Network Working Group D. Gustafson Request for Comments: 3760 Future Foundation Category: Informational M. Just Treasury Board of Canada M. Nystrom RSA Security April 2004 Securely Available Credentials (SACRED) - Credential Server Framework Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved.
AbstractAs the number, and more particularly the number of different types, of devices connecting to the Internet increases, credential mobility becomes an issue for IETF standardization. This document responds to the requirements on protocols for secure exchange of credentials listed in RFC 3157, by presenting an abstract protocol framework. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Functional Overview. . . . . . . . . . . . . . . . . . . . . . 2 2.1. Definitions. . . . . . . . . . . . . . . . . . . . . . . 2 2.2. Credentials. . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Network Architecture . . . . . . . . . . . . . . . . . . 5 3. Protocol Framework . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Credential Upload. . . . . . . . . . . . . . . . . . . . 8 3.2. Credential Download. . . . . . . . . . . . . . . . . . . 10 3.3. Credential Removal . . . . . . . . . . . . . . . . . . . 11 3.4. Credential Management. . . . . . . . . . . . . . . . . . 12 4. Protocol Considerations. . . . . . . . . . . . . . . . . . . . 12 4.1. Secure Credential Formats. . . . . . . . . . . . . . . . 12 4.2. Authentication Methods . . . . . . . . . . . . . . . . . 13 4.3. Transport Protocol Suites. . . . . . . . . . . . . . . . 16 5. Security Considerations. . . . . . . . . . . . . . . . . . . . 17 5.1. Communications Security. . . . . . . . . . . . . . . . . 17 5.2. Systems Security . . . . . . . . . . . . . . . . . . . . 18
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.1. Normative References . . . . . . . . . . . . . . . . . . 20 6.2. Informative References . . . . . . . . . . . . . . . . . 20 7. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 21 8. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 22 RFC3157]. These requirements assume that two distinctly different network architectures will be created to support credential exchange for roaming users: a) Client/Server Credential Exchange b) Peer-to-Peer Credential Exchange This document describes the framework for one or more client/server credential exchange protocols. In all cases, adequate user authentication methods will be used to ensure credentials are not divulged to unauthorized parties. As well, adequate server authentication methods will be used to ensure that each client's authentication information (see Section 2.1) is not compromised, and to ensure that roaming users interact with intended/authorized credential servers.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED" and "MAY" in this document are to be interpreted as described in [RFC2119]. client authentication information: information that is presented by the client to a server to authenticate the client. This may include a password token, a registration string that may have been received out-of-band (and possibly used for initially registering a roaming user) or data signed with a signature key belonging to the client (e.g., as part of TLS [RFC2246] client authentication). credentials: cryptographic objects and related data used to support secure communications over the Internet. Credentials may consist of public/private key pairs, symmetric keys, X.509 public key certificates, attribute certificates, and/or application data. Several standardized formats for the representation of credentials exist, e.g., [PKCS12], [PKCS15] (see "secured credentials" below). passkey: a symmetric key, derived from a password. password: a string of characters known only to a client and used for the purposes of authenticating to a server and/or securing credentials. A user may be required to remember more than one password. password token: a value derived from a password using a one-way function that may be used by a client to authenticate to a server. A password token may be derived from a password using a one-way hash function, for example. secured credentials: a set of one or more credentials that have been cryptographically secured, e.g., encrypted/MACed with a passkey. Secured credentials may be protected using more than one layer of encryption, e.g., the credential is secured with a passkey corresponding to a user's password and also by a key known only to the server (the credential's stored form). During network transfer, the passkey-protected credential may be protected with an additional encryption layer using a symmetric key chosen by the Credential Server (e.g., the transmitted form). strong password protocol: a protocol that authenticates clients to servers securely (see e.g., [SPEKE] for a more detailed definition of this), where the client need only memorize a small secret (a password) and carries no other secret information, and where the server carries a verifier
(password token) which allows it to authenticate the client. A shared secret is negotiated between client and server and is used to protect data subsequently exchanged. Note the distinction between an "account password" and a "credential password." An account password (and corresponding password token) is used to authenticate to a Credential Server and to negotiate a key that provides session level encryption between client and server. A credential password is used to derive a passkey that's used to provide persistent encryption and authentication for a stored credential. Applicable secured credential standards documents (e.g., [PKCS15]) describe the technical details of specific password-based- encryption (pbe) techniques that are used to protect credentials from unauthorized use. Although the same password value may be used to provide both services, it is likely that different, algorithm specific passkeys would be generated from this password (i.e., because of different salt values, etc.). In addition, although it may be more convenient for a user to remember only a single password, differing security policies (e.g., password rules) between the credential server and the credential issuers may result in a user having to remember multiple passwords.
