cleanup: Rename a few badly-chosen variables.

[tripe] / doc / protocol.ms

.\" -*-nroff-*-
.so tmac.rfc
.
.TL "Straylight/Edgeware"               "Mark Wooding"
.TL "Request for Comments:  XXXX"       "Straylight/Edgeware"
.TL ""                                  "11 April 2003"
.
.TT XXXX Wooding "April 2003" \
  "TrIPE: The Trivial IP Encryption Protocol"
.
.TI 0 "Status of this Memo"
.P
This memo defines an Experimental Protocol for the Internet community.
This memo does not specify an Internet standard of any kind.  Discussion
and suggestions for improvement are requested.  Distribution of this
memo is unlimited.
.P
.T0 "Introduction"
.P
TrIPE is a simple protocol which enables IP datagrams (or other data) to
be exchanged between a pair of hosts over a hostile network while
maintaining the properties of secrecy and authenticity; i.e., that the
content of the datagrams cannot be determined by eavesdroppers on the
network, and that either endpoint can determine whether a datagram
received is an unaltered copy of one that was sent by the other.
.P
While similar services are provided by other protocols (e.g., [IPSEC]),
they tend to be very complicated and difficult to analyze (see, for
example, [IPSEC-EVAL]).  By contrast, TrIPE attempts to get away with
doing as little as possible.  There are no negotiations to decide which
ciphers are to be used: these things are defined in the protocol
specification.  There is only one key-exchange algorithm defined.
.P
In addition to making analysis easier, a simpler protocol also helps
reduce the complexity of implementations: this makes implementation
errors less likely, and makes auditing an implementation for security
holes a more realistic proposition.
.P
.T0 "Interpretation"
.P
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 [REQ].
.P
.T0 "Protocol overview"
.P
The TrIPE protocol sets up a secure point-to-point channel between two
peer hosts, through which IP datagrams may be passed securely.
.P
All TrIPE messages are sent as UDP datagrams.  No UDP port has been
registered for TrIPE yet.
.P
When a pair of peer hosts are made aware of each other, they begin a key
negotiation, using an authenticated Diffie-Hellman key exchange
protocol.  This enables them to agree a 
.I keyset :
a collection of short-term symmetric keys and other parameters (such as
sequence numbering spaces).  Keysets expire after a fixed amount of
time, or after they have been used to encrypt a given amount of data,
whichever happens first.  Before the current keyset expires, a new key
negotation is started, so that the peers can seamlessly start using the
new keys before the old ones become invalid.
.P
.T1 "Keysets"
.P
A
.I keyset
is a collection of symmetric keys and associated state information.  The
items required, and the symbolic names by which they are described in
this document are:
.UL
.LI
.B "Incoming and outgoing encryption keys"
.K c (
and
.K' c "" )
.LE
.P
.T1 "Key exchange"
.
.
.P
.T0 "Data representation and notation"
.
.de SD
.P
.LS
..
.de SM
.br
.B "\\$1" \c
.if !'\\$2'.' \ \\$2\c
.if !'\\$3'' \{\
: 
.I "\\$3" \c
.\}
..
.de SR
.br
.B "\\$1" \c
.if !'\\$2'.' \ \\$2\c
.if !'\\$3'' : \\$3
..
.de ST
.SM "\\$1" "\\$2" "\\$3"
.LS 2n
..
.P
We need to deal with a number of data items during the protocol.  
.if t \{\
Object names are given in
.I italics .
.\}
A plain name indicates `our' value; a `primed' name (e.g.,
.I alpha' )
indicates the peer's corresponding value.  If a compound data item name
is primed, toggle the primed-ness of the components.
.P
Data objects are given types which determine their representation in
protocol messages.  Type names are given in 
.B UPPERCASE .
.P
.T1 "Atomic data items"
.DL
.DI OCTET
A single octet, representing a value between 0 and 255.
.DI U16
A pair of octets, representing a value between 0 and 65535.  The more
significant octet appears first.
.DI U32
Four octets, representing a value between 0 and 4294967295.  More
significant octets appear first.
.DI "STRING \fIn\fR\fB"
A string of
.I n
octets.  A
.B STRING
does not have a numeric value.
.DI MP
A nonnegative multiprecision integer.  Let
.I n
be the integer to be represented, and let
.I z
be the number of octets required to represent
.I n
in base-256 format with no leading zeroes, except that if
.I n 
= 0
then 
.I z
= 1; in other cases,
.I z 
is the unique integer such that
.ie t 256\*(^(\fIz\fP\-1\*(^) \(<= \fIn\fP < 256\*(^(\fIz\fP\*(^).
.el 256^{z-1} <= n < 256^z.
The encoding for
.I n
then consists of two octets encoding
.I z
as a
.B U16
followed by the
.I z
octets which are the base-256 digits of
.I n ,
most significant first.
.LE
.P
.T1 "Structured data items"
.P
A
.I structure
is a compound object which is simply the concatenation of a number of
its component objects.  Structures are used directly as messages, and
indirectly as things to be hashed or encrypted.
.P
The notation
.SD
.  ST STRUCT . kx-cookie
.    SR OCTET . 0x11
.    SM MP . c
.    SM STRING 20 hash
.  LE
.LE
.P
indicates that
.I msg-cookie
consists of the two items
.
.f
.
.SD
.  ST STRUCT . msg-packet
.    SR OCTET . 0x00
.    br
payload
.  LE
.LE
.
.SD
.  ST STRUCT . kx-prechal
.    SR OCTET . 0x10
.    SM MP . c
.  LE
.LE
.
.SD
.  ST STRUCT . kx-cookie
.    SR OCTET . 0x11
.    SM MP . c
.    ST HASH .
.      SR TEXT . "tripe-cookie"
.      SM MP . c'
.    LE
.  LE
.LE
.P
.T0 "Security considerations"
.P
This memo describes a cryptographic protocol for ensuring secrecy and
integrity of communications between network hosts.  From this point of
view, it is entirely about security.
.P
Before deploying TrIPE on their own systems, administrators ought to
satisfy themselves that the cryptographic algorithms used are
sufficiently strong for their purposes, and that their implementation of
the TrIPE software has come from a trustworthy source.  They should also
ensure that they have adequate procedures in place for transporting
public keys without a risk of them being modified by adversaries.
.P
.T0 "References"
.BS IPSEC-EVAL
.BR IPSEC
Kent, S., Atkinson, R., `Security Architecture for the Internet
Protocol', RFC 2401, November 1998.
.
.BR IPSEC-EVAL
Ferguson, N., Schneier, B., `A Cryptographic Evaluation of IPsec',
December 1999.
.
.BR REQ
Bradner, S., `Key words for use in RFCs to Indicate Requirement Levels',
BCP 14, RFC 2119, March 1997.
.BE