]> Cisco Systems, Inc.

510 McCarthy Drive Milpitas CA 95035 USA pritikin@cisco.com

AKAYLA, Inc.

7150 Moorland Drive Clarksville MD 21029 USA peter@akayla.com

Aruba Networks

1322 Crossman Avenue Sunnyvale CA 94089-1113 USA dharkins@arubanetworks.com

Security PKIX pki,est This document profiles certificate enrollment for clients using Certificate Management over CMS (CMC) messages over a secure transport. This profile, called Enrollment over Secure Transport (EST), describes a simple, yet functional, certificate management protocol targeting Public Key Infrastructure (PKI) clients that need to acquire client certificates and associated Certification Authority (CA) certificates. It also supports client-generated public/private key pairs as well as key pairs generated by the CA.

This document profiles certificate enrollment for clients using Certificate Management over CMS (CMC) messages over a secure transport. Enrollment over Secure Transport (EST) describes the use of Transport Layer Security (TLS) 1.1 , 1.2 , or any future version) and Hypertext Transfer Protocol (HTTP) to provide an authenticated and authorized channel for Simple Public Key Infrastructure (PKI) Requests and Responses . Architecturally, the EST service is located between a Certification Authority (CA) and a client. It performs several functions traditionally allocated to the Registration Authority (RA) role in a PKI. The nature of communication between an EST server and a CA is not described in this document. EST adopts the Certificate Management Protocol (CMP) model for CA certificate rollover, but it does not use the CMP message syntax or protocol. EST servers are extensible in that new functions may be defined to provide additional capabilities not specified in CMC, and this document defines two such extensions: one for requesting Certificate Signing Request attributes and another for requesting server-generated keys. EST specifies how to transfer messages securely via HTTP over TLS (HTTPS) , where the HTTP headers and media types are used in conjunction with TLS. HTTPS operates over TCP; this document does not specify EST over HTTP/Datagram Transport Layer Security/User Datagram Protocol (HTTP/DTLS/UDP). With a suitable specification for combining HTTP, DTLS, and UDP, there are no EST requirements that would prevent it from working over such a stack. shows how the layers build upon each other.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . It is assumed that the reader is familiar with the terms and concepts described in Public Key Cryptography Standard (PKCS) #10 , HTTPS , CMP , CMC , and TLS . In addition to the terms defined in the terminology section of CMC , the following terms are defined for clarity: For certificate issuing services, the EST CA is reached through the EST server; the CA could be logically "behind" the EST server or embedded within it. Any trust anchor (TA) that is not authoritative for the PKI hierarchy for which the EST server is providing services. Any TA that is explicitly configured on the client or server for use during EST TLS authentication; for example, a TA that is manually configured on the EST client or bootstrapped as described in . (See more details in Sections and .) Any third-party TA that is available on the client or server for use during TLS authentication but is not specifically indicated for use during EST TLS authentication; for example, TAs commonly used by web browsers to authenticate web servers or TAs used by servers to authenticate manufacturer-installed client credentials (such as certificates populated into cable modems or routers in the factory). The authorization model for these TAs is different from the authorization model for Explicit Trust Anchors. (See more details in Sections , , and ). Certificate-less TLS cipher suites provide a way to perform mutual authentication in situations where neither the client nor server have certificates or are willing to use them. The credential used for authentication is a word, phrase, code, or key that is shared between the client and server. The credential must be uniquely shared between the client and server in order to provide authentication of an individual client to an individual server.

This section provides an informative overview of the operational scenarios to better introduce the reader to the protocol discussion. Both the EST clients and server are configured with information that provides the basis for mutual authentication and for authorization. The specific initialization data depends on the methods available in the client and server, but it can include shared secrets, network service names and locations (e.g., a Uniform Resource Identifier (URI) ), trust anchor information (e.g., a CA certificate or a hash of a TA's certificate), and enrollment keys and certificates. Depending on an enterprise's acquisition and network management practices, some initialization may be performed by the vendor prior to delivery of client hardware and software. In that case, the client vendor may provide data, such as trust anchors, to the enterprise via a secure procedure. The distribution of this initial information is out of scope. Distribution of trust anchors and other certificates can be effected via the EST server. However, nothing can be inferred about the authenticity of this data until an out-of-band mechanism is used to verify them. Sections - very closely mirror the text of the Scenarios Appendix of with such modifications as are appropriate for this profile. Sections -, below, enumerate the set of EST functions (see ) and provide an informative overview of EST's capabilities. The general client/server interaction proceeds as follows: The client initiates a TLS-secured HTTP session with an EST server. A specific EST service is requested based on a portion of the URI used for the session. The client and server authenticate each other. The client verifies that the server is authorized to serve this client. The server verifies that the client is authorized to make use of this server and the request that the client has made. The server acts upon the client request.

The EST client can request a copy of the current EST CA certificate(s) from the EST server. The EST client is assumed to perform this operation before performing other operations. Throughout this document we assume the EST CA has a certificate that is used by the client to verify signed objects issued by the CA, e.g., certificates and certificate revocation lists (CRLs), and that a different certificate than the one used to verify signatures on certificates and CRLs is used when EST protocol communication requires additional encryption. The EST client authenticates and verifies the authorization scope of the EST server when requesting the current CA certificate(s). As detailed in Sections and , available options include: Verifying the EST server's HTTPS URI against the EST server's certificate using Implicit TAs (similar to a common HTTPS exchange). This allows the EST server and client to leverage existing TAs that might be known to the EST client. The client can leverage a previously distributed trust anchor specific to the EST server. This allows the EST client to use an existing, potentially older, CA certificate to request a current CA certificate. For bootstrapping, the EST client can rely upon manual authentication performed by the end-user as detailed in . The client can leverage the binding of a shared credential to a specific EST server with a certificate-less TLS cipher suite. Client authentication is not required for this exchange, so it is trivially supported by the EST server.

After authenticating an EST server and verifying that it is authorized to provide services to the client, an EST client can acquire a certificate for itself by submitting an enrollment request to that server. The EST server authenticates and authorizes the EST client as specified in Sections , , and . The methods described in the normative text that are discussed in this overview include: TLS with a previously issued client certificate (e.g., an existing certificate issued by the EST CA); TLS with a previously installed certificate (e.g., manufacturer-installed certificate or a certificate issued by some other party); Certificate-less TLS (e.g., with a shared credential distributed out-of-band); HTTP-based with a username/password distributed out-of-band.

If the EST client has a previously installed certificate issued by a third-party CA, this certificate can be used to authenticate the client's request for a certificate from the EST server (if that CA is recognized by the EST server). An EST client responds to the EST server's TLS certificate request message with the existing certificate already held by the client. The EST server will verify the client's existing certificate and authorize the client's request as described in .

