draft-ietf-secsh-architecture-15.txt   draft-ietf-secsh-architecture-16.txt 
Network Working Group T. Ylonen Network Working Group T. Ylonen
Internet-Draft SSH Communications Security Corp Internet-Draft SSH Communications Security Corp
Expires: March 31, 2004 D. Moffat, Ed. Expires: December 1, 2004 C. Lonvick, Ed.
Sun Microsystems, Inc Cisco Systems, Inc
Oct 2003 June 2, 2004
SSH Protocol Architecture SSH Protocol Architecture
draft-ietf-secsh-architecture-15.txt draft-ietf-secsh-architecture-16.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
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. This document describes the services over an insecure network. This document describes the
architecture of the SSH protocol, as well as the notation and architecture of the SSH protocol, as well as the notation and
terminology used in SSH protocol documents. It also discusses the SSH terminology used in SSH protocol documents. The SSH protocol
algorithm naming system that allows local extensions. The SSH consists of three major components: The Transport Layer Protocol
protocol consists of three major components: The Transport Layer provides server authentication, confidentiality, and integrity with
Protocol provides server authentication, confidentiality, and perfect forward secrecy. Details of these protocols are described in
integrity with perfect forward secrecy. The User Authentication separate documents.
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 Table of Contents
1. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Specification of Requirements . . . . . . . . . . . . . . . 3 3. Specification of Requirements . . . . . . . . . . . . . . . . 3
4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1 Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1 Host Keys . . . . . . . . . . . . . . . . . . . . . . . . 4
4.2 Extensibility . . . . . . . . . . . . . . . . . . . . . . . 5 4.2 Extensibility . . . . . . . . . . . . . . . . . . . . . . 5
4.3 Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 5 4.3 Policy Issues . . . . . . . . . . . . . . . . . . . . . . 6
4.4 Security Properties . . . . . . . . . . . . . . . . . . . . 6 4.4 Security Properties . . . . . . . . . . . . . . . . . . . 6
4.5 Packet Size and Overhead . . . . . . . . . . . . . . . . . . 6 4.5 Packet Size and Overhead . . . . . . . . . . . . . . . . . 7
4.6 Localization and Character Set Support . . . . . . . . . . . 7 4.6 Localization and Character Set Support . . . . . . . . . . 7
5. Data Type Representations Used in the SSH Protocols . . . . 8 5. Data Type Representations Used in the SSH Protocols . . . . . 8
6. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . 10 6. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . 10
7. Message Numbers . . . . . . . . . . . . . . . . . . . . . . 11 7. Message Numbers . . . . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 11 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . 12 9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9.1 Pseudo-Random Number Generation . . . . . . . . . . . . . . 12 9.1 Pseudo-Random Number Generation . . . . . . . . . . . . . 13
9.2 Transport . . . . . . . . . . . . . . . . . . . . . . . . . 13 9.2 Transport . . . . . . . . . . . . . . . . . . . . . . . . 13
9.2.1 Confidentiality . . . . . . . . . . . . . . . . . . . . . . 13 9.2.1 Confidentiality . . . . . . . . . . . . . . . . . . . 13
9.2.2 Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 16 9.2.2 Data Integrity . . . . . . . . . . . . . . . . . . . . 16
9.2.3 Replay . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 9.2.3 Replay . . . . . . . . . . . . . . . . . . . . . . . . 16
9.2.4 Man-in-the-middle . . . . . . . . . . . . . . . . . . . . . 17 9.2.4 Man-in-the-middle . . . . . . . . . . . . . . . . . . 17
9.2.5 Denial-of-service . . . . . . . . . . . . . . . . . . . . . 19 9.2.5 Denial-of-service . . . . . . . . . . . . . . . . . . 19
9.2.6 Covert Channels . . . . . . . . . . . . . . . . . . . . . . 19 9.2.6 Covert Channels . . . . . . . . . . . . . . . . . . . 20
9.2.7 Forward Secrecy . . . . . . . . . . . . . . . . . . . . . . 20 9.2.7 Forward Secrecy . . . . . . . . . . . . . . . . . . . 20
9.3 Authentication Protocol . . . . . . . . . . . . . . . . . . 20 9.3 Authentication Protocol . . . . . . . . . . . . . . . . . 20
9.3.1 Weak Transport . . . . . . . . . . . . . . . . . . . . . . . 21 9.3.1 Weak Transport . . . . . . . . . . . . . . . . . . . . 21
9.3.2 Debug messages . . . . . . . . . . . . . . . . . . . . . . . 21 9.3.2 Debug Messages . . . . . . . . . . . . . . . . . . . . 21
9.3.3 Local security policy . . . . . . . . . . . . . . . . . . . 21 9.3.3 Local Security Policy . . . . . . . . . . . . . . . . 22
9.3.4 Public key authentication . . . . . . . . . . . . . . . . . 22 9.3.4 Public Key Authentication . . . . . . . . . . . . . . 22
9.3.5 Password authentication . . . . . . . . . . . . . . . . . . 22 9.3.5 Password Authentication . . . . . . . . . . . . . . . 22
9.3.6 Host based authentication . . . . . . . . . . . . . . . . . 23 9.3.6 Host Based Authentication . . . . . . . . . . . . . . 23
9.4 Connection protocol . . . . . . . . . . . . . . . . . . . . 23 9.4 Connection Protocol . . . . . . . . . . . . . . . . . . . 23
9.4.1 End point security . . . . . . . . . . . . . . . . . . . . . 23 9.4.1 End Point Security . . . . . . . . . . . . . . . . . . 23
9.4.2 Proxy forwarding . . . . . . . . . . . . . . . . . . . . . . 23 9.4.2 Proxy Forwarding . . . . . . . . . . . . . . . . . . . 23
9.4.3 X11 forwarding . . . . . . . . . . . . . . . . . . . . . . . 24 9.4.3 X11 Forwarding . . . . . . . . . . . . . . . . . . . . 24
Normative References . . . . . . . . . . . . . . . . . . . . 24 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
Informative References . . . . . . . . . . . . . . . . . . . 25 10.1 Normative References . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 27 10.2 Informative References . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . 28 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 27
Intellectual Property and Copyright Statements . . . . . . . . 28
1. Contributors 1. Contributors
The major original contributors of this document were: Tatu Ylonen, The major original contributors of this document were: Tatu Ylonen,
Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH Communications Tero Kivinen, Timo J. Rinne, Sami Lehtinen (all of SSH
Security Corp), and Markku-Juhani O. Saarinen (University of Communications Security Corp), and Markku-Juhani O. Saarinen
Jyvaskyla) (University of Jyvaskyla). Darren Moffit was the original editor of
this document and also made very substantial contributions.
The document editor is: Darren.Moffat@Sun.COM. Comments on this Additional contributors to this document include [need list].
internet draft should be sent to the IETF SECSH working group, Listing their names here does not mean that they endorse this
details at: http://ietf.org/html.charters/secsh-charter.html document, but that they have contributed to it.
Comments on this internet draft should be sent to the IETF SECSH
working group, details at:
http://ietf.org/html.charters/secsh-charter.html Note: This paragraph
will be removed before this document progresses to become an RFC.
2. Introduction 2. 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 The Transport Layer Protocol [SSH-TRANS] provides server o The Transport Layer Protocol [SSH-TRANS] provides server
authentication, confidentiality, and integrity. It may optionally authentication, confidentiality, and integrity. It may optionally
also provide compression. The transport layer will typically be also provide compression. The transport layer will typically be
run over a TCP/IP connection, but might also be used on top of any run over a TCP/IP connection, but might also be used on top of any
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requirements. They are to be interpreted as described in [RFC2119]. requirements. They are to be interpreted as described in [RFC2119].
4. Architecture 4. Architecture
4.1 Host Keys 4.1 Host Keys
Each server host SHOULD have a host key. Hosts MAY have multiple Each server host SHOULD have a host key. Hosts MAY have multiple
host keys using multiple different algorithms. Multiple hosts MAY 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 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 (DSS least one key using each REQUIRED public key algorithm (DSS
[FIPS-186]). [FIPS-186-2]).
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 client is really talking to the correct server. For this to be
possible, the client must have a priori knowledge of the server's possible, the client must have a priori knowledge of the server's
public host key. 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 third-party coordination. The downside is that the database of
name-to-key associations may become burdensome to maintain. name-to-key associations may become burdensome to maintain.
o The host name-to-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, certification authority (CA). The client only knows the CA root
and can verify the validity of all host keys certified by accepted key, and can verify the validity of all host keys certified by
CAs. accepted CAs.
