This code system http://terminology.hl7.org/CodeSystem/restful-security-service defines the following codes:

Code Display Definition
OAuth OAuth OAuth (unspecified version see oauth.net).
SMART-on-FHIR SMART-on-FHIR OAuth2 using SMART-on-FHIR profile (see http://docs.smarthealthit.org/).
NTLM NTLM Microsoft NTLM Authentication.
Basic Basic Basic authentication defined in HTTP specification.
Kerberos Kerberos see http://www.ietf.org/rfc/rfc4120.txt.
Certificates Certificates SSL where client must have a certificate registered with the server.
<status value="active"/><experimental value="false"/><date value="2020-12-28T16:55:11+11:00"/><publisher value="HL7 (FHIR Project)"/><contact><telecom><system value="url"/><value value="http://hl7.org/fhir"/></telecom><telecom><system value="email"/><value value="fhir@lists.hl7.org"/></telecom></contact><description value="Types of security services used with FHIR."/><caseSensitive value="true"/><valueSet value="http://hl7.org/fhir/ValueSet/restful-security-service"/><content value="complete"/><concept><code value="OAuth"/><display value="OAuth"/><definition value="OAuth (unspecified version see oauth.net)."/></concept><concept><code value="SMART-on-FHIR"/><display value="SMART-on-FHIR"/><definition value="OAuth2 using SMART-on-FHIR profile (see http://docs.smarthealthit.org/)."/></concept><concept><code value="NTLM"/><display value="NTLM"/><definition value="Microsoft NTLM Authentication."/></concept><concept><code value="Basic"/><display value="Basic"/><definition value="Basic authentication defined in HTTP specification."/></concept><concept><code value="Kerberos"/><display value="Kerberos"/><definition value="see http://www.ietf.org/rfc/rfc4120.txt."/></concept><concept><code value="Certificates"/><display value="Certificates"/><definition value="SSL where client must have a certificate registered with the server."/></concept></CodeSystem>