The EST client and EST server can be mutually authenticated using a certificate-less TLS cipher suite (see ).

The EST server can optionally also request that the EST client submit a username/password using the HTTP Basic or Digest authentication methods (see ). This approach is desirable if the EST client cannot be authenticated during the TLS handshake (see ) or the EST server policy requires additional authentication information; see . In all cases, HTTP&nbhy;based client authentication is only to be performed over a TLS&nbhy;protected transport (see ).

An EST client can renew/rekey its existing client certificate by submitting a re-enrollment request to an EST server. When the current EST client certificate can be used for TLS client authentication (), the client presents this certificate to the EST server for client authentication. When the to be reissued EST client certificate cannot be used for TLS client authentication, any of the authentication methods used for initial enrollment can be used. For example, if the client has an alternative certificate issued by the EST CA that can be used for TLS client authentication, then it can be used. The certificate request message includes the same Subject and SubjectAltName as the current certificate. Name changes are requested as specified in .

The EST client can request a server-generated certificate and key pair (see ).

Full PKI Request messages can be transported via EST using the Full CMC Request function. This affords access to functions not provided by the Simple Enrollment functions. Full PKI Request messages are defined in Sections 3.2 and 4.2 of . See for a discussion of how EST provides a transport for these messages.

Prior to sending an enrollment request to an EST server, an EST client can query the EST server for a set of additional attributes that the client is requested to use in a subsequent enrollment request. These attributes can provide additional descriptive information that the EST server cannot access itself, such as the Media Access Control (MAC) address of an interface of the EST client. Alternatively, these attributes can indicate the kind of enrollment request, such as a specific elliptic curve or a specific hash function that the client is expected to use when generating the CSR.

provides an expansion of , describing how the layers are used. Each aspect is described in more detail in the sections that follow.

Specifying HTTPS as the secure transport for enrollment messages introduces two "layers" to communicate authentication and control messages: TLS and HTTP. The TLS layer provides integrity and confidentiality during transport. The proof-of-identity is supplied by TLS handshake authentication and optionally also by the HTTP layer headers. The message type and control/error messages are included in the HTTP headers. CMC (, Section 3.1) notes that "the Simple PKI Request MUST NOT be used if a proof-of-identity needs to be included". Since the TLS and HTTP layers can provide proof-of-identity for EST clients and servers, the Simple PKI message types are used. The TLS layer certificate exchange provides a method for authorizing client enrollment requests using existing certificates. Such certificates may have been issued by the CA (from which the client is requesting a certificate), or they may have been issued under a distinct PKI (e.g., an IEEE 802.1AR Initial Device Identifier (IDevID) credential). Proof-of-possession (POP) is a distinct issue from proof-of-identity and is included in the Simple PKI message type as described in . A method of linking proof-of-identity and proof&nbhy;of&nbhy;possession is described in . This document also defines transport for CMC that complies with the CMC Transport Protocols. CMC's POP and proof&nbhy;of&nbhy;identity mechanisms are defined in CMC, but the mechanisms here can also be used in conjunction with those mechanisms in "Full PKI" messages. During protocol exchanges, different certificates can be used. The following table provides an informative overview. End-entities can have one or more certificates of each type listed in and use one or more trust anchor databases of each type listed in .

The EST client MUST be capable of generating and parsing Simple PKI messages (see ). Generating and parsing Full PKI messages is OPTIONAL (see ). The client MUST also be able to request CA certificates from the EST server and parse the returned "bag" of certificates (see ). Requesting CSR attributes and parsing the returned list of attributes is OPTIONAL (see ). Details of the EST client application configuration are out of scope of the protocol discussion but are necessary for understanding the prerequisites of initiating protocol operations. The EST client is RECOMMENDED to be configured with TA databases for or with a secret key for . Implementations conforming to this standard MUST provide the ability to designate Explicit TAs. For human usability reasons, a "fingerprint" of an Explicit TA database entry can be configured for bootstrapping as discussed in . Configuration of an Implicit TA database, perhaps by its inclusion within the EST client distribution or available from the operating system, provides flexibility along with the caveats detailed in . Implementations conforming to this standard MUST provide the ability to disable use of any Implicit TA database. The EST client is configured with sufficient information to form the EST server URI. This can be the full operation path segment (e.g., https://www.example.com/.well-known/est/ or https://www.example.com/.well-known/est/arbitraryLabel1), or the EST client can be configured with a tuple composed of the authority portion of the URI along with the OPTIONAL label (e.g., "www.example.com:80" and "arbitraryLabel1") or just the authority portion of the URI.

HTTP is used to transfer EST messages. URIs are defined for handling each media type (i.e., message type) as described in . HTTP is also used for client authentication services when TLS client authentication is not available, due to the lack of a client certificate suitable for use by TLS (see ). HTTP authentication can also be used in addition to TLS client authentication if the EST server wishes additional authentication information, as noted in . Registered media types are used to convey EST messages as specified in . HTTP 1.1 and above support persistent connections. As described in Section 8.1 of RFC 2616, persistent connections may be used to reduce network and processing loads associated with multiple HTTP requests. EST does not require or preclude persistent HTTP connections.

The HTTP Status value is used to communicate success or failure of an EST function. HTTP authentication is used by a client when requested by the server. The media types specified in the HTTP Content-Type header indicate which EST message is being transferred. Media types used by EST are specified in . HTTP redirections (3xx status codes) to the same web origin (see ) SHOULD be handled by the client without user input so long as all applicable security checks (Sections and ) have been enforced on the initial connection. The client initiates a new TLS connection and performs all applicable security checks when redirected to other web origin servers. Redirections to other web origins require the EST client to obtain user input for non-GET or HEAD requests as specified in . Additionally, if the client has already generated a CSR that includes linking identity and POP information, then the CSR will need to be recreated to incorporate the tls-unique from the new, redirected session. Note: the key pair need not be regenerated. These are processing and interface burdens on the client. EST server administrators are advised to take this into consideration.

