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Network Working Group T. Ylonen
Internet-Draft T. Kivinen
Expires: January 12, 2004 SSH Communications Security Corp
M. Saarinen
University of Jyvaskyla
T. Rinne
S. Lehtinen
SSH Communications Security Corp
July 14, 2003
SSH Protocol Architecture
draft-ietf-secsh-architecture-14.txt
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
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 12, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
SSH is a protocol for secure remote login and other secure network
services over an insecure network. This document describes the
architecture of the SSH protocol, as well as the notation and
terminology used in SSH protocol documents. It also discusses the
SSH algorithm naming system that allows local extensions. The SSH
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protocol consists of three major components: The Transport Layer
Protocol provides server authentication, confidentiality, and
integrity with perfect forward secrecy. The User Authentication
Protocol authenticates the client to the server. The Connection
Protocol multiplexes the encrypted tunnel into several logical
channels. Details of these protocols are described in separate
documents.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Specification of Requirements . . . . . . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Extensibility . . . . . . . . . . . . . . . . . . . . . . . 6
3.3 Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 6
3.4 Security Properties . . . . . . . . . . . . . . . . . . . . 7
3.5 Packet Size and Overhead . . . . . . . . . . . . . . . . . . 7
3.6 Localization and Character Set Support . . . . . . . . . . . 8
4. Data Type Representations Used in the SSH Protocols . . . . 9
5. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . 11
6. Message Numbers . . . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . 13
8.1 Pseudo-Random Number Generation . . . . . . . . . . . . . . 13
8.2 Transport . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.2.1 Confidentiality . . . . . . . . . . . . . . . . . . . . . . 14
8.2.2 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 17
8.2.3 Replay . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.2.4 Man-in-the-middle . . . . . . . . . . . . . . . . . . . . . 18
8.2.5 Denial-of-service . . . . . . . . . . . . . . . . . . . . . 20
8.2.6 Covert Channels . . . . . . . . . . . . . . . . . . . . . . 21
8.2.7 Forward Secrecy . . . . . . . . . . . . . . . . . . . . . . 21
8.3 Authentication Protocol . . . . . . . . . . . . . . . . . . 21
8.3.1 Weak Transport . . . . . . . . . . . . . . . . . . . . . . . 22
8.3.2 Debug messages . . . . . . . . . . . . . . . . . . . . . . . 22
8.3.3 Local security policy . . . . . . . . . . . . . . . . . . . 23
8.3.4 Public key authentication . . . . . . . . . . . . . . . . . 23
8.3.5 Password authentication . . . . . . . . . . . . . . . . . . 24
8.3.6 Host based authentication . . . . . . . . . . . . . . . . . 24
8.4 Connection protocol . . . . . . . . . . . . . . . . . . . . 24
8.4.1 End point security . . . . . . . . . . . . . . . . . . . . . 24
8.4.2 Proxy forwarding . . . . . . . . . . . . . . . . . . . . . . 24
8.4.3 X11 forwarding . . . . . . . . . . . . . . . . . . . . . . . 25
9. Intellectual Property . . . . . . . . . . . . . . . . . . . 25
10. Additional Information . . . . . . . . . . . . . . . . . . . 26
References . . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 29
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Full Copyright Statement . . . . . . . . . . . . . . . . . . 31
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1. Introduction
SSH is a protocol for secure remote login and other secure network
services over an insecure network. It consists of three major
components:
o The Transport Layer Protocol [SSH-TRANS] provides server
authentication, confidentiality, and integrity. It may
optionally also provide compression. The transport layer will
typically be run over a TCP/IP connection, but might also be
used on top of any other reliable data stream.
o The User Authentication Protocol [SSH-USERAUTH] authenticates
the client-side user to the server. It runs over the transport
layer protocol.
o The Connection Protocol [SSH-CONNECT] multiplexes the encrypted
tunnel into several logical channels. It runs over the user
authentication protocol.
The client sends a service request once a secure transport layer
connection has been established. A second service request is sent
after user authentication is complete. This allows new protocols
to be defined and coexist with the protocols listed above.
The connection protocol provides channels that can be used for a
wide range of purposes. Standard methods are provided for setting
up secure interactive shell sessions and for forwarding
("tunneling") arbitrary TCP/IP ports and X11 connections.
2. Specification of Requirements
All documents related to the SSH protocols shall use the keywords
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
requirements. They are to be interpreted as described in [RFC-
2119].
3. Architecture
3.1 Host Keys
Each server host SHOULD have a host key. Hosts MAY have multiple
host keys using multiple different algorithms. Multiple hosts MAY
share the same host key. If a host has keys at all, it MUST have
at least one key using each REQUIRED public key algorithm
(currently DSS [FIPS-186]).
The server host key is used during key exchange to verify that the
client is really talking to the correct server. For this to be
possible, the client must have a priori knowledge of the server's
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public host key.
Two different trust models can be used:
o The client has a local database that associates each host name
(as typed by the user) with the corresponding public host key.
This method requires no centrally administered infrastructure,
and no third-party coordination. The downside is that the
database of name-to-key associations may become burdensome to
maintain.
o The host name-to-key association is certified by some trusted
certification authority. The client only knows the CA root
key, and can verify the validity of all host keys certified by
accepted CAs.
The second alternative eases the maintenance problem, since
ideally only a single CA key needs to be securely stored on the
client. On the other hand, each host key must be appropriately
certified by a central authority before authorization is
possible. Also, a lot of trust is placed on the central
infrastructure.
