Internet DRAFT - draft-bider-ssh-quic
draft-bider-ssh-quic
Internet Engineering Task Force d. bider
Internet-Draft Bitvise Limited
Intended status: Informational 2 December 2020
Expires: 5 June 2021
QUIC-based UDP Transport for Secure Shell (SSH)
draft-bider-ssh-quic-09
Abstract
The Secure Shell protocol (SSH) [RFC4251] is widely used for purposes
including secure remote administration, file transfer using SFTP and
SCP, and encrypted tunneling of TCP connections. Because it is based
on TCP, SSH suffers similar problems as motivate the HTTP protocol to
transition to UDP-based QUIC [QUIC]. These include: unauthenticated
network intermediaries can trivially disconnect SSH sessions; SSH
connections are lost when mobile clients change IP addresses;
performance limitations in OS-based TCP stacks; many round-trips to
establish a connection; duplicate flow control on the level of the
connection as well as channels. This memo specifies SSH key exchange
over UDP and leverages QUIC to provide a UDP-based transport.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 June 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Terminology . . . . . . . . . . . . . . . . 3
2. SSH/QUIC key exchange . . . . . . . . . . . . . . . . . . . . 3
2.1. Distinguishing SSH key exchange from QUIC datagrams . . . 3
2.2. Wire Encoding . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Obfuscated Envelope . . . . . . . . . . . . . . . . . . . 4
2.3.1. Obfuscation Keyword . . . . . . . . . . . . . . . . . 5
2.4. Packet Size Limits . . . . . . . . . . . . . . . . . . . 6
2.5. Required QUIC Versions and TLS Cipher Suites . . . . . . 6
2.6. Random Elements . . . . . . . . . . . . . . . . . . . . . 6
2.7. Errors in Key Exchange . . . . . . . . . . . . . . . . . 8
2.7.1. "disc-reason" Extension Pair . . . . . . . . . . . . 8
2.7.2. "err-desc" Extension Pair . . . . . . . . . . . . . . 9
2.8. SSH_QUIC_INIT . . . . . . . . . . . . . . . . . . . . . . 9
2.8.1. Extensibility . . . . . . . . . . . . . . . . . . . . 13
2.9. SSH_QUIC_REPLY . . . . . . . . . . . . . . . . . . . . . 14
2.9.1. Error Reply . . . . . . . . . . . . . . . . . . . . . 17
2.9.2. Extensibility . . . . . . . . . . . . . . . . . . . . 17
2.10. SSH_QUIC_CANCEL . . . . . . . . . . . . . . . . . . . . . 19
2.10.1. Extensibility . . . . . . . . . . . . . . . . . . . 19
3. Key Exchange Methods . . . . . . . . . . . . . . . . . . . . 20
3.1. Required Key Exchange Methods . . . . . . . . . . . . . . 21
3.2. Example 1: "curve25519-sha256" . . . . . . . . . . . . . 22
3.3. Example 2: "diffie-hellman-group14-sha256" . . . . . . . 22
4. SSH_MSG_EXT_INFO and the SSH Version String . . . . . . . . . 23
4.1. "ssh-version" . . . . . . . . . . . . . . . . . . . . . . 24
4.2. "no-flow-control" . . . . . . . . . . . . . . . . . . . . 24
4.3. "delay-compression" . . . . . . . . . . . . . . . . . . . 24
5. QUIC Session Setup . . . . . . . . . . . . . . . . . . . . . 25
5.1. Shared Secrets . . . . . . . . . . . . . . . . . . . . . 25
6. Adaptation of SSH to QUIC Streams . . . . . . . . . . . . . . 26
6.1. SSH/QUIC Packet Format . . . . . . . . . . . . . . . . . 26
6.1.1. Compression . . . . . . . . . . . . . . . . . . . . . 26
6.2. Use of QUIC Streams . . . . . . . . . . . . . . . . . . . 27
6.3. Packet Sequence Numbers . . . . . . . . . . . . . . . . . 27
6.4. Channel IDs . . . . . . . . . . . . . . . . . . . . . . . 27
6.5. Disconnection . . . . . . . . . . . . . . . . . . . . . . 28
6.6. Prohibited SSH Packets . . . . . . . . . . . . . . . . . 28
6.7. Global SSH Packets . . . . . . . . . . . . . . . . . . . 28
6.8. SSH Channel Packets . . . . . . . . . . . . . . . . . . . 29
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6.9. Closing a Channel . . . . . . . . . . . . . . . . . . . . 31
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9. Security Considerations . . . . . . . . . . . . . . . . . . . 32
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Normative References . . . . . . . . . . . . . . . . . . 32
10.2. Informative References . . . . . . . . . . . . . . . . . 33
Appendix A. Generating Random Lengths . . . . . . . . . . . . . 34
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
THIS DOCUMENT IS AN EARLY VERSION AND IS A WORK IN PROGRESS.
NON-LATEST DRAFT VERSIONS MUST BE DISREGARDED.
IMPLEMENTATION AT THIS STAGE IS EXPERIMENTAL.
CONTACT THE AUTHOR IF YOU INTEND TO IMPLEMENT.
This memo specifies SSH key exchange over UDP, and then leverages
QUIC to provide a UDP-based transport for SSH. QUIC's use of the TLS
handshake is replaced with a one-roundtrip SSH/QUIC key exchange.
