draft-ietf-secsh-architecture-05.txt   draft-ietf-secsh-architecture-06.txt 
Network Working Group T. Ylonen Network Working Group T. Ylonen
INTERNET-DRAFT T. Kivinen INTERNET-DRAFT T. Kivinen
draft-ietf-secsh-architecture-05.txt M. Saarinen draft-ietf-secsh-architecture-06.txt M. Saarinen
Expires in six months T. Rinne Expires in six months T. Rinne
S. Lehtinen S. Lehtinen
SSH Communications Security SSH Communications Security
11 May 2000 21 Nov, 2000
SSH Protocol Architecture SSH Protocol Architecture
Status of This memo Status of This memo
This document is an Internet-Draft and is in full conformance This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026. with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
skipping to change at page 1, line 38 skipping to change at page 1, line 38
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
Abstract Abstract
SSH is a protocol for secure remote login and other secure network ser- SSH is a protocol for secure remote login and other secure network ser-
vices over an insecure network. This document describes the architecture vices over an insecure network. This document describes the architecture
of the SSH protocol, and the notation and terminology used in SSH proto- of the SSH protocol, as well as the notation and terminology used in SSH
col documents. It also discusses the SSH algorithm naming system that protocol documents. It also discusses the SSH algorithm naming system
allows local extensions. The SSH protocol consists of three major com- that allows local extensions. The SSH protocol consists of three major
ponents: Transport layer protocol provides server authentication, confi- components: The Transport Layer Protocol provides server authentication,
dentiality, and integrity with perfect forward secrecy. User authentica- confidentiality, and integrity with perfect forward secrecy. The User
tion protocol authenticates the client to the server. Connection proto- Authentication Protocol authenticates the client to the server. The Con-
col multiplexes the encrypted tunnel into several logical channels. nection Protocol multiplexes the encrypted tunnel into several logical
Details of these protocols are described in separate documents. channels. Details of these protocols are described in separate docu-
ments.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Specification of Requirements . . . . . . . . . . . . . . . . . 2 2. Specification of Requirements . . . . . . . . . . . . . . . . . 2
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Extensibility . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Extensibility . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 4 3.3. Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Security Properties . . . . . . . . . . . . . . . . . . . . 5 3.4. Security Properties . . . . . . . . . . . . . . . . . . . . 5
3.5. Packet Size and Overhead . . . . . . . . . . . . . . . . . . 5 3.5. Packet Size and Overhead . . . . . . . . . . . . . . . . . . 5
3.6. Localization and Character Set Support . . . . . . . . . . . 6 3.6. Localization and Character Set Support . . . . . . . . . . . 6
4. Data Type Representations Used in the SSH Protocols . . . . . . 7 4. Data Type Representations Used in the SSH Protocols . . . . . . 7
4.1. Encoding of Network Addresses . . . . . . . . . . . . . . . 8
5. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Message Numbers . . . . . . . . . . . . . . . . . . . . . . . . 9 6. Message Numbers . . . . . . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 9
9. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction 1. Introduction
SSH is a protocol for secure remote login and other secure network SSH is a protocol for secure remote login and other secure network
services over an insecure network. It consists of three major services over an insecure network. It consists of three major
components: components:
o Transport layer protocol [SSH-TRANS] provides server authentication, o The Transport Layer Protocol [SSH-TRANS] provides server
confidentiality, and integrity. It may optionally also provide authentication, confidentiality, and integrity. It may optionally
compression. The transport layer will typically be run over a TCP/IP also provide compression. The transport layer will typically be run
connection, but might also be used on top of any other reliable data over a TCP/IP connection, but might also be used on top of any other
stream. reliable data stream.
o User authentication protocol [SSH-USERAUTH] authenticates the client- o The User Authentication Protocol [SSH-USERAUTH] authenticates the
side user to the server. It runs over the transport layer protocol. client-side user to the server. It runs over the transport layer
protocol.
o Connection protocol [SSH-CONN] multiplexes the encrypted tunnel into o The Connection Protocol [SSH-CONN] multiplexes the encrypted tunnel
several logical channels. It runs over the user authentication into several logical channels. It runs over the user authentication
protocol. protocol.
The client sends a service request once a secure transport layer The client sends a service request once a secure transport layer
connection has been established. A second service request is sent after connection has been established. A second service request is sent after
user authentication is complete. This allows new protocols to be defined user authentication is complete. This allows new protocols to be defined
and coexist with the protocols listed above. and coexist with the protocols listed above.