The second alternative eases the maintenance problem, since The second alternative eases the maintenance problem, since ideally
ideally only a single CA key needs to be securely stored on the only a single CA key needs to be securely stored on the client. On
client. On the other hand, each host key must be appropriately the other hand, each host key must be appropriately certified by a
certified by a central authority before authorization is possible. central authority before authorization is possible. Also, a lot of
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 to the host for the first association is not checked when connecting to the host for the first
time. This allows communication without prior communication of host time. This allows communication without prior communication of host
keys or certification. The connection still provides protection keys or certification. The connection still provides protection
against passive listening; however, it becomes vulnerable to active against passive listening; however, it becomes vulnerable to active
man-in-the-middle attacks. Implementations SHOULD NOT normally allow man-in-the-middle attacks. Implementations SHOULD NOT normally allow
such connections by default, as they pose a potential security such connections by default, as they pose a potential security
problem. However, as there is no widely deployed key infrastructure problem. However, as there is no widely deployed key infrastructure
available on the Internet yet, this option makes the protocol much available on the Internet yet, this option makes the protocol much
more usable during the transition time until such an infrastructure more usable during the transition time until such an infrastructure
emerges, while still providing a much higher level of security than emerges, while still providing a much higher level of security than
that offered by older solutions (e.g. telnet [RFC-854] and rlogin that offered by older solutions (e.g. telnet [RFC0854] and rlogin
[RFC-1282]). [RFC1282]).
Implementations SHOULD try to make the best effort to check host 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 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 without checking the first time a host is connected, save the key in
a local database, and compare against that key on all future a local database, and compare against that key on all future
connections to that host. connections to that 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
the SHA-1 hash of the public key. Such fingerprints can easily be from the SHA-1 hash of the public key. Such fingerprints can easily
verified by using telephone or other external communication channels. be 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 The members of this Working Group believe that 'ease of use' is
security solutions, and no improvement in security is gained if the critical to end-user acceptance of security solutions, and no
new solutions are not used. Thus, providing the option not to check improvement in security is gained if the new solutions are not used.
the server host key is believed to improve the overall security of Thus, providing the option not to check the server host key is
the Internet, even though it reduces the security of the protocol in believed to improve the overall security of the Internet, even though
configurations where it is allowed. it reduces the security of the protocol in configurations where it is
allowed.
4.2 Extensibility 4.2 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 and/or key exchange methods. Central registration of all extensions
is cumbersome, especially for experimental or classified features. is cumbersome, especially for experimental or classified features.
On the other hand, having no central registration leads to conflicts On the other hand, having no central registration leads to conflicts
in method identifiers, making interoperability difficult. in method identifiers, making interoperability difficult.
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The protocol allows full negotiation of encryption, integrity, key The protocol allows full negotiation of encryption, integrity, key
exchange, compression, and public key algorithms and formats. exchange, compression, and public key algorithms and formats.
Encryption, integrity, public key, and compression algorithms can be Encryption, integrity, public key, and compression algorithms can be
different for each direction. different for each direction.
The following policy issues SHOULD be addressed in the configuration The following policy issues SHOULD be addressed in the configuration
mechanisms of each implementation: mechanisms of each implementation:
o Encryption, integrity, and compression algorithms, separately for o Encryption, integrity, and compression algorithms, separately for
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 o The authentication methods that are to be required by the server
for each user. The server's policy MAY require multiple for each user. The server's policy MAY require multiple
authentication for some or all users. The required algorithms MAY authentication for some or all users. The required algorithms MAY
depend on the location where the user is trying to log in from. 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 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 example, the policy SHOULD NOT allow the server to start sessions
or run commands on the client machine, and MUST NOT allow or run commands on the client machine, and MUST NOT allow
connections to the authentication agent unless forwarding such connections to the authentication agent unless forwarding such
connections has been requested. Other issues, such as which TCP/ connections has been requested. Other issues, such as which TCP/
IP ports can be forwarded and by whom, are clearly issues of local IP ports can be forwarded and by whom, are clearly issues of local
policy. Many of these issues may involve traversing or bypassing policy. Many of these issues may involve traversing or bypassing
firewalls, and are interrelated with the local security policy. firewalls, and are interrelated with the local security policy.
4.4 Security Properties 4.4 Security Properties
The primary goal of the SSH protocol is improved security on the The primary goal of the SSH protocol is to improve security on the
Internet. It attempts to do this in a way that is easy to deploy, Internet. It attempts to do this in a way that is easy to deploy,
even at the cost of absolute security. even at the cost of absolute security.
o All encryption, integrity, and public key algorithms used are o All encryption, integrity, and public key algorithms used are
well-known, well-established algorithms. well-known, well-established algorithms.
o All algorithms are used with cryptographically sound key sizes o All algorithms are used with cryptographically sound key sizes
that are believed to provide protection against even the strongest that are believed to provide protection against even the strongest
cryptanalytic attacks for decades. cryptanalytic attacks for decades.
o All algorithms are negotiated, and in case some algorithm is o All algorithms are negotiated, and in case some algorithm is
broken, it is easy to switch to some other algorithm without broken, it is easy to switch to some other algorithm without
modifying the base protocol. modifying the base protocol.
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 easier. The particular case where this comes up is verifying that
the server host key really belongs to the desired host; the protocol the server host key really belongs to the desired host; the protocol
allows the verification to be left out (but this is NOT RECOMMENDED). allows the verification to be left out, but this is NOT RECOMMENDED.
This is believed to significantly improve usability in the short This is believed to significantly improve usability in the short
term, until widespread Internet public key infrastructures emerge. term, until widespread Internet public key infrastructures emerge.
4.5 Packet Size and Overhead 4.5 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 is in the order headers, padding, and Message Authentication Code (MAC). The minimum
of 28 bytes (depending on negotiated algorithms). The increase is packet size is in the order of 28 bytes (depending on negotiated
negligible for large packets, but very significant for one-byte algorithms). The increase is negligible for large packets, but very
packets (telnet-type sessions). There are, however, several factors significant for one-byte packets (telnet-type sessions). There are,
that make this a non-issue in almost all cases: 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 o The minimum size of a TCP/IP header is 32 bytes. Thus, the
increase is actually from 33 to 51 bytes (roughly). increase is actually from 33 to 51 bytes (roughly).
o The minimum size of the data field of an Ethernet packet is 46 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 bytes [RFC0894]. Thus, the increase is no more than 5 bytes.
Ethernet headers are considered, the increase is less than 10 When Ethernet headers are considered, the increase is less than 10
percent. percent.
o The total fraction of telnet-type data in the Internet is o The total fraction of telnet-type data in the Internet is
negligible, even with increased packet sizes. negligible, 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 a significant effect is PPP [RFC1134] over slow modem lines (PPP
compresses the TCP/IP headers, emphasizing the increase in packet compresses the TCP/IP headers, emphasizing the increase in packet
size). However, with modern modems, the time needed to transfer is in size). However, with modern modems, the time needed to transfer is
the order of 2 milliseconds, which is a lot faster than people can in the order of 2 milliseconds, which is a lot faster than people can
type. type.
There are also issues related to the maximum packet size. To There are also issues related to the maximum packet size. To
minimize delays in screen updates, one does not want excessively minimize delays in screen updates, one does not want excessively
large packets for interactive sessions. The maximum packet size is large packets for interactive sessions. The maximum packet size is
negotiated separately for each channel. negotiated separately for each channel.
4.6 Localization and Character Set Support 4.6 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
data MUST be explicitly specified. In most places, ISO 10646 with the data MUST be explicitly specified. In most places, ISO 10646
UTF-8 encoding is used [RFC-2279]. When applicable, a field is also with UTF-8 encoding is used [RFC2279]. When applicable, a field is
provided for a language tag [RFC-3066]. also provided for a language tag [RFC3066].
One big issue is the character set of the interactive session. There One big issue is the character set of the interactive session. There
is no clear solution, as different applications may display data in is 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 employed in the client, and the character set to be used is
effectively determined by the terminal emulation. Thus, no place is effectively determined by the terminal emulation. Thus, no place is
provided for directly specifying the character set or encoding for provided for directly specifying the character set or encoding for
terminal session data. However, the terminal emulation type (e.g. terminal session data. However, the terminal emulation type (e.g.
