draft-ietf-secsh-architecture-06.txt   draft-ietf-secsh-architecture-07.txt 
Network Working Group T. Ylonen Network Working Group T. Ylonen
INTERNET-DRAFT T. Kivinen INTERNET-DRAFT T. Kivinen
draft-ietf-secsh-architecture-06.txt M. Saarinen draft-ietf-secsh-architecture-07.txt M. Saarinen
Expires in six months T. Rinne Expires: 9 July, 2001 T. Rinne
S. Lehtinen S. Lehtinen
SSH Communications Security SSH Communications Security
21 Nov, 2000 9 January, 2001
SSH Protocol Architecture SSH Protocol Architecture
Status of This memo Status of This Memo
This document is an Internet-Draft and is in full conformance This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026. with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as other groups may also distribute working documents as
Internet-Drafts. Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six Internet-Drafts are draft documents valid for a maximum of six
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2. Specification of Requirements . . . . . . . . . . . . . . . . . 2 2. Specification of Requirements . . . . . . . . . . . . . . . . . 2
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Host Keys . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Extensibility . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Extensibility . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 4 3.3. Policy Issues . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Security Properties . . . . . . . . . . . . . . . . . . . . 5 3.4. Security Properties . . . . . . . . . . . . . . . . . . . . 5
3.5. Packet Size and Overhead . . . . . . . . . . . . . . . . . . 5 3.5. Packet Size and Overhead . . . . . . . . . . . . . . . . . . 5
3.6. Localization and Character Set Support . . . . . . . . . . . 6 3.6. Localization and Character Set Support . . . . . . . . . . . 6
4. Data Type Representations Used in the SSH Protocols . . . . . . 7 4. Data Type Representations Used in the SSH Protocols . . . . . . 7
5. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Message Numbers . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Message Numbers . . . . . . . . . . . . . . . . . . . . . . . . 9
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 10
9. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . . 10 9. Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction 1. Introduction
SSH is a protocol for secure remote login and other secure network SSH is a protocol for secure remote login and other secure network
services over an insecure network. It consists of three major services over an insecure network. It consists of three major
components: components:
o 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 run also provide compression. The transport layer will typically be run
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All documents related to the SSH protocols shall use the keywords All documents related to the SSH protocols shall use the keywords
"MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
NOT", "RECOMMENDED, "MAY", and "OPTIONAL" to describe requirements. NOT", "RECOMMENDED, "MAY", and "OPTIONAL" to describe requirements.
They are to be interpreted as described in [RFC-2119]. They are to be interpreted as described in [RFC-2119].
3. Architecture 3. Architecture
3.1. Host Keys 3.1. Host Keys
Each server host MUST have a host key. Hosts MAY have multiple host Each server host SHOULD have a host key. Hosts MAY have multiple host
keys using multiple different algorithms. Multiple hosts MAY share the keys using multiple different algorithms. Multiple hosts MAY share the
same host key. Every host MUST have at least one key using each REQUIRED same host key. If a host has keys at all, it MUST have at least one key
public key algorithm (currently DSS [FIPS-186]). using each REQUIRED public key algorithm (currently DSS [FIPS-186]).
The server host key is used during key exchange to verify that the The server host key is used during key exchange to verify that the
client is really talking to the correct server. For this to be possible, client is really talking to the correct server. For this to be possible,
the client must have a priori knowledge of the server's public host key. the client must have a priori knowledge of the server's public host key.
Two different trust models can be used: Two different trust models can be used:
o The client has a local database that associates each host name (as o The client has a local database that associates each host name (as
typed by the user) with the corresponding public host key. This typed by the user) with the corresponding public host key. This
method requires no centrally administered infrastructure, and no method requires no centrally administered infrastructure, and no
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time. This allows communication without prior communication of host keys time. This allows communication without prior communication of host keys
or certification. The connection still provides protection against or certification. The connection still provides protection against
passive listening; however, it becomes vulnerable to active man-in-the- passive listening; however, it becomes vulnerable to active man-in-the-
middle attacks. Implementations SHOULD NOT normally allow such middle attacks. Implementations SHOULD NOT normally allow such
connections by default, as they pose a potential security problem. connections by default, as they pose a potential security problem.