The connection protocol provides channels that can be used for a wide The connection protocol provides channels that can be used for a wide
range of purposes. Standard methods are provided for setting up secure range of purposes. Standard methods are provided for setting up secure
interactive shell sessions and for forwarding ("tunneling") arbitrary interactive shell sessions and for forwarding ("tunneling") arbitrary
TCP/IP ports and X11 connections. TCP/IP ports and X11 connections.
2. Specification of Requirements 2. Specification of Requirements
All of the documents related to the SSH protocols shall use the keywords All documents related to the SSH protocols shall use the keywords
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
NOT", "RECOMMENDED, "MAY", and "OPTIONAL" to describe requirements. NOT", "RECOMMENDED, "MAY", and "OPTIONAL" to describe requirements.
They are to be interpreted as described in [RFC-2119]. They are to be interpreted as described in [RFC-2119].
3. Architecture 3. Architecture
3.1. Host Keys 3.1. Host Keys
Each server host MUST have a host key. Hosts MAY have multiple host Each server host MUST have a host key. Hosts MAY have multiple host
keys using multiple different algorithms. Multiple hosts MAY share the keys using multiple different algorithms. Multiple hosts MAY share the
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The server host key is used during key exchange to verify that the 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, 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. the client must have a priori knowledge of the server's public host key.
Two different trust models can be used: Two different trust models can be used:
o The client has a local database that associates each host name (as o The client has a local database that associates each host name (as
typed by the user) with the corresponding public host key. This typed by the user) with the corresponding public host key. This
method requires no centrally administered infrastructure, and no method requires no centrally administered infrastructure, and no
third-party coordination. The downside is that the database of name- third-party coordination. The downside is that the database of name-
key associations may become burdensome to maintain. to-key associations may become burdensome to maintain.
o The host name - key association is certified by some trusted o The host name-to-key association is certified by some trusted
certification authority. The client only knows the CA root key, and certification authority. The client only knows the CA root key, and
can verify the validity of all host keys certified by accepted CAs. can verify the validity of all host keys certified by accepted CAs.
The second alternative eases the maintenance problem, since ideally The second alternative eases the maintenance problem, since ideally
only a single CA key needs to be securely stored on the client. On 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 the other hand, each host key must be appropriately certified by a
central authority before authorization is possible. Also, a lot of central authority before authorization is possible. Also, a lot of
trust is placed on the central infrastructure. trust is placed on the central infrastructure.
The protocol provides the option that the server name - host key The protocol provides the option that the server name - host key
association is not checked when connecting the host for the first time. association is not checked when connecting to the host for the first
This allows communication without prior communication of host keys or time. This allows communication without prior communication of host keys
certification. The connection still provides protection against passive or certification. The connection still provides protection against
listening; however, it becomes vulnerable to active man-in-the-middle passive listening; however, it becomes vulnerable to active man-in-the-
attacks. Implementations SHOULD NOT normally allow such connections by middle attacks. Implementations SHOULD NOT normally allow such
default, as they pose a potential security problem. However, as there is connections by default, as they pose a potential security problem.
no widely deployed key infrastructure available on the Internet yet, However, as there is no widely deployed key infrastructure available on
this option makes the protocol much more usable during the transition the Internet yet, this option makes the protocol much more usable during
time until such an infrastructure emerges, while still providing a much the transition time until such an infrastructure emerges, while still
higher level of security than that offered by older solutions (e.g. providing a much higher level of security than that offered by older
telnet [RFC-854] and rlogin [RFC-1282]). solutions (e.g. telnet [RFC-854] and rlogin [RFC-1282]).
Implementations SHOULD try to make best effort to check host keys. An Implementations SHOULD try to make the best effort to check host keys.
example of a possible strategy is to only accept a host key without 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 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 database, and compare against that key on all future connections to that
host. host.
Implementations MAY provide additional methods for verifying the Implementations MAY provide additional methods for verifying the
correctness of host keys, e.g. a hexadecimal fingerprint derived from correctness of host keys, e.g. a hexadecimal fingerprint derived from
the SHA-1 hash of the public key. Such fingerprints can easily be the SHA-1 hash of the public key. Such fingerprints can easily be
verified by using telephone or other external communication channels. verified by using telephone or other external communication channels.
All implementations SHOULD provide an option to not accept host keys All implementations SHOULD provide an option to not accept host keys
that cannot be verified. that cannot be verified.