protocol, a secured credential is an opaque (and at least partially privacy and integrity protected) data object that can be used by a network connected device. Once downloaded, clients must be able to recover their credentials from this opaque format. At a minimum, all supported credential formats SHOULD provide privacy and integrity protection for private keys, secret keys, and any other data objects that must be protected from disclosure or modification. Typically, these security capabilities are part of the basic credential format such that the credential (e.g., a data file) is protected when stored on hard drives, flexible diskettes, etc. During network transmission, the secured credential is protected with a second (outer) encryption layer. The outer encryption layer is created using a session-level encryption key that was derived during the mutual authentication process. Effectively, secured credentials traverse an "encrypted tunnel" that provides an additional layer of privacy protection for credentials (and any other) information exchanged.
Credential Store - The repository for secured credentials. There might be access control features but those generally aren't sufficient in themselves for securing credentials. The credential server may be capable of splitting credentials across multiple credential stores for redundancy or to provide additional levels of protection for user credentials. Protocol 1 - The protocol used to authenticate the client and credential server, and download and upload user credentials from a credential server. Protocol 2 - The protocol used by the Credential Server to store and retrieve user credentials (LDAP, LDAP/SSL, or other). Protocol 3 - The protocol used by the client to store and retrieve user credentials from the credential store (LDAP, LDAP/SSL, or other). This framework describes the high level design for protocol 1. Protocols 2 and 3 are closely related (but out of scope for this document) and could be implemented using standard protocols, such as LDAP or secure LDAP, or other standard or proprietary protocols. Note also that any administrator-credential server protocols are assumed to be server vendor specific and are not the subject of SACRED standardization efforts at this time. Clients are not precluded from exchanging credentials directly with a credential store (or any other server of it's choosing). However, mutual authentication with roaming users and a consistent level of protection for credential data while stored on network servers and while in transit is provided by SACRED protocols exchanged with the credential server. Depending on credential server design, user credentials may flow through the credential server to the credential store or directly between the client and the credential store. Also, users may upload their credentials to several credential servers to obtain enhanced levels of availability. Coordination (automatic replication) of user information or credential data among several credential servers is currently beyond the scope of this document.
The client/server credential exchange protocol is based on three basic and abstract operations; "GET", "PUT", and "DELETE". The secured credential exchange protocol is accomplished as follows: connect - the client initiates a connection to a credential server for the purpose of secure credential exchange. mutual authentication/key negotiation - using a strong password protocol (or equivalent) the client authenticates to the server, the server authenticates to the client, and a session level encryption key is negotiated. The details of the mutual authentication protocol exchange are dependent upon the particular authentication method used. In all cases, the end result is to authenticate the client to the server and server to the client, and establish a strong, shared secret between the two parties. client request(s) - the SACRED client issues one or more high level credential exchange requests (e.g., GET, PUT, or DELETE). server response(s) - the SACRED credential server responds to each request, either performing the operation successfully or indicating an appropriate error. close - the client indicates it has no more requests for the server at this time. The security context between client and server is no longer needed. Close is a logical, session management operation. disconnect - the parties disconnect the transport level connection between client and server. Note that "connect" and "disconnect" are logical, transport-layer dependent operations that enclose the protocol exchange between the two communicating processes. Each high-level credential exchange operation is made up of a series of request-response pairs. The client initiates each request, which the server processes before returning an appropriate response. Each request must complete (server reports success or failure) before the client issues the next request. The server SHOULD be willing to service at least one upload or download request following successful mutual authentication but either party can terminate the logical connection at any time.
