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Network Working Group                                           G. MeyerRequest for Comments: 1968                                Spider SystemsCategory: Standards Track                                      June 1996The PPP Encryption Control Protocol (ECP)Status of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   The Point-to-Point Protocol (PPP) [1] provides a standard method for   transporting multi-protocol datagrams over point-to-point links.  PPP   also defines an extensible Link Control Protocol.   This document defines a method for negotiating data encryption over   PPP links.Conventions   The following language conventions are used in the items of   specification in this document:   o  MUST -- the item is an absolute requirement of the specification.      MUST is only used where it is actually required for interopera-      tion, not to try to impose a particular method on implementors      where not required for interoperability.   o  SHOULD -- the item should be followed for all but exceptional cir-      cumstances.   o  MAY or optional -- the item is truly optional and may be followed      or ignored according to the needs of the implementor.      The words "should" and "may" are also used, in lower case, in      their more ordinary senses.Meyer                       Standards Track                     [Page 1]

RFC 1968                     PPP Encryption                    June 1996Table of Contents1. Introduction ...........................................22. Encryption Control Protocol (ECP) ......................22.1 Sending Encrypted Datagrams .......................33. Additional Packets .....................................43.1 Reset-Request and Reset-Ack .......................54. ECP Configuration Options ..............................64.1 Proprietary Encryption OUI ........................74.2 Publicly Available Encryption Types ...............84.3 Negotiating an Encryption Algorithm ...............95. Security Considerations ................................101. Introduction   In order to establish communications over a PPP link, each end of the   link must first send LCP packets to configure and test the data link   during Link Establishment phase.  After the link has been   established, optional facilities may be negotiated as needed.   One such facility is data encryption.  A wide variety of encryption   methods may be negotiated, although typically only one method is used   in each direction of the link.   A different encryption algorithm may be negotiated in each direction,   for speed, cost, memory or other considerations.2. Encryption Control Protocol (ECP)   The Encryption Control Protocol (ECP) is responsible for configuring   and enabling data encryption algorithms on both ends of the point-   to-point link.   ECP uses the same packet exchange mechanism as the Link Control   Protocol (LCP).  ECP packets may not be exchanged until PPP has   reached the Network-Layer Protocol phase.  ECP packets received   before this phase is reached should be silently discarded.   The Encryption Control Protocol is exactly the same as LCP [1] with   the following exceptions:      Frame Modifications         The packet may utilise any modifications to the basic frame         format which have been negotiated during the Link Establishment         phase.Meyer                       Standards Track                     [Page 2]

RFC 1968                     PPP Encryption                    June 1996      Data Link Layer Protocol Field         Exactly one ECP packet is encapsulated in the PPP Information         field, where the PPP Protocol field indicates type hex 8053         (Encryption Control Protocol).         When individual link data encryption is used in a multiple link         connection to a single destination [2], the PPP Protocol field         indicates type hex 8055 (Individual link Encryption Control         Protocol).      Code field         ECP uses (decimal) codes 1 through 7 (Configure-Request,         Configure-Ack, Configure-Nak, Configure-Reject, Terminate-         Request, Terminate-Ack and Code-Reject); And may also use code         14 (Reset-Request) and code 15 (Reset-Ack).  Other codes should         be treated as unrecognised and should result in Code-Rejects.      Negotiation         ECP packets may not be exchanged until PPP has reached the         Network-Layer Protocol phase.  An implementation should be         prepared to wait for Authentication and Link Quality         Determination to finish before timing out waiting for a         Configure-Ack or other response.         An implementation MUST NOT transmit data until ECP negotiation         has completed successfully.  If ECP negotiation is not         successful the link SHOULD be brought down.      Configuration Option Types         ECP has a distinct set of Configuration Options.2.1 Sending Encrypted Datagrams   Before any encrypted packets may be communicated, PPP must reach the   Network-Layer Protocol phase, and the Encryption Control Protocol   must reach the Opened state.   An encrypted packet is encapsulated in the PPP Information field,   where the PPP Protocol field indicates type hex 0053 (Encrypted   datagram).   When using multiple PPP links to a single destination [2], there are   two methods of employing data encryption:Meyer                       Standards Track                     [Page 3]

