ETSI TS 133 122 V15.3.0 (2019-04)

TECHNICAL SPECIFICATION
LTE;
5G;
Security aspects of Common API Framework (CAPIF)
for 3GPP northbound APIs
(3GPP TS 33.122 version 15.3.0 Release 15)

3GPP TS 33.122 version 15.3.0 Release 15 1 ETSI TS 133 122 V15.3.0 (2019-04)

Reference
RTS/TSGS-0333122vf30
Keywords
5G,LTE,SECURITY
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ETSI
3GPP TS 33.122 version 15.3.0 Release 15 2 ETSI TS 133 122 V15.3.0 (2019-04)
Intellectual Property Rights
Essential patents
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Foreword
This Technical Specification (TS) has been produced by ETSI 3rd Generation Partnership Project (3GPP).
The present document may refer to technical specifications or reports using their 3GPP identities, UMTS identities or
GSM identities. These should be interpreted as being references to the corresponding ETSI deliverables.
The cross reference between GSM, UMTS, 3GPP and ETSI identities can be found under
http://webapp.etsi.org/key/queryform.asp.
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
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3GPP TS 33.122 version 15.3.0 Release 15 3 ETSI TS 133 122 V15.3.0 (2019-04)
Contents
Intellectual Property Rights . 2
Foreword . 2
Modal verbs terminology . 2
Foreword . 5
1 Scope . 6
2 References . 6
3 Definitions, symbols and abbreviations . 6
3.1 Definitions . 6
3.2 Symbols . 6
3.3 Abbreviations . 6
4 Security requirements . 7
4.1 General . 7
4.2 Common security requirements . 7
4.3 Security requirements on the CAPIF-1/1e reference points . 7
4.4 Security requirements on the CAPIF-2/2e reference points . 8
4.5 Security requirements on the CAPIF-3/4/5 reference points . 8
5 Functional security model . 8
6 Security procedures . 9
6.1 Security procedures for API invoker onboarding . 9
6.2 Security procedures for CAPIF-1 reference point . 11
6.3 Security procedures for CAPIF-1e reference point . 11
6.3.1 Authentication and authorization . 11
6.3.1.1 General . 11
6.3.1.2 Security method negotiation. 11
6.3.1.3 API discovery . 12
6.3.1.4 Topology hiding . 12
6.4 Security procedures for CAPIF-2 reference point . 12
6.5 Security procedures for CAPIF-2e reference point . 13
6.5.1 General . 13
6.5.2 Authentication and authorization . 13
6.5.2.1 Method 1 – Using TLS-PSK . 13
6.5.2.2 Method 2 – Using PKI . 14
6.5.2.3 Method 3 – TLS with OAuth token . 15
6.6 Security procedures for CAPIF-3/4/5 reference points . 16
6.7 Security procedures for updating security method . 17
6.8 Security procedure for API invoker offboarding . 17
Annex A (normative): Key derivation functions . 19
A.1 AEFPSK derivation function. 19
Annex B (informative): Security flows . 20
B.1 Onboarding . 20
B.2 Authentication and authorization . 21
Annex C (normative): Access token profile for ‘Method 3 - TLS with OAuth token’ . 24
C.1 General . 24
C.2 Access token profile . 24
C.2.1 . General
.......................................................................................................................................................................... 24
C.2.2 Token claims . 24
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C.3 Obtaining tokens . 25
C.3.1 General . 25
C.3.2 Access token request . 25
C.3.3 Access token response . 26
C.4 Refreshing an access token . 26
C.5 Using the token to access API exposing functions . 26
C.6 Token revocation . 26
C.7 Token validation . 26
C.7.1 Access token validation . 26
Annex D (informative): Change history . 27
History . 28

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3GPP TS 33.122 version 15.3.0 Release 15 5 ETSI TS 133 122 V15.3.0 (2019-04)
Foreword
This Technical Specification has been produced by the 3rd Generation Partnership Project (3GPP).
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an
identifying change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
z the third digit is incremented when editorial only changes have been incorporated in the document.
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3GPP TS 33.122 version 15.3.0 Release 15 6 ETSI TS 133 122 V15.3.0 (2019-04)
1 Scope
The present document specifies the security architecture i.e., the security features and the security mechanisms for the
common API framework (CAPIF) as per the architecture and procedures defined in 3GPP TS 23.222 [3].
