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INTERNET DRAFT Andrew Nourse draft-nourse-scep-14.txt Cheryl Madson expires 1 Jun 2007 David McGrew (revised 1 Dec 2006) Xiaoyi Liu Cisco Systems Category: Informational 1-Dec-2006 Cisco Systems' Simple Certificate Enrollment Protocol(SCEP): Status of this Memo Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Abstract This document specifies the Simple Certificate Enrollment Protocol, a PKI communication protocol which leverages existing technology by using PKCS#7 and PKCS#10. SCEP is the evolution of the enrollment protocol developed by Verisign, Inc. for Cisco Systems, Inc. It now enjoys wide support in both client and CA implementations. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2 2. The Goal of SCEP . . . . . . . . . . . . . . . . . . . . . 3 2.1 SCEP Entity types . . . . . . . . . . . . . . . . . . . . 3 2.2 SCEP Operations Overview . . . . . . . . . . . . . . . . . 7 2.3 PKI Operation Transactional Behavior . . . . . . . . . . . 10 2.4 Security . . . . . . . . . . . . . . . . . . . . . . . . . 12 3. Transport Protocol . . . . . . . . . . . . . . . . . . . . 13 4. Secure Transportation: PKCS #7 . . . . . . . . . . . . . . 14 4.1 SCEP Message Format . . . . . . . . . . . . . . . . . . . 14 Liu/Madson/McGrew/Nourse [Page 2] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 4.2 Signed Transaction Attributes . . . . . . . . . . . . . . 15 5. SCEP Transaction Specification . . . . . . . . . . . . . . 16 5.1 Certificate Enrollment . . . . . . . . . . . . . . . . . . 16 5.2 Poll for Requester Initial Certificate . . . . . . . . . . 22 5.3 Certificate Access . . . . . . . . . . . . . . . . . . . . 26 5.4 CRL Access . . . . . . . . . . . . . . . . . . . . . . . 27 5.5 Get Certificate Authority Certificate . . . . . . . . . . 31 5.6 Get Certificate Authority Certificate Chain . . . . . . . 33 6. Security Considerations . . . . . . . . . . . . . . . . . 33 7. Intellectual Propoerty . . . . . . . . . . . . . . . . . . 33 8. References . . . . . . . . . . . . . . . . . . . . . . . . 33 Appendix A. Cisco Requester Subject Name Definition . . . . . . 34 Appendix B. IPSEC Client Enrollment Certificate Request . . . . 35 Appendix C. Private OID Definitions . . . . . . . . . . . . . 36 Appendix D. Obtaining CRL by LDAP Query . . . . . . . . . . . . 36 Appendix E. SCEP State Transitions . . . . . . . . . . . . . . 37 Appendix F. CA Capabilities . . . . . . . . . . . . . . . . . . 40 Appendix G. Certificate Renewal and CA Key Rollover . . . . . . 41 Appendix H. PKIOperation via HTTP POST Message. . . . . . . . . 42 Appendix Y. Author Contact Information. . . . . . . . . . . . . 43 Appendix Z. Copyright Section . . . . . . . . . . . . . . . . . 43 Section 1. Introduction Public key technology is becoming more widely deployed and is becoming the basis for standards based security, such as the Internet Engineering Task Force's IPSEC and IKE protocols. With the use of public key certificates in network security protocols comes the need for a certificate management protocol that Public Key Infrastructure (PKI) clients and Certificate Authority servers can use to support certificate life cycle operations such as certificate enrollment and revocation, and certificate and CRL access. In the following, Section 2 gives an overview of the PKI operations, and Section 2.4 describes the security goals of the protocol and the mechanisms used to achieve them. The transport protocol and the use of the PKCS#7 data format are described at Section 3 and Section 4, respectively. The last section, Section 5, specifies each PKI operation in terms of the message formats and the data structures of each operation. The appendices provide detailed specifications and examples. Requester subject names are specified in Appendix A, attribute OIDs are specified in Appendix C , and the SCEP state transitions are described in Appendix E. An example of a certificate enrollment request is provided in Appendix B, and an example LDAP query URL encoding is provided in Appendix D. The authors would like to thank Peter William of ValiCert, Inc. (formerly of Verisign, Inc) and Alex Deacon of Verisign, Inc. and Christopher Welles of IRE, Inc. for their contributions to this protocol and to this document. Liu/Madson/McGrew/Nourse [Page 3] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 2.0 The Goal of SCEP The goal of SCEP is to support the secure issuance of certificates to network devices in a scalable manner, using existing technology whenever possible. The protocol supports the following operations: CA and RA public key distribution Certificate enrollment Certificate revocation Certificate query CRL query Certificate and CRL access can be achieved by using the LDAP protocol (as specified in Appendix D), or by using the query messages defined in SCEP. The use of HTTP certificate and CRL access, and the support of CDP as specified in RFC2459, will be specified in a future version of this document. In Section 2.1, we first define PKI entity types as well as the properties of each entity type. In Section 2.2, the PKI operations are described at functional level. Section 2.3 describes the transaction behavior of each PKI operations. The complete PKI messages are covered in Section 5. 2.1 SCEP Entity types The entity types defined in SCEP are the "requester" type (i.e., IPSEC clients), the Certificate Authority (CA) entity type, and the Registration Authority entity type (RA). A requester is sometimes called a "SCEP client" in the following. 2.1.1 Requesters A requester is an entity whose name is defined in a certificate subject name field and optionally, in SubjectAltName, a X.509 certificate V3 extension. As a requester, a SCEP client is identified by a subject name consisting of the following naming attributes: Fully qualified domain name, for example, router.cisco.com IP address, Serial number, and/or x.500 distinguished name The fully qualified domain name is required for a requester that intends to use the certificate for ISAKMP. The IP address, serial number, and x.500 distinguished name are optional name attributes. In the certificate enrollment request, the PKCS#10 subject field contains the required and optional name attributes. The distinguished name, if any, should be the subject name field, while any domain name, serial number, or IP address supplied should be in the subjectAltName field. The subject name field may be empty (if there is no distinguished name) or the subjectAltName may be omitted, but not both. It is important to note that a client named as Alice.cisco.com is different than a client named as Alice.cisco.com plus the IP address name attribute 117.96.1.219. From CA point of view, the Distinguished names assigned in these two cases are distinct names. Liu/Madson/McGrew/Nourse [Page 4] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Entity names which are specified as in the IPSEC profile (i.e., FQDN, IP address and User FQDN) must be presented in certificate's SubjectAltName extension. Multiple IPSEC entity names, (if any) are encoded as multiple values of a single SubjectAltName extension. The CA has the authority to assign a distinguished name to a requester, whether or not one was included in the request. The assigned DN should contain the SCEP client names as the relative DN. The attribute identifiers and an example of SCEP client subject name are specified in Appendix A. Appendix B has an example from Cisco VPN Client enrollment request. 2.1.1.1 Local Key/Certificate/CRL Storage and Certificate-name uniqueness A requester is required to generate asymmetric key pairs and to provide storage to store its private keys. If the requester does not have enough permanent memory to save its certificate, then it should be able to query its own certificate from the CA or an LDAP server, once the certificate has been issued. The public key pairs can be generated with a specific key usage. The key usage is conveyed to the CA through the certificate enrollment request. All current SCEP client implementations expect that there will be only one pair of keys for a given subject name and key usage combination and CA, at any time. This property is called the certificate-name uniqueness property, and it implies that a CA that implements SCEP will enforce the unique mapping between a SCEP client subject name and its key pairs with a given key usage. At any time, if the subject name is changed, or if the key is updated, the existing certificate would have to be revoked before a new one could be issued. It is desirable that the CA enforce certificate-name uniqueness, but it is not mandatory. However a CA that does not enforce uniqueness must provide some other mechanism to prevent the re-transmission of an enrollment request by a SCEP client from creating a second certificate or certificate request, nor can the second request merely be rejected. If a client times out from polling for a pending request it can resynchronize by reissuing the original request with the original subject name, key, and transaction ID. This should return the status of the original transaction, including the certificate if it was granted. It should not create a new transaction unless the original cert has been revoked, or the transaction arrives more than halfway through the validity time of the original certificate. An enrollment request that occurs more than halfway through the validity time of an existing certificate for the same subject name and key usage MAY be interpreted as a re-enrollment or renewal request and accepted. A new certificate with new validity dates may be issued, even though the old one is still valid, if the CA policy permits, as described in 2.1.1.3. See also appendix G. 2.1.1.2 Requester authentication As with every protocol that uses public-key cryptography, the association between the public keys used in the protocol and the identities with which they are associated must be authenticated in a Liu/Madson/McGrew/Nourse [Page 5] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 cryptographically secure manner. This requirement is needed to prevent a "man in the middle" attack, in which an adversary that can manipulate the data as it travels between the protocol participants can subvert the security of the protocol. To satisfy this requirement, SCEP provides two authentication methods: manual authentication, and