However, as there is no widely deployed key infrastructure available on However, as there is no widely deployed key infrastructure available on
the Internet yet, this option makes the protocol much more usable during the Internet yet, this option makes the protocol much more usable during
the transition time until such an infrastructure emerges, while still the transition time until such an infrastructure emerges, while still
providing a much higher level of security than that offered by older providing a much higher level of security than that offered by older
solutions (e.g. telnet [RFC-854] and rlogin [RFC-1282]). solutions (e.g. telnet [RFC-854] and rlogin [RFC-1282]).
Implementations SHOULD try to make the best effort to check host keys. Implementations SHOULD try to make the best effort to check host keys.
An example of a possible strategy is to only accept a host key without An example of a possible strategy is to only accept a host key without
checking the first time a host is connected, save the key in a local
checking the first time a host is connected, save the key in a local
database, and compare against that key on all future connections to that database, and compare against that key on all future connections to that
host. host.
Implementations MAY provide additional methods for verifying the Implementations MAY provide additional methods for verifying the
correctness of host keys, e.g. a hexadecimal fingerprint derived from correctness of host keys, e.g. a hexadecimal fingerprint derived from
the SHA-1 hash of the public key. Such fingerprints can easily be the SHA-1 hash of the public key. Such fingerprints can easily be
verified by using telephone or other external communication channels. verified by using telephone or other external communication channels.
All implementations SHOULD provide an option to not accept host keys All implementations SHOULD provide an option to not accept host keys
that cannot be verified. that cannot be verified.
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delays in screen updates, one does not want excessively large packets delays in screen updates, one does not want excessively large packets
for interactive sessions. The maximum packet size is negotiated for interactive sessions. The maximum packet size is negotiated
separately for each channel. separately for each channel.
3.6. Localization and Character Set Support 3.6. Localization and Character Set Support
For the most part, the SSH protocols do not directly pass text that For the most part, the SSH protocols do not directly pass text that
would be displayed to the user. However, there are some places where would be displayed to the user. However, there are some places where
such data might be passed. When applicable, the character set for the such data might be passed. When applicable, the character set for the
data MUST be explicitly specified. In most places, ISO 10646 with UTF-8 data MUST be explicitly specified. In most places, ISO 10646 with UTF-8
encoding is used [RFC-2044]. When applicable, a field is also provided encoding is used [RFC-2279]. When applicable, a field is also provided
for a language tag [RFC-1766]. for a language tag [RFC-1766].
One big issue is the character set of the interactive session. There is One big issue is the character set of the interactive session. There is
no clear solution, as different applications may display data in no clear solution, as different applications may display data in
different formats. Different types of terminal emulation may also be different formats. Different types of terminal emulation may also be
employed in the client, and the character set to be used is effectively employed in the client, and the character set to be used is effectively
determined by the terminal emulation. Thus, no place is provided for determined by the terminal emulation. Thus, no place is provided for
directly specifying the character set or encoding for terminal session directly specifying the character set or encoding for terminal session
data. However, the terminal emulation type (e.g. "vt100") is data. However, the terminal emulation type (e.g. "vt100") is
transmitted to the remote site, and it implicitly specifies the transmitted to the remote site, and it implicitly specifies the
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For localization purposes, the protocol attempts to minimize the number For localization purposes, the protocol attempts to minimize the number
of textual messages transmitted. When present, such messages typically of textual messages transmitted. When present, such messages typically
relate to errors, debugging information, or some externally configured relate to errors, debugging information, or some externally configured
data. For data that is normally displayed, it SHOULD be possible to data. For data that is normally displayed, it SHOULD be possible to
fetch a localized message instead of the transmitted message by using a fetch a localized message instead of the transmitted message by using a
numerical code. The remaining messages SHOULD be configurable. numerical code. The remaining messages SHOULD be configurable.