The protocol provides the option that the server name - host key
association is not checked when connecting to the host for the
first time. This allows communication without prior communication
of host keys or certification. The connection still provides
protection against passive listening; however, it becomes
vulnerable to active man-in-the-middle attacks. Implementations
SHOULD NOT normally allow such connections by default, as they
pose a potential security problem. However, as there is no widely
deployed key infrastructure available on the Internet yet, this
option makes the protocol much more usable during the transition
time until such an infrastructure emerges, while still providing a
much higher level of security than that offered by older solutions
(e.g. telnet [RFC-854] and rlogin [RFC-1282]).
Implementations SHOULD try to make the best effort to check host
keys. An example of a possible strategy is to only accept a host
key without checking the first time a host is connected, save the
key in a local database, and compare against that key on all
future connections to that host.
Implementations MAY provide additional methods for verifying the
correctness of host keys, e.g. a hexadecimal fingerprint derived
from the SHA-1 hash of the public key. Such fingerprints can
easily be verified by using telephone or other external
communication channels.
All implementations SHOULD provide an option to not accept host
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keys that cannot be verified.
We believe that ease of use is critical to end-user acceptance of
security solutions, and no improvement in security is gained if
the new solutions are not used. Thus, providing the option not to
check the server host key is believed to improve the overall
security of the Internet, even though it reduces the security of
the protocol in configurations where it is allowed.
3.2 Extensibility
We believe that the protocol will evolve over time, and some
organizations will want to use their own encryption,
authentication and/or key exchange methods. Central registration
of all extensions is cumbersome, especially for experimental or
classified features. On the other hand, having no central
registration leads to conflicts in method identifiers, making
interoperability difficult.
We have chosen to identify algorithms, methods, formats, and
extension protocols with textual names that are of a specific
format. DNS names are used to create local namespaces where
experimental or classified extensions can be defined without fear
of conflicts with other implementations.
One design goal has been to keep the base protocol as simple as
possible, and to require as few algorithms as possible. However,
all implementations MUST support a minimal set of algorithms to
ensure interoperability (this does not imply that the local policy
on all hosts would necessary allow these algorithms). The
mandatory algorithms are specified in the relevant protocol
documents.
Additional algorithms, methods, formats, and extension protocols
can be defined in separate drafts. See Section Algorithm Naming
(Section 5) for more information.
3.3 Policy Issues
The protocol allows full negotiation of encryption, integrity, key
exchange, compression, and public key algorithms and formats.
Encryption, integrity, public key, and compression algorithms can
be different for each direction.
The following policy issues SHOULD be addressed in the
configuration mechanisms of each implementation:
o Encryption, integrity, and compression algorithms, separately
for each direction. The policy MUST specify which is the
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preferred algorithm (e.g. the first algorithm listed in each
category).
o Public key algorithms and key exchange method to be used for
host authentication. The existence of trusted host keys for
different public key algorithms also affects this choice.
o The authentication methods that are to be required by the
server for each user. The server's policy MAY require multiple
authentication for some or all users. The required algorithms
MAY depend on the location where the user is trying to log in
from.
o The operations that the user is allowed to perform using the
connection protocol. Some issues are related to security; for
example, the policy SHOULD NOT allow the server to start
sessions or run commands on the client machine, and MUST NOT
allow connections to the authentication agent unless forwarding
such connections has been requested. Other issues, such as
which TCP/IP ports can be forwarded and by whom, are clearly
issues of local policy. Many of these issues may involve
traversing or bypassing firewalls, and are interrelated with
the local security policy.
3.4 Security Properties
The primary goal of the SSH protocol is improved security on the
Internet. It attempts to do this in a way that is easy to deploy,
even at the cost of absolute security.
o All encryption, integrity, and public key algorithms used are
well-known, well-established algorithms.
o All algorithms are used with cryptographically sound key sizes
that are believed to provide protection against even the
strongest cryptanalytic attacks for decades.
o All algorithms are negotiated, and in case some algorithm is
broken, it is easy to switch to some other algorithm without
modifying the base protocol.
Specific concessions were made to make wide-spread fast deployment
easier. The particular case where this comes up is verifying that
the server host key really belongs to the desired host; the
protocol allows the verification to be left out (but this is NOT
RECOMMENDED). This is believed to significantly improve usability
in the short term, until widespread Internet public key
infrastructures emerge.
3.5 Packet Size and Overhead
Some readers will worry about the increase in packet size due to
new headers, padding, and MAC. The minimum packet size is in the
order of 28 bytes (depending on negotiated algorithms). The
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increase is negligible for large packets, but very significant for
one-byte packets (telnet-type sessions). There are, however,
several factors that make this a non-issue in almost all cases:
o The minimum size of a TCP/IP header is 32 bytes. Thus, the
increase is actually from 33 to 51 bytes (roughly).
o The minimum size of the data field of an Ethernet packet is 46
bytes [RFC-894]. Thus, the increase is no more than 5 bytes.
When Ethernet headers are considered, the increase is less than
10 percent.
o The total fraction of telnet-type data in the Internet is
negligible, even with increased packet sizes.
The only environment where the packet size increase is likely to
have a significant effect is PPP [RFC-1134] over slow modem lines
(PPP compresses the TCP/IP headers, emphasizing the increase in
packet size). However, with modern modems, the time needed to
transfer is in the order of 2 milliseconds, which is a lot faster
than people can type.
There are also issues related to the maximum packet size. To
minimize delays in screen updates, one does not want excessively
large packets for interactive sessions. The maximum packet size
is negotiated separately for each channel.
3.6 Localization and Character Set Support
For the most part, the SSH protocols do not directly pass text
that would be displayed to the user. However, there are some
places where such data might be passed. When applicable, the
character set for the data MUST be explicitly specified. In most
places, ISO 10646 with UTF-8 encoding is used [RFC-2279]. When
applicable, a field is also provided for a language tag [RFC-
1766].