RFC 1968                     PPP Encryption                    June 1996   o  The first method is to encrypt the data prior to sending it out      through the multiple links.      The PPP Protocol field MUST indicate type hex 0053.   o  The second is to treat each link as a separate connection, that      may or may not have encryption enabled.      On links which have negotiated encryption, the PPP Protocol field      MUST be type hex 0055 (Individual link encrypted datagram).   Only one encryption algorithm in each direction is in use at a time,   and that is negotiated prior to sending the first encrypted frame.   The PPP Protocol field of the encrypted datagram indicates that the   frame is encrypted, but not the algorithm with which it was   encrypted.   The maximum length of an encrypted packet transmitted over a PPP link   is the same as the maximum length of the Information field of a PPP   encapsulated packet.  If the encryption algorithm is likely to   increase the size of the message beyond that, multilink should also   be negotiated to allow fragmentation of the frames (even if only   using a single link).   If the encryption algorithm carries history between frames, the   encryption algorithm must supply a way of determining if it is   passing data reliably, or it must require the use of a reliable   transport such as LAPB [3].   Compression may also be negotiated using the Compression Control   Protocol [5].  To ensure interoperability, plain text MUST be:   o  First compressed.   o  Then encrypted.   This order has been chosen since it should result in smaller output   and more secure encryption.3. Additional Packets   The Packet format and basic facilities are already defined for LCP   [1].   Up-to-date values of the ECP Code field are specified in the most   recent "Assigned Numbers" RFC [4].  This specification concerns the   following values:Meyer                       Standards Track                     [Page 4]

RFC 1968                     PPP Encryption                    June 1996         14      Reset-Request         15      Reset-Ack3.1 Reset-Request and Reset-Ack   Description      ECP includes Reset-Request and Reset-Ack Codes in order to provide      a mechanism for indicating a decryption failure in one direction      of a decrypted link without affecting traffic in the other      direction.  Some encryption algorithms may not require this      mechanism.      Individual algorithms need to specify a mechanism for determining      how to detect a decryption failure.  On initial detection of a      decryption failure, an ECP implementation SHOULD transmit an ECP      packet with the Code field set to 14 (Reset-Request).  The Data      field may be filled with any desired data.      Once a Reset-Request has been sent, any encrypted packets received      are discarded.  Further Reset-Requests MAY be sent with the same      Identifier, until a valid Reset-Ack is received.      When the link is busy, one decryption error is usually followed by      several more before the Reset-Ack can be received.  It is      undesirable to transmit Reset-Requests more frequently than the      round-trip-time of the link, since this will result in redundant      Reset-Requests and Reset-Acks being transmitted and processed.      The receiver MAY elect to limit transmission of Reset-Requests (to      say one per second) while a Reset-Ack is outstanding.      Upon reception of a Reset-Request, the transmitting encrypter is      reset to an initial state.  An ECP packet MUST be transmitted with      the Code field set to 15 (Reset-Ack), the Identifier field copied      from the Reset-Request packet, and the Data field filled with any      desired data.      On receipt of a Reset-Ack, the receiving decrypter is reset to an      initial state.  Since there may be several Reset-Acks in the pipe,      the decrypter MUST be reset for each Reset-Ack which matches the      currently expected identifier.      A summary of the Reset-Request and Reset-Ack packet formats is      shown below.  The fields are transmitted from left to right.Meyer                       Standards Track                     [Page 5]