The SSH Authentication Protocol [RFC4252] is then conducted over QUIC
stream 0, and the SSH Connection Protocol [RFC4254] is modified to
use QUIC streams.
1.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. SSH/QUIC key exchange
2.1. Distinguishing SSH key exchange from QUIC datagrams
UDP datagrams which form the SSH/QUIC key exchange are sent between
the same client and server IP addresses and ports as QUIC datagrams.
It is therefore necessary for clients and servers to distinguish SSH
key exchange datagrams from QUIC datagrams.
A distinction is allowed by that SSH/QUIC only requires the sending
of QUIC Short Header Packets. Therefore, all UDP datagrams where the
first byte has its high bit set MUST be handled as part of an SSH/
QUIC key exchange.
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2.2. Wire Encoding
This memo uses wire encoding types "byte", "uint32", "uint64",
"mpint" and "string" with meanings as described in [RFC4251].
This memo defines the following new wire encoding type.
"short-str" is a shorter version of "string", encoded as follows:
byte n = short-str-len (unsigned, 0..255)
byte[n] short-str-value
Figure 1
2.3. Obfuscated Envelope
Since SSH servers are commonly used for remote administration, they
are a high-value target for password guessing. One of the most
common complaints from SSH server administrators is the high
frequency of password guessing connections from random clients.
Experience shows that obfuscating the SSH protocol with an
obfuscation keyword is a valuable measure which thwarts password
guessing. This increases practical security of the SSH ecosystem
even if obfuscation does not thwart narrowly targeted attacks. In
the same way, glass windows and low fences are generally useful, even
if they're not impenetrable barriers. They reduce the number of
threats that must be focused on, and allow a greater focus on real
threats.
Every SSH/QUIC connection is parameterized by an obfuscation keyword.
The obfuscation keyword is processed according to Section 2.3.1.
The obfuscation keyword is a common secret shared between a server
and its clients. It has similar characteristics as a TCP or UDP port
number, but is unlimited in possible values. An obfuscation keyword
is chosen by the server administrator. For a client to successfully
connect, it needs to be told the obfuscation keyword, just like it
needs to know the server address and port number. If a client is not
told the obfuscation keyword, it can assume it's empty, just like it
can assume a default, most commonly used port.
An SSH/QUIC server SHOULD allow the administrator to configure an
obfuscation keyword for each interface and port on which the server
is accepting SSH/QUIC connections. An SSH/QUIC client MUST allow the
user to configure an obfuscation keyword separately for outgoing
connections to each server address and port.
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The obfuscation keyword MUST be optional for users to configure. If
a user does not configure it, the obfuscated envelope is applied as
if the obfuscation keyword was an empty character sequence.
All SSH/QUIC key exchange packets are sent as UDP datagrams in the
following obfuscated envelope:
byte[16] obfs-nonce - high bit of first byte MUST be set
byte[] obfs-payload
byte[16] obfs-tag
Figure 2
The field "obfs-nonce" contains random bytes generated by the sender
of the UDP datagram. The high bit of the first byte of "obfs-nonce"
MUST be set to distinguish the packet from QUIC datagrams. See
Section 2.1.
The field "obfs-payload" contains the SSH/QUIC key exchange packet
encrypted using AEAD_AES_256_GCM [RFC5116]. The AEAD is invoked as
follows:
* The secret key K is a SHA-256 digest of the obfuscation keyword,
processed according to Section 2.3.1.
* The nonce N is the field "obfs-nonce".
* The plaintext P is the unencrypted packet payload.
* Associated data A is empty.
* The ciphertext C is stored in "obfs-payload".
The length of encrypted "obfs-payload" is implied by the UDP datagram
length, and is calculated by subtracting the fixed lengths of "obfs-
nonce" and "obfs-tag".
The field "obfs-tag" stores the GCM tag. Receivers MUST check the
tag and MUST ignore datagrams where the GCM tag is invalid.
2.3.1. Obfuscation Keyword
The obfuscation keyword is a sequence of Unicode code points entered
by a user. Applications MUST permit users to enter any Unicode
string allowed by the FreeformClass string class defined in
[RFC8264].
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Before calculating the digest of the obfuscation keyword,
applications MUST process the obfuscation keyword as follows:
1. Process the input according to the OpaqueString profile defined
in [RFC8265].
2. Remove any leading and trailing sequences consisting only of
characters CHARACTER TABULATION (U+0009), LINE FEED (U+000A),
CARRIAGE RETURN (U+000D) and/or SPACE (U+0020). This minimizes
user copy-and-paste errors, where the user is likely to copy
leading and trailing whitespace which is not part of the
obfuscation keyword. Note that the previous step, the
OpaqueString profile, already converted any non-ASCII whitespace
to SPACE (U+0020).
3. Encode the result as a sequence of bytes using UTF-8.
2.4. Packet Size Limits
Clients and servers MUST accept SSH_QUIC_INIT, SSH_QUIC_REPLY and
SSH_QUIC_CANCEL packets with unencrypted "obfs-payload" sizes at
least up to 32768 bytes. This corresponds to minimum SSH packet size
limits which implementations must support as per [RFC4253],
Section 6.1.
2.5. Required QUIC Versions and TLS Cipher Suites
Clients and servers are REQUIRED to implement QUIC protocol version 1
once it is standardized in [QUIC] and [QUIC-TLS].