We believe that ease of use is critical to end-user acceptance of 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 security solutions, and no improvement in security is gained if the new
solutions are not used. Thus, providing the option not to check the solutions are not used. Thus, providing the option not to check the
the server host key is believed to improve overall security of the server host key is believed to improve the overall security of the
Internet, even though it reduces the security of the protocol in Internet, even though it reduces the security of the protocol in
configurations where it is allowed. configurations where it is allowed.
3.2. Extensibility 3.2. Extensibility
We believe that the protocol will evolve over time, and some We believe that the protocol will evolve over time, and some
organizations will want to use their own encryption, authentication organizations will want to use their own encryption, authentication
and/or key exchange methods. Central registration of all extensions is and/or key exchange methods. Central registration of all extensions is
cumbersome, especially for experimental or classified features. On the cumbersome, especially for experimental or classified features. On the
other hand, having no central registration leads to conflicts in method other hand, having no central registration leads to conflicts in method
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each direction. The policy MUST specify which is the preferred each direction. The policy MUST specify which is the preferred
algorithm (e.g. the first algorithm listed in each category). algorithm (e.g. the first algorithm listed in each category).
o Public key algorithms and key exchange method to be used for host o Public key algorithms and key exchange method to be used for host
authentication. The existence of trusted host keys for different authentication. The existence of trusted host keys for different
public key algorithms also affects this choice. public key algorithms also affects this choice.
o The authentication methods that are to be required by the server for o The authentication methods that are to be required by the server for
each user. The server's policy MAY require multiple authentication each user. The server's policy MAY require multiple authentication
for some or all users. The required algorithms MAY depend on the for some or all users. The required algorithms MAY depend on the
location from where the user is trying to log in from. location where the user is trying to log in from.
o The operations that the user is allowed to perform using the o The operations that the user is allowed to perform using the
connection protocol. Some issues are related to security; for connection protocol. Some issues are related to security; for
example, the policy SHOULD NOT allow the server to start sessions or example, the policy SHOULD NOT allow the server to start sessions or
run commands on the client machine, and MUST NOT allow connections to run commands on the client machine, and MUST NOT allow connections to
the authentication agent unless forwarding it has been requested. the authentication agent unless forwarding such connections has been
Other issues, such as which TCP/IP ports can be forwarded and by requested. Other issues, such as which TCP/IP ports can be forwarded
whom, are clear local policy issues. Many of these issues may and by whom, are clearly issues of local policy. Many of these issues
involve traversing or bypassing firewalls, and are interrelated with may involve traversing or bypassing firewalls, and are interrelated
the local security policy. with the local security policy.
3.4. Security Properties 3.4. Security Properties
The primary goal of the SSH protocols is improved security on the 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 Internet. It attempts to do this in a way that is easy to deploy, even
at the cost of absolute security. at the cost of absolute security.
o All encryption, integrity, and public key algorithms used are well- o All encryption, integrity, and public key algorithms used are well-
known, well-established algorithms. known, well-established algorithms.
o All algorithms are used with cryptographically sound key sizes that o All algorithms are used with cryptographically sound key sizes that
are believed to provide protection against even the strongest are believed to provide protection against even the strongest
cryptanalytic attacks for decades. cryptanalytic attacks for decades.
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Specific concessions were made to make wide-spread fast deployment Specific concessions were made to make wide-spread fast deployment
easier. The particular case where this comes up is verifying that the easier. The particular case where this comes up is verifying that the
server host key really belongs to the desired host; the protocol allows 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 the verification to be left out (but this is NOT RECOMMENDED). This is
believed to significantly improve usability in the short term, until believed to significantly improve usability in the short term, until
widespread Internet public key infrastructures emerge. widespread Internet public key infrastructures emerge.
3.5. Packet Size and Overhead 3.5. Packet Size and Overhead
Some readers will worry about the increase in packet size due to new 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 headers, padding, and MAC. The minimum packet size is in the order of
bytes (depending on negotiated algorithms). The increase is negligible 28 bytes (depending on negotiated algorithms). The increase is
for large packets, but very significant for one-byte packets (telnet- negligible for large packets, but very significant for one-byte packets
type sessions). There are, however, several factors that make this a (telnet-type sessions). There are, however, several factors that make
non-issue in almost all cases: this a non-issue in almost all cases:
o The minimum size of a TCP/IP header is 32 bytes. Thus, the increase o The minimum size of a TCP/IP header is 32 bytes. Thus, the increase
is actually from 33 to 51 bytes (roughly). is actually from 33 to 51 bytes (roughly).
o The minimum size of the data field of an ethernet packet is 46 bytes 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 [RFC-894]. Thus, the increase is no more than 5 bytes. When Ethernet
ethernet headers are considered, the increase is by less than 10 headers are considered, the increase is less than 10 percent.
percent.
o The total fraction of telnet-type data in the Internet is negligible, o The total fraction of telnet-type data in the Internet is negligible,
even with increased packet sizes. even with increased packet sizes.
