Network Working Group T. Ylonen
INTERNET-DRAFT T. Kivinen
draft-ietf-secsh-architecture-05.txt M. Saarinen
Expires in six months T. Rinne
S. Lehtinen
SSH Communications Security
11 May 2000
SSH Protocol Architecture
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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Abstract
SSH is a protocol for secure remote login and other secure network ser-
vices over an insecure network. This document describes the architecture
of the SSH protocol, and the notation and terminology used in SSH proto-
col documents. It also discusses the SSH algorithm naming system that
allows local extensions. The SSH protocol consists of three major com-
ponents: Transport layer protocol provides server authentication, confi-
dentiality, and integrity with perfect forward secrecy. User authentica-
tion protocol authenticates the client to the server. Connection proto-
col multiplexes the encrypted tunnel into several logical channels.
Details of these protocols are described in separate documents.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Specification of Requirements . . . . . . . . . . . . . . . . . 2
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Extensibility . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Security Properties . . . . . . . . . . . . . . . . . . . . 5
3.5. Packet Size and Overhead . . . . . . . . . . . . . . . . . . 5
3.6. Localization and Character Set Support . . . . . . . . . . . 6
4. Data Type Representations Used in the SSH Protocols . . . . . . 7
4.1. Encoding of Network Addresses . . . . . . . . . . . . . . . 8
5. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Message Numbers . . . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 10
9. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
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 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 User authentication protocol [SSH-USERAUTH] authenticates the client-
side user to the server. It runs over the transport layer protocol.
o Connection protocol [SSH-CONN] 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.
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2. Specification of Requirements
All of the 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 MUST have a host key. Hosts MAY have multiple host
keys using multiple different algorithms. Multiple hosts MAY share the
same host key. Every host 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 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-
key associations may become burdensome to maintain.
o The host name - 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 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 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
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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 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
the server host key is believed to improve 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'' 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:
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o Encryption, integrity, and compression algorithms, separately for
each direction. The policy MUST specify which is the 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 from 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 it has been requested.
Other issues, such as which TCP/IP ports can be forwarded and by
whom, are clear local policy issues. 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 protocols 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 in the order of 28
bytes (depending on negotiated algorithms). The increase is negligible
for large packets, but very significant for one-byte packets (telnet-
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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 by no more than 5 bytes. When
ethernet headers are considered, the increase is by 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
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 on the order
of 2ms, 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-2044]. When applicable, a field is also be
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
specifying the character set or encoding for terminal session data
directly. 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, character set
for the terminal session is considered primarily a client 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
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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 by using a numeric
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) [RFC1700].
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.
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. Terminating null character SHOULD
normally not 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 one in the most significant bit of the first byte of the data
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partition of. If the most significant bit would be set for a
positive number, the number MUST be preceded by a zero byte.
Unnecessary leading zero or 255 bytes 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
4.1. Encoding of Network Addresses
Network addresses are encoded as strings. DNS names MUST NOT be used, as
DNS is an insecure protocol.
If an address contains a colon (':', ascii 58), it is interpreted as an
IPv6 address. The encoding of IPv6 addresses is described in [RFC-1884].
IPv4 addresses are expressed in the standard dot-separated decimal
format (e.g. 127.0.0.1).
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
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 IANA (Internet Assigned Numbers Authority). Examples include
`3des-cbc', `sha-1', `hmac-sha1', and `zlib' (the quotes are not part
of the name). Additional names of this format may be registered with
IANA; see Section ``IANA Considerations''. Names of this format MUST
NOT be used without first registering with IANA. 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.fi". The format of the part
preceding the at sign is not specified; it MUST consist of US-ASCII
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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 manages its local namespace.
6. Message Numbers
SSH packets have message numbers in the range 1-255. These numbers have
been allocated as follows:
Transport layer protocol:
1-19 Transport layer generic (e.g. disconnect, ignore, debug,
etc)
20-29 Algorithm negotiation
30-49 Key exchange method specific (numbers can be reused for
different authentication methods)
User authentication protocol:
50-59 User authentication generic
60-79 User authentication method specific (numbers can be reused
for different authentication methods)
Connection protocol:
80-89 Connection protocol generic
90-127 Channel related messages
Reserved for client protocols:
128-191 Reserved
Local extensions:
192-255 Local extensions
7. IANA Considerations
Allocation of the following types of names in the SSH protocols is
assigned to IANA:
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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The IANA-allocated 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.