In the following sections, secured credentials and related values are represented using the following notation: SC-x is the secured credential file, which includes a format identifier field and credential data. The credential data is an opaque, encrypted data object (e.g., PKCS#15 or PKCS#12 file). The format identifier is needed to correctly parse the credential data. Name-x is an account-defined selector or locator (a user friendly name) that is used to indicate a specific secured credential. The name of each credential stored under a given user account MUST be unique e.g., there may be one credential called "financial" and another called "healthcare", etc. At a minimum, credential names MUST be unique across a given account/user name. When no name is supplied for a GET operation, all credentials stored for the given username will be returned. ID-x is a distinct credential version indicator that MAY be used to request a conditional GET/PUT/DELETE operation. This credential-ID value SHOULD contain the server's "last- modified" date and time (e.g., the time that this particular credential version was stored on the server) and MAY contain additional information such as a sequence number or a (complete or partial) credential fingerprint that is used to ensure the credential-ID is unique from other credential versions stored under the same user account and credential name. All named credentials may be accessed by authenticating under a single username. If a user needs or prefers to use more than one distinct authentication password (and/or authentication method) to protect access to several secured credentials, he/she SHOULD register those credentials under distinct user/account names, one for each different authentication method used.
The framework for the credential upload, as implemented using the PUT operation, is: - The client and server establish a mutually authenticated session and negotiate a shared secret. - The client will then issue a PUT message that contains the upload credential and related data fields. - The server will respond to the PUT, indicating the credential was successfully stored on the server or that an error occurred. The client's PUT request MAY contain an optional identifier (credential-ID) field. If present, the new credential will only be stored if a credential with the same name and credential-ID is currently stored on the server (e.g., a logical REPLACE operation is performed). The server MUST return an error if a client attempts to replace a credential that does not exist on the server. The credential server's response to a PUT request MUST contain a credential version identifier (credential-ID) for the newly stored credential that MAY be used by clients to optimize subsequent download operations and avoid credential version mismatches.
At least two well-defined credential formats are available today: [PKCS12] and [PKCS15]. Other optional credential formats may also be supported if necessary. For example, additional credential formats might be defined for use with specific (compatible) client devices. Each credential format MUST provide adequate privacy protection for user credentials when they are stored on flexible diskettes, hard disks, etc. Throughout this document, the credential is treated as an opaque (encrypted) data object and, as such, the credential format does not affect the basic credential exchange protocol. RFC3157], Section 3.1, Vulnerabilities. At a minimum, each SACRED authentication method SHOULD ensure that: - The server authenticates the client - The client authenticates the server - The client and server securely negotiate (or derive) a cryptographically strong, secret key (e.g., a session key). - The exchange of one or more user credentials is protected using this session key.
It is expected that all SACRED client/server protocols will provide each of these basic security functions. Some existing authentication protocols that might be used for this purpose include: - Strong password protocols - TLS Sections 4.2.1 and 4.2.2 provide some guidance about when to use these authentication methods based on the generic security capabilities they provide and the security elements (passwords, key pairs, user certificates, CA certificates) that must be available to the SACRED client. RFC2945], [BM92], [BM94], and [SPEKE] MAY be used to provide mutual authentication and privacy for SACRED protocols. All strong password protocols require that user-specific values (i.e., a passtoken and related values) be configured within the server. Only a party who knows the password can calculate the verifier value. It must be securely delivered to the server at a time when the client establishes a relationship with the server. At connect time, messages are exchanged between the two parties and complementary algorithms are used to compute a shared common value known only to the legitimate user and the server. Both parties derive a strong (symmetric) key that may be used to secure communications between the two parties.
