Network Working Group S. Bellovin Internet-Draft Columbia University Intended status: Standards Track March 11, 2012 Expires: September 12, 2012 Hashed Password Exchange draft-bellovin-hpw-01.txt Abstract Many systems (e.g., cryptographic protocols relying on symmetric cryptography) require that plaintext passwords be stored. Given how often people reuse passwords on different systems, this poses a very serious risk if a single machine is compromised. We propose a scheme to derive passwords limited to a single machine from a typed password, and explain how a protocol definition can specify this scheme. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Bellovin Expires September 12, 2012 [Page 1] Internet-Draft Hashed Password Exchange March 2012 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction Today, despite the lessons of more than 30 years [[cite Morris and Thomson]], many systems store plaintext passwords. This is often done for good reasons, such as authenticating some cryptographic exchanges or as a convenience to users with many passwords; see, for example, the password store in many browsers or the Keychain in MacOS. That said, this practice does pose a security risk to users, since their passwords are in danger if the system is compromised. The big problem is not compromise of the actual password used on that system; while regrettable, it is inherent in the service definition. Rather, the problem is that users tend to reuse passwords on different systems. If a password is compromised on one machine, the user is at risk on many different systems. Accordingly, we describe a scheme for storing a single-site-only password, derived from the user's typed password; a compromise of a service thus affects just that service. To accomplish this, we specify a "Hashed Password Exchange" standard, or rather, a metastandard. Rather than specifying a precise way to store and use hashed passwords, we give rules for specifying hashed passwords for use in a given protocol or application. We take advantage of the fact that unlike 1979, when users used very dumb terminals to transmit passwords directly to the receiving applications, most passwords these days are entered into user- controlled software; these programs in turn transmit the passwords to the verifying applications. There is thus intelligence on the user's side; we will use this to irreversibly transform the entered password into some other string. By the same token, the receiving system must apply the same transform to the authenticator supplied at user enrollment time or password change time. Because two independent pieces of software must apply the same transformation, the algorithm must be precisely specified in standards documents. Note that defeating guessing attacks on a captured password file is not the primary goal of this work. That goal, though laudable, ignores changes in technology and environment since the Morris and Thompson paper; today, far more passwords are lost to keystroke loggers, phishing attacks, direct compromise of the server itself, or (as was a problem even 30+ years ago) online guessing attacks. Our scheme helps against this last attack, in that generation of the guesses becomes more expensive; against the other threats, password strength is completely irrelevant. We also note that today, people have very many different passwords. It is impossible to remember Bellovin Expires September 12, 2012 [Page 2] Internet-Draft Hashed Password Exchange March 2012 large numbers of strong passwords; absent use of a password generator and manager, there *will* be reuse across different services. 1.1. Requirements Notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Definitions and Goals We use the following definitions: Username An arbitrary string, the syntax of which is application- dependent, employed by both the user and the verifying system to uniquely identify a given user. Entered Password The authenticator typed by the user to his or her own software. The usual quality rules (length, special characters, etc.) can be applied; that is out of the scope of this standard. Effective Password The actual, over-the-wire, string transmitted by the user's software. Service A particular application on a particular machine or cluster of machines appearing as a single machine Hostname The hostname as supplied by the user. Service URI A URI [RFC3986] for which this effective password should be valid. Only the scheme name, userinfo, and host name portions are discussed here; use of path information is protocol-dependent. In the userinfo field, only the username is used. An example is given below. Our scheme has the following goals: 1. No two users of a given service should have the same effecive password, even if the entered passwords are the same. 2. No two effective passwords for the same user should be the same for different services, even if the entered passwords are the same. 3. It should be infeasible to invert the hashing function to retrieve the entered password from an effective password and service URI. Bellovin Expires September 12, 2012 [Page 3] Internet-Draft Hashed Password Exchange March 2012 4. It should be computationally expensive to mount dictionary attacks on compromised effective passwords. 