2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
[2] 3GPP TS 33.310: "Network Domain Security (NDS); Authentication Framework (AF)".
[3] 3GPP TS 23.222: "Common API Framework for 3GPP Northbound APIs".
[4] IETF RFC 6749: "The OAuth 2.0 Authorization Framework".
[5] IETF RFC 6750: "The OAuth 2.0 Authorization Framework: Bearer Token Usage".
[6] IETF RFC 7519: "JSON Web Token (JWT)".
[7] IETF RFC 7515: "JSON Web Signature (JWS)".
[8] 3GPP TS 33.220: "Generic Authentication Architecture (GAA); Generic Bootstrapping
Architecture (GBA)".
[9] IETF RFC 5246: "The Transport Layer Security (TLS) Protocol Version 1.2".
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the terms and definitions given in 3GPP TR 21.905 [1] and the following
apply. A term defined in the present document takes precedence over the definition of the same term, if any, in 3GPP
TR 21.905 [1].
3.2 Symbols
For the purposes of the present document, the following symbols apply:
AEF Pre-Shared Key for AEF
PSK
3.3 Abbreviations
For the purposes of the present document, the abbreviations given in 3GPP TR 21.905 [1] and the following apply. An
abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in
3GPP TR 21.905 [1].
AEF API Exposing Function
API Application Programming Interface
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CAPIF Common API Framework
JSON JavaScript Object Notation
JWT JSON Web Token
KDF Key Derivation Function
PKI Public Key Infrastructure
PSK Pre-Shared Key
TLS Transport Layer Security
4 Security requirements
4.1 General
Architectural requirements pertaining to CAPIF security are found in 3GPP TS 23.222 [3]. The following are CAPIF
derived security requirements.
4.2 Common security requirements
Security requirements that are applicable to all CAPIF entities are:
- [CAPIF-SEC-4.2-a] The CAPIF shall provide mechanisms to hide the topology of the PLMN trust domain from
the API invokers accessing the service APIs from outside the PLMN trust domain.
- [CAPIF-SEC-4.2-b] The CAPIF shall provide mechanisms to hide the topology of the 3rd party API provider
trust domain from the API invokers accessing the service APIs from outside the 3rd party API provider trust
domain.
- [CAPIF-SEC-4.2-c] The CAPIF shall provide authorization mechanism for service APIs from the 3rd party API
providers.
- [CAPIF-SEC-4.2-d] The CAPIF shall support a common security mechanism for all API implementations to
provide confidentiality and integrity protection.
- [CAPIF-SEC-4.2-e] API invoker authentication and authorization shall support all deployment models listed in
3GPP TS 23.222 [3].
- [CAPIF-SEC-4.2-f] The API invoker and CAPIF should enforce the result of the authentication for the duration
of communications (e.g. by integrity protection or implicit authentication by encryption with a key that is
derived from the authentication and is unknown to the adversary).
4.3 Security requirements on the CAPIF-1/1e reference points
The CAPIF-1/1e reference points between the API invoker and the CAPIF core function shall fulfil the following
requirements:
- [CAPIF-SEC-4.3-a] Mutual authentication between the API invoker and the CAPIF Core function shall be
supported.
- [CAPIF-SEC-4.3-b] The transport of messages over the CAPIF-1 and CAPIF-1e reference points shall be
integrity protected.
- [CAPIF-SEC-4.3-c] The transport of messages over the CAPIF-1 and CAPIF-1e reference points shall be
protected from replay attacks.
- [CAPIF-SEC-4.3-d] The transport of messages over the CAPIF-1 and CAPIF-1e reference points shall be
confidentiality protected.
- [CAPIF-SEC-4.3-e] Privacy of the 3GPP user over the CAPIF-1 and CAPIF-1e reference points shall be
protected.
- [CAPIF-SEC-4.3-f] The CAPIF core function shall authorize the API invoker prior to the API invoker accessing
the AEF.
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- [CAPIF-SEC-4.3-g] The CAPIF core function shall authorize the API invoker prior to accessing the discover
service API.
- [CAPIF-SEC-4.3-h] The CAPIF core function shall authenticate the API invoker's onboarding request.