The EST server MUST support the use of the path-prefix of "/.well-known/" as defined in and the registered name of "est". Thus, a valid EST server URI path begins with "https://www.example.com/.well-known/est". Each EST operation is indicated by a path-suffix that indicates the intended operation:

The operation path () is appended to the path-prefix to form the URI used with HTTP GET or POST to perform the desired EST operation. An example valid URI absolute path for the "/cacerts" operation is "/.well-known/est/cacerts". To retrieve the CA's certificates, the EST client would use the following HTTP request&nbhy;line:

Likewise, to request a new certificate in this example scheme, the EST client would use the following request-line:

The use of distinct operation paths simplifies implementation for servers that do not perform client authentication when distributing /&wj;cacerts responses. An EST server MAY provide service for multiple CAs as indicated by an OPTIONAL additional path segment between the registered application name and the operation path. To avoid conflict, the CA label MUST NOT be the same as any defined operation path segment. The EST server MUST provide services regardless of whether the additional path segment is present. The following are three example valid URIs: https://www.example.com/.well-known/est/cacerts https://www.example.com/.well-known/est/arbitraryLabel1/cacerts https://www.example.com/.well-known/est/arbitraryLabel2/cacerts In this specification, the distinction between enroll and renew/rekey is explicitly indicated by the HTTP URI. When requesting /fullcmc operations, CMC uses the same messages for certificate renewal and certificate rekey. An EST server can provide additional services using other URIs.

The EST server MAY request HTTP-based client authentication. This request can be in addition to successful TLS client authentication () if EST server policy requires additional authentication. (For example, the EST server may require that an EST client "knows" a password in addition to "having" an existing client certificate.) Or, HTTP-based client authentication can be an EST server policy-specified fallback in situations where the EST client did not successfully complete the TLS client authentication. (This might arise if the EST client is enrolling for the first time or if the certificates available to an EST client cannot be used for TLS client authentication.) HTTP Basic and Digest authentication MUST only be performed over TLS 1.1 or later versions. NULL and anon cipher suites MUST NOT be used because they do not provide confidentiality or support mutual certificate-based or certificate-less authentication, respectively. As specified in "Certificate Management over CMS (CMC): Transport Protocols", the server "MUST NOT assume client support for any type of HTTP authentication such as cookies, Basic authentication, or Digest authentication". Clients SHOULD support the Basic and Digest authentication mechanism. Servers that wish to use Basic and Digest authentication reject the HTTP request using the HTTP-defined WWW-Authenticate response-header (, Section 14.47). The client is expected to retry the request, including the appropriate Authorization Request header (, Section 3.2.2), if the client is capable of using the Basic or Digest authentication. If the client is not capable of retrying the request or it is not capable of Basic or Digest authentication, then the client MUST terminate the connection. A client MAY set the username to the empty string ("") if it is presenting a password that is not associated with a username. Support for HTTP-based client authentication has security ramifications as discussed in . The client MUST NOT respond to the server's HTTP authentication request unless the client has authorized the EST server (as per ).

This document uses existing media types for the messages as specified by FTP and HTTP , application/pkcs10 , and CMC . For consistency with , each distinct EST message type uses an HTTP Content-Type header with a specific media type. The EST messages and their corresponding media types for each operation are:

TLS provides authentication, which in turn enables authorization decisions. The EST server and EST client are responsible for ensuring that an acceptable cipher suite is negotiated and that mutual authentication has been performed. TLS authentication is most commonly enabled with the use of certificates . Alternately, certificate-less TLS authentication, where neither the client nor server present a certificate, is also an acceptable method for EST mutual authentication (). The EST server MUST be authenticated during the TLS handshake unless the client is requesting Bootstrap Distribution of CA certificates or Full CMC. HTTPS specifies how HTTP messages are carried over TLS. HTTPS MUST be used. TLS 1.1 (or a later version) MUST be used for all EST communications. TLS session resumption SHOULD be supported. TLS channel-binding information can be inserted into a certificate request, as detailed in , in order to provide the EST server with assurance that the authenticated TLS client has access to the private key for the certificate being requested. The EST server MUST implement .

TLS server authentication with certificates MUST be supported. The EST client authenticates the EST server as defined for the cipher suite negotiated. The following text provides details assuming a certificate-based cipher suite, such as the TLS 1.1 mandatory cipher suite (TLS_RSA_WITH_3DES_EDE_CBC_SHA). Certificate validation MUST be performed as per . The EST server certificate MUST conform to the certificate profile. The client validates the TLS server certificate using the EST client Explicit and, if enabled, Implicit TA database(s). The client MUST maintain a distinction between the use of Explicit and Implicit TA databases during authentication in order to support proper authorization. The EST client MUST perform authorization checks as specified in . If certificate validation fails, the client MAY follow the procedure outlined in for Bootstrap Distribution of CA certificates.

TLS client authentication is the RECOMMENDED method for identifying EST clients. HTTP-based client authentication MAY be used. The EST server authenticates the EST client as defined for the cipher suite negotiated. The following text provides details assuming a certificate-based cipher suite such as the TLS 1.1 mandatory cipher suite (TLS_RSA_WITH_3DES_EDE_CBC_SHA). The EST server MUST support certificate-based client authentication. Generally, the client will use an existing certificate for renew or rekey operations. If the certificate to be renewed or rekeyed is appropriate for the negotiated cipher suite, then the client MUST use it for the TLS handshake, otherwise the client SHOULD use an alternate certificate that is suitable for the cipher suite and contains the same subject identity information. When requesting an enroll operation, the client MAY use a client certificate issued by a third party to authenticate itself. Certificate validation MUST be performed as per . The EST client certificate MUST conform to the certificate profile. The server validates the TLS client certificate using the EST server Explicit and, if enabled, Implicit TA database(s). The server MUST maintain a distinction between the use of Explicit and Implicit TA databases during authentication in order to support proper authorization. The EST server MUST perform authorization checks as specified in . If a client does not support TLS client authentication, then it MUST support HTTP-based client authentication or certificate-less TLS authentication.

Certificate-less TLS cipher suites provide a way to perform mutual authentication in situations where neither the client nor server have certificates, do not desire to use certificates, or do not have the trust anchors necessary to verify a certificate. The client and server MAY negotiate a certificate-less cipher suite for mutual authentication. When using certificate-less mutual authentication in TLS for enrollment, the cipher suite MUST be based on a protocol that is resistant to dictionary attack and MUST be based on a zero knowledge protocol. Transport Layer Security-Secure Remote Password (TLS-SRP) cipher suites, i.e., those with _SRP_ in the name, listed in Section 2.7 of are suitable for this purpose. lists the characteristics of a cipher suite that are suitable for use in certificate-less mutual authentication for enrollment. Successful authentication using a certificate-less cipher suite proves knowledge of a pre-shared secret that implicitly authorizes a peer in the exchange.

As defined in Section 2.1 of CMC, proof-of-possession (POP) "refers to a value that can be used to prove that the private key corresponding to the public key is in the possession of and can be used by an end-entity". The signed enrollment request provides a signature-based proof&nbhy;of&nbhy;possession. The mechanism described in strengthens this by optionally including "Direct"-based proof&nbhy;of&nbhy;possession by including TLS session-specific information within the data covered by the enrollment request signature (thus linking the enrollment request to the authenticated end point of the TLS connection).