authentication based on pre-shared secret. In the manual mode, the requester is required to wait until its identity can be verified by the CA operator using any reliable out-of-band method. To prevent a "man-in-the-middle" attack, a SHA-1, SHA-256, SHA-512, or MD5 `fingerprint' generated on the PKCS#10 (before PKCS #7 enveloping and signing) must be compared out-of-band between the server and the requester. SCEP clients and CAs (or RAs, if appropriate) must display this fingerprint to the operator to enable this verification if manual mode is used. Failing to provide this information leaves the protocol vulnerable to attack by sophisticated adversaries. When utilizing a pre-shared secret scheme, the server should distribute a shared secret to the requester which can uniquely associate the enrollment request with the given end entity. The distribution of the secret must be private: only the end entity should know this secret. The actual binding mechanism between the requester and the secret is subject to the server policy and implementation. When creating the enrollment request, the requester is asked to provide a challenge password. When using the pre-shared secret scheme, the requester must enter the re-distributed secret as the password. In the manual authentication case, the challenge password only used to authenticate a request for the certificate's revokation. This challenge password is included as a PKCS#10 attribute, and is sent to the server as encrypted data. The PKCS#7 envelope protects the privacy of the challenge password with DES encryption. 2.1.1.3 Requester Uses Existing CA-Issued or Self-Signed Certificates In this protocol, the communication between the requester and the certificate authority is secured by using PKCS#7 as the messaging protocol. PKCS#7, however, is a data format which assumes the communicating entities already possess the peer's certificates and requires both parties use the issuer names and issuer assigned certificate serial numbers to identify the certificate in order to verify the signature and decrypt the message. If the requesting system already has a certificate issued by the CA, that certificate may be presented as credentials for the renewal of that certificate if the CA supports the "Renewal" capability and the CA policy permits the certificate to be renewed. If the requester has no certificate issued by the CA, or if the CA does not support and permit renewal, the requestor must generate a self-signed certificate with the requester subject name (the same name later used in the PKCS#10) as both issuer and subject name. During the certificate enrollment, the requester will first post itself as the signing authority by attaching the self-signed certificate to the signed certificate request. When the Certificate Authority makes the envelope on the issued certificate using the public key included in the self-signed certificate, it should use the same issuer name and serial number as conveyed in the self-signed certificate to inform the end entity on which private key should be used to open the envelope. Liu/Madson/McGrew/Nourse [Page 6] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Note that when a client enrolls for separate encryption and signature certificates, it may use the signature certificate to sign both requests, and then expect its signature key to be used to encrypt both responses. In any case, the recipientinfo on the envelope should reflect the key used to encrypt the request. 2.1.1.4 Trusted CA Store To support interoperability between IPSEC peers whose certificates are issued by different CA, SCEP allows the users to configure multiple trusted certificates. Trusted certificates are have been configured as such in the client, based on some out-of-band means such as a "fingerprint". These trusted certificates are used to verify certificate chains that end in those certificates. 2.1.2 Certificate Authority A Certificate Authority(CA) is an entity whose name is defined in the certificate issuer name field. Before any PKI operations can begin, the CA generates its own public key pair and creates a self-signed CA certificate, or causes another CA to issue a certificate to it. Associated with the CA certificate is a fingerprint which will be used by the requester to authenticate the received CA certificate if it is self-signed. The fingerprint is created by calculating a SHA-1, SHA-256, SHA-512, or MD5 hash on the whole CA certificate. Before any requester can start its enrollment, this CA certificate has to be configured at the entity side securely. For IPSEC clients, the client certificates must have SubjectAltName extension. To utilize LDAP as a CRL query protocol, the certificates must have a CRL Distribution Point. Key usage is optional. Without key usage, the public key is assumed as a general purpose public key and it can be used for all the purposes. A Certificate Authority may enforce certain name policy. When using X.500 directory name as the subject name, all the name attributes specified in the PKCS#10 request should be included as Relative DN. All the name attributes as defined in RFC2459 should be specified in the SubjectAltName. An example is provided in Appendix A. If there is no LDAP query protocol support, the Certificate Authority should answer certificate and CRL queries, and to this end it should be online all the time. The updating of the CA's public key is addressed in Appendix G. 2.1.3 Registration Authorities In an environment where an RA is present, a requester performs enrollment through the RA. In order to setup a secure channel with an RA using PKCS#7, the RA certificate(s) have to be obtained by the client in addition to the CA certificate(s). In the following, the CA and RA are specified as one entity in the context of PKI operation definitions. Liu/Madson/McGrew/Nourse [Page 7] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 2.2 SCEP Operations Overview In this section, we give a high level overview of the PKI operations as defined in SCEP. 2.2.1 Requester Initialization The requester initialization includes the key pair generation and the configuring of the required information to communicate with the certificate authority. 2.2.1.1 Key Pairs Before a requester can start PKI transaction, it must have at least one asymmetric key pair, using the selected algorithm (the RSA algorithm is required in SCEP, and is the only algorithm in current implementations). Key pairs may be intended for particular purposes, such as encryption only, or signing only. The usage of any associated certificate can be restricted by adding key usage and extended key usage attributes to the PKCS#10. 2.2.1.2 Required Information A requester is required to have the following information configured before starting any PKI operations: 1. the certificate authority IP address or fully-qualified domain name, 2. the certificate authority HTTP CGI script path, and the HTTP proxy information in case there is no direct Internet connection to the server, 3. If CRLs are being published by the CA to an LDAP directory server, and there is a CRL Distribution Point containing only an X.500 directory name, then the client will need to know the LDAP server fully-qualified domain name or IP address. CRL Distribution Points are discussed in more detail in RFC 2459. 2.2.2 CA/RA Certificate Distribution Before any PKI operation can be started, the requester needs to get the CA/RA certificates. At this time, since no public key has been Liu/Madson/McGrew/Nourse [Page 8] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 exchanged between the requester and the CA/RA, the message to get the CA/RA certificate cannot be secured using PKCS#7. Instead, the CA/RA certificate distribution is implemented as a clear HTTP Get operation. After the requester gets the CA certificate, it has to authenticate the CA certificate by comparing the finger print with the CA/RA operator. Since the RA certificates are signed by the CA, there is no need to authenticate the RA certificates. This operation is defined as a transaction consisting of one HTTP Get message and one HTTP Response message: REQUESTER CA SERVER Get CA/RA Cert: HTTP Get message -----------------------------> CA/RA Cert download: HTTP Response message <--------------------------------------- Compute finger print and call CA operator. Receive call and check finger print If an RA is in use, a degenerated PKCS#7 with a certificate chain consisting of both RA and CA certificates is sent back to the end entity. Otherwise the CA certificate is directly sent back as the HTTP response payload. 2.2.3 Certificate Enrollment A requester starts an enrollment transaction by creating a certificate request using PKCS#10 and sends it to the CA/RA enveloped using the PKCS#7. After the CA/RA receives the request, it will either automatically approve the request and send the certificate back, or it will require the requester to wait until the operator can manually authenticate the identity of the requester. Two attributes are included in the PKCS#10 certificate request - a Challenge Password attribute and an optional ExtensionReq attribute which will be a sequence of extensions the requester would like to be included in its V3 certificate extensions. The Challenge Password may be used to authenticate either the enrollment request itself, or a verbal revocation request for the issued certificate in the event of key compromise or other reason. In the automatic mode, the transaction consists of one PKCSReq PKI Message, and one CertRep PKI message. In the manual mode, the requester enters into polling mode by periodically sending a GetCertInitial PKI message to the server, until the server operator completes the manual authentication, after which the CA will respond to GetCertInitial by returning the issued certificate. A CA MAY run in automatic mode for preapproved requests, and manual mode for the rest. A request with a non-null password is not necessarily a pre-approved request. It is up to the CA server to decide. Polling mode is entered whenever the server returns a PENDING response. Liu/Madson/McGrew/Nourse [Page 9] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 The transaction in automatic mode: REQUESTER CA SERVER PKCSReq: PKI cert. enrollment msg --------------------------------> CertRep: pkiStatus = SUCCESS certificate attached <------------------------------ Receive issued certificate. The transaction in manual mode: REQUESTER CA SERVER PKCSReq: PKI cert. enrollment msg --------------------------------> CertRep: pkiStatus = PENDING <------------------------------ GetCertInitial: polling msg --------------------------------> CertRep: pkiStatus = PENDING <------------------------------ ................. CertRep: pkiStatus = SUCCESS certificate attached <------------------------------ Receive issued certificate. 