One big issue is the character set of the interactive session.
There is no clear solution, as different applications may display
data in different formats. Different types of terminal emulation
may also be employed in the client, and the character set to be
used is effectively determined by the terminal emulation. Thus,
no place is provided for directly specifying the character set or
encoding for terminal session data. However, the terminal
emulation type (e.g. "vt100") is transmitted to the remote site,
and it implicitly specifies the character set and encoding.
Applications typically use the terminal type to determine what
character set they use, or the character set is determined using
some external means. The terminal emulation may also allow
configuring the default character set. In any case, the character
set for the terminal session is considered primarily a client
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local issue.
Internal names used to identify algorithms or protocols are
normally never displayed to users, and must be in US-ASCII.
The client and server user names are inherently constrained by
what the server is prepared to accept. They might, however,
occasionally be displayed in logs, reports, etc. They MUST be
encoded using ISO 10646 UTF-8, but other encodings may be required
in some cases. It is up to the server to decide how to map user
names to accepted user names. Straight bit-wise binary comparison
is RECOMMENDED.
For localization purposes, the protocol attempts to minimize the
number of textual messages transmitted. When present, such
messages typically relate to errors, debugging information, or
some externally configured data. For data that is normally
displayed, it SHOULD be possible to fetch a localized message
instead of the transmitted message by using a numerical code. The
remaining messages SHOULD be configurable.
4. Data Type Representations Used in the SSH Protocols
byte
A byte represents an arbitrary 8-bit value (octet) [RFC-1700].
Fixed length data is sometimes represented as an array of
bytes, written byte[n], where n is the number of bytes in the
array.
boolean
A boolean value is stored as a single byte. The value 0
represents FALSE, and the value 1 represents TRUE. All non-
zero values MUST be interpreted as TRUE; however, applications
MUST NOT store values other than 0 and 1.
uint32
Represents a 32-bit unsigned integer. Stored as four bytes in
the order of decreasing significance (network byte order). For
example, the value 699921578 (0x29b7f4aa) is stored as 29 b7 f4
aa.
uint64
Represents a 64-bit unsigned integer. Stored as eight bytes in
the order of decreasing significance (network byte order).
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string
Arbitrary length binary string. Strings are allowed to contain
arbitrary binary data, including null characters and 8-bit
characters. They are stored as a uint32 containing its length
(number of bytes that follow) and zero (= empty string) or more
bytes that are the value of the string. Terminating null
characters are not used.
Strings are also used to store text. In that case, US-ASCII is
used for internal names, and ISO-10646 UTF-8 for text that
might be displayed to the user. The terminating null character
SHOULD NOT normally be stored in the string.
For example, the US-ASCII string "testing" is represented as 00
00 00 07 t e s t i n g. The UTF8 mapping does not alter the
encoding of US-ASCII characters.
mpint
Represents multiple precision integers in two's complement
format, stored as a string, 8 bits per byte, MSB first.
Negative numbers have the value 1 as the most significant bit
of the first byte of the data partition. If the most
significant bit would be set for a positive number, the number
MUST be preceded by a zero byte. Unnecessary leading bytes
with the value 0 or 255 MUST NOT be included. The value zero
MUST be stored as a string with zero bytes of data.
By convention, a number that is used in modular computations in
Z_n SHOULD be represented in the range 0 <= x < n.
Examples:
value (hex) representation (hex)
---------------------------------------------------------------
0 00 00 00 00
9a378f9b2e332a7 00 00 00 08 09 a3 78 f9 b2 e3 32 a7
80 00 00 00 02 00 80
-1234 00 00 00 02 ed cc
-deadbeef 00 00 00 05 ff 21 52 41 11
name-list
A string containing a comma separated list of names. A name
list is represented as a uint32 containing its length (number
of bytes that follow) followed by a comma-separated list of
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zero or more names. A name MUST be non-zero length, and it
MUST NOT contain a comma (','). Context may impose additional
restrictions on the names; for example, the names in a list may
have to be valid algorithm identifier (see Algorithm Naming
below), or [RFC-1766] language tags. The order of the names in
a list may or may not be significant, also depending on the
context where the list is is used. Terminating NUL characters
are not used, neither for the individual names, nor for the
list as a whole.
Examples:
value representation (hex)
---------------------------------------
(), the empty list 00 00 00 00
("zlib") 00 00 00 04 7a 6c 69 62
("zlib", "none") 00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65
5. Algorithm Naming
The SSH protocols refer to particular hash, encryption, integrity,
compression, and key exchange algorithms or protocols by names.
There are some standard algorithms that all implementations MUST
support. There are also algorithms that are defined in the
protocol specification but are OPTIONAL. Furthermore, it is
expected that some organizations will want to use their own
algorithms.
In this protocol, all algorithm identifiers MUST be printable US-
ASCII non-empty strings no longer than 64 characters. Names MUST
be case-sensitive.
There are two formats for algorithm names:
o Names that do not contain an at-sign (@) are reserved to be
assigned by IETF consensus (RFCs). Examples include `3des-
cbc', `sha-1', `hmac-sha1', and `zlib' (the quotes are not part
of the name). Names of this format MUST NOT be used without
first registering them. Registered names MUST NOT contain an
at-sign (@) or a comma (,).
o Anyone can define additional algorithms by using names in the
format name@domainname, e.g. "ourcipher-cbc@ssh.com". The
format of the part preceding the at sign is not specified; it
MUST consist of US-ASCII characters except at-sign and comma.
The part following the at-sign MUST be a valid fully qualified
internet domain name [RFC-1034] controlled by the person or
organization defining the name. It is up to each domain how it
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manages its local namespace.
