Network Working Group                                          I. Brown
draft-brown-pgp-pfs-03.txt                        Hidden Footprints Ltd
Updates: RFC 2440                                               A. Back
Category: INTERNET-DRAFT                         Zero-Knowledge Systems
Expires: 5 April 2002                                         B. Laurie
                                                       A.L. Digital Ltd
                                                           October 2001

                Forward Secrecy Extensions for OpenPGP

Status of This Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   Copyright (C) Internet Society 2001.  All rights reserved.
   Reproduction or translation of the complete document, but not of
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   The confidentiality of encrypted data depends on the secrecy of the
   key needed to decrypt it. If one key is able to decrypt large
   quantities of data, its compromise will be disastrous. This memo
   describes three methods for limiting this vulnerability for OpenPGP
   messages: reducing the lifetime of confidentiality keys; one-time
   keys; and the additional use of lower-layer security services.

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Table of Contents

    1.          Introduction                                          2
    2.          Short-lifetime encryption keys                        3
    2.1         Key generation and distribution                       3
    2.2         Key surrender                                         4
    3.          One-time keys                                         4
    4.          Secure and decentralised e-mail transport             6
    5.          Security considerations                               6
    6.          Acknowledgements                                      6
    7.          Authors                                               7
    8.          References                                            7
    9.          Full Copyright Statement                              8

1.  Introduction

    OpenPGP systems [1] allow two strangers to communicate privately.
    Each user has a public key that is widely disseminated, and a
    private key that they keep secret. A message encrypted with a
    public key can only be decrypted with the related private key.
    The confidentiality of all messages encrypted with a public key
    rests on the secrecy of the associated private key.

    Online systems such as IPSEC [2] can negotiate new keys for
    every communication using an algorithm like Diffie-Hellman [3].
    If a key is compromised, only the specific session it protected
    will be revealed to an attacker. This desirable property is
    called perfect forward secrecy. The security of previous or
    future encrypted sessions is not affected. Keys are securely
    deleted after use. Without these keys, there is no way captured
    ciphertext can be decrypted.

    It is more difficult to make store and forward systems like e-mail
    forward secret, as they rarely make direct connections between a
    message sender and its recipient. In a typical e-mail encryption
    system, users create a long-term key pair and publish the public
    key in a directory, on their Web page, or via other methods. While
    the use of long-term keys reduces the administrative burden of key
    distribution, the practice introduces vulnerabilities. If a public
    key is used for several years, as is common with OpenPGP systems,
    compromise of the private key will allow an attacker to decrypt any
    message captured during that time.

    This memo describes several methods of reducing the vulnerabilities
    introduced by use of long-term keys. They are a series of options
    that MAY be implemented by OpenPGP clients for increased security.

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    this document are to be interpreted as described in RFC 2119.

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2.  Short-lifetime encryption keys

    Using a series of encryption keys, each with a short lifetime,
    reduces the information revealed by the compromise of any one
    private key because each key protects less data. If expired keys
    are securely deleted, attackers will never be able to retrieve
    them to decrypt captured ciphertext. Therefore when a public
    encryption key expires, an OpenPGP client MUST securely wipe the
    corresponding private key [4].

    Deletion should take place once all messages that could have been
    sent before expiry have been received and decrypted. For example,
    as a user logs on, their mail client SHOULD retrieve and decrypt
    all messages from their mail server before deleting any
    newly-expired private keys. A "panic mode" MAY bypass this step.

    PGP clients are able to group "subkeys" together under a long-term
    signature key to signify their common ownership by one principal.
    To simplify key management, short lifetime keys SHOULD be created
    as subkeys of their owner's long-term signature key.

    Clients receiving messages encrypted with an expired key MAY warn
    the sender that they should not use that public key again.

    Clients receiving messages encrypted with a revoked key MUST warn
    the sender that they should not use that public key again. Any
    relevant key revocation certificates MUST be included in the

    Some OpenPGP systems currently store original message ciphertext
    and decrypt only for display. While this protects messages on
    disk, it means that keys must be stored until all messages they
    protect are deleted. We must assume that an attacker has copies
    of message ciphertext sent over an insecure network such as the
    Internet. These messages remain vulnerable until the corresponding
    private key is deleted.

    Messages therefore MAY be stored temporarily encrypted with a
    short-lifetime key, but are unreadable once it has been deleted.
    Clients MUST allow messages to be stored encrypted under a long-
    term storage key. A mail client MAY implement its own secure
    storage facilities, or use those provided by other software.
    Messages SHOULD NOT be encrypted-to-self using a long-term public

2.1 Key generation and distribution

    There is a trade-off for the user: the cost of generating and
    distributing a new encryption key against the security advantage
    obtained by earlier key expiry.