3. The Hashed Password Scheme Fundamentally, we calculate the effective password by iterating HMAC [RFC2104], using the entered password as the key and the service URI as the data. This meets all four of our goals: 1. Since the username is part of the service URI, different users will have different URIs, and hence different effective passwords. 2. Since the hostname is part of the URI, different services for any given user will have different URIs, and hence different effective passwords. 3. For any reasonable underlying hash function, it is believed to be infeasible to invert HMAC; see [RFC2104] for details. (Arguably, HMAC is overkill. Nevertheless, it is a well-studied, well- understood mechanism for combining known plaintext with a secret key. We see little benefit to concocting some other scheme.) 4. By iterating a sufficient number of times, dictionary attacks can be made arbitrarily expensive. (Although guessing attacks can be made arbitrarily cheap today by use of cloud services or botnets, we prefer to look at it somewhat differently. Whatever the resources the attacker has, his or her effective guessing rate is cut by a factor of the iteration count.) We do not use a salt in this scheme. The primary purposes of a salt are to achieve our first and second goals, which we achieve in other ways. A salt also protects against precomputation of possible passwords of known users in anticipation of a later password file compromise. Our use of service-, host-, and user-specific hashed passwords provides the same protection against untargeted guessing attacks; furthermore, and as noted, guessing attacks are not the primary threat today. Since the salt must be used in calculating the effective password, it would have to be known to the user as well as the server, and users typically have multiple devices on which they enter passwords. Using a salt would require that users know it and reenter it, which we regard as of limited benefit and highly user- hostile: people will *not* tolerate copying random strings or numbers onto multiple platforms, especially phones and the like. Usernames and the hostname portions of service URIs must be canonicalized before applying HMAC. Legal characters in a username are upper and lower case US-ASCII letters, period, hyphen, Bellovin Expires September 12, 2012 [Page 4] Internet-Draft Hashed Password Exchange March 2012 underscore, and digits. All other characters MUST be percent- encoded, per section 2.1 of [RFC3986]. Hostnames MUST be canonicalized per [RFC5890][RFC5891] and converted to lower case. How usernames and hostnames are entered is application- and implementation-dependent, and not part of this specification. The hostname used is either the string users type or unambiguously derivable from it per specified rules. The URI scheme name is given by the protocol specification and MUST NOT be entered directly by the user. The iteration count is protocol- and use-dependent, and given in the protocol specification. The effective password, then, is calculated by iterating HMAC some number of times over the message scheme://username@hostname with the entered password as the key. 3.1. Examples ipsec://someuser@gw.example.net imap://someuser@mail.example.com submission://someuser@mail.example.com Note that although someuser can specify the same entered password for both 'imap' and 'submission' on mail.example.com, the effective passwords will be different. 4. Specifying Hashed Password Exchange The following elements must be in any protocol specification that uses Hashed Password Exchange. o The scheme name MUST be specified. Generally, this will be taken from the IANA name assigned to the port, but this is not required. Thus, a mail submission URI (TCP port 587) might use the scheme name "submission". o The rules for deriving the hostname from what users enter MUST be specified. They may be as simple as "use the name the user specifies, e.g., imap.example.com", or they may account for common alternatives: "If the specified host name does not begin with 'www.', prepend it; thus, both 'example.com' and 'www.example.com' would use the hostname 'www.example.com' in forming the URI. Bellovin Expires September 12, 2012 [Page 5] Internet-Draft Hashed Password Exchange March 2012 o The iteration count MUST be specified. The value -- typically in the hundreds of thousands with today's technology -- SHOULD be different for different services, and MAY be adjusted based on the platforms on which the calculations are typically done. Note that the iteration is done at password change time rather than run- time, so expense is not a major concern. (Just how long the iterations should take will depend on the protocol designers' understanding of likely platforms and usage patterns. Something that will be run exclusively on fast devices and with stored hashed passwords should use a higher count; something where run- time user password entry on a slow device is considered likely should use a lower count.) o To support internationalized, non-ASCII passwords, we adopt the specification text from [RFC6124]. The input password string SHOULD be processed according to the rules of the [RFC4103] profile of [RFC3454] A password SHOULD be considered a "stored string" per [RFC3454] and unassigned code points are therefore prohibited. The output is the binary representation of the processed UTF-8 [RFC3629] character string. Prohibited output and unassigned code points encountered in SASLprep preprocessing SHOULD cause a preprocessing failure and