- [CAPIF-SEC-4.3-i] The CAPIF core function shall authenticate the API invoker's offboarding request.
4.4 Security requirements on the CAPIF-2/2e reference points
The CAPIF-2/2e reference points between the API invoker and API exposing function shall fulfil the following
requirements:
- [CAPIF-SEC-4.4-a] Mutual authentication between the API invoker and the API exposing function shall be
supported.
- [CAPIF-SEC-4.4-b] The transport of messages over the CAPIF-2 and CAPIF-2e reference points shall be
integrity protected.
- [CAPIF-SEC-4.4-c] The transport of messages over the CAPIF-2 and CAPIF-2e reference points shall be
protected from replay attacks.
- [CAPIF-SEC-4.4-d] The transport of messages over the CAPIF-2 and CAPIF-2e reference points shall be
confidentiality protected.
- [CAPIF-SEC-4.4-e] Privacy of the 3GPP user over the CAPIF-2 and CAPIF-2e reference points shall be
protected.
- [CAPIF-SEC-4.4-f] The API exposing function shall determine whether API invoker is authorized to access
service API.
4.5 Security requirements on the CAPIF-3/4/5 reference points
The security requirements for CAPIF-3/4/5 reference points are:
- [CAPIF-SEC-4.5-a] The transport of messages over the CAPIF-3/4/5 reference points shall be integrity
protected.
- [CAPIF-SEC-4.5-b] The transport of messages over the CAPIF-3/4/5 reference points shall be confidentiality
protected.
- [CAPIF-SEC-4.5-c] The transport of messages over the CAPIF-3/4/5 reference points shall be protected from
replay attacks.
- [CAPIF-SEC-4.5-d] The CAPIF core function shall be able to authenticate the service API publishers to publish
and manage the service API information.
- [CAPIF-SEC-4.5-e] The CAPIF core function shall be able to authorize the service API publishers to publish
and manage the service API information.
- [CAPIF-SEC-4.5-f] The CAPIF core function shall be able to request explicit grant of new API invoker’s
onboarding.
5 Functional security model
Figure 5-1 shows the functional security model for the CAPIF architecture. The interfaces CAPIF-1, CAPIF-1e,
CAPIF-2, CAPIF-2e, CAPIF-3, CAPIF-4 and CAPIF-5 are defined in 3GPP TS 23.222 [3] and support the CAPIF
functionality defined in 3GPP TS 23.222 [3]. CAPIF-1, CAPIF-2, CAPIF-3, CAPIF-4 and CAPIF-5 are interfaces that
lie within the PLMN trust domain while the CAPIF-1e and CAPIF-2e interfaces are CAPIF core and AEF access points
for API Invokers outside of the PLMN trust domain.
Security for the CAPIF-1, CAPIF-2, CAPIF-3, CAPIF-4 and CAPIF-5 interfaces support TLS as specified in IETF RFC
5246 [9],and are defined in subclauses 6.2, 6.4 and 6.6 of the present document. Security for the CAPIF-1e and CAPIF-
2e interfaces support TLS [9], and are defined in subclause 6.3 and subclause 6.5, respectively.
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Authentication and authorization are required for both API invokers that lie within the PLMN trust domain and API
invokers that lie outside of the PLMN trust domain. For an API invoker that is outside of the PLMN trust domain, the
CAPIF core function in coordination with the API exposing function utilizes the CAPIF-1e, CAPIF-2e and the CAPIF-3
interfaces to onboard, authenticate and authorize the API invoker prior to granting access to CAPIF services. Security
flow diagrams for onboarding security, CAPIF-1e security and CAPIF-2e security can be found in Annex B. When the
API invoker is within the PLMN trust domain, the CAPIF core function in coordination with the API exposing function
perform authentication and authorization of the API invoker via the CAPIF-1, the CAPIF-2 and the CAPIF-3 interfaces
prior to granting access to CAPIF services. Authentication and authorization of API invokers (both internal and external
to the PLMN trust domain) is specified in clause 6 of the present document.