6. Message Numbers
SSH packets have message numbers in the range 1 to 255. These
numbers have been allocated as follows:
Transport layer protocol:
1 to 19 Transport layer generic (e.g. disconnect, ignore, debug,
etc.)
20 to 29 Algorithm negotiation
30 to 49 Key exchange method specific (numbers can be reused for
different authentication methods)
User authentication protocol:
50 to 59 User authentication generic
60 to 79 User authentication method specific (numbers can be
reused for different authentication methods)
Connection protocol:
80 to 89 Connection protocol generic
90 to 127 Channel related messages
Reserved for client protocols:
128 to 191 Reserved
Local extensions:
192 to 255 Local extensions
7. IANA Considerations
Allocation of the following types of names in the SSH protocols is
assigned by IETF consensus:
o encryption algorithm names,
o MAC algorithm names,
o public key algorithm names (public key algorithm also implies
encoding and signature/encryption capability),
o key exchange method names, and
o protocol (service) names.
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These names MUST be printable US-ASCII strings, and MUST NOT
contain the characters at-sign ('@'), comma (','), or whitespace
or control characters (ASCII codes 32 or less). Names are case-
sensitive, and MUST NOT be longer than 64 characters.
Names with the at-sign ('@') in them are allocated by the owner of
DNS name after the at-sign (hierarchical allocation in [RFC-
2343]), otherwise the same restrictions as above.
Each category of names listed above has a separate namespace.
However, using the same name in multiple categories SHOULD be
avoided to minimize confusion.
Message numbers (see Section Message Numbers (Section 6)) in the
range of 0..191 should be allocated via IETF consensus; message
numbers in the 192..255 range (the "Local extensions" set) are
reserved for private use.
8. Security Considerations
In order to make the entire body of Security Considerations more
accessible, Security Considerations for the transport,
authentication, and connection documents have been gathered here.
The transport protocol [1] provides a confidential channel over an
insecure network. It performs server host authentication, key
exchange, encryption, and integrity protection. It also derives a
unique session id that may be used by higher-level protocols.
The authentication protocol [2] provides a suite of mechanisms
which can be used to authenticate the client user to the server.
Individual mechanisms specified in the in authentication protocol
use the session id provided by the transport protocol and/or
depend on the security and integrity guarantees of the transport
protocol.
The connection protocol [3] specifies a mechanism to multiplex
multiple streams [channels] of data over the confidential and
authenticated transport. It also specifies channels for accessing
an interactive shell, for 'proxy-forwarding' various external
protocols over the secure transport (including arbitrary TCP/IP
protocols), and for accessing secure 'subsystems' on the server
host.
8.1 Pseudo-Random Number Generation
This protocol binds each session key to the session by including
random, session specific data in the hash used to produce session
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keys. Special care should be taken to ensure that all of the
random numbers are of good quality. If the random data here
(e.g., DH parameters) are pseudo-random then the pseudo-random
number generator should be cryptographically secure (i.e., its
next output not easily guessed even when knowing all previous
outputs) and, furthermore, proper entropy needs to be added to the
pseudo-random number generator. RFC 1750 [1750] offers
suggestions for sources of random numbers and entropy.
Implementors should note the importance of entropy and the well-
meant, anecdotal warning about the difficulty in properly
implementing pseudo-random number generating functions.
The amount of entropy available to a given client or server may
sometimes be less than what is required. In this case one must
either resort to pseudo-random number generation regardless of
insufficient entropy or refuse to run the protocol. The latter is
preferable.
8.2 Transport
8.2.1 Confidentiality
It is beyond the scope of this document and the Secure Shell
Working Group to analyze or recommend specific ciphers other than
the ones which have been established and accepted within the
industry. At the time of this writing, ciphers commonly in use
include 3DES, ARCFOUR, twofish, serpent and blowfish. AES has
been accepted by The published as a US Federal Information
Processing Standards [FIPS-197] and the cryptographic community as
being acceptable for this purpose as well has accepted AES. As
always, implementors and users should check current literature to
ensure that no recent vulnerabilities have been found in ciphers
used within products. Implementors should also check to see which
ciphers are considered to be relatively stronger than others and
should recommend their use to users over relatively weaker
ciphers. It would be considered good form for an implementation
to politely and unobtrusively notify a user that a stronger cipher
is available and should be used when a weaker one is actively
chosen.
The "none" cipher is provided for debugging and SHOULD NOT be used
except for that purpose. It's cryptographic properties are
sufficiently described in RFC 2410, which will show that its use
does not meet the intent of this protocol.
The relative merits of these and other ciphers may also be found
in current literature. Two references that may provide
information on the subject are [SCHNEIER] and
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[KAUFMAN,PERLMAN,SPECINER]. Both of these describe the CBC mode
of operation of certain ciphers and the weakness of this scheme.
Essentially, this mode is theoretically vulnerable to chosen
cipher-text attacks because of the high predictability of the
start of packet sequence. However, this attack is still deemed
difficult and not considered fully practicable especially if
relatively longer block sizes are used.
Additionally, another CBC mode attack may be mitigated through the
insertion of packets containing SSH_MSG_IGNORE. Without this
technique, a specific attack may be successful. For this attack
(commonly known as the Rogaway attack
[ROGAWAY],[DAI],[BELLARE,KOHNO,NAMPREMPRE]) to work, the attacker
would need to know the IV of the next block that is going to be
encrypted. In CBC mode that is the output of the encryption of
the previous block. If the attacker does not have any way to see
the packet yet (i.e it is in the internal buffers of the ssh
implementation or even in the kernel) then this attack will not
work. If the last packet has been sent out to the network (i.e
the attacker has access to it) then he can use the attack.