Each category of names listed above has a separate namespace. However,
using the same name in multiple categories SHOULD be avoided to minimize
confusion.
8. Security Considerations
Special care should be taken to ensure that all of the random numbers
are of good quality. The random numbers SHOULD be produced with safe
mechanisms discussed in [RFC1750].
When displaying text, such as error or debug messages to the user, the
client software SHOULD replace any control characters (except tab,
carriage return and newline) with safe sequences to avoid attacks by
sending terminal control characters.
Not using MAC or encryption SHOULD be avoided. The user authentication
protocol is subject to man-in-the-middle attacks if the encryption is
disabled. The SSH protocol does not protect against message alteration
if no MAC is used.
9. Trademark Issues
SSH is a registered trademark and Secure Shell is a trademark of SSH
Communications Security Corp. SSH Communications Security Corp permits
the use of these trademarks as the name of this standard and protocol,
and permits their use to describe that a product conforms to this
standard, provided that the following acknowledgement is included where
the trademarks are used: ``SSH is a registered trademark and Secure
Shell is a trademark of SSH Communications Security Corp
(www.ssh.com)''. These trademarks may not be used as part of a product
name or in otherwise confusing manner without prior written permission
of SSH Communications Security Corp.
10. References
[FIPS-186] Federal Information Processing Standards Publication (FIPS
PUB) 186, Digital Signature Standard, 18 May 1994.
[RFC-854] Postel, J. and Reynolds, J., "Telnet Protocol Specification",
May 1983.
[RFC-894] Hornig, C., "A Standard for the Transmission of IP Datagrams
over Ethernet Networks", April 1984.
[RFC-1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
November 1987.
[RFC-1134] Perkins, D., "The Point-to-Point Protocol: A Proposal for
Multi-Protocol Transmission o Datagrams Over Point-to-Point Links",
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 10]
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November 1989.
[RFC-1282] Kantor, B., "BSD Rlogin", December 1991.
[RFC-1700] Reynolds, J. and Postel, J., "Assigned Numbers", October 1994
(also STD 2).
[RFC-1750] Eastlake, D., Crocker, S., and Schiller, J., "Randomness
Recommendations for Security", December 1994.
[RFC-1766] Alvestrand, H., "Tags for the Identification of Languages",
March 1995.
[RFC-2044] Yergeau, F., "UTF-8, a Transformation Format of Unicode and
ISO 10646", October 1996.
[RFC-2119] Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", March 1997
[Schneier] Schneier, B., "Applied Cryptography Second Edition", John
Wiley & Sons, New York, NY, 1995.
[SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol", Internet
Draft, draft-ietf-secsh-transport-07.txt
[SSH-USERAUTH] Ylonen, T., et al, "SSH Authentication Protocol",
Internet Draft, draft-ietf-secsh-userauth-07.txt
[SSH-CONNECT] Ylonen, T., et al, "SSH Connection Protocol", Internet
Draft, draft-ietf-secsh-connect-07.txt
11. Authors' Addresses
Tatu Ylonen
SSH Communications Security Corp
Fredrikinkatu 42
FIN-00100 HELSINKI
Finland
E-mail: ylo@ssh.com
Tero Kivinen
SSH Communications Security Corp
Fredrikinkatu 42
FIN-00100 HELSINKI
Finland
E-mail: kivinen@ssh.com
Markku-Juhani O. Saarinen
University of Jyvaskyla
Timo J. Rinne
SSH Communications Security Corp
Fredrikinkatu 42
T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne and S. Lehtinen [page 11]
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FIN-00100 HELSINKI
Finland
E-mail: tri@ssh.com
Sami Lehtinen
SSH Communications Security Corp
Fredrikinkatu 42
FIN-00100 HELSINKI
Finland
E-mail: sjl@ssh.com
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