2.2.4 Requester Certificate Revocation A requester should be able to revoke its own certificate. Currently the revocation is implemented as a manual process. In order to revoke a certificate, the requester makes a phone call to the CA server operator. The operator will come back asking the ChallengePassword (which has been sent to the server as an attribute of the PKCS#10 certificate request). If the ChallengePassword matches, the certificate is revoked. The reason of the revocation is documented by CA/RA. 2.2.5 Certificate Access There are two methods to query certificates. The first method is to use LDAP as a query protocol. Using LDAP to query assumes the client understand the LDAP scheme supported by the CA. The SCEP client assumes that the subject DN name in the certificate is used as the URL to query the certificate. The standard attributes (userCertificate and caCertificate) are used as filter. For the environment where LDAP is not available, a certificate query message is defined to retrieve the certificates from the CA. To query a certificate from the certificate authority, a requester sends a request consisting of the certificate's issuer name and the serial number. This assumes that the requester has saved the issuer Liu/Madson/McGrew/Nourse [Page 10] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 name and the serial number of the issued certificate from the previous enrollment transaction. The transaction to query a certificate consists of one GetCert PKI message and one CertRep PKI message: REQUESTER CA SERVER GetCert: PKI cert query msg -------------------------------> CertRep: pkiStatus = SUCCESS certificate attached <----------------------------- Receive the certificate. 2.2.6 CRL Distribution The CA/RA will not "push" the CRL to the end entities. The query of the CRL can only be initialized by the requester. There are three methods to query CRL. The CRL may be retrieved by a simple HTTP GET. If the CA supports this method, it should encode the URL into a CRL Distribution Point extension in the certificates it issues. Support for this method should be incorporated in new and updated clients, but may not be in older versions. The second method is to query CRL using LDAP. This assumes the CA server supports CRL LDAP publishing and issues the CRL Distribution Point in the certificate. The CRL Distribution Point is encoded as a DN. Please refer to Appendix D for the examples of CRL Distribution Point. The third method is implemented for the CA which does not support LDAP CRL publishing or does not implement the CRL Distribution Point. In this case, a CRL query is composed by creating a message consists of the CA issuer name and the CA's certificate serial number. This method is deprecated because it does not scale well and requires the CA to be a high-availability service. The message is sent to the CA in the same way as the other SCEP requests: The transaction to query CRL consists of one GetCRL PKI message and one CertRep PKI message which have no certificates but CRL. REQUESTER CA SERVER GetCRL: PKI CRL query msg ----------------------------------> CertRep: CRL attached <-------------------------------- 2.3 PKI Operation Transactional Behavior As described before, a PKI operation is a transaction consisting of the messages exchanged between a requester and the CA/RA. This section will specify the transaction behavior on both the requester and the Liu/Madson/McGrew/Nourse [Page 11] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 certificate authority server. Because the protocol is basically a two way communication protocol without a confirmation message from the initiating side, state and state resynchronization rules have to be defined, in case any error happens at either side. Before the state transition can be defined, the notion of transaction identifier has to be defined first. 2.3.1 Transaction Identifier A transaction identifier is a string generated by the entity when starting a transaction. Since all the PKI operations defined in this protocol are initiated by the requester, it is the responsibility of the requester to generate a unique string as the transaction identifier. All the PKI messages exchanged for a given PKI transaction must carry the same transaction identifier. The transaction identifier is generated as a SHA-1, SHA-256, SHA-512 or MD5 hash on the public key value for which the enrollment request is made. This allows the SCEP client to reuse the same transaction identifier if it is reissuing a request for the same certificate (i.e. a certificate with the same subject, issuer, and key). The SCEP protocol requires that transaction identifiers be unique, so that queries can be matched up with transactions. For this reason, in those cases in which separate signing and encryption certificates are issued to the same requester, the keys must be different. 2.3.2 State Transitions in Certificate Enrollment The requester state transitions during enrollment operation are indicated in the diagram below: +-<------+ | | GetCertInitial triggered by timeout or | | manual authentication | | [CERT-NONEXISTANT] ------> [CERT-REQ-PENDING] ---> [CERT-ISSUED] | PKCSReq | CertRep with SUCCESS | | | | +--------<-------------------+ request rejected, timeout, or error As described in the section 2.2.3, certificate enrollment starts at the state CERT-NONEXISTANT. Sending PKCSReq changes the state to CERT-REQ-PENDING. Receiving CertRep with SUCCESS status changes the state to CERT-ISSUED. In the case the server sending back the response with pending status, the requester will keep polling certificate response by sending GetCertInitial to the server, until either a CertRep with SUCCESS status is received, or the maximum polling number has been exceeded. If an error or timeout occurs in the CERT-REQ-PENDING state, the end entity will transition to the CERT-NONEXISTANT state. Liu/Madson/McGrew/Nourse [Page 12] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 The client administrator will, eventually, start up another enrollment request. It is important to note that, as long as the requester does not change its subject name or keys, the same transaction id will be used in the "new" transaction. This is important because based on this transaction id, the certificate authority server can recognize this as an existing transaction instead of a new one. 2.3.3 Transaction Behavior of Certificate/CRL Access There is no state maintained during certificate access and CRL access transaction. When using the certificate query and CRL query messages defined in this protocol, the transaction identifier is still required so that the requester can match the response message with the upstanding request message. When using LDAP to query the certificate and the CRL, the behavior is specified by the LDAP protocol. 2.4 Security The security goals of SCEP are that no adversary can: o subvert the public key/identity binding from that intended, o discover the identity information in the enrollment requests and issued certificates, o cause the revocation of certificates with any non-negligible probability. Here an adversary is any entity other than the requester and the CA (and optionally the RA) participating in the protocol that is computationally limited, but that can manipulate data during transmission (that is, a man-in-the-middle). The precise meaning of 'computationally limited' depends on the implementer's choice of cryptographic hash functions and ciphers. The required algorithms are RSA, DES and MD5. Depending on the CA Capabilities, Triple-DES may be used instead of DES, and SHA-1, SHA-256, or SHA-512 may be used instead of MD5. [See Appendix F]. The first and second goals are met through the use of PKCS#7 and PKCS#10 encryption and digital signatures using authenticated public keys. The CA's public key is authenticated via the checking of the CA fingerprint, as specified in Section 2.1.2, and the SCEP client's public key is authenticated through the manual authentication or pre-shared secret authentication, as specified in Section 2.1.1.2. The third goal is met through the use of a Challenge Password for revocation, that is chosen by the SCEP client and communicated to the CA protected by the PKCS#7 encryption, as specified in Section 2.2.4. The motivation of the first security goal is straightforward. The motivation for the second security goal is to protect the identity information in the enrollment requests and certificates. For example, two IPSEC hosts behind a firewall may need to exchange certificates, and may need to enroll certificates with a CA that is outside of a firewall. Liu/Madson/McGrew/Nourse [Page 13] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Most networks with firewalls seek to prevent IP addresses and DNS information from the trusted network leaving that network. The second goal enables the hosts in this example to enroll with a CA outside the firewall without revealing this information. The motivation for the third security goal is to protect the SCEP clients from denial of service attacks. Section 3 Transport Protocol In the SCEP protocol, HTTP is used as the transport protocol for the PKI messages. 