In the optimal case an implementor would need to add an extra
packet only if the packet has been sent out onto the network and
there are no other packets waiting for transmission. Implementors
may wish to check to see if there are any unsent packets awaiting
transmission, but unfortunately it is not normally easy to obtain
this information from the kernel or buffers. If there are not,
then a packet containing SSH_MSG_IGNORE SHOULD be sent. If a new
packet is added to the stream every time the attacker knows the IV
that is supposed to be used for the next packet, then the attacker
will not be able to guess the correct IV, thus the attack will
never be successfull.
As an example, consider the following case:
Client Server
------ ------
TCP(seq=x, len=500) ->
contains Record 1
[500 ms passes, no ACK]
TCP(seq=x, len=1000) ->
contains Records 1,2
ACK
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1. The Nagle algorithm + TCP retransmits mean that the two
records get coalesced into a single TCP segment
2. Record 2 is *not* at the beginning of the TCP segment and
never will be, since it gets ACKed.
3. Yet, the attack is possible because Record 1 has already been
seen.
As this example indicates, it's totally unsafe to use the
existence of unflushed data in the TCP buffers proper as a guide
to whether you need an empty packet, since when you do the second
write(), the buffers will contain the un-ACKed Record 1.
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On the other hand, it's perfectly safe to have the following
situation:
Client Server
------ ------
TCP(seq=x, len=500) ->
contains SSH_MSG_IGNORE
TCP(seq=y, len=500) ->
contains Data
Provided that the IV for second SSH Record is fixed after the data for
the Data packet is determined -i.e. you do:
read from user
encrypt null packet
encrypt data packet
8.2.2 Data Integrity
This protocol does allow the Data Integrity mechanism to be
disabled. Implementors SHOULD be wary of exposing this feature
for any purpose other than debugging. Users and administrators
SHOULD be explicitly warned anytime the "none" MAC is enabled.
So long as the "none" MAC is not used, this protocol provides data
integrity.
Because MACs use a 32 bit sequence number, they might start to
leak information after 2**32 packets have been sent. However,
following the rekeying recommendations should prevent this attack.
The transport protocol [1] recommends rekeying after one gigabyte
of data, and the smallest possible packet is 16 bytes. Therefore,
rekeying SHOULD happen after 2**28 packets at the very most.
8.2.3 Replay
The use of a MAC other than 'none' provides integrity and
authentication. In addition, the transport protocol provides a
unique session identifier (bound in part to pseudo-random data
that is part of the algorithm and key exchange process) that can
be used by higher level protocols to bind data to a given session
and prevent replay of data from prior sessions. For example, the
authentication protocol uses this to prevent replay of signatures
from previous sessions. Because public key authentication
exchanges are cryptographically bound to the session (i.e., to the
initial key exchange) they cannot be successfully replayed in
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other sessions. Note that the session ID can be made public
without harming the security of the protocol.
If two session happen to have the same session ID [hash of key
exchanges] then packets from one can be replayed against the
other. It must be stressed that the chances of such an occurrence
are, needless to say, minimal when using modern cryptographic
methods. This is all the more so true when specifying larger hash
function outputs and DH parameters.
Replay detection using monotonically increasing sequence numbers
as input to the MAC, or HMAC in some cases, is described in RFC
2085 [2085], RFC 2246 [2246], RFC 2743 [2743], RFC 1964 [1964],
RFC 2025 [2025], and RFC 1510 [1510]. The underlying construct is
discussed in RFC 2104 [2104]. Essentially a different sequence
number in each packet ensures that at least this one input to the
MAC function will be unique and will provide a nonrecurring MAC
output that is not predictable to an attacker. If the session
stays active long enough, however, this sequence number will wrap.
This event may provide an attacker an opportunity to replay a
previously recorded packet with an identical sequence number but
only if the peers have not rekeyed since the transmission of the
first packet with that sequence number. If the peers have
rekeyed, then the replay will be detected as the MAC check will
fail. For this reason, it must be emphasized that peers MUST
rekey before a wrap of the sequence numbers. Naturally, if an
attacker does attempt to replay a captured packet before the peers
have rekeyed, then the receiver of the duplicate packet will not
be able to validate the MAC and it will be discarded. The reason
that the MAC will fail is because the receiver will formulate a
MAC based upon the packet contents, the shared secret, and the
expected sequence number. Since the replayed packet will not be
using that expected sequence number (the sequence number of the
replayed packet will have already been passed by the receiver)
then the calculated MAC will not match the MAC received with the
packet.
8.2.4 Man-in-the-middle
This protocol makes no assumptions nor provisions for an
infrastructure or means for distributing the public keys of hosts.
It is expected that this protocol will sometimes be used without
first verifying the association between the server host key and
the server host name. Such usage is vulnerable to man-in-the-
middle attacks. This section describes this and encourages
administrators and users to understand the importance of verifying
this association before any session is initiated.
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There are three cases of man-in-the-middle attacks to consider.
The first is where an attacker places a device between the client
and the server before the session is initiated. In this case, the
attack device is trying to mimic the legitimate server and will
offer its public key to the client when the client initiates a
session. If it were to offer the public key of the server, then
it would not be able to decrypt or sign the transmissions between
the legitimate server and the client unless it also had access to
the private-key of the host. The attack device will also,
simultaneously to this, initiate a session to the legitimate
server masquerading itself as the client. If the public key of
the server had been securely distributed to the client prior to
that session initiation, the key offered to the client by the
attack device will not match the key stored on the client. In
that case, the user SHOULD be given a warning that the offered
host key does not match the host key cached on the client. As
described in Section 3.1 of [ARCH], the user may be free to accept
the new key and continue the session. It is RECOMMENDED that the
warning provide sufficient information to the user of the client
device so they may make an informed decision. If the user chooses
to continue the session with the stored public-key of the server
(not the public-key offered at the start of the session), then the
session specific data between the attacker and server will be
different between the client-to-attacker session and the attacker-
to-server sessions due to the randomness discussed above. From
this, the attacker will not be able to make this attack work since
the attacker will not be able to correctly sign packets containing
this session specific data from the server since he does not have
the private key of that server.