Clients and servers are REQUIRED to implement the TLS cipher suites
TLS_AES_128_GCM_SHA256 and TLS_AES_256_GCM_SHA384 [RFC8446]. Other
cipher suites are optional.
The requirement to implement any particular QUIC protocol version or
TLS cipher suite expires on the 5-year anniversary of the publishing
of this memo. At that point, implementers SHOULD consult any new
standards documents if available, or survey the practical use of SSH/
QUIC for implementation guidance.
2.6. Random Elements
Unlike SSH over TCP, the packets SSH_QUIC_INIT and SSH_QUIC_REPLY do
not provide a "cookie" field for random data. Instead, clients and
servers MUST insert random data using the extensibility mechanisms
described for each SSH key exchange packet.
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At the very minimum, clients and servers MUST insert at least 16
Random Bytes or at least one Random Name, in locations as described
for SSH_QUIC_INIT (Section 2.8.1) and SSH_QUIC_REPLY (Section 2.9.2).
If at all possible, the random data MUST come from a
cryptographically strong random source. Implementations that are
unable to meet this requirement MUST still insert the minimum amount
of random data, as unpredictably as they are able. Compromising on
this requirement reduces the security of any sessions created on the
basis of such SSH_QUIC_INIT and SSH_QUIC_REPLY.
Lengths of Random Names and Random Bytes SHOULD be chosen at random
such that lengths in the shorter end of the range are significantly
more probable, but long lengths are still selected. See Appendix A.
Random Bytes
Random Bytes are generated with values 0..255, in a range of lengths
as specified for the particular usage context.
Random Name
A Random Name is generated in one of two forms: Assigned Form or
Private Form. One of the two forms is randomly chosen so that
Assigned Form, which is shorter, is more likely. The maximum length
of a Random Name is 64 bytes.
Assigned Form
A Random Name in Assigned Form is generated as a string of random
characters with ASCII values 33..126 (inclusive), except @ and the
comma (","). Other characters MUST NOT be included. To avoid
collisions as effectively as a random UUID, a Random Name in Assigned
Form MUST contain at least 20 random characters if using the complete
character set. A Random Name in Assigned Form MUST then be of length
20..64 bytes.
Implementations MAY remove up to 7 characters from the character set
-- reducing it to 85..91 characters -- without increasing the minimum
length. If the character set is further reduced to 69..84
characters, implementations MUST generate at least 21 random
characters instead.
Example Random Names in Assigned Form:
d`kbi>AGrj~r{3lo_Q4r
wNT)=/8C<(DB1|tr:>1f[xq>9bG
u7^dE'\EE_}N}^"J5syI?/8jIxup#s7BM:]>{IT_p3Z~<KLa]bIW643XYh07jqZu
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Figure 3
Private Form
Implementations MAY generate a Random Name in Private Form by first
generating a Random Name in Assigned Form, then appending a domain
name suffix which the implementer controls. A Random Name generated
this way MUST NOT exceed 64 bytes. Example Random Names in Private
Form:
(qKR8W%&zJu;$RQkWa[b@bitvise.com
BDPhhC_vI?+8$e_CGty->wJDYIBX.4zzQ$@denisbider.com
?`z4bb/}</P[pRJ=SvcCV<k0eUPDIHid#e1giY>&Wuf6O7CE?cA`$j"@bider.us
Figure 4
Alternately, implementations MAY generate a Random Name in Anonymous
Form with the format "(local)@(domain).example.com". In this case,
both "(local)" and "(domain)" are replaced by random ASCII characters
from the set A..Z, a..z, and 0..9. This is to ensure that the suffix
has valid domain name syntax.
To avoid collisions as effectively as a random UUID, a Random Name in
Anonymous Form MUST contain at least 22 random characters. A Random
Name in Anonymous Form MUST then be of length 35..64 bytes.
2.7. Errors in Key Exchange
To assist users, clients and servers SHOULD report key exchange
errors as follows:
1. If a server cannot send a successful SSH_QUIC_REPLY, it SHOULD
send an Error Reply. See Section 2.9.1.
2. If a client receives an invalid SSH_QUIC_REPLY, it SHOULD send an
SSH_QUIC_CANCEL. See Section 2.10.
Both packet types use the following extension pairs.
2.7.1. "disc-reason" Extension Pair
"ext-pair-name" contains "disc-reason".
"ext-pair-data" encodes a uint32 with the SSH disconnect reason code.
Reason codes are defined in the table "Disconnect Messages Reason
Codes and Descriptions" in the IANA registry "Secure Shell (SSH)
Protocol Parameters" [IANA-SSH].
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2.7.2. "err-desc" Extension Pair
"ext-pair-name" contains "err-desc".
"ext-pair-data" encodes a human-readable error description in any
language intended to be relevant to the user, encoded as UTF-8.
Receivers that process error descriptions MUST validate that the
description is valid UTF-8. If a description is long, receivers
SHOULD truncate it to a reasonable length depending on the processing
context. For example, a debug log file can record a full 32 kB error
description, while a production log file SHOULD truncate it to a much
shorter length.