3.1 HTTP "GET" and "POST" Message Format The following is the syntax definition of a HTTP GET message sent from a requester to a certificate authority server: Request = "GET " CGI-PATH CGI-PROG "?operation=" OPERATION "&message=" MESSAGE where: CGI-PATH defines the actual CGI path to invoke the CGI program which parses the request. CGI-PROG is set to be the string "pkiclient.exe". This is intended to be the program that the CA will use to handle the SCEP transactions, though the CA may ignore CGI-PROG and use only the CGI-PATH. OPERATION is set to be the string "PKIOperation" when the GET message carries a PKI message to request certificates or CRL; OPERATION is set to be the string "GetCACaps", "GetCACert", "GetNextCACert" or "GetCACertChain" when the GET operation is used to get CA capabilities, CA/RA certificate, the replacement CA/RA certificates for when the current ones expire, or the CA Cert chain (respectively). When OPERATION is "PKIOperation", MESSAGE is a base64-encoded PKI message, When OPERATION is GetCACert, MESSAGE is a CRL distribution point in URI format, otherwise, MESSAGE is a string which represents the certificate authority issuer identifier. SCEP uses the HTTP "GET" and "POST" messages to request information from the CA. Requests for CA certificates or capabilities are sent in the clear, using "GET", with the OPERATION and MESSAGE fields identifying the requested data. CRLs may also be requested in the clear if the CA supports it. Other types of requests are sent using the PKCS#7 data format. These may be issued by means of a GET operation with OPERATION and MESSAGE parameters in the Request-URL. OPERATION identifies the type of GET operation, and MESSAGE is actually the PKCS#7 message Base64-Encoded. For example. a requester may submit a message via HTTP to the server as follows: GET /cgi-bin/pkiclient.exe?operation=PKIOperation&message=MIAGCSqGSIb3D QEHA6CAMIACAQAxgDCBzAIBADB2MGIxETAPBgNVBAcTCE ......AAAAAA== Liu/Madson/McGrew/Nourse [Page 13a] If supported by the CA, the message may also be sent via HTTP POST: POST /cgi-bin/pkiclient.exe?operation=PKIOperation This is further described in Appendix H. To determine if the CA supports POST, use the GetCACaps message described in Appendix F. 3.2 Response Message Format For each GET operation, the CA/RA server will return a MIME object via HTTP. For a GET operation with PKIOperation as its type, the response is tagged as having a Content Type of application/x-pki-message. The body of this message is a BER encoded binary PKI message. The following is an example of the response: "Content-Type:application/x-pki-message\n\n" In the case of GET operation with a type of GetCACert the MIME content type returned will depend on whether or not an RA is in use. If there is no RA, only the CA certificate is sent back in the response, and the response has the content type tagged as application/x-x509-ca-cert. the body of the response is a DER encoded binary X.509 certificate. For example: "Content-Type:application/x-x509-ca-cert\n\n" If there is an RA, the RA certificates are sent back together with the CA certificates, a certificate-only PKCS#7 SignedData is sent back in the response where the SignerInfo is empty. Section 5 has the detailed definition of the message format in this case. The content type is application/x-x509-ca-ra-cert. The response to GetNextCACert is always a certificates-only PKCS#7 SignedData with a content type of application/x-x509-ca-ra-cert. If there is an RA, The signer is the current RA certificate. Otherwise, the signer is the current CA certificate. If the CA supports it, PKIOperation may also be done via an HTTP POST. This is described in Appendix H. Liu/Madson/McGrew/Nourse [Page 14] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Section 4 Secure Transportation: PKCS#7 PKCS#7 is a general enveloping mechanism that enables both signed and encrypted transmission of arbitrary data. It is widely implemented and included in the RSA tool kit. In this section, the general PKCS#7 enveloped PKI message format is specified. The complete PKCS#7 message format for each PKI transaction will be covered in Section 5. 4.1 SCEP Message Format As a transaction message, a SCEP message has a set of transaction specific attributes and an information portion. Employing PKCS#7 data format, the transaction specific attributes are encoded as a set of authenticated attributes of the SignedData. The information portion will first be encrypted to become Enveloped Data, and then the digest of the enveloped information portion is included as one of the message digest attributes and being signed together with the other transaction specific attributes. By applying both enveloping and signing transformations, a SCEP message is protected both for the integrity of its end-end-transition information and the confidentiality of its information portion. The advantage of this technique over the conventional transaction message format is that, the signed transaction type information and the status of the transaction can be determined prior to invoke security handling procedures specific to the information portion being processed. The following is an example of a SCEP message with its enveloped and signed data portion represented by pkcsPKISigned and pkcsPKIEnveloped. The out-most of any PKI message is a blob of ContentInfo, with its content type set to SignedData and the actual signed data as the content. Liu/Madson/McGrew/Nourse [Page 15] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 pkiMessage ContentInfo ::= { contentType {pkcs-7 signedData(2)} content pkcsPKISigned } pkcsPKISigned SignedData ::= { version 1 digestAlgorithm { iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} -- data content identifier content pkcsPKIEnvelope -- enveloped information portion } certificates -- signer certificate chain signerInfo -- including signed transaction info and the digest -- of the enveloped information portion as the -- authenticated attributes } pkcsPKIEnveloped EnvelopedData ::= { version 0 recipientInfos -- information required to open the envelop encryptedContentInfo { contentType {pkcs-7 1} -- data content identifier contentEncryptionAlgorithm encryptedContent -- encrypted information portion } } 4.2 Signed Transaction Attributes The following transaction attributes are encoded as authenticated attributes. Please refer to Appendix B for the OID definitions. transactionID PrintableString -- Decimal value as a string messageType PrintableString -- Decimal value as a string pkiStatus PrintableString -- Decimal value as a string failinfo PrintableString -- Decimal value as a string senderNonce Octet String recipientNonce Octet String where: The transactionID is an attribute which uniquely identify a transaction. This attribute is required in all PKI messages. The messageType attribute specify the type of operation performed by the transaction. This attribute is required in all PKI messages. Currently, the following message types are defined: PKCSReq (19) -- Permits use of PKCS#10 certificate request CertRep (3) -- Response to certificate or CRL request GetCertInitial (20) -- Certificate polling in manual enrollment GetCert (21) -- Retrieve a certificate GetCRL (22) -- Retrieve a CRL Liu/Madson/McGrew/Nourse [Page 16] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 All response message will include transaction status information which is defined as pkiStatus attribute: SUCCESS (0) -- request granted FAILURE (2) -- request rejected PENDING (3) -- request pending for manual approval. If the status in the response is FAILURE, the failinfo attribute will contain one of the following failure reasons: badAlg (0) -- Unrecognized or unsupported algorithm ident badMessageCheck (1) -- integrity check failed badRequest (2) -- transaction not permitted or supported badTime (3) -- Message time field was not sufficiently close to the system time badCertId (4) -- No certificate could be identified matching the provided criteria The attributes of senderNonce and recipientNonce are the 16 byte random numbers generated for each transaction to prevent the replay attack. When a requester sends a PKI message to the server, a senderNonce is included in the message. After the server processes the request, it will send back the requester senderNonce as the recipientNonce and generates another nonce as the senderNonce in the response message. Because the proposed pki protocol is a two-way communication protocol, it is clear that the nonce can only be used by the requester to prevent the replay. The server has to employ extra state related information to prevent a replay attack. Section 5. SCEP Transaction Specification In this section each SCEP transaction is specified in terms of the complete messages exchanged during the transaction. 5.1 Certificate Enrollment The certificate enrollment transaction consists of one PKCSReq message sent to the certificate authority from a requester, and one CertRep message sent back from the server. The pkiStatus returned in the response message is either SUCCESS, or FAILURE, or PENDING. The information portion of a PKCSReq message is a PKCS#10 certificate request, which contains the subject Distinguished Name, the subject public key, and two attributes, a ChallengePassword attribute to be used for revocation, and an optional ExtensionReq attribute which will be a sequence of extensions the requester expects to be included in its V3 certificate extensions. One of the extension attribute specifies the key usage. If the request is granted, the pkiStatus is set to SUCCESS, and the certificate is returned in CertRep; if the request is rejected, the Liu/Madson/McGrew/Nourse [Page 17] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 pkiStatus is set to FAILURE; if the server requires manual approval of the request, the pkiStatus is set to PENDING. The messages exchanged in the manual authentication mode is further specified in Section 5.2. Precondition: Both the requester and the certificate authority have completed their initialization process. The requester has already been configured with the CA/RA certificate. Postcondition: Either the certificate is received by the requester, or the end entity is notified to do the manual authentication, or the request is rejected. 5.1.1 PKCSReq Message Format A PKCSReq message is created by following the steps defined below: 1. Create a PKCS#10 certificate request which is signed by the end entity's private key, corresponding to the public key included in the PKCS#10 certificate request. This constitutes the information portion of PKCSReq. 2. Encrypt the PKCS#10 certificate request using a randomly generated content-encryption key. This content-encryption key is then encrypted by the CA's public key and included in the recipientInfo. This step completes the "envelope" for the PKCS#10 certificate request. 3. Generate a unique string as the transaction id. 4. Generate a 16 byte random number as senderNonce. 5. Generate message digest on the enveloped PKCS#10 certificate request using the selected digest algorithm. 