The second case that should be considered is similar to the first
case in that it also happens at the time of connection but this
case points out the need for the secure distribution of server
public keys. If the server public keys are not securely
distributed then the client cannot know if it is talking to the
intended server. An attacker may use social engineering
techniques to pass off server keys to unsuspecting users and may
then place a man-in-the-middle attack device between the
legitimate server and the clients. If this is allowed to happen
then the clients will form client-to-attacker sessions and the
attacker will form attacker-to-server sessions and will be able to
monitor and manipulate all of the traffic between the clients and
the legitimate servers. Server administrators are encouraged to
make host key fingerprints available for checking by some means
whose security does not rely on the integrity of the actual host
keys. Possible mechanisms are discussed in Section 3.1 of [SSH-
ARCH] and may also include secured Web pages, physical pieces of
paper, etc. Implementors SHOULD provide recommendations on how
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best to do this with their implementation. Because the protocol
is extensible, future extensions to the protocol may provide
better mechanisms for dealing with the need to know the server's
host key before connecting. For example, making the host key
fingerprint available through a secure DNS lookup, or using
kerberos over gssapi during key exchange to authenticate the
server are possibilities.
In the third man-in-the-middle case, attackers may attempt to
manipulate packets in transit between peers after the session has
been established. As described in the Replay part of this
section, a successful attack of this nature is very improbable.
As in the Replay section, this reasoning does assume that the MAC
is secure and that it is infeasible to construct inputs to a MAC
algorithm to give a known output. This is discussed in much
greater detail in Section 6 of RFC 2104. If the MAC algorithm has
a vulnerability or is weak enough, then the attacker may be able
to specify certain inputs to yield a known MAC. With that they
may be able to alter the contents of a packet in transit.
Alternatively the attacker may be able to exploit the algorithm
vulnerability or weakness to find the shared secret by reviewing
the MACs from captured packets. In either of those cases, an
attacker could construct a packet or packets that could be
inserted into an SSH stream. To prevent that, implementors are
encouraged to utilize commonly accepted MAC algorithms and
administrators are encouraged to watch current literature and
discussions of cryptography to ensure that they are not using a
MAC algorithm that has a recently found vulnerability or weakness.
In summary, the use of this protocol without a reliable
association of the binding between a host and its host keys is
inherently insecure and is NOT RECOMMENDED. It may however be
necessary in non-security critical environments, and will still
provide protection against passive attacks. Implementors of
protocols and applications running on top of this protocol should
keep this possibility in mind.
8.2.5 Denial-of-service
This protocol is designed to be used over a reliable transport.
If transmission errors or message manipulation occur, the
connection is closed. The connection SHOULD be re-established if
this occurs. Denial of service attacks of this type ("wire
cutter") are almost impossible to avoid.
In addition, this protocol is vulnerable to Denial of Service
attacks because an attacker can force the server to go through the
CPU and memory intensive tasks of connection setup and key
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exchange without authenticating. Implementors SHOULD provide
features that make this more difficult. For example, only
allowing connections from a subset of IPs known to have valid
users.
8.2.6 Covert Channels
The protocol was not designed to eliminate covert channels. For
example, the padding, SSH_MSG_IGNORE messages, and several other
places in the protocol can be used to pass covert information, and
the recipient has no reliable way to verify whether such
information is being sent.
8.2.7 Forward Secrecy
It should be noted that the Diffie-Hellman key exchanges may
provide perfect forward secrecy (PFS). PFS is essentially defined
as the cryptographic property of a key-establishment protocol in
which the compromise of a session key or long-term private key
after a given session does not cause the compromise of any earlier
session. [ANSI T1.523-2001] SSHv2 sessions resulting from a key
exchange using diffie-hellman-group1-sha1 are secure even if
private keying/authentication material is later revealed, but not
if the session keys are revealed. So, given this definition of
PFS, SSHv2 does have PFS. It is hoped that all other key exchange
mechanisms proposed and used in the future will also provide PFS.
This property is not commuted to any of the applications or
protocols using SSH as a transport however. The transport layer
of SSH provides confidentiality for password authentication and
other methods that rely on secret data.
Of course, if the DH private parameters for the client and server
are revealed then the session key is revealed, but these items can
be thrown away after the key exchange completes. It's worth
pointing out that these items should not be allowed to end up on
swap space and that they should be erased from memory as soon as
the key exchange completes.
8.3 Authentication Protocol
The purpose of this protocol is to perform client user
authentication. It assumes that this run over a secure transport
layer protocol, which has already authenticated the server
machine, established an encrypted communications channel, and
computed a unique session identifier for this session.
Several authentication methods with different security
characteristics are allowed. It is up to the server's local
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policy to decide which methods (or combinations of methods) it is
willing to accept for each user. Authentication is no stronger
than the weakest combination allowed.
The server may go into a "sleep" period after repeated
unsuccessful authentication attempts to make key search more
difficult for attackers. Care should be taken so that this
doesn't become a self-denial of service vector.
8.3.1 Weak Transport
If the transport layer does not provide confidentiality,
authentication methods that rely on secret data SHOULD be
disabled. If it does not provide strong integrity protection,
requests to change authentication data (e.g. a password change)
SHOULD be disabled to prevent an attacker from modifying the
ciphertext without being noticed, or rendering the new
authentication data unusable (denial of service).