The only environment where the packet size increase is likely to have The only environment where the packet size increase is likely to have a
significant effect is PPP [RFC-1134] over slow modem lines (PPP significant effect is PPP [RFC-1134] over slow modem lines (PPP
compresses the TCP/IP headers, emphasizing the increase in packet size). compresses the TCP/IP headers, emphasizing the increase in packet size).
However, with modern modems, the time needed to transfer is on the order However, with modern modems, the time needed to transfer is in the order
of 2ms, which is a lot faster than people can type. of 2 milliseconds, which is a lot faster than people can type.
There are also issues related to the maximum packet size. To minimize There are also issues related to the maximum packet size. To minimize
delays in screen updates, one does not want excessively large packets delays in screen updates, one does not want excessively large packets
for interactive sessions. The maximum packet size is negotiated for interactive sessions. The maximum packet size is negotiated
separately for each channel. separately for each channel.
3.6. Localization and Character Set Support 3.6. Localization and Character Set Support
For the most part, the SSH protocols do not directly pass text that 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 would be displayed to the user. However, there are some places where
such data might be passed. When applicable, the character set for the 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 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 encoding is used [RFC-2044]. When applicable, a field is also provided
provided for a language tag [RFC-1766]. for a language tag [RFC-1766].
One big issue is the character set of the interactive session. There is One big issue is the character set of the interactive session. There is
no clear solution, as different applications may display data in no clear solution, as different applications may display data in
different formats. Different types of terminal emulation may also be different formats. Different types of terminal emulation may also be
employed in the client, and the character set to be used is effectively employed in the client, and the character set to be used is effectively
determined by the terminal emulation. Thus, no place is provided for determined by the terminal emulation. Thus, no place is provided for
specifying the character set or encoding for terminal session data directly specifying the character set or encoding for terminal session
directly. However, the terminal emulation type (e.g. "vt100") is data. However, the terminal emulation type (e.g. "vt100") is
transmitted to the remote site, and it implicitly specifies the transmitted to the remote site, and it implicitly specifies the
character set and encoding. Applications typically use the terminal character set and encoding. Applications typically use the terminal
type to determine what character set they use, or the character set is type to determine what character set they use, or the character set is
determined using some external means. The terminal emulation may also determined using some external means. The terminal emulation may also
allow configuring the default character set. In any case, character set allow configuring the default character set. In any case, the character
for the terminal session is considered primarily a client local issue. set for the terminal session is considered primarily a client local
issue.
Internal names used to identify algorithms or protocols are normally Internal names used to identify algorithms or protocols are normally
never displayed to users, and must be in US-ASCII. never displayed to users, and must be in US-ASCII.
The client and server user names are inherently constrained by what the The client and server user names are inherently constrained by what the
server is prepared to accept. They might, however, occasionally be server is prepared to accept. They might, however, occasionally be
displayed in logs, reports, etc. They MUST be encoded using ISO 10646 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 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. the server to decide how to map user names to accepted user names.
Straight bit-wise binary comparison is RECOMMENDED. Straight bit-wise binary comparison is RECOMMENDED.
For localization purposes, the protocol attempts to minimize the number For localization purposes, the protocol attempts to minimize the number
of textual messages transmitted. When present, such messages typically of textual messages transmitted. When present, such messages typically
relate to errors, debugging information, or some externally configured relate to errors, debugging information, or some externally configured
data. For data that is normally displayed, it SHOULD be possible to data. For data that is normally displayed, it SHOULD be possible to
fetch a localized message instead of the transmitted by using a numeric fetch a localized message instead of the transmitted message by using a
code. The remaining messages SHOULD be configurable. numerical code. The remaining messages SHOULD be configurable.
4. Data Type Representations Used in the SSH Protocols 4. Data Type Representations Used in the SSH Protocols
byte byte
A byte represents an arbitrary 8-bit value (octet) [RFC1700]. A byte represents an arbitrary 8-bit value (octet) [RFC1700].