Server policy will determine whether clients are required to use the mechanism specified in this section. This specification provides a method of linking identity and proof-of-possession by including information specific to the current authenticated TLS session within the signed certification request. The client can determine if the server requires the linking of identity and POP by examining the CSR Attributes Response (see ). Regardless of the CSR Attributes Response, clients SHOULD link identity and POP by embedding tls-unique information in the certification request. If tls-unique information is included by the client, the server MUST verify it. The EST server MAY reject requests without tls-unique information as indicated by server policy. Linking identity and proof-of-possession proves to the server that the authenticated TLS client has possession of the private key associated with the certification request, and that the client was able to sign the certification request after the TLS session was established. This is an alternative to the "Linking Identity and POP Information" method defined by Section 6 of that is available if Full PKI messages are used. The client generating the CSR obtains the tls-unique value from the TLS subsystem as described in Channel Bindings for TLS . The EST client operations between obtaining the tls-unique value through generation of the CSR that contains the current tls-unique value and the subsequent verification of this value by the EST server are the "phases of the application protocol during which application-layer authentication occurs"; these operations are protected by the synchronization interoperability mechanism described in the "Channel Bindings for TLS" interoperability notes in Section 3.1 of . When performing renegotiation, TLS "secure_renegotiation" MUST be used. The tls-unique value is base64 encoded as specified in Section 4 of , and the resulting string is placed in the certification request challenge-password field (, Section 5.4.1). The challenge-password field is limited to 255 bytes (Section 7.4.9 of indicates that no existing cipher suite would result in an issue with this limitation). If the challenge-password attribute is absent, the client did not include the optional channel-binding information (the presence of the challenge-password attribute indicates the inclusion of tls-unique information). If the EST server makes use of a back-end infrastructure for processing, it is RECOMMENDED that the results of this verification be communicated. (For example, this communication might use the CMC "RA POP Witness Control" in a CMC Full PKI Request message. Or, an EST server might TLS-authenticate an EST client as being a trusted infrastructure element that does not forward invalid requests. A detailed discussion of back-end processing is out of scope.) When rejecting requests, the EST server response is as described for all enroll responses (). If a Full PKI Response is included, the CMCFailInfo MUST be set to popFailed. If a human-readable reject message is included, it SHOULD include an informative text message indicating that the linking of identity and POP information is required.

The client MUST check EST server authorization before accepting any server responses or responding to HTTP authentication requests. The EST client authorization method depends on which method was used to authenticate the server. When the Explicit TA database is used to authenticate the EST server, then applies. When the Implicit TA database is used to authenticate the EST server, then applies. Successful authentication using a certificate-less cipher suite implies authorization of the server. The client MAY perform bootstrapping as specified in even if these checks fail.

When the EST client Explicit TA database is used to validate the EST server certificate, the client MUST check either the configured URI or the most recent HTTP redirection URI against the server's identity according to the rules specified in , Section 6.4, or the EST server certificate MUST contain the id-kp-cmcRA extended key usage extension.

When the EST client Implicit TA database is used to validate the EST server certificate, the client MUST check the configured URI and each HTTP redirection URI according to the rules specified in , Section 6.4. The provisioned URI or the most recent HTTP redirection URI provides the basis for authorization, and the server's authenticated identity confirms it is the authorized server.

The decision to issue a certificate to a client is always controlled by local CA policy. The EST server configuration reflects this CA policy. This document does not specify any constraints on such policy. EST provides the EST server access to each client's authenticated identity -- e.g., the TLS client's certificate in addition to any HTTP user authentication credentials -- to help in implementing such policy. If the client's certificate was issued by the EST CA, and it includes the id-kp-cmcRA extended key usage extension, then the client is a Registration Authority (RA) as described in and . In this case, the EST server SHOULD apply authorization policy consistent with an RA client. For example, when handling /&wj;simpleenroll requests, the EST server could be configured to accept POP linking information that does not match the current TLS session because the authenticated EST client RA has verified this information when acting as an EST server (as specified in ). More specific RA mechanisms are available if the EST client uses /fullcmc methods.

Before processing a request, an EST server determines if the client is authorized to receive the requested services. Likewise, the client determines if it will make requests to the EST server. These authorization decisions are described in the next two sections. Assuming that both sides of the exchange are authorized, then the actual operations are as described in subsequent sections.

The EST client can request a copy of the current CA certificates. This function is generally performed before other EST functions.

It is possible that the client was not configured with an Implicit TA database that allows a bootstrap installation of the Explicit TA database as described in . This section describes an alternate method by which minimally configured EST clients can populate their Explicit TA database. If the EST client application does not specify either an Explicit TA database or an Implicit TA database, then the initial TLS server authentication and authorization will fail. The client MAY provisionally continue the TLS handshake to completion for the purposes of accessing the /cacerts or /&wj;fullcmc method. If the EST client continues with an unauthenticated connection, the client MUST extract the HTTP content data from the response (Sections or ) and engage a human user to authorize the CA certificate using out-of-band data such as a CA certificate "fingerprint" (e.g., a SHA-256 or SHA-512 hash on the whole CA certificate). In a /&wj;fullcmc response, it is the Publish Trust Anchors control (CMC, Section 6.15) within the Full PKI Response that must be accepted manually. It is incumbent on the user to properly verify the TA information, or to provide the "fingerprint" data during configuration that is necessary to verify the TA information. HTTP authentication requests MUST NOT be responded to if the server has not been authenticated as specified in Section or if the optional certificate-less authentication is used as specified in Section . The EST client uses the /cacerts response to establish an Explicit Trust Anchor database for subsequent TLS authentication of the EST server. EST clients MUST NOT engage in any other protocol exchange until after the /cacerts response has been accepted and a new TLS session has been established (using TLS certificate-based authentication).

EST clients request the EST CA TA database information of the CA (in the form of certificates) with an HTTPS GET message using an operation path of "/cacerts". EST clients and servers MUST support the /cacerts function. Clients SHOULD request an up-to-date response before stored information has expired in order to ensure the EST CA TA database is up to date. The EST server SHOULD NOT require client authentication or authorization to reply to this request. The client MUST authenticate the EST server, as specified in if certificate-based authentication is used or if the optional certificate-less authentication is used, and check the server's authorization as given in , or follow the procedure outlined in .