API invoker
n
i
API invoker
CAPIF-1e
a
m
o
D
TLS
t
s
TLS
u
r
TLS
T
N
M
L CAPIF-2
TLS
P
CAPIF-1
CAPIF-2e
CAPIF APIs Service APIs
ServSeicerv AicePI sAPIs
CAPIF-3
TLS
API exposing function
CAPIF-4
TLS
API publishing function
CAPIF core function
CAPIF-5
API management function
TLS
API provider domain
Figure 5-1: CAPIF functional security model
6 Security procedures
6.1 Security procedures for API invoker onboarding
The API invoker and the CAPIF core function shall follow the procedure in this subclause to secure and authenticate the
onboarding of the API invoker to the CAPIF core function. The API invoker and the CAPIF core function shall
establish a secure session using TLS as specified in IETF RFC 5246 [9].
With a secure session established, the API Invoker sends an Onboard API Invoker Request message to the CAPIF core
function. The Onboard API Invoker Request message carries an onboard credential obtained during pre-provisioning of
the onboard enrolment information, which may be an OAuth 2.0 [4] access token. When the OAuth 2.0 token based
mechanism is used as the onboarding credential, the access token shall be encoded as JSON web token as specified in
IETF RFC 7519 [6], shall include the JSON web signature as specified in IETF RFC 7515 [7], and shall be validated
per OAuth 2.0 [4], IETF RFC 7519 [6] and IETF RFC 7515 [7]. Other credentials may also be used (e.g. message
digest).
Figure 6.1-1 details the security information flow for the API invoker onboarding procedure. The OAuth 2.0 token
based authentication credential is shown in this example.
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CAPIF core function
API invoker API provider domain
1. Onboarding enrollment information
[CCF(Address,Root CA Certifcate),
OAuth 2.0 Access token]
4. Verify OAuth
access token,
generate API invoker
Profile.
5. Onboard API Invoker Response
[API Invoker ID, API Invoker Certificate, (service API authentication and Authorization
information), (Onboard_Secret)]

Figure 6.1-1: Security procedure for API invoker onboarding
1. As a prerequisite to the onboarding procedure, the API invoker obtains onboarding enrolment information from
the API provider domain. The onboarding enrolment information is used to authenticate and establish a secure
TLS communication with the CAPIF core function during the onboarding process. The enrolment information
includes details of the CAPIF core function (Address, and Root CA certificate) and includes an onboarding
credential (the OAuth 2.0 [4] access token).
NOTE 1: The procedure used to obtain the enrolment information by the API invoker is out of scope of the present
document.
2. The API invoker and CAPIF core function shall establish a secure session based on TLS (Server side certificate
authentication). The API invoker shall use the enrolment information obtained in step 1 to establish the TLS
session with the CAPIF core function.
3. After successful establishment of the TLS session, the API invoker shall send an Onboard API invoker request
message to the CAPIF core function along with the enrolment credential (OAuth 2.0 [4] access token). The API
invoker generates the key pair {Private Key, Public key} and provides the public key along with the Onboard
API invoker request.
4. The CAPIF core function shall validate the enrolment credential (OAuth 2.0 [4] access token). If validation of
the credential (the OAuth 2.0 [4] access token in this example) is successful, the CAPIF core function shall
generate an API invoker's profile as specified in TS 23.222 [3] which may contain the selected method for AEF
authentication and authorization between the API Invoker and the AEF (see subclause 6.5.2). The CAPIF core
function may generate API invoker's certificate on its own, for the assigned API invoker identity and public key.
This certificate shall be used by the API invoker for subsequent authentication procedures with the CAPIF core
function and may be used for establishing a secure connection and authentication with the API Exposing
Function. The CAPIF core function may optionally generate an Onboard_Secret if the subscribed Service API
uses Method 3 (as specified in clause 6.5.2.3 of the present document) for CAPIF-2e security. The
Onboard_Secret value remains the same during the lifetime of the onboarding, and shall be bound to the CAPIF
core function specific API Invoker ID.
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NOTE 2: When API invoker's client certificate is issued by the third party, then in Step 3 the API invoker can
additionally include the certificate in Onboard API Invoker request message. If the CAPIF core function
trusts the issuer of the API invoker's client certificate, then the CAPIF Core Function includes the
provided certificate in the API invoker's profile, in step 4. It is up to the CAPIF domain policy to accept
the client certificates issued by third party.