The assumption as stated above that the Authentication Protocol
only run over a secure transport that has previously authenticated
the server is very important to note. People deploying SSH are
reminded of the consequences of man-in-the-middle attacks if the
client does not have a very strong a priori association of the
server with the host key of that server. Specifically for the
case of the Authentication Protocol the client may form a session
to a man-in-the-middle attack device and divulge user credentials
such as their username and password. Even in the cases of
authentication where no user credentials are divulged, an attacker
may still gain information they shouldn't have by capturing key-
strokes in much the same way that a honeypot works.
8.3.2 Debug messages
Special care should be taken when designing debug messages. These
messages may reveal surprising amounts of information about the
host if not properly designed. Debug messages can be disabled
(during user authentication phase) if high security is required.
Administrators of host machines should make all attempts to
compartmentalize all event notification messages and protect them
from unwarranted observation. Developers should be aware of the
sensitive nature of some of the normal event messages and debug
messages and may want to provide guidance to administrators on
ways to keep this information away from unauthorized people.
Developers should consider minimizing the amount of sensitive
information obtainable by users during the authentication phase in
accordance with the local policies. For this reason, it is
RECOMMENDED that debug messages be initially disabled at the time
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of deployment and require an active decision by an administrator
to allow them to be enabled. It is also RECOMMENDED that a
message expressing this concern be presented to the administrator
of a system when the action is taken to enable debugging messages.
8.3.3 Local security policy
Implementer MUST ensure that the credentials provided validate the
professed user and also MUST ensure that the local policy of the
server permits the user the access requested. In particular,
because of the flexible nature of the SSH connection protocol, it
may not be possible to determine the local security policy, if
any, that should apply at the time of authentication because the
kind of service being requested is not clear at that instant. For
example, local policy might allow a user to access files on the
server, but not start an interactive shell. However, during the
authentication protocol, it is not known whether the user will be
accessing files or attempting to use an interactive shell, or even
both. In any event, where local security policy for the server
host exists, it MUST be applied and enforced correctly.
Implementors are encouraged to provide a default local policy and
make its parameters known to administrators and users. At the
discretion of the implementors, this default policy may be along
the lines of 'anything goes' where there are no restrictions
placed upon users, or it may be along the lines of 'excessively
restrictive' in which case the administrators will have to
actively make changes to this policy to meet their needs.
Alternatively, it may be some attempt at providing something
practical and immediately useful to the administrators of the
system so they don't have to put in much effort to get SSH
working. Whatever choice is made MUST be applied and enforced as
required above.
8.3.4 Public key authentication
The use of public-key authentication assumes that the client host
has not been compromised.
This risk can be mitigated by the use of passphrases on private
keys; however, this is not an enforceable policy. The use of
smartcards, or other technology to make passphrases an enforceable
policy is suggested.
The server could require both password and public-key
authentication, however, this requires the client to expose its
password to the server (see section on password authentication
below.)
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8.3.5 Password authentication
The password mechanism as specified in the authentication protocol
assumes that the server has not been compromised. If the server
has been compromised, using password authentication will reveal a
valid username / password combination to the attacker, which may
lead to further compromises.
This vulnerability can be mitigated by using an alternative form
of authentication. For example, public-key authentication makes
no assumptions about security on the server.
8.3.6 Host based authentication
Host based authentication assumes that the client has not been
compromised. There are no mitigating strategies, other than to
use host based authentication in combination with another
authentication method.
8.4 Connection protocol
8.4.1 End point security
End point security is assumed by the connection protocol. If the
server has been compromised, any terminal sessions, port
forwarding, or systems accessed on the host are compromised.
There are no mitigating factors for this.
If the client end point has been compromised, and the server fails
to stop the attacker at the authentication protocol, all services
exposed (either as subsystems or through forwarding) will be
vulnerable to attack. Implementors SHOULD provide mechanisms for
administrators to control which services are exposed to limit the
vulnerability of other services.
These controls might include controlling which machines and ports
can be target in 'port-forwarding' operations, which users are
allowed to use interactive shell facilities, or which users are
allowed to use exposed subsystems.
8.4.2 Proxy forwarding
The SSH connection protocol allows for proxy forwarding of other
protocols such as SNMP, POP3, and HTTP. This may be a concern for
network administrators who wish to control the access of certain
applications by users located outside of their physical location.
Essentially, the forwarding of these protocols may violate site
specific security policies as they may be undetectably tunneled
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through a firewall. Implementors SHOULD provide an administrative
mechanism to control the proxy forwarding functionality so that
site specific security policies may be upheld.
In addition, a reverse proxy forwarding functionality is
available, which again can be used to bypass firewall controls.
As indicated above, end-point security is assumed during proxy
forwarding operations. Failure of end-point security will
compromise all data passed over proxy forwarding.
8.4.3 X11 forwarding
Another form of proxy forwarding provided by the ssh connection
protocol is the forwarding of the X11 protocol. If end-point
security has been compromised, X11 forwarding may allow attacks
against the X11 server. Users and administrators should, as a
matter of course, use appropriate X11 security mechanisms to
prevent unauthorized use of the X11 server. Implementors,
administrators and users who wish to further explore the security
mechanisms of X11 are invited to read [SCHEIFLER] and analyze
previously reported problems with the interactions between SSH
forwarding and X11 in CERT vulnerabilities VU#363181 and VU#118892
[CERT].
X11 display forwarding with SSH, by itself, is not sufficient to
correct well known problems with X11 security [VENEMA]. However,
X11 display forwarding in SSHv2 (or other, secure protocols),
combined with actual and pseudo-displays which accept connections
only over local IPC mechanisms authorized by permissions or ACLs,
does correct many X11 security problems as long as the "none" MAC
is not used. It is RECOMMENDED that X11 display implementations
default to allowing display opens only over local IPC. It is
RECOMMENDED that SSHv2 server implementations that support X11
forwarding default to allowing display opens only over local IPC.