"vt100") is transmitted to the remote site, and it implicitly "vt100") is transmitted to the remote site, and it implicitly
specifies the character set and encoding. Applications typically use specifies the character set and encoding. Applications typically use
the terminal type to determine what character set they use, or the the terminal type to determine what character set they use, or the
character set is determined using some external means. The terminal character set is determined using some external means. The terminal
emulation may also allow configuring the default character set. In emulation may also allow configuring the default character set. In
any case, the character set for the terminal session is considered any case, the character set for the terminal session is considered
primarily a client local issue. 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.
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For localization purposes, the protocol attempts to minimize the For localization purposes, the protocol attempts to minimize the
number of textual messages transmitted. When present, such messages number of textual messages transmitted. When present, such messages
typically relate to errors, debugging information, or some externally typically relate to errors, debugging information, or some externally
configured data. For data that is normally displayed, it SHOULD be configured data. For data that is normally displayed, it SHOULD be
possible to fetch a localized message instead of the transmitted possible to fetch a localized message instead of the transmitted
message by using a numerical code. The remaining messages SHOULD be message by using a numerical code. The remaining messages SHOULD be
configurable. configurable.
5. Data Type Representations Used in the SSH Protocols 5. Data Type Representations Used in the SSH Protocols
byte byte
A byte represents an arbitrary 8-bit value (octet) [RFC-1700]. A byte represents an arbitrary 8-bit value (octet). Fixed length
Fixed length data is sometimes represented as an array of bytes, data is sometimes represented as an array of bytes, written
written byte[n], where n is the number of bytes in the array. 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
skipping to change at page 9, line 28 skipping to change at page 9, line 32
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 the value 1 as the most significant bit of the first byte of 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 the data partition. If the most significant bit would be set for
positive number, the number MUST be preceded by a zero byte. a positive number, the number MUST be preceded by a zero byte.
Unnecessary leading bytes with the value 0 or 255 MUST NOT be Unnecessary leading bytes with the value 0 or 255 MUST NOT be
included. The value zero MUST be stored as a string with zero included. The value zero MUST be stored as a string with zero
bytes of data. 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)
--------------------------------------------------------------- ---------------------------------------------------------------
skipping to change at page 9, line 45 skipping to change at page 10, line 4
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
name-list name-list
A string containing a comma separated list of names. A 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 is represented as a uint32 containing its length (number of bytes
that follow) followed by a comma-separated list of zero or more that follow) followed by a comma-separated list of zero or more
names. A name MUST be non-zero length, and it MUST NOT contain a names. A name MUST be non-zero length, and it MUST NOT contain a
comma (','). Context may impose additional restrictions on the comma (','). Context may impose additional restrictions on the
names; for example, the names in a list may have to be valid names; for example, the names in a list may have to be valid
algorithm identifier (see Algorithm Naming below), or [RFC-3066] algorithm identifier (see Section 6 below), or [RFC3066] language
language tags. The order of the names in a list may or may not be 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 significant, also depending on the context where the list is is
used. Terminating NUL characters are not used, neither for the used. Terminating NUL characters are not used, neither for the
individual names, nor for the list as a whole. individual names, nor for the list as a whole.
Examples: Examples:
value representation (hex) value representation (hex)
--------------------------------------- ---------------------------------------
(), the empty list 00 00 00 00 (), the empty list 00 00 00 00
("zlib") 00 00 00 04 7a 6c 69 62 ("zlib") 00 00 00 04 7a 6c 69 62
("zlib", "none") 00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65 ("zlib", "none") 00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65
skipping to change at page 10, line 45 skipping to change at page 10, line 50
assigned by IETF consensus (RFCs). Examples include `3des-cbc', assigned by IETF consensus (RFCs). Examples include `3des-cbc',
`sha-1', `hmac-sha1', and `zlib' (the quotes are not part of the `sha-1', `hmac-sha1', and `zlib' (the quotes are not part of the
name). Names of this format MUST NOT be used without first name). Names of this format MUST NOT be used without first
registering them. Registered names MUST NOT contain an at-sign registering them. Registered names MUST NOT contain an at-sign
(@) or a comma (,). (@) or a comma (,).
o Anyone can define additional algorithms by using names in the o Anyone can define additional algorithms by using names in the
format name@domainname, e.g. "ourcipher-cbc@example.com". The format name@domainname, e.g. "ourcipher-cbc@example.com". The
format of the part preceding the at sign is not specified; it MUST 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 consist of US-ASCII characters except at-sign and comma. The part
following the at-sign MUST be a valid fully qualified internet following the at-sign MUST be a valid fully qualified internet
domain name [RFC-1034] controlled by the person or organization domain name [RFC1034] controlled by the person or organization
defining the name. It is up to each domain how it manages its defining the name. It is up to each domain how it manages its
local namespace. local namespace.
7. Message Numbers 7. Message Numbers
SSH packets have message numbers in the range 1 to 255. These numbers SSH packets have message numbers in the range 1 to 255. These
have been allocated as follows: numbers have been allocated as follows:
Transport layer protocol: Transport layer protocol:
1 to 19 Transport layer generic (e.g. disconnect, ignore, debug, 1 to 19 Transport layer generic (e.g. disconnect, ignore, debug,
etc.) etc.)
20 to 29 Algorithm negotiation 20 to 29 Algorithm negotiation
30 to 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:
skipping to change at page 11, line 39 skipping to change at page 11, line 41
Reserved for client protocols: Reserved for client protocols:
128 to 191 Reserved 128 to 191 Reserved
Local extensions: Local extensions:
192 to 255 Local extensions 192 to 255 Local extensions
8. IANA Considerations 8. IANA Considerations
The initial state of the IANA registry is detailed in [SSH-NUMBERS]. This document is part of a set. The instructions for IANA for the
SSH protocol as defined in this document, [SSH-USERAUTH],
[SSH-TRANS], and [SSH-CONNECT], are detailed in [SSH-NUMBERS]. The
following is a brief summary for convenience, but note well that
[SSH-NUMBERS] is the actual initial instructions to the IANA, which
may be superceded in the future.
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 by IETF consensus: assigned by IETF consensus:
o SSH encryption algorithm names, o SSH encryption algorithm names,
o SSH MAC algorithm names, o SSH MAC algorithm names,
o SSH public key algorithm names (public key algorithm also implies o SSH public key algorithm names (public key algorithm also implies
encoding and signature/encryption capability), encoding and signature/encryption capability),
o SSH key exchange method names, and o SSH key exchange method names, and
o SSH protocol (service) names. o SSH protocol (service) names.
These names MUST be printable US-ASCII strings, and MUST NOT contain These names MUST be printable US-ASCII strings, and MUST NOT contain
the characters at-sign ('@'), comma (','), or whitespace or control the characters at-sign ('@'), comma (','), or whitespace or control
characters (ASCII codes 32 or less). Names are case-sensitive, and characters (ASCII codes 32 or less). Names are case-sensitive, and
MUST NOT be longer than 64 characters. MUST NOT be longer than 64 characters.
Names with the at-sign ('@') in them are allocated by the owner of Names with the at-sign ('@') in them are allocated by the owner of
DNS name after the at-sign (hierarchical allocation in [RFC-2343]), DNS name after the at-sign (hierarchical allocation in [RFC2434]),
otherwise the same restrictions as above. otherwise the same restrictions as above.
Each category of names listed above has a separate namespace. Each category of names listed above has a separate namespace.
However, using the same name in multiple categories SHOULD be avoided However, using the same name in multiple categories SHOULD be avoided
to minimize confusion. to minimize confusion.
Message numbers (see Section Message Numbers (Section 7)) in the Message numbers (see Section Message Numbers (Section 7)) in the
range of 0..191 are allocated via IETF consensus; message numbers in range of 0..191 are allocated via IETF consensus as described in
the 192..255 range (the "Local extensions" set) are reserved for [RFC2434]. Message numbers in the 192..255 range (the "Local
private use. extensions" set) are reserved for private use.
9. Security Considerations 9. Security Considerations
In order to make the entire body of Security Considerations more In order to make the entire body of Security Considerations more
accessible, Security Considerations for the transport, accessible, Security Considerations for the transport,
authentication, and connection documents have been gathered here. authentication, and connection documents have been gathered here.