5. The CAPIF core function shall respond with an Onboard API invoker response message. The response shall
include the CAPIF core function assigned API invoker ID, AEF Authentication and authorization information (if
generated in step 4), API invoker's certificate and the API invoker Onboard_Secret (if generated by the CAPIF
core function).
6.2 Security procedures for CAPIF-1 reference point
TLS, as specified in IETF RFC 5246 [9], shall be used to provide integrity protection, replay protection and
confidentiality protection. The support of TLS is mandatory and optional to use based on the domain administrator's
policy to protect interfaces within the trusted domain.
The procedure in subclause 6.3 of the present document shall be followed unless the security of CAPIF-1 reference
point is provided by other means.
6.3 Security procedures for CAPIF-1e reference point
6.3.1 Authentication and authorization
6.3.1.1 General
For authentication of the CAPIF-1e reference point, mutual authentication based on client and server certificates shall
be performed between the CAPIF core function and the API invoker, as specified in IETF RFC 5246 [9] using TLS.
Certificate based authentication shall follow the profiles given in 3GPP TS 33.310 [2], subclauses 6.1.3a and 6.1.4a.
The structure of the PKI used for the certificate is out of scope of the present document.
TLS [9] shall be used to provide integrity protection, replay protection and confidentiality protection for CAPIF-1e
interface. The support of TLS on CAPIF-1e interface is mandatory. Security profiles for TLS implementation and usage
shall follow the provisions given in TS 33.310 [2], Annex E.
6.3.1.2 Security method negotiation
The API invoker and the CAPIF core function shall negotiate a security method that shall be used by the API invoker
and the API exposing function for CAPIF-2e interface authentication and protection. After successful mutual
authentication on CAPIF-1e interface, based on the API invoker's subscribed service APIs, access scenarios (whether
the API invoker access the AEF prior to service API invocation or upon the service API invocation) and AEF
capabilities, the CAPIF core function shall choose the security method and sends the chosen security methods along
with the information required for authentication of the API invoker at the AEF to the API invoker. The information may
include the validity time of the CAPIF-2e credentials. This is depicted in figure 6.3.1-1.
Pre-conditions:
1. The API invoker is onboarded with the CAPIF core function.
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API invoker CAPIF core function
1. TLS establishment
3. select a security
method
4. Security Method Response (AEF details, Selected security Method and Security
Information)
Figure 6.3.1-1: Selection of security method to be used in CAPIF-2/2e reference point
1. Mutual authentication based on client and server certificates shall be established using TLS between the API
invoker and the CAPIF core function. The client certificate that was provided to the API invoker as the result of
successful onboarding is used based on the description in subclause 6.1 of the present document.
2. The API invoker may send CAPIF-2/2e security capability information to the CAPIF core function in the
Security Method Request message, indicating the list of security methods that the API invoker supports over
CAPIF-2/2e reference point for each AEF.
3. The CAPIF core function shall select a security method to be used over CAPIF-2/2e reference point for each
requested AEF, taking into account the information from the API invoker in step 2, access scenarios and AEF
capabilities.
4. The CAPIF core function shall send Security Method Response message to the API invoker, indicating the
selected security method for each AEF, any security information related to the security method. The API invoker
shall use this method in the subsequent communication establishment with the API exposing function over
CAPIF-2/2e reference point, as described in subclause 6.5 of the present document.
6.3.1.3 API discovery
After successful authentication between API invoker and CAPIF core function, the CAPIF core function shall decide
whether the API invoker is authorized to perform discovery based on API invoker ID and discovery policy.
6.3.1.4 Topology hiding
When topology hiding is enabled, the CAPIF core function shall respond to service APIs discovery requests with AEF
information, which exposes the service API and acts as topology hiding entity.
6.4 Security procedures for CAPIF-2 reference point
TLS, as specified in IETF RFC 5246 [9], shall be used to provide integrity protection, replay protection and
confidentiality protection. The support of TLS is mandatory and optional to use based on the domain administrator's
policy to protect interfaces within the trusted domain.
The procedure in subclause 6.5 of the present document shall be followed unless the security of CAPIF-2 reference
point is provided by other means.
If the domain administrator's policy to authorize the API invoker's service API invocation requests is set, the API
invoker's authorization shall be performed according to the authorization mechanisms specified for CAPIF-2e reference
point in subclause 6.5 of the present document.