If successful, the server response MUST have an HTTP 200 response code. Any other response code indicates an error and the client MUST abort the protocol. A successful response MUST be a certs-only CMC Simple PKI Response, as defined in , containing the certificates described in the following paragraph. The HTTP content-type of "application/pkcs7-mime" is used. The Simple PKI Response is sent with a Content&nbhy;Transfer&nbhy;Encoding of "base64" . The EST server MUST include the current root CA certificate in the response. The EST server MUST include any additional certificates the client would need to build a chain from an EST CA-issued certificate to the current EST CA TA. For example, if the EST CA is a subordinate CA, then all the appropriate subordinate CA certificates necessary to build a chain to the root EST CA are included in the response. The EST server SHOULD include the three "Root CA Key Update" certificates OldWithOld, OldWithNew, and NewWithOld in the response chain. These are defined in Section 4.4 of CMP. The EST client MUST be able to handle these certificates in the response. The EST CA's most recent self-signed certificate (e.g., NewWithNew certificate) is self-signed and has the latest NotAfter date. If the EST server does not include these in the response, then after the current EST CA certificate expires, the EST clients will need to be reinitialized with the PKI using the Bootstrap Distribution of CA certificates method, which involves user interaction. After out-of-band validation occurs, all the other certificates MUST be validated using normal certificate path validation (using the most recent CA certificate as the TA) before they can be used to build certificate paths during certificate validation. The EST client MUST store the extracted EST CA certificate as an Explicit TA database entry for subsequent EST server authentication. The EST client SHOULD disable use of Implicit TA database entries for this EST server now that an Explicit TA database entry is available. If the client disables the Implicit TA database, and if the EST server certificate was verified using an Implicit TA database entry, then the client MUST include the "Trusted CA Indication" extension in future TLS sessions . This indicates to the server that only an EST server certificate authenticatable by the Explicit TA database entry is now acceptable (otherwise, the EST server might continue to use a server certificate that is only verifiable by a now disabled Implicit TA). The EST client SHOULD also make the CA Certificate response information available to the end-entity software for use when validating peer certificates.

EST clients request a certificate from the EST server with an HTTPS POST using the operation path value of "/simpleenroll". EST clients request a renew/rekey of existing certificates with an HTTP POST using the operation path value of "/simplereenroll". EST servers MUST support the /simpleenroll and /simplereenroll functions. It is RECOMMENDED that a client obtain the current CA certificates, as described in , before performing certificate request functions. This ensures that the client will be able to validate the EST server certificate. The client MUST authenticate the EST server as specified in if certificate-based authentication is used or if the optional certificate-less authentication is used. The client MUST verify the authorization of the EST server as specified in . The server MUST authenticate the client as specified in if certificate-based authentication is used or if the optional certificate-less authentication is used. The server MUST verify client authorization as specified in . The EST server MUST check the tls-unique value, as described in , if one is submitted by the client. The server MAY accept a certificate request for manual authorization checking by an administrator. ( describes the use of an HTTP 202 response to the EST client if this occurs.)

When HTTPS POSTing to /simpleenroll, the client MUST include a Simple PKI Request as specified in CMC, Section 3.1 (i.e., a PKCS #10 Certification Request). The Certification Signing Request (CSR) signature provides proof&nbhy;of&nbhy;possession of the client-possessed private key to the EST server. If the CSR KeyUsage extension indicates that the private key can be used to generate digital signatures, then the client MUST generate the CSR signature using the private key. If the key can be used to generate digital signatures but the requested CSR KeyUsage extension prohibits generation of digital signatures, then the CSR signature MAY still be generated using the private key, but the key MUST NOT be used for any other signature operations (this is consistent with the recommendations concerning submission of proof&nbhy;of&nbhy;possession to an RA or CA as described in ). The use of /fullcmc operations provides access to more advanced proof-of-possession methods that are used when the key pair cannot be used for digital signature generation (see ). The HTTP content-type of "application/pkcs10" is used here. The format of the message is as specified in with a Content-Transfer-Encoding of "base64" . If the EST client authenticated using a previously installed certificate issued by a third-party CA (see ), the client MAY include the ChangeSubjectName attribute, as defined in , in the CSR to request that the subjectName and SubjectAltName be changed in the new certificate. The EST client MAY request additional certificates even when using an existing certificate in the TLS client authentication. For example, the client can use an existing certificate for TLS client authentication when requesting a certificate that cannot be used for TLS client authentication.

EST clients renew/rekey certificates with an HTTPS POST using the operation path value of "/simplereenroll". A certificate request employs the same format as the "simpleenroll" request, using the same HTTP content-type. The request Subject field and SubjectAltName extension MUST be identical to the corresponding fields in the certificate being renewed/rekeyed. The ChangeSubjectName attribute, as defined in , MAY be included in the CSR to request that these fields be changed in the new certificate. If the Subject Public Key Info in the certification request is the same as the current client certificate, then the EST server renews the client certificate. If the public key information in the certification request is different than the current client certificate, then the EST server rekeys the client certificate.

If the enrollment is successful, the server response MUST contain an HTTP 200 response code with a content-type of "application/pkcs7-mime". A successful response MUST be a certs-only CMC Simple PKI Response, as defined in , containing only the certificate that was issued. The HTTP content-type of "application/pkcs7-mime" with an smime-type parameter "certs-only" is used, as specified in . The server MUST answer with a suitable 4xx or 5xx HTTP error code when a problem occurs. A Simple PKI Response with an HTTP content-type of "application/pkcs7-mime" (see ) MAY be included in the response data to convey an error response. If the content-type is not set, the response data MUST be a plaintext human-readable error message containing explanatory information describing why the request was rejected (for example, indicating that CSR attributes are incomplete). If the server responds with an HTTP 202, this indicates that the request has been accepted for processing but that a response is not yet available. The server MUST include a Retry-After header as defined for HTTP 503 responses. The server also MAY include informative human-readable content. The client MUST wait at least the specified "retry-after" time before repeating the same request. The client repeats the initial enrollment request after the appropriate "retry-after" interval has expired. The client SHOULD log or inform the end-user of this event. The server is responsible for maintaining all states necessary to recognize and handle retry operations as the client is stateless in this regard; it simply sends the same request repeatedly until it receives a different response code. All other return codes are handled as specified in HTTP . If the client closes the TLS connections while waiting for the Retry-After time to expire, then the client initiates a new TLS connection and performs all applicable security checks. If the client has already generated a CSR that includes linking identity and POP information (Section 3.5), then the CSR will need to be recreated to incorporate the tls-unique from the new, redirected session. Note: the key pair need not be regenerated. These are processing and interface burdens on the client. EST server administrators are advised to take this into consideration. The EST client MAY also make the certificate response, and associated private key, available to end-entity software for use as an end&nbhy;entity certificate.

An EST client can request a certificate from an EST server with an HTTPS POST using the operation path value of "/fullcmc". Support for the /fullcmc function is OPTIONAL for both clients and servers.