The transport protocol [1] provides a confidential channel over an The transport protocol [SSH-TRANS] provides a confidential channel
insecure network. It performs server host authentication, key over an insecure network. It performs server host authentication,
exchange, encryption, and integrity protection. It also derives a key exchange, encryption, and integrity protection. It also derives
unique session id that may be used by higher-level protocols. a unique session id that may be used by higher-level protocols.
The authentication protocol [2] provides a suite of mechanisms which The authentication protocol [SSH-USERAUTH] provides a suite of
can be used to authenticate the client user to the server. mechanisms which can be used to authenticate the client user to the
Individual mechanisms specified in the in authentication protocol use server. Individual mechanisms specified in the in authentication
the session id provided by the transport protocol and/or depend on protocol use the session id provided by the transport protocol and/or
the security and integrity guarantees of the transport protocol. depend on the security and integrity guarantees of the transport
protocol.
The connection protocol [3] specifies a mechanism to multiplex The connection protocol [SSH-CONNECT] specifies a mechanism to
multiple streams [channels] of data over the confidential and multiplex multiple streams (channels) of data over the confidential
authenticated transport. It also specifies channels for accessing an and authenticated transport. It also specifies channels for
interactive shell, for 'proxy-forwarding' various external protocols accessing an interactive shell, for 'proxy-forwarding' various
over the secure transport (including arbitrary TCP/IP protocols), and external protocols over the secure transport (including arbitrary
for accessing secure 'subsystems' on the server host. TCP/IP protocols), and for accessing secure 'subsystems' on the
server host.
9.1 Pseudo-Random Number Generation 9.1 Pseudo-Random Number Generation
This protocol binds each session key to the session by including This protocol binds each session key to the session by including
random, session specific data in the hash used to produce session random, session specific data in the hash used to produce session
keys. Special care should be taken to ensure that all of the random 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 numbers are of good quality. If the random data here (e.g.,
parameters) are pseudo-random then the pseudo-random number generator Diffie-Hellman (DH) parameters) are pseudo-random then the
should be cryptographically secure (i.e., its next output not easily pseudo-random number generator should be cryptographically secure
guessed even when knowing all previous outputs) and, furthermore, (i.e., its next output not easily guessed even when knowing all
proper entropy needs to be added to the pseudo-random number previous outputs) and, furthermore, proper entropy needs to be added
generator. RFC 1750 [1750] offers suggestions for sources of random to the pseudo-random number generator. [RFC1750] offers suggestions
numbers and entropy. Implementors should note the importance of for sources of random numbers and entropy. Implementors should note
entropy and the well-meant, anecdotal warning about the difficulty in the importance of entropy and the well-meant, anecdotal warning about
properly implementing pseudo-random number generating functions. the difficulty in properly implementing pseudo-random number
generating functions.
The amount of entropy available to a given client or server may The amount of entropy available to a given client or server may
sometimes be less than what is required. In this case one must sometimes be less than what is required. In this case one must
either resort to pseudo-random number generation regardless of either resort to pseudo-random number generation regardless of
insufficient entropy or refuse to run the protocol. The latter is insufficient entropy or refuse to run the protocol. The latter is
preferable. preferable.
9.2 Transport 9.2 Transport
9.2.1 Confidentiality 9.2.1 Confidentiality
It is beyond the scope of this document and the Secure Shell Working It is beyond the scope of this document and the Secure Shell Working
Group to analyze or recommend specific ciphers other than the ones Group to analyze or recommend specific ciphers other than the ones
which have been established and accepted within the industry. At the which have been established and accepted within the industry. At the
time of this writing, ciphers commonly in use include 3DES, ARCFOUR, time of this writing, ciphers commonly in use include 3DES, ARCFOUR,
twofish, serpent and blowfish. AES has been accepted by The twofish, serpent and blowfish. AES has been published by The US
published as a US Federal Information Processing Standards [FIPS-197] Federal Information Processing Standards as [FIPS-197] and the
and the cryptographic community as being acceptable for this purpose cryptographic community has accepted AES as well. As always,
as well has accepted AES. As always, implementors and users should implementors and users should check current literature to ensure that
check current literature to ensure that no recent vulnerabilities no recent vulnerabilities have been found in ciphers used within
have been found in ciphers used within products. Implementors should products. Implementors should also check to see which ciphers are
also check to see which ciphers are considered to be relatively considered to be relatively stronger than others and should recommend
stronger than others and should recommend their use to users over their use to users over relatively weaker ciphers. It would be
relatively weaker ciphers. It would be considered good form for an considered good form for an implementation to politely and
implementation to politely and unobtrusively notify a user that a unobtrusively notify a user that a stronger cipher is available and
stronger cipher is available and should be used when a weaker one is should be used when a weaker one is actively chosen.
actively chosen.
The "none" cipher is provided for debugging and SHOULD NOT be used The "none" cipher is provided for debugging and SHOULD NOT be used
except for that purpose. It's cryptographic properties are except for that purpose. It's cryptographic properties are
sufficiently described in RFC 2410, which will show that its use does sufficiently described in [RFC2410], which will show that its use
not meet the intent of this protocol. does not meet the intent of this protocol.
The relative merits of these and other ciphers may also be found in The relative merits of these and other ciphers may also be found in
current literature. Two references that may provide information on current literature. Two references that may provide information on
the subject are [SCHNEIER] and [KAUFMAN,PERLMAN,SPECINER]. Both of the subject are [SCHNEIER] and [KAUFMAN,PERLMAN,SPECINER] Both of
these describe the CBC mode of operation of certain ciphers and the these describe the CBC mode of operation of certain ciphers and the
weakness of this scheme. Essentially, this mode is theoretically weakness of this scheme. Essentially, this mode is theoretically
vulnerable to chosen cipher-text attacks because of the high vulnerable to chosen cipher-text attacks because of the high
predictability of the start of packet sequence. However, this attack predictability of the start of packet sequence. However, this attack
is still deemed difficult and not considered fully practicable is deemed difficult and not considered fully practicable especially
especially if relatively longer block sizes are used. if relatively longer block sizes are used.
Additionally, another CBC mode attack may be mitigated through the Additionally, another CBC mode attack may be mitigated through the
insertion of packets containing SSH_MSG_IGNORE. Without this insertion of packets containing SSH_MSG_IGNORE. Without this
technique, a specific attack may be successful. For this attack technique, a specific attack may be successful. For this attack
(commonly known as the Rogaway attack (commonly known as the Rogaway attack
[ROGAWAY],[DAI],[BELLARE,KOHNO,NAMPREMPRE]) to work, the attacker [ROGAWAY],[DAI][BELLARE,KOHNO,NAMPREMPRE],) to work, the attacker
would need to know the IV of the next block that is going to be would need to know the Initialization Vector (IV) of the next block
encrypted. In CBC mode that is the output of the encryption of the that is going to be encrypted. In CBC mode that is the output of the
previous block. If the attacker does not have any way to see the encryption of the previous block. If the attacker does not have any
packet yet (i.e it is in the internal buffers of the ssh way to see the packet yet (i.e., it is in the internal buffers of the
implementation or even in the kernel) then this attack will not work. SSH implementation or even in the kernel) then this attack will not
If the last packet has been sent out to the network (i.e the attacker work. If the last packet has been sent out to the network (i.e., the
has access to it) then he can use the attack. attacker has access to it) then he can use the attack.
In the optimal case an implementor would need to add an extra packet 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 only if the packet has been sent out onto the network and there are
no other packets waiting for transmission. Implementors may wish to no other packets waiting for transmission. Implementors may wish to
check to see if there are any unsent packets awaiting transmission, check to see if there are any unsent packets awaiting transmission,
but unfortunately it is not normally easy to obtain this information but unfortunately it is not normally easy to obtain this information
from the kernel or buffers. If there are not, then a packet 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 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 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 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. guess the correct IV, thus the attack will never be successful.
As an example, consider the following case: As an example, consider the following case:
Client Server Client Server
------ ------ ------ ------
TCP(seq=x, len=500) -> TCP(seq=x, len=500) ---->
contains Record 1 contains Record 1
[500 ms passes, no ACK] [500 ms passes, no ACK]
TCP(seq=x, len=1000) -> TCP(seq=x, len=1000) ---->
contains Records 1,2 contains Records 1,2
ACK ACK
1. The Nagle algorithm + TCP retransmits mean that the two records 1. The Nagle algorithm + TCP retransmits mean that the two records
get coalesced into a single TCP segment get coalesced into a single TCP segment.