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6.5 Security procedures for CAPIF-2e reference point
6.5.1 General
Based on the selected security method by the CAPIF Core Function (c.f., subclause 6.3.1), one of the methods specified
in subclause 6.5.2 shall be used by the API invoker and the API exposing function for CAPIF-2e interface
authentication and protection.
6.5.2 Authentication and authorization
6.5.2.1 Method 1 – Using TLS-PSK
The API invoker and the API exposing function shall follow the procedure in this sub-clause to establish dedicated
secure session using TLS connection based on Pre-Shared Key (PSK). CAPIF-1e authentication shall be used to
bootstrap a Pre-Shared key for authenticating a TLS connection for CAPIF-2e. It is assumed that both the API invoker
and the CAPIF core function are pre-provisioned with certificates. The TLS profile as specified in Annex E of TS
33.310 [2] shall be used.
Figure 6.5.2.1-1 details the message flow between the API invoker, the CAPIF core function and the API exposing
function, to establish secure CAPIF-2e interface using a pre-shared key for authentication.
API exposing
API invoker CAPIF core function
function
1. CAPIF-1e Authentication and secure session establishment
2. Derive secret AEFPS K 2. Derive secret AEFPS K
3. Authentication Initiation Request (API Invoker ID)
4A. Request security information
4B. Response (AEF PS K)
5. Authentication Initiation Response
6. TLS connection established (TLS-PSK, AEF PS K)

Figure 6.5.2.1-1: CAPIF-2e interface authentication and protection using TLS-PSK
1. CAPIF-1e authentication and secure session is established as specified in subclause 6.3.1 of the present
document. The CAPIF core function shall provide the validity timer value for the key AEF
PSK.
2. After successful establishment of TLS on CAPIF-1e, the API invoker and the CAPIF core function shall derive
the key AEF .
PSK
The Key AEF shall be bound to an AEF and shall be derived as specified in Annex A. The API invoker and
PSK
the CAPIF core function starts the validity timer for the key AEF .
PSK
3. The API Invoker shall send Authentication Initiation Request to the AEF, including the CAPIF core function
assigned API invoker ID.
Steps 1 and 2 of this procedure may be skipped if the API invoker is already in possession of a valid key
AEF . In this case, the API invoker begins the procedure at step 3.
PSK
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3GPP TS 33.122 version 15.3.0 Release 15 14 ETSI TS 133 122 V15.3.0 (2019-04)
NOTE: Void.
4. The AEF shall request for security information from the CAPIF Core Function to perform authentication and
secure interface establishment with the API invoker, if the AEF does not have a valid key. The CAPIF Core
Function provides the security information related to the chosen security method (TLS-PSK: AEF ) to the
PSK
AEF over CAPIF-3 reference point. The CAPIF core function shall provide the remaining validity timer value
for the key AEF .
PSK
5. After fetching the relevant security information (AEF ) for the authentication, the AEF shall send
PSK
Authentication Initiation Response message to API invoker to initiate the TLS session establishment. The AEF
starts the validity timer based on the value received from the CAPIF core function in step 4.
6. The API Invoker and the AEF shall perform mutual authentication using the key AEF and establish TLS
PSK
session over the CAPIF-2e.
After successful establishment of TLS on CAPIF-2e reference point, the API exposing function shall authorize the API
invoker's service API invocation request based on authorization information obtained from CAPIF core function as
specified in subclause 8.16 of TS 23.222 [3].
6.5.2.2 Method 2 – Using PKI
The API invoker and the API exposing function shall follow the procedure in this subclause to establish dedicated
secure session over CAPIF-2e using TLS based on certificate based mutual authentication. It is assumed that both API
invoker and API exposing function are pre-provisioned with certificates.
Figure 6.5.2.2-1 details the message flow between the API invoker, the CAPIF core function and the API exposing
function related to this security method.