4. Data Type Representations Used in the SSH Protocols 4. Data Type Representations Used in the SSH Protocols
byte byte
A byte represents an arbitrary 8-bit value (octet) [RFC1700]. A byte represents an arbitrary 8-bit value (octet) [RFC-1700].
Fixed length data is sometimes represented as an array of bytes, Fixed length data is sometimes represented as an array of bytes,
written byte[n], where n is the number of bytes in the array. written byte[n], where n is the number of bytes in the array.
boolean boolean
A boolean value is stored as a single byte. The value 0 A boolean value is stored as a single byte. The value 0
represents false, and the value 1 represents true. All non-zero represents FALSE, and the value 1 represents TRUE. All non-zero
values MUST be interpreted as true; however, applications MUST NOT values MUST be interpreted as TRUE; however, applications MUST NOT
store values other than 0 and 1. store values other than 0 and 1.
uint32 uint32
Represents a 32-bit unsigned integer. Stored as four bytes in the Represents a 32-bit unsigned integer. Stored as four bytes in the
order of decreasing significance (network byte order). order of decreasing significance (network byte order).
For example, the value 699921578 (0x29b7f4aa) is stored as 29 b7 For example, the value 699921578 (0x29b7f4aa) is stored as 29 b7
f4 aa. f4 aa.
uint64
Represents a 64-bit unsigned integer. Stored as eight bytes in
the order of decreasing significance (network byte order).
string string
Arbitrary length binary string. Strings are allowed to contain Arbitrary length binary string. Strings are allowed to contain
arbitrary binary data, including null characters and 8-bit arbitrary binary data, including null characters and 8-bit
characters. They are stored as a uint32 containing its length characters. They are stored as a uint32 containing its length
(number of bytes that follow) and zero (= empty string) or more (number of bytes that follow) and zero (= empty string) or more
bytes that are the value of the string. Terminating null bytes that are the value of the string. Terminating null
characters are not used. characters are not used.
Strings are also used to store text. In that case, US-ASCII is Strings are also used to store text. In that case, US-ASCII is
used for internal names, and ISO-10646 UTF-8 for text that might used for internal names, and ISO-10646 UTF-8 for text that might
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There are also algorithms that are defined in the protocol specification There are also algorithms that are defined in the protocol specification
but are OPTIONAL. Furthermore, it is expected that some organizations but are OPTIONAL. Furthermore, it is expected that some organizations
will want to use their own algorithms. will want to use their own algorithms.
In this protocol, all algorithm identifiers MUST be printable US-ASCII In this protocol, all algorithm identifiers MUST be printable US-ASCII
strings no longer than 64 characters. Names MUST be case-sensitive. strings no longer than 64 characters. Names MUST be case-sensitive.
There are two formats for algorithm names: There are two formats for algorithm names:
o Names that do not contain an at-sign (@) are reserved to be assigned o Names that do not contain an at-sign (@) are reserved to be assigned
by IANA (Internet Assigned Numbers Authority). Examples include by IETF consensus (RFCs). Examples include `3des-cbc', `sha-1',
`3des-cbc', `sha-1', `hmac-sha1', and `zlib' (the quotes are not part `hmac-sha1', and `zlib' (the quotes are not part of the name). Names
of the name). Additional names of this format may be registered with of this format MUST NOT be used without first registering them.
IANA; see Section ``IANA Considerations''. Names of this format MUST Registered names MUST NOT contain an at-sign (@) or a comma (,).
NOT be used without first registering with IANA. Registered names
MUST NOT contain an at-sign (@) or a comma (,).
o Anyone can define additional algorithms by using names in the format o Anyone can define additional algorithms by using names in the format
name@domainname, e.g. "ourcipher-cbc@ssh.fi". The format of the part name@domainname, e.g. "ourcipher-cbc@ssh.com". The format of the part
preceding the at sign is not specified; it MUST consist of US-ASCII preceding the at sign is not specified; it MUST consist of US-ASCII
characters except at-sign and comma. The part following the at-sign characters except at-sign and comma. The part following the at-sign
MUST be a valid fully qualified internet domain name [RFC-1034] MUST be a valid fully qualified internet domain name [RFC-1034]
controlled by the person or organization defining the name. It is up controlled by the person or organization defining the name. It is up
to each domain how it manages its local namespace. to each domain how it manages its local namespace.