Fixed length data is sometimes represented as an array of bytes, Fixed length data is sometimes represented as an array of bytes,
written byte[n], where n is the number of bytes in the array. written byte[n], where n is the number of bytes in the array.
boolean boolean
A boolean value is stored as a single byte. The value 0 A boolean value is stored as a single byte. The value 0
represents false, and the value 1 represents true. All non-zero represents false, and the value 1 represents true. All non-zero
values MUST be interpreted as true; however, applications MUST not values MUST be interpreted as true; however, applications MUST NOT
store values other than 0 and 1. store values other than 0 and 1.
uint32 uint32
Represents a 32-bit unsigned integer. Stored as four bytes in the Represents a 32-bit unsigned integer. Stored as four bytes in the
order of decreasing significance (network byte order). order of decreasing significance (network byte order).
For example, the value 699921578 (0x29b7f4aa) is stored as 29 b7 For example, the value 699921578 (0x29b7f4aa) is stored as 29 b7
f4 aa. f4 aa.
string string
Arbitrary length binary string. Strings are allowed to contain Arbitrary length binary string. Strings are allowed to contain
arbitrary binary data, including null characters and 8-bit arbitrary binary data, including null characters and 8-bit
characters. They are stored as a uint32 containing its length characters. They are stored as a uint32 containing its length
(number of bytes that follow) and zero (= empty string) or more (number of bytes that follow) and zero (= empty string) or more
bytes that are the value of the string. Terminating null bytes that are the value of the string. Terminating null
characters are not used. characters are not used.
Strings are also used to store text. In that case, US-ASCII is 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 used for internal names, and ISO-10646 UTF-8 for text that might
be displayed to the user. Terminating null character SHOULD be displayed to the user. The terminating null character SHOULD
normally not be stored in the string. NOT normally be stored in the string.
For example, the US-ASCII string "testing" is represented as 00 00 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 00 07 t e s t i n g. The UTF8 mapping does not alter the encoding
of US-ASCII characters. of US-ASCII characters.
mpint mpint
Represents multiple precision integers in two's complement format, Represents multiple precision integers in two's complement format,
stored as a string, 8 bits per byte, MSB first. Negative numbers 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 have the value 1 as the most significant bit of the first byte of
partition of. If the most significant bit would be set for a the data partition. If the most significant bit would be set for a
positive number, the number MUST be preceded by a zero byte. positive number, the number MUST be preceded by a zero byte.
Unnecessary leading zero or 255 bytes MUST NOT be included. The Unnecessary leading bytes with the value 0 or 255 MUST NOT be
value zero MUST be stored as a string with zero bytes of data. 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 By convention, a number that is used in modular computations in
Z_n SHOULD be represented in the range 0 <= x < n. Z_n SHOULD be represented in the range 0 <= x < n.
Examples: Examples:
value (hex) representation (hex) value (hex) representation (hex)
--------------------------------------------------------------- ---------------------------------------------------------------
0 00 00 00 00 0 00 00 00 00
9a378f9b2e332a7 00 00 00 08 09 a3 78 f9 b2 e3 32 a7 9a378f9b2e332a7 00 00 00 08 09 a3 78 f9 b2 e3 32 a7
80 00 00 00 02 00 80 80 00 00 00 02 00 80
-1234 00 00 00 02 ed cc -1234 00 00 00 02 ed cc
-deadbeef 00 00 00 05 ff 21 52 41 11 -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 5. Algorithm Naming
The SSH protocols refer to particular hash, encryption, integrity, The SSH protocols refer to particular hash, encryption, integrity,
compression, and key exchange algorithms or protocols by names. There compression, and key exchange algorithms or protocols by names. There
are some standard algorithms that all implementations MUST support. are some standard algorithms that all implementations MUST support.
There are also algorithms that are defined in the protocol specification There are also algorithms that are defined in the protocol specification
but are OPTIONAL. Furthermore, it is expected that some organizations but are OPTIONAL. Furthermore, it is expected that some organizations
will want to use their own algorithms. will want to use their own algorithms.
In this protocol, all algorithm identifiers MUST be printable US-ASCII In this protocol, all algorithm identifiers MUST be printable US-ASCII
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o Anyone can define additional algorithms by using names in the format 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 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 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 characters except at-sign and comma. The part following the at-sign
MUST be a valid fully qualified internet domain name [RFC-1034] MUST be a valid fully qualified internet domain name [RFC-1034]
controlled by the person or organization defining the name. It is up controlled by the person or organization defining the name. It is up
to each domain how it manages its local namespace. to each domain how it manages its local namespace.