API exposing
API invoker CAPIF core function
function
1. Authentication Initiation Request (API Invoker ID)
2A. Request security information
2B. Response (Root Certificate of CA
to validate API invoker certificate)
3. Authentication Initiation Response
4. TLS connection established (TLS-PKI)

Figure 6.5.2.2-1: CAPIF-2e interface authentication and protection using certificate based mutual
authentication
1. The API invoker shall send Authentication Initiation Request to the AEF, including API invoker ID.
2. The AEF shall request for security information from the CAPIF Core Function to perform authentication and
secure interface establishment with the API invoker. The CAPIF Core Function provides the security
information related to the chosen security method (TLS-PKI) to the AEF over CAPIF-3 reference point. CAPIF
core function may return API invoker's root CA certificate for the AEF to validate the API invoker's certificate.
3. After fetching the relevant security information for the authentication, AEF shall send Authentication Initiation
Response message to API invoker to initiate the TLS session establishment procedure.
4. Then the API Invoker and the AEF shall perform mutual authentication using certificates and establish TLS
session over the CAPIF-2e. Certificate based authentication shall follow the profiles given in 3GPP TS 33.310
ETSI
3GPP TS 33.122 version 15.3.0 Release 15 15 ETSI TS 133 122 V15.3.0 (2019-04)
[2], clauses 6.1.3a and 6.1.4a. The structure of the PKI used for the certificate is out of scope of the present
document.
After successful establishment of TLS on CAPIF-2e reference point, the API exposing function shall authorize the API
invoker's service API invocation request based on authorization information obtained from CAPIF core function as
specified in subclause 8.16 of TS 23.222 [3].
6.5.2.3 Method 3 – TLS with OAuth token
This method details establishment of secure channel over CAPIF-1e, CAPIF-2e reference points, and uses the OAuth
2.0 [4] token based mechanism to authorize and honour API invoker's northbound API invocations to the API exposing
function. Figure 6.5.2.3-1 details security information flows between the API invoker, the CAPIF core function and the
API exposing function. It is assumed that the API invoker, the CAPIF core function and the AEF are pre-provisioned
with the appropriate credentials and related information to establish a secure session.
As per OAuth 2.0 [4], the CAPIF core function shall perform the functionalities of the Authorization and token protocol
endpoints, the API invoker shall perform the functions of the resource owner, client and redirection endpoints
functionalities, while the API exposing function shall perform the resource server functions. The API invoker client
(Client endpoint) shall be registered as a confidential client type with an authorization grant type of ‘client credentials'.
The access token shall follow the profile described in annex C.

API exposing
API invoker CAPIF core function
function
1. CAPIF-1e Authentication and secure session establishment
2. OAuth 2.0 based access token request.
3. Verify access token
request
4. Respond with Oauth 2.0 Access Token
5. TLS connection established
6. Invoke Northbound API with Oauth 2.0 Access Token
7. Verify Access
Token, authorization
claims in token,
execute Northbound
8. Response Northbound API
API request.
invocation
Figure 6.5.2.3-1: CAPIF-2e interface authentication and protection using Access Tokens
1. CAPIF-1e authentication and secure session establishment is performed as specified in subclause 6.3.1.
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2. After successful establishment of TLS session over CAPIF-1e, as described in subclause 6.3.1 of the present
document, the API invoker shall send an Access Token Request message to the CAPIF core function as per the
OAuth 2.0 [4] specification.
3. The CAPIF core function shall verify the Access Token Request message per OAuth 2.0 [4] specification.
4. If the CAPIF core function successfully verifies the Access Token Request message, the CAPIF core function
shall generate an access token specific to the API invoker and return it in an Access Token Response message.
Steps 1 to 4 of this procedure may be skipped if the API invoker is already in possession of a valid OAuth access
token. In this case, the API invoker begins the procedure at step 5.
NOTE 1: The API invoker may include the CAPIF core function assigned API invoker ID and the Onboard_Secret
in the OAuth access token request message for the CAPIF core function to validate the access token
request.
NOTE 2: Void.
5. On CAPIF-2e, the API invoker authenticates to the AEF by establishing a TLS session with the API exposing
function based on the authentication and authorization method (i.e. Server (AEF) side certificate authentication
or certificate-based mutual authentication) as indicated by CAPIF core function. The following procedure shall
be performed prior to establishment of TLS session.
The API invoker shall send Authentication Initiation Request to the AEF, including API invoker ID.
The AEF shall request for security information from the CAPIF Core Function to perform authentication and
secure interface estab
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