6. Message Numbers 6. Message Numbers
SSH packets have message numbers in the range 1 to 255. These numbers SSH packets have message numbers in the range 1 to 255. These numbers
have been allocated as follows: have been allocated as follows:
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90 to 127 Channel related messages 90 to 127 Channel related messages
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
7. IANA Considerations
Allocation of the following types of names in the SSH protocols is Allocation of the following types of names in the SSH protocols is
assigned to IANA: assigned by IETF consensus:
o encryption algorithm names, o encryption algorithm names,
o MAC algorithm names, o MAC algorithm names,
o public key algorithm names (public key algorithm also implies o public key algorithm names (public key algorithm also implies
encoding and signature/encryption capability), encoding and signature/encryption capability),
o key exchange method names, and o key exchange method names, and
o protocol (service) names. o protocol (service) names.
The IANA-allocated names MUST be printable US-ASCII strings, and MUST These names MUST be printable US-ASCII strings, and MUST NOT contain the
NOT contain the characters at-sign ('@'), comma (','), or whitespace or characters at-sign ('@'), comma (','), or whitespace or control
control characters (ASCII codes 32 or less). Names are case-sensitive, characters (ASCII codes 32 or less). Names are case-sensitive, and MUST
and MUST NOT be longer than 64 characters. NOT be longer than 64 characters.
Names with the at-sign ('@') in them are allocated by the owner of DNS
name after the at-sign (hierarchical allocation in [RFC-2343]),
otherwise the same restrictions as above.
Each category of names listed above has a separate namespace. However, Each category of names listed above has a separate namespace. However,
using the same name in multiple categories SHOULD be avoided to minimize using the same name in multiple categories SHOULD be avoided to minimize
confusion. confusion.
Message numbers (see Section ``Message Numbers'') in the range of 0..191
should be allocated via IETF consensus; message numbers in the 192..255
range (the "Local extensions" set) are reserved for private use.
8. Security Considerations 8. Security Considerations
Special care should be taken to ensure that all of the random numbers Special care should be taken to ensure that all of the random numbers
are of good quality. The random numbers SHOULD be produced with safe are of good quality. The random numbers SHOULD be produced with safe
mechanisms discussed in [RFC1750]. mechanisms discussed in [RFC-1750].
When displaying text, such as error or debug messages to the user, the When displaying text, such as error or debug messages to the user, the
client software SHOULD replace any control characters (except tab, client software SHOULD replace any control characters (except tab,
carriage return and newline) with safe sequences to avoid attacks by carriage return and newline) with safe sequences to avoid attacks by
sending terminal control characters. sending terminal control characters.
Not using MAC or encryption SHOULD be avoided. The user authentication Not using MAC or encryption SHOULD be avoided. The user authentication
protocol is subject to man-in-the-middle attacks if the encryption is protocol is subject to man-in-the-middle attacks if the encryption is
disabled. The SSH protocol does not protect against message alteration disabled. The SSH protocol does not protect against message alteration
if no MAC is used. if no MAC is used.
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Shell is a trademark of SSH Communications Security Corp Shell is a trademark of SSH Communications Security Corp
(www.ssh.com)''. These trademarks may not be used as part of a product (www.ssh.com)''. These trademarks may not be used as part of a product
name or in otherwise confusing manner without prior written permission name or in otherwise confusing manner without prior written permission
of SSH Communications Security Corp. of SSH Communications Security Corp.
10. References 10. References
[FIPS-186] Federal Information Processing Standards Publication (FIPS [FIPS-186] Federal Information Processing Standards Publication (FIPS
PUB) 186, Digital Signature Standard, 18 May 1994. PUB) 186, Digital Signature Standard, 18 May 1994.
