string or the string it is to be compared against.

compare the metadata member name and method values using Unicode code-point equality.

Condition: when comparing JSON strings with other Unicode strings

Comparisons between the two strings MUST be performed as a Unicode code-point-to-code-point equality comparison.

use PKCE to prevent authorization code injection and misuse.

Condition: when using the authorization code grant

Public clients MUST use PKCE to this end.