If the HTTP POST to /fullcmc is not a valid Full PKI Request, the server MUST reject the message. The HTTP content-type used is "application/pkcs7-mime" with an smime-type parameter "CMC-request", as specified in . The body of the message is the binary value of the encoding of the PKI Request with a Content&nbhy;Transfer&nbhy;Encoding of "base64" .

If the enrollment is successful, the server response MUST include an HTTP 200 response code with a content-type of "application/pkcs7-mime" as specified in . The response data includes either the Simple PKI Response with an smime-type parameter of "certs-only" or the Full PKI Response with an smime-type parameter "CMC-response", as specified in Section 3.2.1 of . The body of the message is the binary value of the encoding of the PKI Response with a Content-Transfer-Encoding of "base64" . When rejecting a request, the server MUST specify either an HTTP 4xx error or an HTTP 5xx error. A CMC response with the content-type of "application/pkcs7-mime" MUST be included in the response data for any CMC error response. All other return codes are handled as specified in or HTTP. For example, a client interprets an HTTP 404 or 501 response to indicate that this service is not implemented.

An EST client may request a private key and associated certificate from an EST server using an HTTPS POST with an operation path value of "/serverkeygen". Support for the /serverkeygen function is OPTIONAL. A client MUST authenticate an EST server, as specified in if certificate-based authentication is used or if the optional certificate-less authentication is used, and check the server's authorization as given in . The EST server MUST authenticate the client, as specified in if certificate-based authenticated is used or if the optional certificate-less authentication is used, and check the client's authorization as given in . The EST server applies whatever authorization or logic it chooses to determine if the private key and certificate should be provided. Cipher suites that have a NULL confidentiality algorithm MUST NOT be used as they will disclose the contents of an unprotected private key. Proper random number and key generation is a server implementation responsibility, and server archiving of generated keys is determined by CA policy. The key pair and certificate are transferred over the TLS session. The cipher suite used to return the private key and certificate MUST offer confidentiality commensurate with the private key being delivered to the client. The EST client MAY request additional certificates even when using an existing certificate in the TLS client authentication. For example, the client can use an existing certificate for TLS client authentication when requesting a certificate that cannot be used for TLS client authentication.

The certificate request is HTTPS POSTed and is the same format as for the "/simpleenroll" and "/simplereenroll" path extensions with the same content-type and transfer encoding. In all respects, the server SHOULD treat the CSR as it would any enroll or re-enroll CSR; the only distinction here is that the server MUST ignore the public key values and signature in the CSR. These are included in the request only to allow re-use of existing codebases for generating and parsing such requests. If the client desires to receive the private key with encryption that exists outside of and in addition to that of the TLS transport used by EST or if server policy requires that the key be delivered in such a form, the client MUST include an attribute in the CSR indicating the encryption key to be used. Both symmetric and asymmetric encryption are supported as described in the following subsections. The client MUST also include an SMIMECapabilities attribute (, Section 2.5) in the CSR to indicate the key encipherment algorithms the client is willing to use. It is strongly RECOMMENDED that the clients request that the returned private key be afforded the additional security of the Cryptographic Message Syntax (CMS) EnvelopedData in addition to the TLS-provided security to protect against unauthorized disclosure.

To specify a symmetric encryption key to be used to encrypt the server-generated private key, the client MUST include a DecryptKeyIdentifier attribute (as defined in Section 2.2.5 of ) specifying the identifier of the secret key to be used by the server to encrypt the private key. While that attribute was originally designated for specifying a firmware encryption key, it exactly mirrors the requirements for specifying a secret key to encrypt a private key. If the server does not have a secret key matching the identifier specified by the client, the request MUST be terminated and an error returned to the client. Distribution of the key specified by the DecryptKeyIdentifier to the key generator and the client is outside the scope of this document.

To specify an asymmetric encryption key to be used to encrypt the server-generated private key, the client MUST include an AsymmetricDecryptKeyIdentifier attribute. The AsymmetricDecryptKeyIdentifier attribute is defined as:

The asymmetric-decrypt-key-identifier attribute values have ASN.1 type AsymmetricDecryptKeyIdentifier (where ASN.1 is defined in )::

If the server does not have a public key matching the identifier specified by the client, the request MUST be terminated and an error returned to the client. Distribution of the key specified by the AsymmetricDecryptKeyIdentifier to the key generator and the client is outside the scope of this document. If the key identified is bound to an X.509 certificate, then the key MUST either explicitly support keyTransport or keyAgreement or its use MUST be unrestricted.

If the request is successful, the server response MUST have an HTTP 200 response code with a content-type of "multipart/mixed" consisting of two parts: one part is the private key data and the other part is the certificate data. The format in which the private key data part is returned is dependent on whether the private key is being returned with additional encryption on top of that provided by TLS. If additional encryption is not being employed, the private key data MUST be placed in an "application/pkcs8". An "application/pkcs8" part consists of the base64-encoded DER-encoded PrivateKeyInfo with a Content-Transfer-Encoding of "base64" . If additional encryption is being employed, the private key is placed inside of a CMS SignedData. The SignedData is signed by the party that generated the private key, which may or may not be the EST server or the EST CA. The SignedData is further protected by placing it inside of a CMS EnvelopedData, as described in Section 4 of . The following list shows how the EncryptedData is used, depending on the type of protection key specified by the client. If the client specified a symmetric encryption key to protect the server-generated private key, the EnvelopedData content is encrypted using the secret key identified in the request. The EnvelopedData RecipientInfo field MUST indicate the key-encryption kekri key management technique. The values are as follows: version is set to 4, key-encryption key identifier (kekid) is set to the value of the DecryptKeyIdentifier from ; keyEncryptionAlgorithm is set to one of the key wrap algorithms that the client included in the SMIMECapabilities accompanying the request; and encryptedKey is the encrypted key. If the client specified an asymmetric encryption key suitable for key transport operations to protect the server-generated private key, the EnvelopedData content is encrypted using a randomly generated symmetric encryption key. The cryptographic strength of the symmetric encryption key SHOULD be equivalent to the client-specified asymmetric key. The EnvelopedData RecipientInfo field MUST indicate the KeyTransRecipientInfo (ktri) key management technique. In KeyTransRecipientInfo, the RecipientIdentifier (rid) is either the subjectKeyIdentifier copied from the attribute defined in or the server determines an associated issuerAndSerialNumber from the attribute; version is derived from the choice of rid , keyEncryptionAlgorithm is set to one of the key wrap algorithms that the client included in the SMIMECapabilities accompanying the request, and encryptedKey is the encrypted key. If the client specified an asymmetric encryption key suitable for key agreement operations to protect the server-generated private key, the EnvelopedData content is encrypted using a randomly generated symmetric encryption key. The cryptographic strength of the symmetric encryption key SHOULD be equivalent to the client-specified asymmetric key. The EnvelopedData RecipientInfo field MUST indicate the KeyAgreeRecipientInfo (kari) key management technique. In the KeyAgreeRecipientInfo type, version, originator, and user keying material (ukm) are as in , and keyEncryptionAlgorithm is set to one of the key wrap algorithms that the client included in the SMIMECapabilities accompanying the request. The recipient's key identifier is either copied from the attribute defined in to subjectKeyIdentifier or the server determines an IssuerAndSerialNumber that corresponds to the value provided in the attribute. In all three additional encryption cases, the EnvelopedData is returned in the response as an "application/pkcs7-mime" part with an smime-type parameter of "server-generated-key" and a Content-Transfer-Encoding of "base64". The certificate data part is an "application/pkcs7-mime" and exactly matches the certificate response to /simpleenroll. When rejecting a request, the server MUST specify either an HTTP 4xx error or an HTTP 5xx error. If the content-type is not set, the response data MUST be a plaintext human-readable error message.