[RFC-854] Postel, J. and Reynolds, J., "Telnet Protocol Specification", [RFC-854] Postel, J. and Reynolds, J: "Telnet Protocol Specification",
May 1983. May 1983.
[RFC-894] Hornig, C., "A Standard for the Transmission of IP Datagrams [RFC-894] Hornig, C: "A Standard for the Transmission of IP Datagrams
over Ethernet Networks", April 1984. over Ethernet Networks", April 1984.
[RFC-1034] Mockapetris, P., "Domain Names - Concepts and Facilities", [RFC-1034] Mockapetris, P: "Domain Names - Concepts and Facilities",
November 1987. November 1987.
[RFC-1134] Perkins, D., "The Point-to-Point Protocol: A Proposal for [RFC-1134] Perkins, D: "The Point-to-Point Protocol: A Proposal for
Multi-Protocol Transmission o Datagrams Over Point-to-Point Links", Multi-Protocol Transmission of Datagrams Over Point-to-Point Links",
November 1989. November 1989.
[RFC-1282] Kantor, B., "BSD Rlogin", December 1991. [RFC-1282] Kantor, B: "BSD Rlogin", December 1991.
[RFC-1700] Reynolds, J. and Postel, J., "Assigned Numbers", October 1994 [RFC-1700] Reynolds, J. and Postel, J: "Assigned Numbers", October 1994
(also STD 2). (also STD 2).
[RFC-1750] Eastlake, D., Crocker, S., and Schiller, J., "Randomness [RFC-1750] Eastlake, D., Crocker, S., and Schiller, J: "Randomness
Recommendations for Security", December 1994. Recommendations for Security", December 1994.
[RFC-1766] Alvestrand, H., "Tags for the Identification of Languages", [RFC-1766] Alvestrand, H: "Tags for the Identification of Languages",
March 1995. March 1995.
[RFC-2044] Yergeau, F., "UTF-8, a Transformation Format of Unicode and [RFC-2279] Yergeau, F: "UTF-8, a transformation format of ISO 10646",
ISO 10646", October 1996. January 1998.
[RFC-2119] Bradner, S., "Key words for use in RFCs to indicate [RFC-2119] Bradner, S: "Key words for use in RFCs to indicate
Requirement Levels", March 1997 Requirement Levels", March 1997.
[Schneier] Schneier, B., "Applied Cryptography Second Edition", John [RFC-2343] Narten, T. and Alvestrand, H: "Guidelines for Writing an IANA
Wiley & Sons, New York, NY, 1995. Considerations Section in RFCs", October 1998.
[SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol", Internet [SSH-TRANS] Ylonen, T., et al: "SSH Transport Layer Protocol", Internet-
Draft, draft-ietf-secsh-transport-07.txt Draft, draft-ietf-secsh-transport-09.txt
[SSH-USERAUTH] Ylonen, T., et al, "SSH Authentication Protocol", [SSH-USERAUTH] Ylonen, T., et al: "SSH Authentication Protocol",
Internet Draft, draft-ietf-secsh-userauth-07.txt Internet-Draft, draft-ietf-secsh-userauth-09.txt
[SSH-CONNECT] Ylonen, T., et al, "SSH Connection Protocol", Internet [SSH-CONNECT] Ylonen, T., et al: "SSH Connection Protocol", Internet-
Draft, draft-ietf-secsh-connect-07.txt Draft, draft-ietf-secsh-connect-09.txt
11. Authors' Addresses
Tatu Ylonen Tatu Ylonen
SSH Communications Security Corp SSH Communications Security Corp
Fredrikinkatu 42 Fredrikinkatu 42
FIN-00100 HELSINKI FIN-00100 HELSINKI
Finland Finland
E-mail: ylo@ssh.com E-mail: ylo@ssh.com
Tero Kivinen Tero Kivinen
SSH Communications Security Corp SSH Communications Security Corp
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