6. Create SignedData by adding the requester's self- or CA-certificate as the signer's public key certificate. Include the message type, transaction id, the senderNonce and the message digest as the authenticated attributes and sign the attributes using the end entity's private key. This completes the SignedData. 7. The SignedData is prepended with the ContenInfo blob which indicates a SignedData object. This final step completes the create of a complete PKCSReq PKI message. In the following, the PKCSReq message is defined following the ASN.1 notation. For readability, the values of a field is either represented by a quoted string which specifies the intended value, or a constant when the value is known. Liu/Madson/McGrew/Nourse [Page 18] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 -- PKCSReq information portion pkcsCertReq CertificationRequest ::= { -- PKCS#10 version 0 subject "the requester's subject name" subjectPublicKeyInfo { algorithm {pkcs-1 1} -- rsa encryption subjectPublicKey "DER encoding of the requester's public key" } attributes { challengePassword {{pkcs-9 7} "password string" } extensions } signatureAlgorithm {pkcs-1 4} -- MD5WithRSAEncryption signature "bit string which is created by signing inner content of the defined pkcsCertReq using requester's private key, corresponding to the public key included in subjectPublicKeyInfo." } -- Enveloped information portion pkcsCertReqEnvelope EnvelopeData ::= { -- PKCS#7 version 0 recipientInfo { version 0 issuerAndSerialNumber { issuer "the CA issuer name" serialNumber "the CA certificate serial number" } keyEncryptionAlgorithm {pkcs-1 1} -- rsa encryption encryptedKey "content-encryption key encrypted by CA public key" } encryptedContentInfo { contentType {pkcs-7 1} -- data content contentEncryptionAlgorithm "object identifier for DES encryption" encryptedContent "encrypted pkcsCertReq using the content- encryption key" } } -- Signed PKCSReq pkcsCertReqSigned SignedData ::= { -- PKCS#7 version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} -- data content identifier content pkcsCertReqEnvelope } certificate { -- requester self-signed or CA-issued certificate version 3 serialNumber "the transaction id associated with enrollment" signature {pkcs-1 4} -- md5WithRSAEncryption Liu/Madson/McGrew/Nourse [Page 19] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 issuer " the requester's subject name" validity { notBefore "a UTC time" notAfter "a UTC time" } subject "the requester's subject name" subjectPublicKeyInfo { algorithm {pkcs-1 1} subjectPublicKey "DER encoding of requester's public key" } signatureAlgorithm {pkcs-1 4} signature "the signature generated by using the requester's private key corresponding to the public key in this certificate." } signerInfo { version 1 issuerAndSerialNumber { issuer "the requester's subject name" serialNumber "the transaction id associated with the enrollment" } digestAlgorithm {iso(0) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} authenticateAttributes { contentType {{pkcs-9 3} {pkcs-7 1}} messageDigest {{pkcs-9 4} "an octet string"} transaction-id {{id-attributes transId(7)} "printable string"} -- this transaction id will be used -- together with the subject name as -- the identifier of the requester's key -- pair during enrollment messageType {{id-attributes messageType(2)} "PKCSReq"} senderNonce {{id-attributes senderNonce(5)} "a random number encoded as a string"} } digestEncryptionAlgorithm {pkcs-1 1} -- rsa encryption encryptedDigest "encrypted digest of the authenticated attributes using requester's private key" } } pkcsReq PKIMessage ::= { contentType {pkcs-7 2} content pkcsCertReqSigned } Liu/Madson/McGrew/Nourse [Page 20] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 5.1.2 CertRep Message Format The response to an SCEP enrollment request is a CertRep message. 5.1.2.1 PENDING Response When the CA is configured to manually authenticate the requester, the CertRep is returned with the attribute pkiStatus set to PENDING. The data portion for this message is null. Only the transaction required attributes are sent back. CertRepSigned SignedData ::= { -- PKCS#7 version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo {contentType {pkcs-7 1} -- empty content } signerInfo { version 1 issuerAndSerialNumber { issuer "name of CA that issued the CA [RA] cert" serialNumber "the serial number of the CA [RA] cert" } digestAlgorithm (iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} authenticateAttributes { contentType {{pkcs-9 3} {pkcs-7 1}} messageDigest {{pkcs-9 4} NULL} messageType {{id-attribute messageType(0)} "CertRep"} transaction-id {{id-attributes transid(7)} "printablestring"} --- same transaction id used in PKCSReq pkiStatus {{id-attributes pkiStatus(3)} "PENDING"} recipientNonce {{id-attributes recipientNonce(6)}<16 bytes>} senderNonce {{id-attributes senderNonce(5)} <16 bytes>} } digestEncrytionAlgorithm {pkcs-1 1} encryptedDigest "encrypted message digest of the authenticated attributes using the CA's [RA's] private key" } } CertRep PKIMessage ::= { contentType {pkcs-7 2} content CertRepSigned } 5.1.2.2 Failure Response In this case, the CertRep sent back to the requester is same as in the PENDING case, except that the pkiStatus attribute is set to FAILURE, and the failInfo attribute should be included: pkistatus {{id-attributes pkiStatus(3)} "FAILURE"} failInfo {{id-attributes failInfo(4)} "the reason to reject"} Liu/Madson/McGrew/Nourse [Page 21] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 5.1.2.3 SUCCESS response In this case, the information portion of CertRep will be a degenerated PKCS#7 which contains the requester's certificate. It is then enveloped and signed as below: pkcsCertRep SignedData ::= { -- PKCS#7 version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { -- empty content since this is degenerated PKCS#7 contentType {pkcs-7 1} } certificates { certificate { -- issued requester's certificate // must be first version 3 serialNumber "issued requester's certificate serial number" signature {pkcs-1 4} -- md5WithRSAEncryption issuer "the certificate authority issuer name" validity { notBefore "UTC time" notAfter "UTC time" } subject "the requester subject name as given in PKCS#10" subjectPublicKeyInfo { algorithm {pkcs-1 1} subjectPublicKey "a DER encoding of requester public key as given in PKCS#10" } extensions " the extensions as given in PKCS#10" signatureAlgorithm {pkcs-1 4} signature " the certificate authority signature" } certificate "the certificate authority certificate" (optional) certificate "the registration authority certificate(s)" (optional) } } pkcsCertRepEnvelope EnvelopedData ::= { -- PKCS#7 version 0 recipientInfo { version 0 issuerAndSerialNumber { -- use issuer name and serial number as -- conveyed in requester's self-signed -- certificate, included in the PKCSReq issuer "the requester's subject name" serialNumber "the serial number defined by the requester in its self-signed certificate" } keyEncryptionAlgorithm {pkcs-1 1} encryptedKey "content-encrypt key encrypted by the requester's public key which is same key as authenticated in the requester's certificate" } Liu/Madson/McGrew/Nourse [Page 22] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 encryptedContentInfo { contentType {pkcs-7 1} -- data content identifier contentEncryptionAlgorithm "OID for DES encryption" encryptedContent "encrypted pkcsCertRep using content encryption key" } } pkcsCertRepSigned SignedData ::= { -- PKCS#7 version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} content pkcsCertRepEnvelope } signerInfo { version 1 issuerAndSerialNumber { issuer "the certificate authority issuer name" serialNumber "the CA certificate's serial number" } digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} authenticateAttributes { contentType {{pkcs-9 3} {pkcs-7 1}} messageDigest {{pkcs-9 4} "a octet string"} messageType {{id-attribute messageType(2)} "CertRep"} transaction-id {{id-attributes transId(7)} "printable string"} -- same transaction id as given in PKCSReq pkiStatus {{id-attributes pkiStatus(3) "SUCCESS"} recipientNonce {{id-attribute recipientNonce(6)}<16 bytes>} senderNonce {{ id-attributes senderNonce(5) <16 bytes>} } digestEncryptionAlgorithm {pkcs-1 1} encryptedDigest "encrypted digest of authenticate attributes using CA's private key " } } CertRep PKIMessage ::= { contentType {pkcs-7 2} content pkcsCertRepSigned } 5.2 Poll for Requester Initial Certificate Either triggered by the PENDING status received from the CertRep, or by the non-response timeout for the previous PKCSReq, a requester will enter the polling state by periodically sending GetCertInitial to the server, until either the request is granted and the certificate is sent back, or the request is rejected, or the configured time limit for polling is exceeded. Liu/Madson/McGrew/Nourse [Page 23] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Since GetCertInitial is part of the enrollment, the messages exchanged during the polling period should carry the same transaction identifier as the previous PKCSReq. PreCondition Either the requester has received a CertRep with pkiStatus set to be PENDING, or the previous PKCSReq has timed out. PostContition The requester has either received the certificate, or be rejected of its request, or the polling period ended as a failure. 