2.8. SSH_QUIC_INIT
A client begins an SSH/QUIC session by sending one or more copies of
SSH_QUIC_INIT. If multiple copies are sent, copies intended for the
same connection MUST be identical. A reasonable strategy is to send
one copy every 50 - 500 ms until the client receives a valid
SSH_QUIC_REPLY or times out. A server MUST remember recently
received SSH_QUIC_INIT packets and send identical SSH_QUIC_REPLY
responses. If different SSH_QUIC_INIT packets are received from the
same client IP address, the server MUST assume they are intended to
begin separate connections, even if they specify the same "client-
connection-id". A server MAY implement throttling of incoming
connections, by IP address or otherwise, where excessive
SSH_QUIC_INIT packets are disregarded. Once a server receives QUIC
data confirming that a client has processed an SSH_QUIC_REPLY, the
server MUST disregard any further identical copies of the same
SSH_QUIC_INIT, at least until the SSH/QUIC session started by such an
SSH_QUIC_INIT ends.
SSH_QUIC_INIT is an obfuscated datagram (Section 2.3) where "obfs-
payload" encrypts the following:
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byte SSH_QUIC_INIT = 1 (see Extensibility)
short-str client-connection-id (MAY be empty)
short-str server-name-indication (MAY be empty)
byte v = nr-quic-versions (MUST NOT be zero)
uint32[v] client-quic-versions
string client-quic-trnsp-params
string client-sig-algs (MUST NOT be empty)
byte f = nr-trusted-fingerprints (MAY be zero)
the following 1 field repeated f times:
short-str trusted-fingerprint (MUST NOT be empty)
byte k = nr-client-kex-algs (MUST NOT be zero)
the following 2 fields repeated k times:
short-str client-kex-alg-name (MUST NOT be empty)
string client-kex-alg-data (MUST NOT be empty)
byte c = nr-cipher-suites (MUST NOT be zero)
the following 1 field repeated c times:
short-str quic-tls-cipher-suite
byte e = nr-ext-pairs (see Extensibility)
the following 2 fields repeated e times:
short-str ext-pair-name (MUST NOT be empty)
string ext-pair-data (MAY be empty)
byte[0..] padding: all 0xFF to minimal obfs-payload size 1200
Figure 5
SSH_QUIC_INIT does not include an SSH version string or compression
negotiation. Instead, clients MUST use SSH_MSG_EXT_INFO for these
purposes. See Section 4.
SSH_QUIC_INIT does not include a "cookie" field for random data.
Clients MUST insert random data using the packet's extensibility
mechanisms. See Section 2.8.1 and Section 2.6.
The field "client-connection-id" contains a QUIC Connection ID of
length 0..20 bytes. The server will use this as the QUIC Destination
Connection ID in QUIC packets sent to the client. Clients MAY send
an empty Connection ID if they are using other means of routing
connections.
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The field "server-name-indication" SHOULD contain the server DNS name
if a DNS name was entered by the user when configuring the
connection. This can be invaluable in hosting environments: it
allows servers to expose to clients multiple distinct identities on
the same network address and port. If non-empty, the field MUST
encode the DNS name entered by the user as a string consisting of
printable US-ASCII characters. Internationalized domain names MUST
be represented in their US-ASCII encoding. If the user connected
directly to an IP address, this field MUST be empty. This avoids
disclosing private information in case of port forwarded connections.
Example non-empty values:
localhost
server.example.com
xn--bcher-kva.example
Figure 6
The fields "client-quic-versions" enumerate QUIC protocol versions
supported by the client. The client MUST send at least one version.
The client MUST send supported versions in the order it prefers the
server to use them.
The field "client-quic-trnsp-params" encodes the client's QUIC
Transport Parameters as defined in [QUIC].
The field "client-sig-algs" MUST contain at least one signature
algorithm supported by the client for server authentication. These
are the same algorithms as used in SSH_MSG_KEXINIT ([RFC4253],
Section 7.1) in the field "server_host_key_algorithms". The client
MUST send signature algorithms in the order it prefers the server to
use them.
The client SHOULD include algorithms in "client-sig-algs" as follows:
* If the client does not yet trust any host key for the server:
"client-sig-algs" SHOULD include all signature algorithms
supported and enabled by the client for use with any server.
* Otherwise, the client already trusts some host keys for the
server. In this case, if the client sends any "trusted-
fingerprint" fields, then "client-sig-algs" SHOULD include all
signature algorithms supported and enabled by the client for use
with any server.
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* Otherwise, the client already trusts some host keys for the
server, but does not send any "trusted-fingerprint" fields. In
this case, "client-sig-algs" MUST include only signature
algorithms associated with the host keys the client already trusts
for this server.
There MAY be zero or more "trusted-fingerprint" fields. Each
"trusted-fingerprint" contains a binary fingerprint of a host key
that is trusted for this connection by the client. The fingerprint
algorithm is left unspecified. The server SHOULD try to match the
fingerprint using all algorithms it supports which produce the
provided fingerprint size. The current recommended fingerprint
algorithm is SHA-256, with fingerprint size 32 bytes. Servers MUST
tolerate the presence of unrecognized fingerprints of any size. The
preference order of trusted fingerprints is dominated by the
preference order of algorithms in "client-sig-algs".
The packet MUST include at least one SSH key exchange algorithm,
encoded as a pair of "client-kex-alg-name" and "client-kex-alg-data"
fields. The field "client-kex-alg-name" MUST specify a key exchange
method which would be valid in the field "kex_algorithms" in
SSH_MSG_KEXINIT under [RFC4253], Section 7.1. In addition, the key
exchange method MUST meet criteria in Section 3.