CA policy may allow inclusion of client-provided attributes in certificates that it issues, and some of these attributes may describe information that is not available to the CA. In addition, a CA may desire to certify a certain type of public key and a client may not have a priori knowledge of that fact. Therefore, clients SHOULD request a list of expected attributes that are required, or desired, by the CA in an enrollment request or if dictated by local policy. The EST server SHOULD NOT require client authentication or authorization to reply to this request. Requesting CSR attributes is optional, but clients are advised that CAs may refuse enrollment requests that are not encoded according to the CA's policy.

The EST client requests a list of CA-desired CSR attributes from the CA by sending an HTTPS GET message to the EST server with an operations path of "/csrattrs".

If locally configured policy for an authenticated EST client indicates a CSR Attributes Response is to be provided, the server response MUST include an HTTP 200 response code. An HTTP response code of 204 or 404 indicates that a CSR Attributes Response is not available. Regardless of the response code, the EST server and CA MAY reject any subsequent enrollment requests for any reason, e.g., incomplete CSR attributes in the request. Responses to attribute request messages MUST be encoded as the content-type of "application/csrattrs" with a Content&nbhy;Transfer&nbhy;Encoding of "base64" . The syntax for application/csrattrs body is as follows:

An EST server includes zero or more OIDs or attributes that it requests the client to use in the certification request. The client MUST ignore any OID or attribute it does not recognize. When the server encodes CSR Attributes as an empty SEQUENCE, it means that the server has no specific additional information it desires in a client certification request (this is functionally equivalent to an HTTP response code of 204 or 404). If the CA requires a particular crypto system or use of a particular signature scheme (e.g., certification of a public key based on a certain elliptic curve, or signing using a certain hash algorithm) it MUST provide that information in the CSR Attribute Response. If an EST server requires the linking of identity and POP information (see ), it MUST include the challengePassword OID in the CSR Attributes Response. The structure of the CSR Attributes Response SHOULD, to the greatest extent possible, reflect the structure of the CSR it is requesting. Requests to use a particular signature scheme (e.g. using a particular hash function) are represented as an OID to be reflected in the SignatureAlgorithm of the CSR. Requests to use a particular crypto system (e.g., certification of a public key based on a certain elliptic curve) are represented as an attribute, to be reflected as the AlgorithmIdentifier of the SubjectPublicKeyInfo, with a type indicating the algorithm and the values indicating the particular parameters specific to the algorithm. Requests for descriptive information from the client are made by an attribute, to be represented as Attributes of the CSR, with a type indicating the extensionRequest and the values indicating the particular attributes desired to be included in the resulting certificate's extensions. The sequence is Distinguished Encoding Rules (DER) encoded and then base64 encoded (Section 4 of ). The resulting text forms the application/csrattr body, without headers. For example, if a CA requests a client to submit a certification request containing the challengePassword (indicating that linking of identity and POP information is requested; see ), an extensionRequest with the Media Access Control (MAC) address () of the client, and to use the secp384r1 elliptic curve and to sign with the SHA384 hash function. Then, it takes the following:

and encodes them into an ASN.1 SEQUENCE to produce: 30 41 06 09 2a 86 48 86 f7 0d 01 09 07 30 12 06 07 2a 86 48 ce 3d 02 01 31 07 06 05 2b 81 04 00 22 30 16 06 09 2a 86 48 86 f7 0d 01 09 0e 31 09 06 07 2b 06 01 01 01 01 16 06 08 2a 86 48 ce 3d 04 03 03 and then base64 encodes the resulting ASN.1 SEQUENCE to produce: MEEGCSqGSIb3DQEJBzASBgcqhkjOPQIBMQcGBSuBBAAiMBYGCSqGSIb3DQEJDjEJ BgcrBgEBAQEWBggqhkjOPQQDAw==

defines an OID that has been registered in an arc delegated by the IANA to the PKIX working group. IANA has registered the following: IANA updated the well-known URI registry with the following filled-in template from . URI suffix: est Change controller: IETF IANA has updated the "Application Media Types" registry with the following filled-in templates from . The media subtype for CSR attributes in a CSR Attributes Response is application/csrattrs. Type name: application Subtype name: csrattrs Required parameters: None Optional parameters: None Encoding considerations: binary; Security Considerations: Clients request a list of attributes that servers wish to be in certification requests. The request/response is normally done in a TLS-protected tunnel. Interoperability considerations: None Published specification: This memo. Applications which use this media type: Enrollment over Secure Transport (EST) Additional information: Magic number(s): None File extension: .csrattrs Person & email address to contact for further information: Dan Harkins <dharkins@arubanetworks.com> Restrictions on usage: None Author: Dan Harkins <dharkins@arubanetworks.com> Intended usage: COMMON Change controller: The IESG <iesg@ietf.org> The application/pkcs7-mime content-type defines the optional "smime&nbhy;type" parameter with a set of specific values. This document adds another value, "server-generated-key", as the parameter value for Server-side Key Generation Response.