5.2.1 GetCertInitial Message Format Since at this time the certificate has not been issued, the requester can only use the requester's subject name, combined with the transaction identifier, to identify the polled certificate request. The certificate authority server must be able to uniquely identify the polled certificate request. A subject name can have more than one outstanding certificate request (with different key usage attributes). -- Information portion pkcsGetCertInitial issuerAndSubject ::= { issuer "the certificate authority issuer name" subject "the requester subject name as given in PKCS#10" } pkcsGetCertInitialEnvelope EnvelopedData ::= { version 0 recipientInfo { version 0 issuerAndSerialNumber { issuer "the CA issuer name" serialNumber "the CA certificate serial number" } keyEncryptionAlgorithm {pkcs-1 1} encryptedKey "content-encrypt key encrypted by CA's public key" } encryptedContentInfo { contentType {pkcs-7 1} -- data content contentEncryptionAlgorithm "OID for DES encryption" encryptedContent "encrypted getCertInital" } } pkcsGetCertInitialSigned SignedData ::= { -- PKCS#7 version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} Liu/Madson/McGrew/Nourse [Page 24] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 content pkcsGetCertIntialEnvelope } certificate { -- the requester's self-signed certificate version 3 serialNumber "the transaction id associated with enrollment" signature {pkcs-1 4} -- md5WithRSAEncryption issuer " the requester's subject name" validity { notBefore "a UTC time" notAfter "a UTC time" } subject "the requester's subject name" subjectPublicKeyInfo { algorithm {pkcs-1 1} subjectPublicKey "DER encoding of requester's public key" } signatureAlgorithm {pkcs-1 4} signature "the signature generated by using the requester's private key corresponding to the public key in this certificate." } signerInfo { version 1 issuerAndSerialNumber { issuer "requester's subject name" serialNumber "the transaction id used in previous PKCSReq" } digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} authenticateAttributes { contentType {{pkcs-9 3} {pkcs-7 1}} messageDigest {{pkcs-9 4} "an octet string"} -- digest of getCertInitial messageType {{id-attribute messageType(2)} "GetCertInitial"} transaction-id {{id-attributes transId(7)} "printable string"} -- same transaction idused in previous PKCSReq senderNonce {{id-attribute senderNonce(3)} 0x<16 bytes>} } digestEncryptionAlgorithm {pkcs-1 1} encryptedDigest "encrypted digest of authenticateAttributes" } } GetCertInitial PKIMessage ::= { contentType {pkcs-7 2} content pkcsGetCertInitialSigned } 5.2.2 GetCertInitial Response Message Format The response messages for GetCertInitial are the same as for PKCSReq. Liu/Madson/McGrew/Nourse [Page 25] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 5.3 Certificate Access The certificate query message defined in this section is an option when the LDAP server is not available to provide the certificate query. A requester should be able to query an issued certificate from the certificate authority, as long as the issuer name and the issuer assigned certificate serial number is known to the requesting end entity. This transaction is not intended to provide the service as a certificate directory service. A more complicated query mechanism would have to be defined in order to allow a requester to query a certificate using various different fields. This transaction consists of one GetCert message sent to the server by a requester, and one CertRep message sent back from the server. PreCondition The queried certificate have been issued by the certificate authority and the issuer assigned serial number is known. PostCondition Either the certificate is sent back or the request is rejected. 5.3.1 GetCert Message Format The queried certificate is identified by its issuer name and the issuer assigned serial number. If this is a query for an arbitrary requester's certificate, the requesting requester should includes its own CA issued certificate in the signed envelope. If this is a query for its own certificate (assume the requester lost the issued certificate, or does not have enough non-volatile memory to save the certificate), then the self-signed certificate has to be included in the signed envelope. pkcsGetCert issuerAndSerialNumber ::= { issuer "the certificate issuer name" serialNumber "the certificate serial number" } pkcsGetCertEnvelope EnvelopedData ::= { version 0 recipientInfo { version 0 issuerAndSerialNumber { issuer "the CA [RA] issuer name" serialNumber "the CA [RA] certificate serial number" } keyEncryptionAlgorithm {pkcs-1 1} encryptedKey "content-encrypt key encrypted by CA [RA] public key" } Liu/Madson/McGrew/Nourse [Page 26] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 encryptedContentInfo { contentType {pkcs-7 1} -- data content contentEncryptionAlgorithm "OID for DES encryption" encryptedContent "encrypted pkcsGetCert using the content encryption key" } } pkcsGetCertSigned SignedData ::= { version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} content pkcsGetCertEnvelope } certificates { certificate "CA issued certificate" or "self-signed certificate" } signerInfo { version 1 issuerAndSerialNumber { issuer "the requester's subject name" serialNumber "requester's certificate serial number" } digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} authenticateAttributes { contentType {{pkcs-9 3} {pkcs-7 1}} messageDigest {{pkcs-9 4} "an octet string"} -- digest of pkcsGetCertEnvelope messageType {{id-attribute messageType(2)} "GetCert"} transaction-id {{id-attributes transId(7)} "printable string"} senderNonce {{id-attribute senderNonce(3)} <16 bytes>} } digestEncryptionAlgorithm {pkcs-1 1} encryptedDigest "encrypted digest of authenticateAttributes" } } GetCert PKIMessage ::= { contentType {pkcs-7 2} content pkcsGetCertSigned } Liu/Madson/McGrew/Nourse [Page 27] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 5.3.2 CertRep Message Format In this case, the CertRep from the server is same as the CertRep for the PKCSReq, except that the server will only either grant the request or reject the request. Also, the recipientInfo should use the CA issuer name and CA assigned serial number to identify the requester's key pair since at this time, the requester has received its own certificate. 5.4 CRL Access The CRL query message defined in this section is an option when the LDAP server is not available to provide the CRL query. In the PKI protocol proposed here, only the requester can initiate the transaction to download CRL. A requester sends GetCRL request to the server and the server sends back CertRep whose information portion is a degenerated PKCS#7 which contains only the most recent CRL. The size of CRL included in the CertRep should be determined by the implementation. PreCondition The certificate authority certificate has been downloaded to the end entity. PostCondition CRL sent back to the requester. 5.4.1 GetCRL Message format The CRL is identified by using both CA's issuer name and the CA certificate's serial number: pkcsGetCRL issuerAndSerialNumber { issuer "the certificate authority issuer name" serialNumber "certificate authority certificate's serial number" } When the CRLDistributionPoint is supported, the pkcsGetCRL is defined as the following: pkcsGetCRL SEQUENCE { crlIssuer issuerAndSerialNumber distributionPoint CE-CRLDistPoints } where CE-CRLDisPoints is defined in X.509, but must contain only one CRL distribution point. Liu/Madson/McGrew/Nourse [Page 28] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 pkcsGetCRLEnvelope EnvelopedData ::= { version 0 recipientInfo { version 0 issuerAndSerialNumber { issuer "the certificate authority (or RA) issuer name" serialNumber "the CA (RA) certificate's serial number" } keyEncryptionAlgorithm {pkcs-1 1} encryptedKey "content-encrypt key encrypted by CA (RA) public key" } encryptedContentInfo { contentType {pkcs-7 1} -- data content contentEncryptionAlgorithm "OID for DES encryption" encryptedContent "encrypted pkcsGetCRL" } } pkcsGetCRLSigned SignedData ::= { version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} content pkcsGetCRLEnvelope } certificates { certificate "CA-issued or self-signed requester's certificate" } signerInfo { version 1 issuerAndSerialNumber { issuer "the requester's issuer name" serialNumber "the requester's certificate serial number" } digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} authenticateAttributes { contentType {{pkcs-9 3} {pkcs-7 1}} messageDigest {{pkcs-9 4} 0x<16/20 bytes>} -- digest of pkcsGetCRLEnvelope messageType {{id-attribute messageType(2)} "GetCRL"} transaction-id {{id-attributes transId(7)} "printable string"} senderNonce {{id-attribute senderNonce(3)} <16 bytes>} } digestEncryptionAlgorithm {pkcs-1 1} encryptedDigest "encrypted digest of authenticateAttributes" } } GetCRL PKIMessage ::= { contentType {pkcs-7 2} content pkcsGetCRLSigned } Liu/Madson/McGrew/Nourse [Page 29] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 5.4.2 CertRep Message Format The CRL is sent back to the requester through CertRep message. The information portion of this message is a degenerated PKCS#7 SignedData which contains only a CRL. pkcsCertRep SignedData ::= { version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} } crl { signature {pkcs-1 4} issuer "the certificate authority issuer name" lastUpdate "UTC time" nextUpdate "UTC time" revokedCertificate { -- the first entry userCertificate "certificate serial number" revocationData "UTC time" .... -- last entry userCertificate "certificate serial number" revocationData "UTC time" } } pkcsCertRepEnvelope EnvelopedData ::= { version 0 recipientInfo { version 0 issuerAndSerialNumber { issuer "the requester's issuer name" serialNumber "the requester certificate serial number" } keyEncryptionAlgorithm {pkcs-1 1} encryptedKey "content-encrypt key encrypted by requester's public key " } encryptedContentInfo { contentType {pkcs-7 1} -- data content contentEncryptionAlgorithm "OID for DES encryption" encryptedContent "encrypted pkcsCertRep using requester's public key" } } Liu/Madson/McGrew/Nourse [Page 30] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 pkcsCertRepSigned SignedData ::= { -- PKCS#7 version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} content pkcsCertRepEnvelope } signerInfo { version 1 issuerAndSerialNumber { issuer "the certificate authority issuer name" serialNumber "the CA certificate's serial number" } digestAlgorithm {iso(1), member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} authenticateAttributes { contentType {{pkcs-9 3} {pkcs-7 1}} messageDigest {{pkcs-9 4} "an octet string"} -- digest of pkcsCertRepEnvelope messageType {{id-attribute messageType(2)} "CertRep"} transaction-id {{id-attributes transId(7)} "printable string"} -- same transaction id as given in PKCSReq pkiStatus {{id-attributes pkiStatus(3) "SUCCESS"} recipientNonce{{id-attribute recipientNonce(6)}<16 bytes>} senderNonce {{id-attribute senderNonce (5) 0x<16 bytes>} } digestEncryptionAlgorithm {pkcs-1 1} encryptedDigest "encrypted digest of authenticatedAttributes using CA private key" } } NOTE:The PKCS#7 EncryptedContent is specified as an octet string, but SCEP entities must also accept a sequence of octet strings as a valid alternate encoding. This alternate encoding must be accepted wherever PKCS #7 Enveloped Data is specified in this document. Liu/Madson/McGrew/Nourse [Page 31] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 5.5 Get Certificate Authority Certificate Before any transaction begins, end entities have to get the CA (and possibly RA) certificate(s) first. Since the requester may have no CA certificates or CA public keys at all, this message can not be encrypted and the response must be authenticated by out-of-band means. These certs are obtained by means of an HTTP GET message. To get the CA certificate, the requester does a "HTTP GET" with a URL that identifies a CGI script on the server and an optional CA issuer identifier as the parameter to the CGI script. The response is either a single X.509 CA certificate ("CA mode"), or a PKCS7 message containing the CA certificate and RA certificates ("RA mode"). The client can determine which mode the CA operates in by which response it gets. Once the CA certificate is received by the requester, a fingerprint is generated using the SHA1, SHA256, SHA512 or MD5 hash algorithm on the whole CA certificate. If the requester does not have a certificate path to a trusted CA certificate, this fingerprint may be used to verify the certificate, by some positive out-of-band means, such as a phone call. 5.5.1 GetCACert HTTP Message Format "GET" CGI-PATH CGI-PROG "?operation=GetCACert" "&message=" CA-IDENT where: CGI-PATH defines the actual CGI path to invoke the CGI program which parses the request. CGI-PROG is set to be the string "pkiclient.exe" and this is expected to be the program that the CA will use to handle the SCEP transactions. CA-IDENT is any string which is understood by the CA. For example, it could be a domain name like ietf.org. If a certificate authority has multiple CA certificates this field can be used to distinguish which is required. Otherwise it may be ignored. 5.5.2 Response The response for GetCACert is different between the case where the CA directly communicated with the requester during the enrollment, and the case where a RA exists and the requester communicates with the RA during the enrollment. 5.5.2.1 CA Certificate Only Response A binary X.509 CA certificate is sent back as a MIME object with a Content-Type of application/x-x509-ca-cert. 5.5.2.2 CA and RA Certificates Response When an RA exists, both CA and RA certificates must be sent back in the response to the GetCACert request. The RA certificate(s) must be signed by the CA. A certificates-only PKCS#7 SignedData is used to carry the certificates to the requester, with a Content-Type of application/x-x509-ca-ra-cert. Liu/Madson/McGrew/Nourse [Page 32] 5.5.3 Get Next Certificate Authority Certificate 5.5.3.1 GetNextCACert HTTP Message Format "GET" CGI-PATH CGI-PROG "?operation=GetNextCACert" "&message=" CA-IDENT The response to this message is a PKCS#7 certificates-only message containing a CA certificate (and possibly RA certificates) to be used when the current CA certificate expires, signed with the current CA cert (or RA certificate, if the CA is in RA mode. Note that a PKCS#7 is returned even in CA mode. 5.5.3.2 GetCACaps HTTP Message Format "GET" CGI-PATH CGI-PROG "?operation=GetCACaps" "&message=" CA-IDENT This message requests capabilities from CA. The response is a list of text capabilities, as defined in Appendix F. Support for this message is optional, but if it is not supported, the client should assume that none of the capabilities in Appendix F are supported. 5.6 Get Certificate Authority Certificate Chain GetCACertChain provides a way to get the entire certificate chain. 5.6.1 GetCACertChain HTTP Message Format "GET" CGI-SCRIPT "?" "operation=GetCACertChain" "&" "message" CA-IDENT where CGI-SCRIPT and CA-IDENT are as described for GetCACert. 5.6.2 Response The response for GetCACertChain is a certificates-only PKCS#7 SignedData to carry the certificates to the requester, with a Content-Type of application/x-x509-ca-ra-cert-chain. 5.6.3 Backwards Compatability Versions of SCEP prior to revision 3 do not support GetCACertChain. Certificate Authorities written to these prior versions will not be able to process the message and may return an HTML error. To avoid this, clients should send the GetCACert message first. If the returned certificate is self-signed or is signed by a Certificate Authority that is trusted by the client, then it is not necessary to send the GetCACertChain message and it should not be sent. If a Certificate Authority is configured with a certificate that is not either self-signed or has a self-signed issuer, then it should support this message. In other words, it should be supported if the CA hierarchy is more than two-deep. An old CA in a two-deep hierarchy might still get this message from a client if the client did not trust either that CA or its issuer. In that event, the certificate cannot be trusted anyway. In any case the CA must not crash or hang upon the receipt of the message and the client must be able to handle whatever error is returned by the CA, including an HTML error or an ungraceful disconnect. Liu/Madson/McGrew/Nourse [Page 33] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 The following is the ASN.1 definition of Cert-Only PKCS#7: certOnly SignedData ::= { version 1 digestAlgorithm {iso(1) member-body(2) US(840) rsadsi(113549) digestAlgorithm(2) 5} contentInfo { contentType {pkcs-7 1} -- data content identifier content -- NULL } certificates -- the RA and CA certificates. } CARACerts PKIMessage ::= { -- special pki message sent in the clear contentType {pkcs-7 2} content certOnly } 6.0 Security Considerations This entire document is about security. Common security considerations such as keeping private keys truly private and using adequate lengths for symmetric and asymmetric keys must be followed in order to maintain the security of this protocol. 7.0 Intellectual Property This protcol includes the optional use of Certificate Revocation List Distribution Point (CRLDP) technology, which is a patented technology of Entrust Technologies, Inc. (Method for Efficient Management of Certificate Revocation Lists and Update Information (U.S. Patent 5,699,431)). Please contact Entrust Technologies, Inc. (www.entrust.com) for more information on licensing CRLDP technology. 8.0 References [PKCS7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315, March 1998. [PKCS10] Kaliski, B., "PKCS #10: Certification Request Syntax Version 1.5", RFC 2314, March 1998. [RFC2459] Housley, R., ec. al., "Internet X.509 Public Key Infrastructure Certificate and CRL Profile", RFC 2459, January 1999. Liu/Madson/McGrew/Nourse [Page 34] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Appendix A: Cisco Requester Subject Name Definition The ip address and the FQDN of a SCEP client should be included in the V3 extension subjectAltName. When the subjectAltName extension attribute is present, both the subjectAltName fields and the subjectName field could have the IP address and the FQDN information. When the X.500 directory is used by the CA to define the name space, the subject name defined above become a RDN which is part of DN binded to the requester's public key in the certificate. A sample of DN assigned by Entrust CA is given below (assume the same ciscoRouterAlice is used as the requester defined subject name): OU = InteropTesting, O = Entrust Technologies, C = CA RDN = {"alice.cisco.com", "172.21.114.67", "22334455"} Liu/Madson/McGrew/Nourse [Page 35] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Appendix B: IPSEC Client Enrollment Certificate Request The following is the certificate enrollment request (PKCS#10) as created by Cisco VPN Client: -----END NEW CERTIFICATE REQUEST----- 0 30 439: SEQUENCE { 4 30 288: SEQUENCE { 8 02 1: INTEGER 0 11 30 57: SEQUENCE { 13 31 55: SET { 15 30 53: SEQUENCE { 17 06 3: OBJECT IDENTIFIER commonName (2 5 4 3) 22 13 46: PrintableString : 'For Xiaoyi, IPSEC attrs in alternate name extn' : } : } : } 70 30 158: SEQUENCE { 73 30 13: SEQUENCE { 75 06 9: OBJECT IDENTIFIER rsaEncryption (1 2 840 113549 1 1 1) 86 05 0: NULL : } 88 03 140: BIT STRING 0 unused bits : 30 81 88 02 81 80 73 DB 1D D5 65 AA EF C7 D4 8E : AA 6E EB 46 AC 91 2A 0F 50 51 17 AD 50 A2 2A F2 : CE BE F1 E4 22 8C D7 61 A1 6C 87 61 62 92 CB A6 : 80 EA B4 0F 09 9D 18 5F 39 A3 02 0E DB 38 4C E4 : 8A 63 2E 72 8B DC BE 9E ED 6C 1A 47 DE 13 1B 0F : 83 29 4D 3E 08 86 FF 08 2B 43 09 EF 67 A7 6B EA : 77 62 30 35 4D A9 0F 0F DF CC 44 F5 4D 2C 2E 19 : E8 63 94 AC 84 A4 D0 01 E1 E3 97 16 CD 86 64 18 : [ Another 11 bytes skipped ] : } 231 A0 63: [0] { 233 30 61: SEQUENCE { 235 06 9: OBJECT IDENTIFIER extensionReq (1 2 840 113549 1 9 14) 246 31 48: SET { 248 30 46: SEQUENCE { 250 30 44: SEQUENCE { 252 06 3: OBJECT IDENTIFIER subjectAltName (2 5 29 17) 257 04 37: OCTET STRING 30 23 87 04 01 02 03 04 81 0D 65 6D 61 69 Liu/Madson/McGrew/Nourse [Page 36] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 6C 40 69 72 65 2E 63 6F 6D 82 0C 66 71 64 6E 2E 69 72 65 2E 63 6F 6D : } : } : } : } : } : } 296 30 13: SEQUENCE { 298 06 9: OBJECT IDENTIFIER md5withRSAEncryption (1 2 840 113549 1 1 4) 309 05 0: NULL : } 311 03 129: BIT STRING 0 unused bits : 19 60 55 45 7F 72 FD 4E E5 3F D2 66 B0 77 13 9A : 87 86 75 6A E1 36 C6 B6 21 71 68 BD 96 F0 B4 60 : 95 8F 12 F1 65 33 16 FD 46 8A 63 19 90 40 B4 B7 : 2C B5 AC 63 17 50 28 F0 CD A4 F0 00 4E D2 DE 6D : C3 4F F5 CB 03 4D C8 D8 31 5A 7C 01 47 D2 2B 91 : B5 48 55 C8 A7 0B DD 45 D3 4A 8D 94 04 3A 6C B0 : A7 1D 64 74 AB 8A F7 FF 82 C7 22 0A 2A 95 FB 24 : 88 AA B6 27 83 C1 EC 5E A0 BA 0C BA 2E 6D 50 C7 : } Appendix C: Private OID Definitions The OIDs used in defining pkiStatus are VeriSign self-maintained OIDs. Please note, work is in progress to replace the VeriSign owned object identifiers with the standard object identifiers. Once the standarlization is completed, this documentation will be updated. id-VeriSign OBJECT_IDENTIFIER ::= {2 16 US(840) 1 VeriSign(113733)} id-pki OBJECT_IDENTIFIER ::= {id-VeriSign pki(1)} id-attributes OBJECT_IDENTIFIER ::= {id-pki attributes(9)} id-messageType OBJECT_IDENTIFIER ::= {id-attributes messageType(2)} id-pkiStatus OBJECT_IDENTIFIER ::= {id-attributes pkiStatus(3)} id-failInfo OBJECT_IDENTIFIER ::= {id-attributes failInfo(4)} id-senderNonce OBJECT_IDENTIFIER ::= {id-attributes senderNonce(5)} id-recipientNonce OBJECT_IDENTIFIER ::= {id-attributes recipientNonce(6)} id-transId OBJECT_IDENTIFIER ::= {id-attributes transId(7)} id-extensionReq OBJECT_IDENTIFIER ::= {id-attributes