RFC 1968                     PPP Encryption                    June 1996        0                   1                   2                   3        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |     Code      |  Identifier   |            Length             |       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       |    Data ...       +-+-+-+-+   Code      14 for Reset-Request;      15 for Reset-Ack.   Identifier      On transmission, the Identifier field MUST be changed whenever the      content of the Data field changes, and whenever a valid reply has      been received for a previous request.  For retransmissions, the      Identifier SHOULD remain unchanged.      On reception, the Identifier field of the Reset-Request is copied      into the Identifier field of the Reset-Ack packet.   Data      The Data field is zero or more octets and contains uninterpreted      data for use by the sender.  The data may consist of any binary      value and may be of any length from zero to the peer's established      MRU minus four.4. ECP Configuration Options   ECP Configuration Options allow negotiation of encryption algorithms   and their parameters.  ECP uses the same Configuration Option format   defined for LCP [1], with a separate set of Options.   Configuration Options, in this protocol, indicate algorithms that the   receiver is willing or able to use to decrypt data sent by the   sender.  Systems may offer to accept several algorithms, and   negotiate a single one that will be used.   Up-to-date values of the ECP Option Type field are specified in the   most recent "Assigned Numbers" RFC [4].  Current values are assigned   as follows:Meyer                       Standards Track                     [Page 6]

RFC 1968                     PPP Encryption                    June 1996         ECP Option      Encryption type         0               OUI         1               DESE   All compliant ECP implementations SHOULD implement ECP option 1 - the   PPP DES Encryption Protocol (DESE) [6].   Vendors who want to use proprietary encryption MAY use the OUI   mechanism to negotiate these without recourse to requesting an   assigned option number from the Internet Assigned Numbers Authority.   All other encryption options are registered by IANA.  At the time of   writing only DESE (option 1) is registered.  Other registered options   may be found by referring to future versions of the Assigned Numbers   RFC.4.1 Proprietary Encryption OUI   Description      This Configuration Option provides a way to negotiate the use of a      proprietary encryption protocol.      Vendor's encryption protocols are distinguished from each other by      means of an Organisationally Unique Identifier (OUI), namely the      first three octets of a Vendor's Ethernet address assigned by IEEE      802.      Since the first matching encryption will be used, it is      recommended that any known OUI encryption options be transmitted      first, before the common options are used.      Before accepting this option, the implementation must verify that      the OUI identifies a proprietary algorithm that the implementation      can decrypt, and that any vendor specific negotiation values are      fully understood.      A summary of the Proprietary Encryption OUI Configuration Option      format is shown below.  The fields are transmitted from left to      right.Meyer                       Standards Track                     [Page 7]

RFC 1968                     PPP Encryption                    June 1996       0                   1                   2                   3       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |     Type      |    Length     |       OUI ...      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+            OUI       |    Subtype    |  Values...      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-   Type       0   Length      >= 6   IEEE OUI      The IEEE OUI is the most significant three octets of an Ethernet      Physical Address, assigned to the vendor by IEEE 802.  This      identifies the option as being proprietary to the indicated      vendor.  The bits within the octet are in canonical order, and the      most significant octet is transmitted first.   Subtype      This field is specific to each OUI, and indicates an encryption      type for that OUI.  There is no standardisation for this field.      Each OUI implements its own values.   Values      This field is zero or more octets, and contains additional data as      determined by the vendor's encryption protocol.4.2 Publicly Available Encryption Types   Description      These Configuration Options provide a way to negotiate the use of      a publicly defined encryption algorithm.      These protocols should be made available to interested parties,      but may have certain licencing or export restrictions associated      with them.  For additional information, refer to the encryption      protocol documents that define each of the encryption types.Meyer                       Standards Track                     [Page 8]