    Key generation is typically a time-consuming operation. The client
    SHOULD minimise the time required by the user to complete this
    operation. This can be achieved, for example, by background key

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    generation, or by using trade-offs that speed up key generation
    with minimal reduction in security. With Elgamal [5], for example,
    the expensive key component to generate is the public prime
    modulus. A group of keys can share a common public modulus with no
    negative security implications other than that the key then
    presents a fatter target for pre-computation attacks. Multiple
    forward secret Elgamal keys MAY therefore use the same prime
    modulus with minimal security reduction.

    Key distribution can be eased by submitting new keys to key
    servers, where they will be available for other users to retrieve.
    Submission and retrieval of generally-available public keys SHOULD
    be performed automatically by software. Expired public encryption
    keys MAY be deleted by users and keyservers to save space.

    If an OpenPGP client has more than one valid encryption key
    available for a given message recipient, the key nearest its
    expiration date MUST be used. This limits the time during which the
    corresponding private key will be vulnerable to attack. The time
    required to deliver a message should be taken into account by
    sender and recipient when checking an expiry date.

    Signature keys that are long-lived and certified by other users
    allow a web of trust to build up. Encryption keys SHOULD be
    certified by a user's long-term signature key to allow their
    verification by other users.

2.2 Key surrender

    Before an OpenPGP client exports a private key as plaintext, the
    associated public key MUST be revoked and redistributed. A "reason
    for revocation" signature subpacket MUST be included in the key
    revocation specifying "Key material has been compromised" (value

    The least compromising key required MUST be the one surrendered.
    The session key used to encrypt an individual message will often be
    sufficient. Otherwise, a subkey should be surrendered before a long-
    term top-level key. Signature keys should not be surrendered unless
    absolutely necessary.

3.  One-time keys

    Taking short-term keys to their logical conclusion, a different key
    could be used to protect every message. Schneier and Hall [6]
    suggested a user could make several public keys available in a
    directory. After a key was retrieved by another user, it would be
    deleted. This requires message senders to have online access to a
    directory. Not all e-mail users have this facility. It also allows
    an attacker to mount a denial of service attack by exhaustively
    requesting new one-time keys from the directory.

    An off-line scheme is more compatible with the store and forward
    nature of e-mail, and resistant to DoS. Every time a user sends a

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    message encrypted with a public key whose signature includes a one-
    time key support subpacket, they SHOULD include a new one-time
    public subkey for the recipient to encrypt any reply with. It MUST
    be sent in the format [primary public signing key | one-time public
    subkey | signature by primary key]. If PGP/MIME [7] support is
    available, new key(s) MUST be sent in a separate application/pgp-
    keys MIME bodypart.

    One-time subkeys MUST NOT be exported by their recipient to a third
    party, particularly a key server.

    Users still MUST possess a relatively long-lived encryption key. If
    Alice were writing to Bob for the first time, she would encrypt her
    message with his long term key. She would also include a newly
    created one-time public subkey. Bob would use this new key the next
    time he wrote to Alice, then delete it. Alice would decrypt the
    message with the associated private subkey, then securely wipe it.

    A "one-time key support" feature subpacket on a public key
    indicates support for one-time keys. These subpackets MUST be
    included in the hashed area of a signature. They are formatted
    as follows:

    Subpacket type: 30
    Contents: Bit 1 (0x2) must be set

    A "one-time key" flag subpacket marker MUST be present in the
    signature of a one-time subkey. These subpackets are formatted as

    Subpacket type: 27
    Contents: Bit 5 (0x20) must be set

    One-time key flag subpackets MUST be included in the hashed area of
    a signature. They are marked as critical so that the entire
    signature will be ignored by non-compliant OpenPGP clients,
    preventing more than one message being encrypted using a one-time

    When encrypting messages to a key with a signature containing a
    one-time feature subpacket, at least one new public encryption
    subkey MUST be included in the message. This key MUST be signed by
    the sender's long-term signature key and include a one-time key
    flag subpacket. The lifetime of a one-time subkey SHOULD be set to
    as short a period as possible given the expected response time of
    the recipient. This minimises the key storage requirements of
    sender and recipient. As a user's collection of private keys grows,
    she may wish to reduce the lifetime of new one-time subkeys.

    A client MUST include further new public encryption subkeys if it
    believes a message will receive multiple replies. Each reply SHOULD
    be encrypted with a different subkey if available.

    Clients MUST delete a one-time subkey after successfully encrypting

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    data using it. They SHOULD use a one-time subkey, if available, in
    preference to a short-lifetime key.