2. Record 2 is *not* at the beginning of the TCP segment and never 2. Record 2 is *not* at the beginning of the TCP segment and never
will be, since it gets ACKed. will be, since it gets ACKed.
3. Yet, the attack is possible because Record 1 has already been 3. Yet, the attack is possible because Record 1 has already been
seen. seen.
As this example indicates, it's totally unsafe to use the existence 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 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 need an empty packet, since when you do the second write(), the
buffers will contain the un-ACKed Record 1. buffers will contain the un-ACKed Record 1.
On the other hand, it's perfectly safe to have the following On the other hand, it's perfectly safe to have the following
situation: situation:
skipping to change at page 16, line 10 skipping to change at page 16, line 10
As this example indicates, it's totally unsafe to use the existence 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 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 need an empty packet, since when you do the second write(), the
buffers will contain the un-ACKed Record 1. buffers will contain the un-ACKed Record 1.
On the other hand, it's perfectly safe to have the following On the other hand, it's perfectly safe to have the following
situation: situation:
Client Server Client Server
------ ------ ------ ------
TCP(seq=x, len=500) -> TCP(seq=x, len=500) ---->
contains SSH_MSG_IGNORE contains SSH_MSG_IGNORE
TCP(seq=y, len=500) -> TCP(seq=y, len=500) ---->
contains Data contains Data
Provided that the IV for second SSH Record is fixed after the data for Provided that the IV for the second SSH Record is fixed after the data for
the Data packet is determined -i.e. you do: the Data packet is determined, then the following should be performed:
read from user read from user
encrypt null packet encrypt null packet
encrypt data packet encrypt data packet
9.2.2 Data Integrity 9.2.2 Data Integrity
This protocol does allow the Data Integrity mechanism to be disabled. This protocol does allow the Data Integrity mechanism to be disabled.
Implementors SHOULD be wary of exposing this feature for any purpose Implementors SHOULD be wary of exposing this feature for any purpose
other than debugging. Users and administrators SHOULD be explicitly other than debugging. Users and administrators SHOULD be explicitly
warned anytime the "none" MAC is enabled. warned anytime the "none" MAC is enabled.
So long as the "none" MAC is not used, this protocol provides data So long as the "none" MAC is not used, this protocol provides data
integrity. integrity.
Because MACs use a 32 bit sequence number, they might start to leak Because MACs use a 32 bit sequence number, they might start to leak
information after 2**32 packets have been sent. However, following information after 2**32 packets have been sent. However, following
the rekeying recommendations should prevent this attack. The the rekeying recommendations should prevent this attack. The
transport protocol [1] recommends rekeying after one gigabyte of transport protocol [SSH-TRANS] recommends rekeying after one gigabyte
data, and the smallest possible packet is 16 bytes. Therefore, of data, and the smallest possible packet is 16 bytes. Therefore,
rekeying SHOULD happen after 2**28 packets at the very most. rekeying SHOULD happen after 2**28 packets at the very most.
9.2.3 Replay 9.2.3 Replay
The use of a MAC other than 'none' provides integrity and The use of a MAC other than 'none' provides integrity and
authentication. In addition, the transport protocol provides a authentication. In addition, the transport protocol provides a
unique session identifier (bound in part to pseudo-random data that unique session identifier (bound in part to pseudo-random data that
is part of the algorithm and key exchange process) that can be used 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 by higher level protocols to bind data to a given session and prevent
replay of data from prior sessions. For example, the authentication replay of data from prior sessions. For example, the authentication
protocol uses this to prevent replay of signatures from previous protocol uses this to prevent replay of signatures from previous
sessions. Because public key authentication exchanges are sessions. Because public key authentication exchanges are
cryptographically bound to the session (i.e., to the initial key cryptographically bound to the session (i.e., to the initial key
exchange) they cannot be successfully replayed in other sessions. exchange) they cannot be successfully replayed in other sessions.
Note that the session ID can be made public without harming the Note that the session ID can be made public without harming the
security of the protocol. security of the protocol.
If two session happen to have the same session ID [hash of key If two sessions happen to have the same session ID (hash of key
exchanges] then packets from one can be replayed against the other. exchanges) then packets from one can be replayed against the other.
It must be stressed that the chances of such an occurrence are, It must be stressed that the chances of such an occurrence are,
needless to say, minimal when using modern cryptographic methods. needless to say, minimal when using modern cryptographic methods.
This is all the more so true when specifying larger hash function This is all the more so true when specifying larger hash function
outputs and DH parameters. outputs and DH parameters.
Replay detection using monotonically increasing sequence numbers as Replay detection using monotonically increasing sequence numbers as
input to the MAC, or HMAC in some cases, is described in [RFC2085] /> input to the MAC, or HMAC in some cases, is described in [RFC2085],
[RFC2246], [RFC2743], [RFC1964], [RFC2025], and [RFC1510]. The [RFC2246], [RFC2743], [RFC1964], [RFC2025], and [RFC1510]. The
underlying construct is discussed in [RFC2104]. Essentially a underlying construct is discussed in [RFC2104]. Essentially a
different sequence number in each packet ensures that at least this different sequence number in each packet ensures that at least this
one input to the MAC function will be unique and will provide a one input to the MAC function will be unique and will provide a
nonrecurring MAC output that is not predictable to an attacker. If nonrecurring MAC output that is not predictable to an attacker. If
the session stays active long enough, however, this sequence number the session stays active long enough, however, this sequence number
will wrap. This event may provide an attacker an opportunity to will wrap. This event may provide an attacker an opportunity to
replay a previously recorded packet with an identical sequence number replay a previously recorded packet with an identical sequence number
but only if the peers have not rekeyed since the transmission of the but only if the peers have not rekeyed since the transmission of the
first packet with that sequence number. If the peers have rekeyed, first packet with that sequence number. If the peers have rekeyed,
skipping to change at page 17, line 43 skipping to change at page 17, line 43
because the receiver will formulate a MAC based upon the packet because the receiver will formulate a MAC based upon the packet
contents, the shared secret, and the expected sequence number. Since contents, the shared secret, and the expected sequence number. Since
the replayed packet will not be using that expected sequence number the replayed packet will not be using that expected sequence number
(the sequence number of the replayed packet will have already been (the sequence number of the replayed packet will have already been
passed by the receiver) then the calculated MAC will not match the passed by the receiver) then the calculated MAC will not match the
MAC received with the packet. MAC received with the packet.
9.2.4 Man-in-the-middle 9.2.4 Man-in-the-middle
This protocol makes no assumptions nor provisions for an This protocol makes no assumptions nor provisions for an
infrastructure or means for distributing the public keys of hosts. It infrastructure or means for distributing the public keys of hosts.
is expected that this protocol will sometimes be used without first It is expected that this protocol will sometimes be used without
verifying the association between the server host key and the server first verifying the association between the server host key and the
host name. Such usage is vulnerable to man-in-the-middle attacks. server host name. Such usage is vulnerable to man-in-the-middle
This section describes this and encourages administrators and users attacks. This section describes this and encourages administrators
to understand the importance of verifying this association before any and users to understand the importance of verifying this association
session is initiated. before any session is initiated.
There are three cases of man-in-the-middle attacks to consider. The 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 first is where an attacker places a device between the client and the
server before the session is initiated. In this case, the attack server before the session is initiated. In this case, the attack
device is trying to mimic the legitimate server and will offer its 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 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 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 to decrypt or sign the transmissions between the legitimate server
and the client unless it also had access to the private-key of the and the client unless it also had access to the private-key of the
host. The attack device will also, simultaneously to this, initiate host. The attack device will also, simultaneously to this, initiate
a session to the legitimate server masquerading itself as the client. a session to the legitimate server masquerading itself as the client.
If the public key of the server had been securely distributed to the 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 prior to that session initiation, the key offered to the
client by the attack device will not match the key stored on 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 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. 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 As described in Section Host Keys (Section 4.1), the user may be free
the new key and continue the session. It is RECOMMENDED that the to accept the new key and continue the session. It is RECOMMENDED
warning provide sufficient information to the user of the client that the warning provide sufficient information to the user of the
device so they may make an informed decision. If the user chooses to client device so they may make an informed decision. If the user
continue the session with the stored public-key of the server (not chooses to continue the session with the stored public-key of the
the public-key offered at the start of the session), then the session server (not the public-key offered at the start of the session), then
specific data between the attacker and server will be different the session specific data between the attacker and server will be
between the client-to-attacker session and the attacker-to-server different between the client-to-attacker session and the
sessions due to the randomness discussed above. From this, the attacker-to-server sessions due to the randomness discussed above.
attacker will not be able to make this attack work since the attacker From this, the attacker will not be able to make this attack work
will not be able to correctly sign packets containing this session since the attacker will not be able to correctly sign packets
specific data from the server since he does not have the private key containing this session specific data from the server since he does
of that server. not have the private key of that server.