extensionReq(8)} Liu/Madson/McGrew/Nourse [Page 37] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Appendix D: CRL Query by means of LDAP In order to retrieve the CRL by means of LDAP, the client needs to know where in the directory it is stored. The certificate must contain a CRL Distribution Point extension encoded as a DN or as an LDAP URI. For example, the certificate issued by Entrust VPN contains the following DN as the CRL distribution point: CN = CRL1, O = cisco, C = US. The asn.1 encoding of this distribution point is: 30 2C 31 0B 30 09 06 03 55 04 06 13 02 55 53 31 0E 30 0C 06 03 55 04 0A 13 05 63 69 73 63 6F 31 0D 30 0B 06 03 55 04 03 13 04 43 52 4C 31 The ldap form would be: ldap://servername/CN=CRL1,O=cisco,C=US Appendix E: SCEP State Transitions SCEP state transitions are based on transaction identifier. The design goal is to ensure the synchronization between the CA and the requester under various error situations. An identity is defined by the combination of FQDN, the IP address and the client serial number. FQDN is the required name attribute. It is important to notice that, a client named as Alice.cisco.com is different from the client named as Alice.cisco.com plus IPAddress 117.96.1.219. Each enrollment transaction is uniquely associated with a transaction identifier. Because the enrollment transaction could be interrupted by various errors, including network connection errors or client reboot, the SCEP client generates a transaction identifier by calculating a hash on the public key value for which the enrollment is requested. This retains the same transaction identifier throughout the enrollment transaction, even if the client has rebooted or timed out, and issues a new enrollment request for the same key pair. It also provides the way for the CA to uniquely identify a transaction in its database. At the requester side, it generates a transaction identifier which is included in PKCSReq. If the CA returns a response of PENDING, the requester will poll by periodically sending out GetCertInitial with the same transaction identifier until either a response other than PENDING is obtained, or the configured maximum time has elapsed. If the client times out or the client reboots, the client administrator will start another enrollment transaction with the same key pair. The second enrollment will have the transaction idenifier. At the server side, instead of accepting the PKCSReq as a new enrollment request, it should respond as if another GetCertInitial message had been sent with that transaction ID. In another word, the second PKCSReq should be taken as a resynchronization message to allow the enrollment resume as the same transaction. It is important to keep the transaction id unique since SCEP requires the same policy and same identity be applied to the same subject name and Liu/Madson/McGrew/Nourse [Page 38] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 key pair binding. In the current implementation, an SCEP client can only assume one identity. At any time, only one key pair, with a given key usage, can be associated with the same identity. The following gives several examples of client to CA transactions. Client actions are indicated in the left column, CA actions are indicated in the right column. A blank action signifies that no message was received. Note that these examples assume that the CA enforces the certificate-name uniqueness property defined in Section 2.1.1.1. The first transaction, for example, would read like this: "Client Sends PKCSReq message with transaction ID 1 to the CA. The CA signs the certificate and constructs a CertRep Message containing the signed certificate with a transaction ID 1. The client receives the message and installs the cert locally." Successful Enrollment Case: no manual authentication PKCSReq (1) ----------> CA Signs Cert Client Installs Cert <---------- CertRep (1) SIGNED CERT Successful Enrollment Case: manual authentication required PKCSReq (10) ----------> Cert Request goes into Queue Client Polls <---------- CertRep (10) PENDING GetCertInitial (10) ----------> Still pending Client Polls <---------- CertRep (10) PENDING GetCertInitial (10) ----------> Still pending Client Polls <---------- CertRep (10) PENDING GetCertInitial (10) ----------> Still pending Client Polls <---------- CertRep (10) PENDING GetCertInitial (10) ----------> Cert has been signed Client Installs Cert <---------- CertRep (10) SIGNED CERT Resync Case - CA Receive and Signs PKCSReq, Client Did not receive CertRep: PKCSReq (3) ----------> Cert Request goes into queue <---------- CertRep (3) PENDING GetCertInitial (3) ----------> <---------- CertRep (3) PENDING GetCertInitial (3) -----------> <----------- CA signed Cert and sent back CertRep(3) (Time Out) PKCSReq (3) ----------> Cert already signed, sent back to client Client Installs Cert <---------- CertRep (3) SIGNED CERT Liu/Madson/McGrew/Nourse [Page 39] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Case when NVRAM is lost and client has to generate a new key pair, there is no change of name information: PKCSReq (4) ----------> CA Signs Cert Client Installs Cert <---------- CertRep (4) SIGNED CERT (Client looses Cert) PKCSReq (5) ----------> There is already a valid cert with this DN. Client Admin Revokes <---------- CertRep (5) OVERLAPPING CERT ERROR PKCSReq (5) ----------> CA Signs Cert Client Installs Cert <---------- CertRep (5) SIGNED CERT Case when client admin resync the enrollment using a different PKCS#10: PKCSReq (6) ----------> CA Signs Cert <---------- CertRep (6) SIGNED CERT (Client timeout and admin starts another enrollment with a different PKCS#10, but the same transaction id) PKCSReq (6) with different PKCS#10 ----------> There is already a valid cert with this entity (by checking FQDN). <---------- CertRep (6) INVALID PKCS#10 CERT ERROR Client admin either revokes the existing cert or corrects the error by enrolling with the same PKCS#10 as the first PKCSReq(6) PKCSReq (6) ----------> CA find the existing Cert Client Installs Cert <---------- CertRep (6) SIGNED CERT Resync case when server is slow in response: PKCSReq (13) ----------> Cert Request goes into Queue <---------- CertRep (13) PENDING GetCertInitial ----------> Still pending <---------- CertRep (13) PENDING GetCertInitial ----------> Still pending <---------- CertRep (13) PENDING GetCertInitial ----------> Still pending <---------- CertRep (13) PENDING GetCertInitial ----------> Still pending (TimeOut) <---------- CertRep (13) PENDING Case 1 PKCSReq (13) ----------> Still pending Client polls <---------- CertRep (13) PENDING CertCertInitial ----------> Cert has been signed Client Installs Cert <---------- CertRep (13) SIGNED CERT Case 2 PKCSReq (13) ----------> Cert has been signed Client Installs Cert <---------- CertRep (13) SIGNED CERT Liu/Madson/McGrew/Nourse [Page 40] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Appendix F. CA Capabilities The response for a GetCACaps message is a list of CA capabilities, in plain text, separated by characters, as follows (quotation marks are NOT sent): Keyword Description "GetNextCACert" CA Supports the GetNextCACert message. "POSTPKIOperation" PKIOPeration messages may be sent via HTTP POST. "Renewal" Clients may use current certificate and key to authenticate an enrollment request for a new certificate. "SHA-512" CA Supports the SHA-512 hashing algorithm in signatures and fingerprints. "SHA-256" CA Supports the SHA-256 hashing algorithm in signatures and fingerprints. "SHA-1" CA Supports the SHA-1 hashing algorithm in signatures and fingerprints. "DES3" CA Supports triple-DES for encryption. The client should use SHA-1, SHA-256, or SHA-512 in preference to MD5 hashing if it is supported by the CA. A client must be able to accept and ignore any unknown keywords that might be sent back by a CA that implements a future version of SCEP. If none of the above capabilities are supported by the CA, no data is returned. The appropriate HTML headers are returned in any case. Example: GET /cgi-bin/pkiclient.exe?operation=GetCACaps&message=myca might return: GetNextCACert POSTPKIOperation This means that the CA supports the GetNextCACert message and allows PKIOperation messages (PKCSreq, GetCert, GetCertInitial...) to be sent using HTTP POST. Liu/Madson/McGrew/Nourse [Page 41] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Appendix G. Certificate Renewal and CA Key Rollover To renew a client certificate, use the PKCSreq message and sign it with the existing client certificate instead of a self-signed certificate. To obtain the new CA certificate prior to the expiration of the current one, use the GetNextCACert message if the CA supports it. To obtain a new client certificate signed by the new CA certificate, use the new CA or RA certificate in the message envelope. Example: GetNextCACert ----------> <---------- CertRep (3) New CA certificate PKCSReq (1) ----------> CA Signs certificate with NEW key Client Stores Cert <---------- CertRep (3) Certificate issued for installation when from NEW CA certificate and keypair. existing cert expires. enveloped for new CA or RA cert and keypair. The CA will use the envelope to determine which key and certificate to use to issue the client certificate. Liu/Madson/McGrew/Nourse [Page 42] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Appendix H. PKIOperation via HTTP POST Message If the remote CA supports it, any of the PKCS#7-encoded SCEP messages may be sent via HTTP POST instead of HTTP GET. This is allowed for any SCEP message except GetCACert, GetCACertChain, GetNextCACert, or GetCACaps. In this form of the message, Base 64 encoding is not used. POST /cgi-bin/pkiclient.exe?operation=PKIOperation The client can verify that the CA supports SCEP messages via POST by looking for the "POSTPKIOperation" capability (See Appendix F). Liu/Madson/McGrew/Nourse [Page 43] Cisco Systems' Simple Certificate Enrollment Protocol Dec 2006 Appendix Y. Author Contact Information Xiaoyi Liu Cheryl Madson Cisco Cisco 510 McCarthy Drive 510 McCarthy Drive Milpitas, CA Milpitas, CA. David McGrew Andrew Nourse Cisco Cisco 170 West Tasman Drive 510 McCarthy Drive San Jose, CA 94134 Milpitas, CA. Appendix Z. Copyright Section Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. This draft expires 1 Jun 2007 [End of draft-nourse-scep-14.txt]



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