If the client wishes to simply advertise its support for a particular
key exchange algorithm, but does not prefer to use it in this
connection, it MAY enumerate the algorithm with empty "client-kex-
alg-data". Otherwise, if the client wishes to allow the algorithm to
be used, it MUST include non-empty "client-kex-alg-data". In this
case, "client-kex-alg-data" contains the client's portion of key
exchange inputs as specified in Section 3. The client MAY send
multiple key exchange algorithms with filled-out "client-kex-alg-
data". The client MUST send these algorithms in the order it prefers
the server to use them.
There MUST be at least one "quic-tls-cipher-suite" field. Each of
these specifies a TLS cipher suite ([RFC8446], Appendix B.4) which is
supported by the client, and which can be used with a version of QUIC
([QUIC], [QUIC-TLS]) supported by the client. The client MUST
enumerate supported cipher suites in the order it prefers the server
to use them.
The client MAY send any number of extensions, encoded as a pair of
"ext-pair-name" and "ext-pair-data" fields. This memo defines no
extensions for SSH_QUIC_INIT, but see Section 2.8.1.
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The "padding" field contains all 0xFF bytes to ensure that the
unencrypted "obfs-payload" for SSH_QUIC_INIT is at least 1200 bytes
in length. Servers MUST ignore smaller SSH_QUIC_INIT packets. This
is REQUIRED to prevent abuse of SSH_QUIC_INIT for Amplified
Reflection DDoS. If the unencrypted size of "obfs-payload" is
already 1200 bytes or larger, the padding MAY be omitted.
2.8.1. Extensibility
Implementations MUST allow room for future extensibility of
SSH_QUIC_INIT in the following manners:
1. By using a different packet type in the first byte -- this is, a
value other than 1 used by SSH_QUIC_INIT. Servers MUST NOT
penalize clients for sending unknown packet types unless there is
another reason to penalize the client, such as a blocked IP
address or the sheer volume of datagrams.
2. By including algorithms in "client-sig-algs" which are unknown to
or not supported by the server. Servers MUST tolerate the
presence of such algorithms.
3. By including fingerprints in "trusted-fingerprints" that use
algorithms or lengths that are unknown to or not supported by the
server. Servers MUST tolerate the presence of such fingerprints.
4. By including SSH key exchange algorithms which are unknown to or
not supported by the server, with algorithm data in a format
that's unknown to or not supported by the server. Servers MUST
tolerate the presence of such algorithms and their data.
5. By including QUIC TLS cipher suites which are unknown to or not
supported by the server. Servers MUST tolerate the presence of
such cipher suites.
6. By including extensions which are unknown to or not supported by
the server, with extension data in a format that's unknown to or
not supported by the server. Servers MUST tolerate the presence
of such extensions and their data.
Experience shows that any extensibility which is not actively
exercised is lost due to implementations that lock down expectations
incorrectly. Therefore, all clients MUST do at least one of the
following, in each SSH_QUIC_INIT packet, at random:
1. In the field "client-sig-algs", include in a random position at
least one Random Name (Section 2.6).
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2. In the fields "client-quic-versions", include in a random
position a version number of the form 0x0A?A?A?A, where ?
indicates a random nibble. See [QUIC], section "Versions". Note
the difference from the random version pattern in the server's
SSH_QUIC_REPLY. Due to the minimal amount of entropy provided by
this rule, this MUST NOT be the only insertion of randomness made
in a packet.
3. Include in a random position at least one host key fingerprint
consisting of 16..255 Random Bytes (Section 2.6).
4. Include in a random position at least one SSH key exchange
algorithm where the field "client-kex-alg-name" contains a Random
Name, and the field "client-kex-alg-data" contains 0..1000 Random
Bytes.
5. In the fields "quic-tls-cipher-suite", include in a random
position at least one entry consisting of 16..255 Random Bytes.
6. In extension pairs, include in a random position at least one
extension where the field "ext-pair-name" contains a Random Name,
and the field "ext-pair-value" contains 0..1000 Random Bytes.
2.9. SSH_QUIC_REPLY
Implementations MUST take care to prevent abuse of the SSH/QUIC key
exchange for Amplified Reflection DDoS attacks. This means:
1. A server MUST NOT send more than one SSH_QUIC_REPLY in response
to any individual SSH_QUIC_INIT.
2. A server MUST NOT respond to any SSH_QUIC_INIT with unencrypted
"obfs-payload" smaller than 1200 bytes.
3. Before sending an SSH_QUIC_REPLY, the server MUST verify that the
reply is shorter than the SSH_QUIC_INIT packet to which it is
replying. If this is not the case, the server MUST send an Error
Reply (Section 2.9.1). Such an Error Reply MUST be shorter than
the SSH_QUIC_INIT packet.