6. Message Numbers 6. Message Numbers
SSH packets have message numbers in the range 1-255. These numbers have SSH packets have message numbers in the range 1 to 255. These numbers
been allocated as follows: have been allocated as follows:
Transport layer protocol: Transport layer protocol:
1-19 Transport layer generic (e.g. disconnect, ignore, debug, 1 to 19 Transport layer generic (e.g. disconnect, ignore, debug,
etc) etc.)
20-29 Algorithm negotiation 20 to 29 Algorithm negotiation
30-49 Key exchange method specific (numbers can be reused for 30 to 49 Key exchange method specific (numbers can be reused for
different authentication methods) different authentication methods)
User authentication protocol: User authentication protocol:
50-59 User authentication generic 50 to 59 User authentication generic
60-79 User authentication method specific (numbers can be reused 60 to 79 User authentication method specific (numbers can be
for different authentication methods) reused for different authentication methods)
Connection protocol: Connection protocol:
80-89 Connection protocol generic 80 to 89 Connection protocol generic
90-127 Channel related messages 90 to 127 Channel related messages
Reserved for client protocols: Reserved for client protocols:
128-191 Reserved 128 to 191 Reserved
Local extensions: Local extensions:
192-255 Local extensions 192 to 255 Local extensions
7. IANA Considerations
Allocation of the following types of names in the SSH protocols is Allocation of the following types of names in the SSH protocols is
assigned to IANA: assigned to IANA:
o encryption algorithm names, o encryption algorithm names,
o MAC algorithm names, o MAC algorithm names,
o public key algorithm names (public key algorithm also implies o public key algorithm names (public key algorithm also implies
encoding and signature/encryption capability), encoding and signature/encryption capability),
o key exchange method names, and o key exchange method names, and
o protocol (service) names. o protocol (service) names.
The IANA-allocated names MUST be printable US-ASCII strings, and MUST The IANA-allocated names MUST be printable US-ASCII strings, and MUST
NOT contain the characters at-sign ('@'), comma (','), or whitespace or NOT contain the characters at-sign ('@'), comma (','), or whitespace or
control characters (ascii codes 32 or less). Names are case-sensitive, control characters (ASCII codes 32 or less). Names are case-sensitive,
and MUST not be longer than 64 characters. and MUST NOT be longer than 64 characters.
Each category of names listed above has a separate namespace. However, Each category of names listed above has a separate namespace. However,
using the same name in multiple categories SHOULD be avoided to minimize using the same name in multiple categories SHOULD be avoided to minimize
confusion. confusion.
8. Security Considerations 8. Security Considerations
Special care should be taken to ensure that all of the random numbers 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 are of good quality. The random numbers SHOULD be produced with safe
mechanisms discussed in [RFC1750]. mechanisms discussed in [RFC1750].
When displaying text, such as error or debug messages to the user, the When displaying text, such as error or debug messages to the user, the
client software SHOULD replace any control characters (except tab, client software SHOULD replace any control characters (except tab,
carriage return and newline) with safe sequences to avoid attacks by carriage return and newline) with safe sequences to avoid attacks by
sending terminal control characters. sending terminal control characters.
skipping to change at page 11, line 35 skipping to change at page 11, line 25
Wiley & Sons, New York, NY, 1995. Wiley & Sons, New York, NY, 1995.
[SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol", Internet [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol", Internet
Draft, draft-ietf-secsh-transport-07.txt Draft, draft-ietf-secsh-transport-07.txt
[SSH-USERAUTH] Ylonen, T., et al, "SSH Authentication Protocol", [SSH-USERAUTH] Ylonen, T., et al, "SSH Authentication Protocol",
Internet Draft, draft-ietf-secsh-userauth-07.txt Internet Draft, draft-ietf-secsh-userauth-07.txt
[SSH-CONNECT] Ylonen, T., et al, "SSH Connection Protocol", Internet [SSH-CONNECT] Ylonen, T., et al, "SSH Connection Protocol", Internet
Draft, draft-ietf-secsh-connect-07.txt Draft, draft-ietf-secsh-connect-07.txt
11. Authors' Addresses
Tatu Ylonen Tatu Ylonen
SSH Communications Security Corp SSH Communications Security Corp
Fredrikinkatu 42 Fredrikinkatu 42
FIN-00100 HELSINKI FIN-00100 HELSINKI
Finland Finland
E-mail: ylo@ssh.com E-mail: ylo@ssh.com
Tero Kivinen Tero Kivinen
SSH Communications Security Corp SSH Communications Security Corp
 End of changes. 

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