The second case that should be considered is similar to the first 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 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 points out the need for the secure distribution of server public
keys. If the server public keys are not securely distributed then keys. If the server public keys are not securely distributed then
the client cannot know if it is talking to the intended server. An the client cannot know if it is talking to the intended server. An
attacker may use social engineering techniques to pass off server attacker may use social engineering techniques to pass off server
keys to unsuspecting users and may then place a man-in-the-middle keys to unsuspecting users and may then place a man-in-the-middle
attack device between the legitimate server and the clients. If this attack device between the legitimate server and the clients. If this
is allowed to happen then the clients will form client-to-attacker is allowed to happen then the clients will form client-to-attacker
sessions and the attacker will form attacker-to-server sessions and sessions and the attacker will form attacker-to-server sessions and
will be able to monitor and manipulate all of the traffic between the will be able to monitor and manipulate all of the traffic between the
clients and the legitimate servers. Server administrators are clients and the legitimate servers. Server administrators are
encouraged to make host key fingerprints available for checking by encouraged to make host key fingerprints available for checking by
some means whose security does not rely on the integrity of the some means whose security does not rely on the integrity of the
actual host keys. Possible mechanisms are discussed in Section 3.1 actual host keys. Possible mechanisms are discussed in Section Host
of [SSH-ARCH] and may also include secured Web pages, physical pieces Keys (Section 4.1) and may also include secured Web pages, physical
of paper, etc. Implementors SHOULD provide recommendations on how pieces of paper, etc. Implementors SHOULD provide recommendations on
best to do this with their implementation. Because the protocol is how best to do this with their implementation. Because the protocol
extensible, future extensions to the protocol may provide better is extensible, future extensions to the protocol may provide better
mechanisms for dealing with the need to know the server's host key mechanisms for dealing with the need to know the server's host key
before connecting. For example, making the host key fingerprint before connecting. For example, making the host key fingerprint
available through a secure DNS lookup, or using kerberos over gssapi available through a secure DNS lookup, or using Kerberos ([RFC1510])
during key exchange to authenticate the server are possibilities. over GSS-API ([RFC1964]) during key exchange to authenticate the
server are possibilities.
In the third man-in-the-middle case, attackers may attempt to In the third man-in-the-middle case, attackers may attempt to
manipulate packets in transit between peers after the session has manipulate packets in transit between peers after the session has
been established. As described in the Replay part of this section, a been established. As described in the Replay part of this section, a
successful attack of this nature is very improbable. As in the successful attack of this nature is very improbable. As in the
Replay section, this reasoning does assume that the MAC is secure and 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 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 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 6 of [RFC2104]. If the MAC algorithm has a vulnerability or is weak
enough, then the attacker may be able to specify certain inputs to 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 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 of a packet in transit. Alternatively the attacker may be able to
exploit the algorithm vulnerability or weakness to find the shared exploit the algorithm vulnerability or weakness to find the shared
secret by reviewing the MACs from captured packets. In either of secret by reviewing the MACs from captured packets. In either of
those cases, an attacker could construct a packet or packets that those cases, an attacker could construct a packet or packets that
could be inserted into an SSH stream. To prevent that, implementors could be inserted into an SSH stream. To prevent that, implementors
are encouraged to utilize commonly accepted MAC algorithms and are encouraged to utilize commonly accepted MAC algorithms and
administrators are encouraged to watch current literature and administrators are encouraged to watch current literature and
discussions of cryptography to ensure that they are not using a MAC discussions of cryptography to ensure that they are not using a MAC
skipping to change at page 20, line 16 skipping to change at page 20, line 20
the recipient has no reliable way to verify whether such information the recipient has no reliable way to verify whether such information
is being sent. is being sent.
9.2.7 Forward Secrecy 9.2.7 Forward Secrecy
It should be noted that the Diffie-Hellman key exchanges may provide It should be noted that the Diffie-Hellman key exchanges may provide
perfect forward secrecy (PFS). PFS is essentially defined as the perfect forward secrecy (PFS). PFS is essentially defined as the
cryptographic property of a key-establishment protocol in which the cryptographic property of a key-establishment protocol in which the
compromise of a session key or long-term private key after a given compromise of a session key or long-term private key after a given
session does not cause the compromise of any earlier session. [ANSI session does not cause the compromise of any earlier session. [ANSI
T1.523-2001] SSHv2 sessions resulting from a key exchange using T1.523-2001] SSH sessions resulting from a key exchange using
diffie-hellman-group1-sha1 are secure even if private keying/ diffie-hellman-group1-sha1 are secure even if private keying/
authentication material is later revealed, but not if the session authentication material is later revealed, but not if the session
keys are revealed. So, given this definition of PFS, SSHv2 does have keys are revealed. So, given this definition of PFS, SSH does have
PFS. It is hoped that all other key exchange mechanisms proposed and PFS. It is hoped that all other key exchange mechanisms proposed and
used in the future will also provide PFS. This property is not used in the future will also provide PFS. This property is not
commuted to any of the applications or protocols using SSH as a commuted to any of the applications or protocols using SSH as a
transport however. The transport layer of SSH provides transport however. The transport layer of SSH provides
confidentiality for password authentication and other methods that confidentiality for password authentication and other methods that
rely on secret data. rely on secret data.
Of course, if the DH private parameters for the client and server are 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 revealed then the session key is revealed, but these items can be
thrown away after the key exchange completes. It's worth pointing thrown away after the key exchange completes. It's worth pointing
skipping to change at page 21, line 28 skipping to change at page 21, line 31
of the consequences of man-in-the-middle attacks if the client does 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 not have a very strong a priori association of the server with the
host key of that server. Specifically for the case of the host key of that server. Specifically for the case of the
Authentication Protocol the client may form a session to a Authentication Protocol the client may form a session to a
man-in-the-middle attack device and divulge user credentials such as man-in-the-middle attack device and divulge user credentials such as
their username and password. Even in the cases of authentication their username and password. Even in the cases of authentication
where no user credentials are divulged, an attacker may still gain where no user credentials are divulged, an attacker may still gain
information they shouldn't have by capturing key-strokes in much the information they shouldn't have by capturing key-strokes in much the
same way that a honeypot works. same way that a honeypot works.
9.3.2 Debug messages 9.3.2 Debug Messages
Special care should be taken when designing debug messages. These Special care should be taken when designing debug messages. These
messages may reveal surprising amounts of information about the host messages may reveal surprising amounts of information about the host
if not properly designed. Debug messages can be disabled (during if not properly designed. Debug messages can be disabled (during
user authentication phase) if high security is required. user authentication phase) if high security is required.
Administrators of host machines should make all attempts to Administrators of host machines should make all attempts to
compartmentalize all event notification messages and protect them compartmentalize all event notification messages and protect them
from unwarranted observation. Developers should be aware of the from unwarranted observation. Developers should be aware of the
sensitive nature of some of the normal event messages and debug sensitive nature of some of the normal event messages and debug
messages and may want to provide guidance to administrators on ways messages and may want to provide guidance to administrators on ways
to keep this information away from unauthorized people. Developers to keep this information away from unauthorized people. Developers
should consider minimizing the amount of sensitive information should consider minimizing the amount of sensitive information
obtainable by users during the authentication phase in accordance obtainable by users during the authentication phase in accordance
with the local policies. For this reason, it is RECOMMENDED that with the local policies. For this reason, it is RECOMMENDED that
debug messages be initially disabled at the time of deployment and debug messages be initially disabled at the time of deployment and
require an active decision by an administrator to allow them to be require an active decision by an administrator to allow them to be
enabled. It is also RECOMMENDED that a message expressing this enabled. It is also RECOMMENDED that a message expressing this
concern be presented to the administrator of a system when the action concern be presented to the administrator of a system when the action
is taken to enable debugging messages. is taken to enable debugging messages.