SSH_QUIC_REPLY is an obfuscated datagram (Section 2.3) where "obfs-
payload" encrypts the following:
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byte SSH_QUIC_REPLY = 2
short-str client-connection-id
short-str server-connection-id (Non-empty except on error)
byte v = nr-quic-versions (MUST NOT be zero)
uint32[v] server-quic-versions
string server-quic-trnsp-params
string server-sig-algs (MUST NOT be empty)
string server-kex-algs (MUST NOT be empty)
byte c = nr-cipher-suites (MUST NOT be zero)
the following 1 field repeated c times:
short-str quic-tls-cipher-suite
byte e = nr-ext-pairs (see Extensibility)
the following 2 fields repeated e times:
short-str ext-pair-name (MUST NOT be empty)
string ext-pair-data (MAY be empty)
string server-kex-alg-data (Non-empty except on error)
Figure 7
SSH_QUIC_REPLY does not include an SSH version string or compression
negotiation. Instead, servers MUST use SSH_MSG_EXT_INFO for these
purposes. See Section 4.
SSH_QUIC_REPLY does not include a "cookie" field for random data.
Servers MUST insert random data using the packet's extensibility
mechanisms. See Section 2.9.2 and Section 2.6.
The field "client-connection-id" encodes the "client-connection-id"
sent by the client in SSH_QUIC_INIT.
The field "server-connection-id" contains a QUIC Connection ID of
length 1..20 bytes. The client will use this as the QUIC Destination
Connection ID in QUIC packets sent to the server. This field MUST be
empty if sending an Error Reply (Section 2.9.1), and MUST NOT be
empty otherwise.
The fields "server-quic-versions" enumerate QUIC protocol versions
supported by the server. The server MUST send at least one version.
The QUIC version used for the connection is the first version
enumerated in "client-quic-versions" which is also present in
"server-quic-versions". If there is no such version, see
Section 2.9.1.
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The field "server-quic-trnsp-params" encodes the server's QUIC
Transport Parameters as defined in [QUIC].
The field "server-sig-algs" MUST contain at least one signature
algorithm supported by the server. The server SHOULD enumerate all
signature algorithms for which it has host keys. These are the same
algorithms as used in SSH_MSG_KEXINIT ([RFC4253], Section 7.1) in the
field "server_host_key_algorithms". In the SSH/QUIC key exchange,
the server MUST use a host key it possesses that (1) matches any
fingerprint enumerated in the "trusted-fingerprint" fields in
SSH_QUIC_INIT; and (2) can be used with the earliest possible
signature algorithm enumerated in "client-sig-algs". If there are
multiple such host keys, the client's preference order in "client-
sig-algs" dominates the preference order of "trusted-fingerprint".
If there is no such host key, the server MUST use any host key that
can be used with the earliest possible signature algorithm enumerated
in "client-sig-algs". If there is no such host key either, see
Section 2.9.1.
The field "server-kex-algs" MUST contain at least one SSH key
exchange algorithm supported by the server. The key exchange
algorithm which is used in the connection is the first algorithm sent
in client's SSH_QUIC_INIT where: (1) the field "client-kex-alg-data"
is non-empty, and (2) the algorithm is also present in "server-kex-
algs". If there is no such key exchange algorithm, see
Section 2.9.1.
There MUST be at least one "quic-tls-cipher-suite" field. Each of
these specifies a TLS cipher suite ([RFC8446], Appendix B.4) which is
supported by the server, and which can be used with a version of QUIC
([QUIC], [QUIC-TLS]) supported by the server. The TLS cipher suite
which is used for the connection is the first suite sent in the
client's SSH_QUIC_INIT where: (1) the cipher suite is supported by
the negotiated QUIC protocol version, and (2) the cipher suite is
present in the server's SSH_QUIC_REPLY. If there is no such cipher
suite, see Section 2.9.1.
The server MAY send any number of extensions, encoded as a pair of
"ext-pair-name" and "ext-pair-data" fields. Some extensions are
defined for use with an Error Reply (see Section 2.9.1). Other
extensions MAY be defined in the future; see Section 2.9.2.
The field "server-kex-alg-data" MUST be empty if the packet is an
Error Reply. Otherwise, this field contains information for the SSH
key exchange method: see Section 3. Generally, this includes the
server's portion of key exchange inputs; the server's host key; and
the server's signature of the calculated exchange hash.
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2.9.1. Error Reply
If a server encounters an error which it is useful and appropriate to
communicate to the client, the server MAY send an "Error Reply"
version of SSH_QUIC_REPLY. Such a reply is created as follows:
* The server includes and populates all fields of SSH_QUIC_REPLY as
it would normally, except that the fields "server-connection-id"
and "server-kex-alg-data" MUST remain empty.
* In the extension pair fields, a "disc-reason" Extension Pair MUST
be included. An "err-desc" Extension Pair MAY also be included.
See Section 2.7.
* Extensibility considerations for SSH_QUIC_REPLY in Section 2.9.2
also apply to an Error Reply.
If the server does not support any of the QUIC protocol versions
enumerated by the client, the server SHOULD send an Error Reply with
the disconnect reason code
SSH_DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED.
In the following circumstances, the server SHOULD send an Error Reply
with the disconnect reason code SSH_DISCONNECT_KEY_EXCHANGE_FAILED:
* If the server could have sent a successful SSH_QUIC_REPLY, but it
would have been larger than the client's SSH_QUIC_INIT, even
though the SSH_QUIC_INIT met or exceeded the minimum length.
* If the server possesses no server host key that can be used with a
signature algorithm enumerated in the client's SSH_QUIC_INIT.
* If the server supports no key exchange algorithms matching the
ones for which the client sent "client-kex-alg-data" in
SSH_QUIC_INIT.