the output SHOULD NOT be used. o The hash function to be used with HMAC MUST be specified. MD5 [RFC1321] is more than sufficient; however, the tradeoff is likely to be between what code is likely to be available in implenetations versus the iteration count. SHA-512 [RFC6234] is much slower than MD5, but since the goal is constant time, this matters very little; thus, MD5 would have a higher iteration count than SHA-512 would for the same protocol. o The encoding rules for sending the effective password over the wire are not crucial but must be specified. The output of HMAC is an arbitrary byte string. Given the length of typical HMAC output and the infrequency with which they are sent, transmission efficiency is not a major concern, so a simple hexadecimal encoding is fine. Implementations MAY specify truncation; however, they SHOULD NOT use effective passwords shorter than 16 octets before encoding. o If the password is not transmitted but is used internally (e.g., as part of a cryptopgrahic exchange), how the effective password is used MUST be specified. Some protocols will use it directly as a key; others will use the hexadecimal ASCII string in place of a password. Bellovin Expires September 12, 2012 [Page 6] Internet-Draft Hashed Password Exchange March 2012 o Some protocols, such as HTTP, permit multiple hosts to appear on a single IP address. For such protocols, the desired hostname must be transmitted prior to or along with the hashed password, to allow the host to calculate the proper hashed password value. How this is done MUST be specified. o If the protocol permits negotiation of authentication methods, a separate code point MUST be assigned to this scheme. How passwords are changed -- that is, how new effective passwords are supplied to the verifying machine -- is beyond the scope of this specification. If the entered password is sent directly at password change time, quality checks can be enforced; however, this exposes entered passwords to attacks who have compromised the verifying machine. This is not a major risk, since the rate of password change is low. Conversely, client-side code (e.g., Javascript) can make advisory recommendations on password strength; while the server cannot enforce this, since it will see only effective passwords, very few users will have the will and the skill to override this. If effective passwords are used only for the usual password verification and not for cryptographic purposes, they should be treated with the care used for ordinary password, i.e., read- protected, hashed, etc. There is little need for extra iterations, though, since the iteration used in calculating them already provides strong protection against dictionary attacks, and it is unlikely that the extra server-side iterations will be significantly larger than the iterations already performed to comply with this specification. As before, there is no need for an additional salt. 5. Related Work A number of papers have described schemes for browser-based password stores that simplify the process of having separate effective passwords for different web sites. Many -- [[Abadi--pwdhash]] [[Halderman et al.]] -- use a cryptographic function of the domain name and a master password to calculate it. [[Abadi-pwdhash]] has many pointers. This work differs in two important ways. First, it applies to more services than just HTTP. Second, it specifies how other protocol specification documents should handle the situation, independent of requirements for password strength. 6. Acknowledgments A number of people made useful comments and suggestions, even if they didn't agree with all parts of this document. They include Martin Bellovin Expires September 12, 2012 [Page 7] Internet-Draft Hashed Password Exchange March 2012 Abadi, Uri Blumenthal, Dan Harkins, Mouse, Yaron Sheffer, Joe Touch, and Sujing Zhou. 7. Security Considerations To be written. 8. Normative References [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April 1992. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3454] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text Conversation", RFC 4103, June 2005. [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010. [RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, August 2010. [RFC6124] Sheffer, Y., Zorn, G., Tschofenig, H., and S. Fluhrer, "An EAP Authentication Method Based on the Encrypted Key Exchange (EKE) Protocol", RFC 6124, February 2011. [RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011. Bellovin Expires September 12, 2012 [Page 8] Internet-Draft Hashed Password Exchange March 2012 Appendix A. Change History A.1. Changes from -00 to -01 Added more text explaining why salting isn't particularly helpful Add the requirement to transmit the hostname for some services Started a related work section Clarified the internationalization requirement Miscellaneous edits Appendix B. Open Issues How should related domains (e.g., www.amazon.com and www.amazon.co.uk) be handled, if the site wishes the same password to work on all of them. A particular case in point is the way the prefix "www." should be handled. Should there be a general rule about the service name appearing in the hostname? Author's Address S.M. Bellovin Columbia University 1214 Amsterdam Avenue MC 0401 New York, NY 10027 US Phone: +1 212 939 7149 EMail: bellovin@acm.org Bellovin Expires September 12, 2012 [Page 9]