4.  Secure and decentralised e-mail transport

    The vast majority of current mail clients deliver messages first to
    a local mail server, which forwards them to their recipient's mail
    server, where they remain until collected. This procedure minimises
    security because it fails to take advantage of mail transport
    protocols such as SMTP [8] over secure transport or network layer
    security links such as TLS [9] or IPSEC [2]. This is particularly
    important given that these protocols allow transient keys to be
    generated and then discarded after each session, providing perfect
    forward secrecy.

    End-to-end security would be better provided if clients delivered
    messages directly to the recipient's mail server. This allows a
    secure link to be set up between the two, providing a second layer
    of forward secrecy. Ideally, as greater numbers of users gain
    permanent Internet connections through cable modems or Digital
    Subscriber Lines, they can run mail servers on their own machines.
    DNS Mail eXchange records [10] can be used to specify a backup mail
    server such as at an ISP for times when the recipient's machine is

    OpenPGP mail clients therefore SHOULD deliver messages directly to
    the recipient's mail server, and SHOULD use any available lower
    layer security services to protect the links used to deliver

    Where OpenPGP keys are used in such services, they SHOULD NOT be
    used to encrypt keying material that can later be decrypted if
    they are compromised. Ideally, they SHOULD be used only to
    authenticate a forward-secret key negotiation protocol such as
    Diffie-Hellman [3]. At the least, new short-lifetime key pairs
    SHOULD be generated for key encryption use.

    Direct delivery of mail can reveal the sender and recipient of
    messages to traffic analysts. Clients MAY use anonymous remailers
    [11] or IP services [12] to mask this information.

5.  Security Considerations

    Users of these extensions must consider the complete security
    environment in which they are operating. Highly-secure
    communications are of limited use between two insecure systems
    vulnerable to crackers, virii, and other methods of message and
    key compromise at source. Bellovin [13] describes a minimum set of
    precautions that should be taken.

6.  Acknowledgements

    Thanks to Nick Bohm, Richard Clayton, Hal Finney and Edwin Woudt
    for suggestions that have been incorporated into this draft.

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7.  Authors' Addresses

    Ian Brown
    Hidden Footprints Ltd.
    Gower Street
    London WC1E 6BT
    United Kingdom

    Phone: +44 20 7679 3704
    Fax: +44 20 7387 1397

    Adam Back
    Zero-Knowledge Systems Inc.
    888 de Maisonneuve East
    6th Floor
    Quebec H2L 4S8


    Ben Laurie
    A.L. Digital Ltd.
    Voysey House
    Barley Mow Passage
    London W4 4GB
    United Kingdom

    Phone: +44 20 8735 0686

8.  References

    [1]  Callas, J., Donnerhacke, L., Finney, H. and Thayer, R.,
         "OpenPGP Message Format", RFC 2440, November 1998.

    [2]  Atkinson, R., "Security Architecture for the Internet
         Protocol", RFC 1825, August 1995.

    [3]  Diffie, W. and Hellman, M., "New directions in cryptography",
         IEEE Transactions on Information Theory 22(6), November 1976,

    [4]  US Department of Defense, "Department of Defense Trusted
         Computer System Evaluation Criteria", DoD 5200.28-STD,
         December 1985.

    [5]  Elgamal, T., "A Public Key Cryptosystem and a Signature Scheme
         Based on Discrete Logarithms", IEEE Transactions on
         Information Theory 31(4), July 1985, 469-472.

    [6]  Schneier, B. and Hall, C., "An Improved E-mail Security
         Protocol", Proc. 13th Annual Computer Security Applications

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         Conference, New York: ACM Press, 1997, pp. 232-238.

    [7]  Elkins, M., Del Torto, D., Levien, R. and Roessler, T., "MIME
         Security with OpenPGP", RFC 3156, August 2001.

    [8]  Postel, J., "Simple Mail Transfer Protocol", RFC 821, August

    [9]  Dierks, T. and Allen, C., "The TLS Protocol", RFC 2246,
         November 1997.

    [10] Mockapetris, P., "Domain Names - Concepts and Facilities", STD
         13, RFC 1034, November 1987.

    [11] Chaum, D., "Untraceable Electronic Mail, Return Addresses, and
         Digital Pseudonyms", Communications of the ACM 24(2) 84-88,
         February 1981.

    [12] Goldberg, I. and Shostack, A., "Freedom Network 1.0
         Architecture and Protocols",

    [13] Bellovin, S., "Can Someone Read My E-Mail?",, 1998.

9.  Full Copyright Statement

    Copyright (C) The Internet Society (2001).  All Rights Reserved.

    This document and translations of it may be copied and furnished to
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    However, this document itself may not be modified in any way, such
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    other than English.

    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on

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