* If the server supports no TLS cipher suites enumerated in the
client's SSH_QUIC_INIT.
Besides "disc-reason", an "err-desc" extension pair SHOULD be
included to describe the specific error.
2.9.2. Extensibility
Implementations MUST allow room for future extensibility of
SSH_QUIC_REPLY in the following manners:
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1. By including algorithms in "server-sig-algs" which are unknown to
or not supported by the client. Clients MUST tolerate the
presence of such algorithms.
2. By including SSH key exchange algorithms which are unknown to or
not supported by the client, with algorithm data in a format
that's unknown to or not supported by the client. Clients MUST
tolerate the presence of such algorithms and their data.
3. By including QUIC TLS cipher suites which are unknown to or not
supported by the client. Clients MUST tolerate the presence of
such cipher suites.
4. By including extensions which are unknown to or not supported by
the client, with extension data in a format that's unknown to or
not supported by the client. Clients MUST tolerate the presence
of such extensions and their data.
Experience shows that any extensibility which is not actively
exercised is lost due to implementations that lock down expectations
incorrectly. Therefore, all servers MUST do at least one of the
following, in each SSH_QUIC_REPLY packet, at random:
1. In the fields "server-quic-versions", include in a random
position a version number of the form 0xFA?A?A?A, where ?
indicates a random nibble. See [QUIC], section "Versions". Note
the difference from the random version pattern in the client's
SSH_QUIC_INIT. Due to the minimal amount of entropy provided by
this rule, this MUST NOT be the only insertion of randomness made
in a packet.
2. In the field "server-sig-algs", include in a random position one
Random Name (Section 2.6).
3. In the field "server-kex-algs", include in a random position one
Random Name (Section 2.6).
4. In the fields "quic-tls-cipher-suite", include in a random
position one entry consisting of 16..64 Random Bytes.
5. In extension pairs, include in a random position one extension
pair where the field "ext-pair-name" contains a Random Name, and
the field "ext-pair-value" contains 0..100 Random Bytes.
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2.10. SSH_QUIC_CANCEL
If a client cannot process the server's successful SSH_QUIC_REPLY,
the client SHOULD report the error to the server using
SSH_QUIC_CANCEL.
A client MUST NOT send an SSH_QUIC_CANCEL in response to an
SSH_QUIC_REPLY which is itself an Error Reply. A client MUST assume
that such a connection was already canceled by the server.
A client SHOULD send two or more copies of SSH_QUIC_CANCEL, in
transmissions separated by a fraction of a second, to increase the
likelihood of successful delivery. The server sends no
acknowledgment to SSH_QUIC_CANCEL. After the server has received
SSH_QUIC_CANCEL, it MUST ignore subsequent copies of SSH_QUIC_CANCEL
for the same connection.
SSH_QUIC_CANCEL is an obfuscated datagram (Section 2.3) where "obfs-
payload" encrypts the following:
byte SSH_QUIC_CANCEL = 3
short-str server-connection-id
byte e = nr-ext-pairs (see Extensibility)
the following 2 fields repeated e times:
short-str ext-pair-name (MUST NOT be empty)
string ext-pair-data (MAY be empty)
Figure 8
The "server-connection-id" field MUST equal the "server-connection-
id" field in the server's SSH_QUIC_REPLY.
In the extension pair fields, a "disc-reason" Extension Pair MUST be
included. An "err-desc" Extension Pair MAY also be included. See
Section 2.7.
2.10.1. Extensibility
Extensibility considerations also apply to SSH_QUIC_CANCEL:
* Clients MAY include extensions which are unknown to or not
supported by the server, with extension data in a format that's
unknown to or not supported by the server.
* Servers MUST tolerate the presence of such extensions and their
data.
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* Clients SHOULD include, in a random position, at least one
extension pair where the field "ext-pair-name" contains a Random
Name, and the field "ext-pair-value" contains 0..300 Random Bytes.
3. Key Exchange Methods
Clients and servers MAY use any key exchange method which is defined
for SSH over TCP, whether it is assigned or private, as long as it
meets all of the following criteria:
1. The algorithm requires exactly one message from the client to the
server, for example SSH_MSG_KEX_ECDH_INIT. We call this message
KEXMSG_CLIENT.
2. The algorithm requires exactly one reply from the server to the
client, for example SSH_MSG_KEX_ECDH_REPLY. We call this message
KEXMSG_SERVER.
3. The algorithm specifies a hash function HASH, for example SHA-
256, SHA-384, or SHA-512.
4. The algorithm specifies calculation of an exchange hash H by
applying HASH to a concatenation of encoded fields.
5. The algorithm uses a server host key to sign H.
6. The algorithm includes the server's public host key, and the
signature of H, in its KEXMSG_SERVER message to the client.
7. The algorithm produces a shared secret K, represented as a signed
(positive or negative) multi-precision integer.
Any such algorithm is modified for use in SSH over QUIC as follows:
1. The field "client-kex-alg-data" in SSH_QUIC_INIT encodes the same
fields, in the same order, as KEXMSG_CLIENT, including the
leading byte for the SSH packet type.
2. The field "server-kex-alg-data" in SSH_QUIC_REPLY encodes the
same fields, in the same order, as KEXMSG_SERVER, including the
leading byte for the SSH packet type.