9.3.3 Local security policy 9.3.3 Local Security Policy
Implementer MUST ensure that the credentials provided validate the Implementer MUST ensure that the credentials provided validate the
professed user and also MUST ensure that the local policy of the professed user and also MUST ensure that the local policy of the
server permits the user the access requested. In particular, because server permits the user the access requested. In particular, because
of the flexible nature of the SSH connection protocol, it may not be of the flexible nature of the SSH connection protocol, it may not be
possible to determine the local security policy, if any, that should possible to determine the local security policy, if any, that should
apply at the time of authentication because the kind of service being apply at the time of authentication because the kind of service being
requested is not clear at that instant. For example, local policy 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 might allow a user to access files on the server, but not start an
interactive shell. However, during the authentication protocol, it is interactive shell. However, during the authentication protocol, it
not known whether the user will be accessing files or attempting to is not known whether the user will be accessing files or attempting
use an interactive shell, or even both. In any event, where local 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 security policy for the server host exists, it MUST be applied and
enforced correctly. enforced correctly.
Implementors are encouraged to provide a default local policy and Implementors are encouraged to provide a default local policy and
make its parameters known to administrators and users. At the make its parameters known to administrators and users. At the
discretion of the implementors, this default policy may be along the discretion of the implementors, this default policy may be along the
lines of 'anything goes' where there are no restrictions placed upon lines of 'anything goes' where there are no restrictions placed upon
users, or it may be along the lines of 'excessively restrictive' in users, or it may be along the lines of 'excessively restrictive' in
which case the administrators will have to actively make changes to which case the administrators will have to actively make changes to
this policy to meet their needs. Alternatively, it may be some this policy to meet their needs. Alternatively, it may be some
attempt at providing something practical and immediately useful to attempt at providing something practical and immediately useful to
the administrators of the system so they don't have to put in much 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 effort to get SSH working. Whatever choice is made MUST be applied
and enforced as required above. and enforced as required above.
9.3.4 Public key authentication 9.3.4 Public Key Authentication
The use of public-key authentication assumes that the client host has The use of public-key authentication assumes that the client host has
not been compromised. It also assumes that the private-key of the not been compromised. It also assumes that the private-key of the
server host has not been compromised. server host has not been compromised.
This risk can be mitigated by the use of passphrases on private keys; This risk can be mitigated by the use of passphrases on private keys;
however, this is not an enforceable policy. The use of smartcards, however, this is not an enforceable policy. The use of smartcards,
or other technology to make passphrases an enforceable policy is or other technology to make passphrases an enforceable policy is
suggested. suggested.
The server could require both password and public-key authentication, The server could require both password and public-key authentication,
however, this requires the client to expose its password to the however, this requires the client to expose its password to the
server (see section on password authentication below.) server (see section on password authentication below.)
9.3.5 Password authentication 9.3.5 Password Authentication
The password mechanism as specified in the authentication protocol The password mechanism as specified in the authentication protocol
assumes that the server has not been compromised. If the server has assumes that the server has not been compromised. If the server has
been compromised, using password authentication will reveal a valid been compromised, using password authentication will reveal a valid
username / password combination to the attacker, which may lead to username / password combination to the attacker, which may lead to
further compromises. further compromises.
This vulnerability can be mitigated by using an alternative form of This vulnerability can be mitigated by using an alternative form of
authentication. For example, public-key authentication makes no authentication. For example, public-key authentication makes no
assumptions about security on the server. assumptions about security on the server.
9.3.6 Host based authentication 9.3.6 Host Based Authentication
Host based authentication assumes that the client has not been Host based authentication assumes that the client has not been
compromised. There are no mitigating strategies, other than to use compromised. There are no mitigating strategies, other than to use
host based authentication in combination with another authentication host based authentication in combination with another authentication
method. method.
9.4 Connection protocol 9.4 Connection Protocol
9.4.1 End point security 9.4.1 End Point Security
End point security is assumed by the connection protocol. If the End point security is assumed by the connection protocol. If the
server has been compromised, any terminal sessions, port forwarding, server has been compromised, any terminal sessions, port forwarding,
or systems accessed on the host are compromised. There are no or systems accessed on the host are compromised. There are no
mitigating factors for this. mitigating factors for this.
If the client end point has been compromised, and the server fails to If the client end point has been compromised, and the server fails to
stop the attacker at the authentication protocol, all services stop the attacker at the authentication protocol, all services
exposed (either as subsystems or through forwarding) will be exposed (either as subsystems or through forwarding) will be
vulnerable to attack. Implementors SHOULD provide mechanisms for vulnerable to attack. Implementors SHOULD provide mechanisms for
administrators to control which services are exposed to limit the administrators to control which services are exposed to limit the
vulnerability of other services. vulnerability of other services.
These controls might include controlling which machines and ports can These controls might include controlling which machines and ports can
be target in 'port-forwarding' operations, which users are allowed to be target in 'port-forwarding' operations, which users are allowed to
use interactive shell facilities, or which users are allowed to use use interactive shell facilities, or which users are allowed to use
exposed subsystems. exposed subsystems.
9.4.2 Proxy forwarding 9.4.2 Proxy Forwarding
The SSH connection protocol allows for proxy forwarding of other The SSH connection protocol allows for proxy forwarding of other
protocols such as SNMP, POP3, and HTTP. This may be a concern for protocols such as SNMP, POP3, and HTTP. This may be a concern for
network administrators who wish to control the access of certain network administrators who wish to control the access of certain
applications by users located outside of their physical location. applications by users located outside of their physical location.
Essentially, the forwarding of these protocols may violate site Essentially, the forwarding of these protocols may violate site
specific security policies as they may be undetectably tunneled specific security policies as they may be undetectably tunneled
through a firewall. Implementors SHOULD provide an administrative through a firewall. Implementors SHOULD provide an administrative
mechanism to control the proxy forwarding functionality so that site mechanism to control the proxy forwarding functionality so that site
specific security policies may be upheld. specific security policies may be upheld.
In addition, a reverse proxy forwarding functionality is available, In addition, a reverse proxy forwarding functionality is available,
which again can be used to bypass firewall controls. which again can be used to bypass firewall controls.
As indicated above, end-point security is assumed during proxy As indicated above, end-point security is assumed during proxy
forwarding operations. Failure of end-point security will compromise forwarding operations. Failure of end-point security will compromise
all data passed over proxy forwarding. all data passed over proxy forwarding.
9.4.3 X11 forwarding 9.4.3 X11 Forwarding
Another form of proxy forwarding provided by the ssh connection Another form of proxy forwarding provided by the SSH connection
protocol is the forwarding of the X11 protocol. If end-point protocol is the forwarding of the X11 protocol. If end-point
security has been compromised, X11 forwarding may allow attacks security has been compromised, X11 forwarding may allow attacks
against the X11 server. Users and administrators should, as a matter against the X11 server. Users and administrators should, as a matter
of course, use appropriate X11 security mechanisms to prevent of course, use appropriate X11 security mechanisms to prevent
unauthorized use of the X11 server. Implementors, administrators and unauthorized use of the X11 server. Implementors, administrators and
users who wish to further explore the security mechanisms of X11 are users who wish to further explore the security mechanisms of X11 are
invited to read [SCHEIFLER] and analyze previously reported problems invited to read [SCHEIFLER] and analyze previously reported problems
with the interactions between SSH forwarding and X11 in CERT with the interactions between SSH forwarding and X11 in CERT
vulnerabilities VU#363181 and VU#118892 [CERT]. vulnerabilities VU#363181 and VU#118892 [CERT].
X11 display forwarding with SSH, by itself, is not sufficient to X11 display forwarding with SSH, by itself, is not sufficient to
correct well known problems with X11 security [VENEMA]. However, X11 correct well known problems with X11 security [VENEMA]. However, X11
display forwarding in SSHv2 (or other, secure protocols), combined display forwarding in SSH (or other, secure protocols), combined with
with actual and pseudo-displays which accept connections only over actual and pseudo-displays which accept connections only over local
local IPC mechanisms authorized by permissions or ACLs, does correct IPC mechanisms authorized by permissions or ACLs, does correct many
many X11 security problems as long as the "none" MAC is not used. It X11 security problems as long as the "none" MAC is not used. It is
is RECOMMENDED that X11 display implementations default to allowing RECOMMENDED that X11 display implementations default to allowing
display opens only over local IPC. It is RECOMMENDED that SSHv2 display opens only over local IPC. It is RECOMMENDED that SSH server
server implementations that support X11 forwarding default to implementations that support X11 forwarding default to allowing
allowing display opens only over local IPC. On single-user systems display opens only over local IPC. On single-user systems it might
it might be reasonable to default to allowing local display opens be reasonable to default to allowing local display opens over TCP/IP.
over TCP/IP.