RFC 1968                     PPP Encryption                    June 1996      A summary of the Encryption Type Configuration Option format is      shown below.  The fields are transmitted from left to right.       0                   1                   2                   3       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |     Type      |    Length     |  Values...      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-    Type       1 to 254, indicating the encryption protocol option       being negotiated.  DESE [6] is option type 1.  Refer to the       latest Assigned Numbers RFC for other encryption protocols.    Length       >= 2   Values      This field is zero or more octets, and contains additional data as      determined by the encryption protocol.4.3 Negotiating an Encryption Algorithm   ECP uses LCP option negotiation techniques to negotiate encryption   algorithms.  In contrast with most other LCP or NCP negotiation of   multiple options, ECP negotiation is expected to converge on a single   mutually agreeable option (encryption algorithm) - or none.   Encryption SHOULD be negotiated in both directions, but the   algorithms MAY be different.   An implementation willing to decrypt using a particular encryption   algorithm (or one of a set of algorithms) offers the algorithm(s) as   an option (or options) in an ECP Configure-Request - call this end   the Decrypter; call the peer the Encrypter.   A Decrypter supporting more than one encryption algorithm may send a   Configure-Request containing either:   o  an ordered list of options, with the most-preferred encryption      algorithm coming first.   o  Or may just offer its preferred (not already Rejected) option.Meyer                       Standards Track                     [Page 9]

RFC 1968                     PPP Encryption                    June 1996   An Encrypter wishing to accept the first option (of several) MAY   Configure-Ack ALL Options to indicate complete acceptance of the   first-listed, preferred, algorithm.   Otherwise, if the Encrypter does not recognise - or is unwilling to   support - an option it MUST send a Configure-Reject for that option.   Where more than one option is offered, the Encrypter SHOULD   Configure-Reject all but a single preferred option.   If the Encrypter Configure-Rejects all offered ECP options - and the   Decrypter has no further (non-rejected) options it can offer in a   Configure-Request - the Encrypter SHOULD take the link down.   If the Encrypter recognises an option, but it is not acceptable due   to values in the request (or optional parameters not in the request),   it MUST send a Configure-Nak with the option modified appropriately.   The Configure-Nak MUST contain only those options that will be   acceptable.  The Decrypter SHOULD send a new Configure-Request with   only the single preferred option, adjusted as specified in the   Configure-Nak.5. Security Considerations   Negotiation of encryption using PPP is designed to provide protection   against eavesdropping on that link.  The strength of the protection   is dependent on the encryption algorithm used and the care with which   any 'secret' used by the encryption algorithm is protected.   It must be recognised that complete security can only be obtained   through end-to-end security between hosts.References   [1]  Simpson, W., Editor; "The Point-to-Point Protocol (PPP)", STD        51,RFC 1661, Daydreamer, July 1994.   [2]  Sklower, K., Lloyd, B., McGregor, G. and and D. Carr, "The PPP        Multilink Protocol (MP)",RFC 1717, University of California,        Berkeley, November 1994.   [3]  Rand, D., "PPP Reliable Transmission",RFC 1663, Novell, July        1994.   [4]  Reynolds, J., and Postel, J.; "ASSIGNED NUMBERS", STD 2,RFC 1700, USC/Information Sciences Institute, October 1994.   [5]  Rand, D., "The PPP Compression Control Protocol (CCP)",RFC1962, Novell, June 1996.Meyer                       Standards Track                    [Page 10]

RFC 1968                     PPP Encryption                    June 1996   [6]  Sklower, K., and G. Meyer, "The PPP DES Encryption Protocol        (DESE)",RFC 1969, University of California, Berkeley, June        1996.Acknowledgements   The style and approach of this proposal owes much to the work on the   Compression CP [5].Chair's Address   The working group can be contacted via the current chair:   Karl Fox   Ascend Communications   3518 Riverside Drive, Suite 101   Columbus, Ohio 43221   EMail: karl@ascend.comAuthor's Address   Gerry Meyer   Spider Systems   Stanwell Street   Edinburgh EH6 5NG   Scotland, UK   Phone: (UK) 131 554 9424   Fax:   (UK) 131 554 0649   EMail: gerry@spider.co.ukMeyer                       Standards Track                    [Page 11]