Support for Basic authentication, as specified in HTTP, allows the server access to a client's cleartext password. This provides support for legacy username/password databases but requires exposing the plaintext password to the EST server. Use of a PIN or one-time password can help mitigate such exposure, but it is RECOMMENDED that EST clients use such credentials only once to obtain a client certificate (that will be used during future interactions with the EST server). When a client uses the Implicit TA database for certificate validation (see ), then authorization proceeds as specified in . In this situation, the client has validated the server as being a responder that is certified by a third party for the URI configured, but it cannot verify that the responder is authorized to act as an RA for the PKI in which the client is trying to enroll. Clients using an Implicit Trust Anchor database are RECOMMENDED to use only TLS-based client authentication (to prevent exposing HTTP&nbhy;based client authentication information). It is RECOMMENDED that such clients include "Linking Identity and POP Information" () in requests (to prevent such requests from being forwarded to a real EST server by a man in the middle). It is RECOMMENDED that the Implicit Trust Anchor database used for EST server authentication be carefully managed to reduce the chance of a third-party CA with poor certification practices from being trusted. Disabling the Implicit Trust Anchor database after successfully receiving the Distribution of CA certificates response () limits any vulnerability to the first TLS exchange. Certificate-less TLS cipher suites that maintain security and perform the mutual authentication necessary for enrollment have the following properties: the only information leaked by an active attack is whether or not a single guess of the secret is correct. any advantage an adversary gains is through interaction and not computation. it is possible to perform countermeasures, such as exponential backoff after a certain number of failed attempts, to frustrate repeated active attacks. Using a certificate-less cipher suite that does not have the properties listed above would render the results of enrollment void and potentially result in certificates being issued to unauthenticated and/or unauthorized entities. When using a certificate-less TLS cipher suite, the shared secret used for authentication and authorization cannot be shared with an entity that is not a party to the exchange: someone other than the client and the server. Any additional sharing of secrets voids the security afforded by a certificate-less cipher suite. Exposure of a shared secret used by a certificate-less cipher suite to a third party enables client impersonation that can result in corruption of a client's trust anchor database. TLS cipher suites that include "_EXPORT_" and "_DES_" in their names MUST NOT be used. These ciphers do not offer a sufficient level of protection; 40-bit crypto in 2013 doesn't offer acceptable protection, and the use of DES is deprecated. As described in CMC, Section 6.7 of , "For keys that can be used as signature keys, signing the certification request with the private key serves as a POP on that key pair". The inclusion of tls-unique within the certification request links the proof-of-possession to the TLS proof-of-identity by enforcing that the POP operation occurred while the TLS session was active. This implies to the server that the authenticated client currently has access to the private key. If the authenticated client is known to have specific capabilities, such as hardware protection for authentication credentials and key storage, this implication is strengthened but not proven. The server-side key generation method allows keys to be transported over the TLS connection to the client without any application-layer protection. The distribution of private key material is inherently risky. Private key distribution uses the encryption mode of the negotiated TLS cipher suite. Keys are not protected by preferred key wrapping methods such as AES Key Wrap or as specified in as encryption of the private key beyond that provided by TLS is optional. It is RECOMMENDED that EST servers not support this operation by default. It is RECOMMENDED that clients not request this service unless there is a compelling operational benefit. Use of an Implicit Trust Anchor database is NOT RECOMMENDED when server&nbhy;side key generation is employed. The use of an encrypted CMS Server&nbhy;Side Key Generation Response is RECOMMENDED. Regarding the CSR attributes that the CA may list for inclusion in an enrollment request, there are no real inherent security issues with the content being conveyed, but an adversary who is able to interpose herself into the conversation could exclude attributes that a server may want, include attributes that a server may not want, and render meaningless other attributes that a server may want. ASN.1 encoding rules (e.g., DER and BER) have a type-length-value structure, and it is easy to construct malicious content with invalid length fields that can cause buffer overrun conditions. ASN.1 encoding rules allow for arbitrary levels of nesting, which may make it possible to construct malicious content that will cause a stack overflow. Interpreters of ASN.1 structures should be aware of these issues and should take appropriate measures to guard against buffer overflows and stack overruns in particular, and malicious content in general.

&RFC2045; &RFC2119; &RFC2585; &RFC2616; &RFC2617; &RFC2633; &RFC2986; &RFC3986; &RFC4086; &RFC4108; &RFC4210; &RFC4346; &RFC4648; &RFC5077; &RFC5246; &RFC5272; &RFC5273; &RFC5274; &RFC5280; &RFC5652; &RFC5746; &RFC5751; &RFC5785; &RFC5929; &RFC5958; &RFC6066; &RFC6125; &RFC6402; &RFC6454; &RFC6838; ITU-T Recommendation X.680 (2008) | ISO/IEC 8824-1:2008 ITU-T Recommendation X.690 (2008) | ISO/IEC 8825-1:2008 &RFC2307; &RFC2818; &RFC2985; &RFC3394; &RFC5054; &RFC5967; &RFC6403; National Institute of Standards and Technology IEEE Standards Association National Institute of Standards and Technology

(Informative) This section expands on the Operational Scenario Overviews by providing detailed examples of the messages at each TLS layer.

The following is an example of a valid /cacerts exchange. During the initial TLS handshake, the client can ignore the optional server-generated "certificate request" and can instead proceed with the HTTP GET request:

In response, the server provides the current CA certificates:

The following is an example of a valid /csrattrs exchange. During this exchange, the EST client authenticates itself using an existing certificate issued by the CA for which the EST server provides services. The initial TLS handshake is identical to the enrollment example handshake. The HTTP GET request:

In response, the server provides suggested attributes that are appropriate for the authenticated client. In this example, the EST server also includes two example attributes that the client would ignore unless the attribute type is known to the client:

The following is an example of a valid /simpleenroll exchange. The data messages for /simplereenroll are similar. During this exchange, the EST client uses an out-of-band distributed username/password to authenticate itself to the EST server. This is the normal HTTP WWW-Authenticate behavior and is included here for informative purposes. When an existing TLS client certificate is used, the server might skip requesting the HTTP WWW-Authenticate header, such as during a /simplereenroll operation. During the initial TLS handshake, the client can ignore the optional server-generated "certificate request" and can instead proceed with the HTTP POST request. In response to the initial HTTP POST attempt, the server requests WWW-Authenticate from the client (this might occur even if the client used a client certificate, as detailed in the normative text above):

In the subsequent HTTP POST, the username/password is included, along with the complete application/pkcs10 content:

The EST server uses the username/password to perform authentication/authorization and responds with the issued certificate:

The following is an example of a valid /serverkeygen exchange. During this exchange, the EST client authenticates itself using an existing certificate issued by the CA the EST server provides services for. The initial TLS handshake is identical to the enrollment example handshake. An example HTTP POSTed message is:

Because the DecryptKeyIdentifier attribute is not included in this request, the response does not include additional encryption beyond the TLS session. The EST server response is:

The editors would like to thank Stephen Kent, Vinod Arjun, Jan Vilhuber, Sean Turner, Russ Housley, and others for their feedback and prototypes of early versions of this document. Our thanks also go the authors of , around whose document we structured part of this specification.