Implementors of the X11 forwarding protocol SHOULD implement the Implementors of the X11 forwarding protocol SHOULD implement the
magic cookie access checking spoofing mechanism as described in magic cookie access checking spoofing mechanism as described in
[ssh-connect] as an additional mechanism to prevent unauthorized use [SSH-CONNECT] as an additional mechanism to prevent unauthorized use
of the proxy. of the proxy.
Normative References 10. References
[SSH-ARCH] 10.1 Normative References
Ylonen, T., "SSH Protocol Architecture", I-D
draft-ietf-architecture-15.txt, Oct 2003.
[SSH-TRANS] [SSH-TRANS]
Ylonen, T., "SSH Transport Layer Protocol", I-D Ylonen, T. and C. Lonvick, "SSH Transport Layer Protocol",
draft-ietf-transport-17.txt, Oct 2003. I-D draft-ietf-transport-18.txt, May 2004.
[SSH-USERAUTH] [SSH-USERAUTH]
Ylonen, T., "SSH Authentication Protocol", I-D Ylonen, T. and C. Lonvick, "SSH Authentication Protocol",
draft-ietf-userauth-18.txt, Oct 2003. I-D draft-ietf-userauth-21.txt, May 2004.
[SSH-CONNECT] [SSH-CONNECT]
Ylonen, T., "SSH Connection Protocol", I-D Ylonen, T. and C. Lonvick, "SSH Connection Protocol", I-D
draft-ietf-connect-18.txt, Oct 2003. draft-ietf-connect-19.txt, May 2004.
[SSH-NUMBERS] [SSH-NUMBERS]
Lehtinen, S. and D. Moffat, "SSH Protocol Assigned Ylonen, T. and C. Lonvick, "SSH Protocol Assigned
Numbers", I-D draft-ietf-secsh-assignednumbers-05.txt, Oct Numbers", I-D draft-ietf-assignednumbers-06.txt, May 2004.
2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
Informative References 10.2 Informative References
[FIPS-186] [FIPS-186-2]
Federal Information Processing Standards Publication, Federal Information Processing Standards Publication,
"FIPS PUB 186, Digital Signature Standard", May 1994. "FIPS PUB 186-2, Digital Signature Standard (DSS)",
January 2000.
[FIPS-197] [FIPS-197]
National Institue of Standards and Technology, "FIPS 197, National Institute of Standards and Technology, "FIPS 197,
Specification for the Advanced Encryption Standard", Specification for the Advanced Encryption Standard",
November 2001. November 2001.
[ANSI T1.523-2001] [ANSI T1.523-2001]
American National Standards Insitute, Inc., "Telecom American National Standards Institute, Inc., "Telecom
Glossary 2000", February 2001. Glossary 2000", February 2001.
[SCHEIFLER] [SCHEIFLER]
Scheifler, R., "X Window System : The Complete Reference Scheifler, R., "X Window System : The Complete Reference
to Xlib, X Protocol, Icccm, Xlfd, 3rd edition.", Digital to Xlib, X Protocol, Icccm, Xlfd, 3rd edition.", Digital
Press ISBN 1555580882, Feburary 1992. Press ISBN 1555580882, February 1992.
[RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol
Specification", STD 8, RFC 854, May 1983. Specification", STD 8, RFC 854, May 1983.
[RFC0894] Hornig, C., "Standard for the transmission of IP datagrams [RFC0894] Hornig, C., "Standard for the transmission of IP datagrams
over Ethernet networks", STD 41, RFC 894, April 1984. over Ethernet networks", STD 41, RFC 894, April 1984.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1134] Perkins, D., "Point-to-Point Protocol: A proposal for [RFC1134] Perkins, D., "Point-to-Point Protocol: A proposal for
multi-protocol transmission of datagrams over multi-protocol transmission of datagrams over
Point-to-Point links", RFC 1134, November 1989. Point-to-Point links", RFC 1134, November 1989.
[RFC1282] Kantor, B., "BSD Rlogin", RFC 1282, December 1991. [RFC1282] Kantor, B., "BSD Rlogin", RFC 1282, December 1991.
[RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network [RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993. Authentication Service (V5)", RFC 1510, September 1993.
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
October 1994.
[RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness [RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994. Recommendations for Security", RFC 1750, December 1994.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
1964, June 1996. 1964, June 1996.
[RFC2025] Adams, C., "The Simple Public-Key GSS-API Mechanism [RFC2025] Adams, C., "The Simple Public-Key GSS-API Mechanism
(SPKM)", RFC 2025, October 1996. (SPKM)", RFC 2025, October 1996.
[RFC2085] Oehler, M. and R. Glenn, "HMAC-MD5 IP Authentication with [RFC2085] Oehler, M. and R. Glenn, "HMAC-MD5 IP Authentication with
Replay Prevention", RFC 2085, February 1997. Replay Prevention", RFC 2085, February 1997.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: [RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104, Keyed-Hashing for Message Authentication", RFC 2104,
February 1997. February 1997.
[RFC2246] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
and P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, RFC 2246, January 1999.
January 1999.
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998. 10646", RFC 2279, January 1998.
[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and [RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
Its Use With IPsec", RFC 2410, November 1998. Its Use With IPsec", RFC 2410, November 1998.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434, IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998. October 1998.
[RFC2743] Linn, J., "Generic Security Service Application Program [RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000. Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[SCHNEIER] [SCHNEIER]
Schneier, B., "Applied Cryptography Second Edition: Schneier, B., "Applied Cryptography Second Edition:
protocols algorithms and source in code in C", 1996. protocols algorithms and source in code in C", 1996.
[KAUFMAN,PERLMAN,SPECINER] [KAUFMAN,PERLMAN,SPECINER]
Kaufman, C., Perlman, R. and M. Speciner, "Network Kaufman, C., Perlman, R. and M. Speciner, "Network
Security: PRIVATE Communication in a PUBLIC World", 1995. Security: PRIVATE Communication in a PUBLIC World", 1995.
[CERT] CERT Coordination Center, The., "http://www.cert.org/nav/ [CERT] CERT Coordination Center, The.,
index_red.html". "http://www.cert.org/nav/index_red.html".
[VENEMA] Venema, W., "Murphy's Law and Computer Security", [VENEMA] Venema, W., "Murphy's Law and Computer Security",
Proceedings of 6th USENIX Security Symposium, San Jose CA Proceedings of 6th USENIX Security Symposium, San Jose CA
http://www.usenix.org/publications/library/proceedings/ http://www.usenix.org/publications/library/proceedings/
sec96/venema.html, July 1996. sec96/venema.html, July 1996.
[ROGAWAY] Rogaway, P., "Problems with Proposed IP Cryptography", [ROGAWAY] Rogaway, P., "Problems with Proposed IP Cryptography",
Unpublished paper http://www.cs.ucdavis.edu/~rogaway/ Unpublished paper http://www.cs.ucdavis.edu/~rogaway/
papers/draft-rogaway-ipsec-comments-00.txt, 1996. papers/draft-rogaway-ipsec-comments-00.txt, 1996.
skipping to change at page 27, line 35 skipping to change at page 27, line 34
Authors' Addresses Authors' Addresses
Tatu Ylonen Tatu Ylonen
SSH Communications Security Corp SSH Communications Security Corp
Fredrikinkatu 42 Fredrikinkatu 42
HELSINKI FIN-00100 HELSINKI FIN-00100
Finland Finland
EMail: ylo@ssh.com EMail: ylo@ssh.com
Darren J. Moffat (editor) Chris Lonvick (editor)
Sun Microsystems, Inc Cisco Systems, Inc
17 Network Circle 12515 Research Blvd.
Menlo Park CA 94025 Austin 78759
USA USA
EMail: Darren.Moffat@Sun.COM EMail: clonvick@cisco.com
Intellectual Property Statement Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and IETF's procedures with respect to rights in standards-track and
skipping to change at page 28, line 34 skipping to change at page 28, line 34
this standard. Please address the information to the IETF Executive this standard. Please address the information to the IETF Executive
Director. Director.
The IETF has been notified of intellectual property rights claimed in The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this regard to some or all of the specification contained in this
document. For more information consult the online list of claimed document. For more information consult the online list of claimed
rights. rights.
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
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