On single-user systems it might be reasonable to default to
allowing local display opens over TCP/IP.
Implementors of the X11 forwarding protocol SHOULD implement the
magic cookie access checking spoofing mechanism as described in
[ssh-connect] as an additional mechanism to prevent unauthorized
use of the proxy.
9. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described
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in this document or the extent to which any license under such
rights might or might not be available; neither does it represent
that it has made any effort to identify any such rights.
Information on the IETF's procedures with respect to rights in
standards-track and standards-related documentation can be found
in BCP-11. Copies of claims of rights made available for
publication 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
Secretariat.
The IETF has been notified of intellectual property rights claimed
in regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
10. Additional Information
The current document editor is: Darren.Moffat@Sun.COM. Comments
on this internet draft should be sent to the IETF SECSH working
group, details at: http://ietf.org/html.charters/secsh-
charter.html
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[FIPS-197] National Institue of Standards and
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[SCHEIFLER] Scheifler, R., "X Window System : The
Complete Reference to Xlib, X
Protocol, Icccm, Xlfd, 3rd edition.",
Digital Press ISBN 1555580882,
Feburary 1992.
[RFC0854] Postel, J. and J. Reynolds, "Telnet
Protocol Specification", STD 8, RFC
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Internet-Draft SSH Protocol Architecture July 2003
854, May 1983.
[RFC0894] Hornig, C., "Standard for the
transmission of IP datagrams over
Ethernet networks", STD 41, RFC 894,
Apr 1984.
[RFC1034] Mockapetris, P., "Domain names -
concepts and facilities", STD 13, RFC
1034, Nov 1987.
[RFC1134] Perkins, D., "Point-to-Point Protocol:
A proposal for multi-protocol
transmission of datagrams over Point-
to-Point links", RFC 1134, Nov 1989.
[RFC1282] Kantor, B., "BSD Rlogin", RFC 1282,
December 1991.
[RFC1510] Kohl, J. and C. Neuman, "The Kerberos
Network Authentication Service (V5)",
RFC 1510, September 1993.
[RFC1700] Reynolds, J. and J. Postel, "Assigned
Numbers", STD 2, RFC 1700, October
1994.
[RFC1750] Eastlake, D., Crocker, S. and J.
Schiller, "Randomness Recommendations
for Security", RFC 1750, December
1994.
[RFC1766] Alvestrand, H., "Tags for the
Identification of Languages", RFC
1766, March 1995.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-
API Mechanism", RFC 1964, June 1996.
[RFC2025] Adams, C., "The Simple Public-Key GSS-
API Mechanism (SPKM)", RFC 2025,
October 1996.
[RFC2085] Oehler, M. and R. Glenn, "HMAC-MD5 IP
Authentication with Replay
Prevention", RFC 2085, February 1997.
[RFC2104] Krawczyk, H., Bellare, M. and R.
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Internet-Draft SSH Protocol Architecture July 2003
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.
[RFC2246] Dierks, T. and C. Allen, "The TLS
Protocol Version 1.0", RFC 2246,
January 1999.
[RFC2279] Yergeau, F., "UTF-8, a transformation
format of ISO 10646", RFC 2279,
January 1998.
[RFC2410] Glenn, R. and S. Kent, "The NULL
Encryption Algorithm and Its Use With
IPsec", RFC 2410, November 1998.
[RFC2434] Narten, T. and H. Alvestrand,
"Guidelines for Writing an IANA
Considerations Section in RFCs", BCP
26, RFC 2434, October 1998.
[RFC2743] Linn, J., "Generic Security Service
Application Program Interface Version
2, Update 1", RFC 2743, January 2000.
[SSH-ARCH] Ylonen, T., "SSH Protocol
Architecture", I-D draft-ietf-
architecture-14.txt, July 2003.
[SSH-TRANS] Ylonen, T., "SSH Transport Layer
Protocol", I-D draft-ietf-transport-
16.txt, July 2003.
[SSH-USERAUTH] Ylonen, T., "SSH Authentication
Protocol", I-D draft-ietf-userauth-
17.txt, July 2003.
[SSH-CONNECT] Ylonen, T., "SSH Connection Protocol",
I-D draft-ietf-connect-17.txt, July
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[SSH-NUMBERS] Lehtinen, S. and D. Moffat, "SSH
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July 2003.
[SCHNEIER] Schneier, B., "Applied Cryptography
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[VENEMA] Venema, W., "Murphy's Law and Computer
Security", Proceedings of 6th USENIX
Security Symposium, San Jose CA
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, July 1996.
[ROGAWAY] Rogaway, P., "Problems with Proposed
IP Cryptography", Unpublished paper
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, 1996.
[DAI] Dai, W., "An attack against SSH2
protocol", Email to the SECSH Working
Group ietf-ssh@netbsd.org
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archive/secsh/2002-02.mail, Feb 2002.
[BELLARE,KOHNO,NAMPREMPRE] Bellaire, M., Kohno, T. and C.
Namprempre, "Authenticated Encryption
in SSH: Fixing the SSH Binary Packet
Protocol", , Sept 2002.
Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: ylo@ssh.com
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Internet-Draft SSH Protocol Architecture July 2003
Tero Kivinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: kivinen@ssh.com
Markku-Juhani O. Saarinen
University of Jyvaskyla
Timo J. Rinne
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: tri@ssh.com
Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
HELSINKI FIN-00100
Finland
EMail: sjl@ssh.com
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Ylonen, et. al. Expires January 12, 2004 [Page 31]
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