client and server. Following server authentication and security context setup, all client requests and server responses exchanged are integrity and privacy protected. Protocol designers and implementors should be aware that the flexibility of the certificate-based TLS server authentication method creates security risks that need to be mitigated. Specifically, the need to ensure the user is connected to the intended credential server (secure site), and no other. The TLS v1.0 standard [RFC2246] identifies the basis for managing this risk in section F.3 (see also Section 5.2 in this document): "Implementations and users must be careful when deciding which certificates and certificate authorities are acceptable; a dishonest certificate authority can do tremendous damage." Note also that a faulty implementation of (increasingly complex) TLS server certificate chain processing, by the SACRED client, could lead to similar compromise, allowing successful credential server masquerade or man-in-the-middle attacks. An engineering approach that provides an enhanced or augmented server authentication method may be warranted for SACRED protocol designs. It is also important to understand that simple layering of independently developed security protocols (e.g., using BEEP or similar layering techniques) produces a complex, multilayer security protocol that might be easily defeated by a combination-specific attack that is able to expose and exploit known weaknesses of the individual protocol(s). When necessary, and after a TLS session has been established between the two parties, the credential server can request that the client provide her user id and password information to authenticate the remote user. Preferably, client and server can cooperate to perform an authentication operation that allows the server to authenticate the client (and perhaps vice-versa) in a "zero knowledge manner". In such cases, the client need not have a security credential. TLS with Client Authentication (cTLS) TLS provides an optional, secure session capability (sometimes called client-side TLS) whereby the TLS server can request client authentication by verifying the client's digital signature. In order to use cTLS to provide mutual authentication, the client must also be configured with at least one security credential that is acceptable to the TLS server for remote client authentication purposes.
BEEP provides the necessary user authentication/session security and session management capabilities needed to support SACRED credential exchange operations. RFC1945] and [RFC2616]. HTTP provides general purpose typing and negotiation of data representation, allowing systems to be built independently of the data objects being transferred. HTTP support is available in a wide variety of server and client platforms, including portable devices that apply to roaming environments (laptop PCs, PDAs, mobile phones, etc.). HTTP is layered over TCP and can be used, optionally, with TLS to provide authenticated, session level security. Either or both TLS authentication options, sTLS or cTLS, may be used whenever TLS is supported. RFC3157] Security Considerations.
As well, the interface should encourage the entering of the password in the appropriate interface field so that protections can be properly enforced. For example, a user should be guided to not mistakenly enter their password in the "username" field (since their password would likely be echoed to the screen in this case, and might not be encrypted when communicated to the server). This might be accomplished via the automatic insertion of the user name or several user name choices in the appropriate on-screen dialog field, for example.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3157] Arsenault, A. and S. Farrell, "Securely Available Credentials - Requirements", RFC 3157, August 2001. [BM92] Bellovin, S. and M. Merritt, "Encrypted Key Exchange: Password-based protocols secure against dictionary attacks", Proceedings of the IEEE Symposium on Research in Security and Privacy, May 1992. [BM94] Bellovin, S. and M. Merritt, "Augmented Encrypted Key Exchange: a Password-Based Protocol Secure Against Dictionary Attacks and Password File Compromise, ATT Labs Technical Report, 1994. [PKCS12] "PKCS 12 v1.0: Personal Information Exchange Syntax", RSA Laboratories, June 24, 1999. [PKCS15] "PKCS #15 v1.1: Cryptographic Token Information Syntax Standard", RSA Laboratories, June 2000. [RFC1945] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer Protocol-- HTTP/1.0", RFC 1945, May 1996.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frysyk, H., Masinter, L., Leach, M. and T. Berners-Lee, "Hypertext Transfer Protocol - HTTP/1.1", RFC 2616, June 1999. [RFC2945] Wu, T., "The SRP Authentication and Key Exchange System", RFC 2945, September 2000. [RFC3080] Rose, M., "The Blocks Extensible Exchange Protocol Core", RFC 3080, March 2001. [RFC3081] Rose, M., "Mapping the BEEP Core onto TCP", RFC 3081, March 2001. [SPEKE] Jablon, D., "Strong Password-Only Authenticated Key Exchange", September 1996.
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. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- firstname.lastname@example.org. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.