3. The calculation of H specified by the algorithm is not performed.
Instead, H is calculated by applying the hash function HASH to a
concatenation of the following:
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byte[8] "SSH/QUIC"
string Unencrypted "obfs-payload" content of SSH_QUIC_INIT
string Unencrypted "obfs-payload" content of SSH_QUIC_REPLY,
excluding the entire field "server-kex-alg-data"
The fields of "server-kex-alg-data", excluding signature field
mpint K
Figure 9
When a field is excluded as above, the entire encoding of the field
is omitted: both the encoding of the content and the encoding of the
length.
The SSH packet type byte is included:
* To ensure there are at least two fields in the encoded content.
This avoids situations where an outer string (the field "client-
kex-alg-data") would contain a single inner string (from
KEXMSG_CLIENT). This could confuse implementers to incorrectly
encode a single string only.
* For future consistency. The packet type byte may be useful for
multiple-roundtrip key exchange methods, for example those using
GSS-API [RFC4462]. Such key exchange methods are not currently
defined for SSH/QUIC, but can be.
3.1. Required Key Exchange Methods
Clients and servers are REQUIRED to implement the key exchange method
"curve25519-sha256" [RFC8731]. All other key exchange methods are
optional.
Clients and servers MAY permit the user to disable a required key
exchange method. However, required methods MUST be enabled by
default.
The requirement to implement any particular key exchange method
expires on the 5-year anniversary of the publishing of this memo. At
that point, implementers SHOULD consult any new standards documents
if available, or survey the practical use of SSH/QUIC for
implementation guidance.
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3.2. Example 1: "curve25519-sha256"
When using the SSH key exchange method "curve25519-sha256", the
SSH_QUIC_INIT field "client-kex-alg-data" is derived from
SSH_MSG_KEX_ECDH_INIT ([RFC5656], Section 4) and contains the
following:
byte SSH_MSG_KEX_ECDH_INIT = 30
string Q_C, client's ephemeral public key octet string
Figure 10
The SSH_QUIC_REPLY field "server-kex-alg-data" is derived from
SSH_MSG_KEX_ECDH_REPLY and contains the following:
byte SSH_MSG_KEX_ECDH_REPLY = 31
string K_S, server's public host key
string Q_S, server's ephemeral public key octet string
string the signature on the exchange hash
Figure 11
The shared secret K is calculated as in [RFC8731]. Then the exchange
hash H is calculated by applying SHA-256 to a concatenation of the
following:
string Content of SSH_QUIC_INIT
string Content of SSH_QUIC_REPLY, except "server-kex-alg-data"
byte SSH_MSG_KEX_ECDH_REPLY = 31
string K_S, server's public host key
string Q_S, server's ephemeral public key octet string
mpint K
Figure 12
3.3. Example 2: "diffie-hellman-group14-sha256"
When using the SSH key exchange method "diffie-hellman-
group14-sha256" [RFC8268], the SSH_QUIC_INIT field "client-kex-alg-
data" is derived from SSH_MSG_KEXDH_INIT ([RFC4253], Section 8) and
contains the following:
byte SSH_MSG_KEXDH_INIT = 30
mpint e
Figure 13
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The SSH_QUIC_REPLY field "server-kex-alg-data" is derived from
SSH_MSG_KEXDH_REPLY and contains the following:
byte SSH_MSG_KEXDH_REPLY = 31
string server public host key and certificates (K_S)
mpint f
string signature of H
Figure 14
The shared secret K is calculated as in [RFC4253]. Then the exchange
hash H is calculated by applying SHA-256 to a concatenation of the
following:
string Content of SSH_QUIC_INIT
string Content of SSH_QUIC_REPLY, except "server-kex-alg-data"
byte SSH_MSG_KEXDH_REPLY = 31
string server public host key and certificates (K_S)
mpint f
mpint K
Figure 15
4. SSH_MSG_EXT_INFO and the SSH Version String
A common user complaint to SSH application authors is that SSH over
TCP sends the application version in plain text. The application
version cannot be omitted, otherwise implementations cannot support a
number of behaviors which other software versions implement
incorrectly.
A prominent example is the order of arguments in the SFTP request
SSH_FXP_SYMLINK. To send a request that will have the desired
effects, the client MUST consult the server's version string to know
whether the server uses the standard order of fields, or a reverse
order used by OpenSSH.
SSH over QUIC removes the version string from the SSH key exchange.
Instead, all clients and servers are REQUIRED to send and accept
SSH_MSG_EXT_INFO [RFC8308], and to include the "ssh-version"
extension defined here.
Clients MUST send SSH_MSG_EXT_INFO as the very first SSH packet over
QUIC stream 0. The client MUST include the "ssh-version" extension
in this SSH_MSG_EXT_INFO.
Servers MUST send SSH_MSG_EXT_INFO either:
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1. as the very first SSH packet over QUIC stream 0, and/or
2. immediately preceding the server's SSH_MSG_USERAUTH_SUCCESS.
A server MUST include the "ssh-version" extension in at least one of
its SSH_MSG_EXT_INFO. If the server sends SSH_MSG_EXT_INFO at both
opportunities, it MAY omit "ssh-version" at the first opportunity,
but only if it will send it in the second opportunity. The second
SSH_MSG_EXT_INFO sent by the server MAY change a previously sent
"ssh-version" extension value to include more specific detail. For
example, the server MAY send a more accurate server software version
when the client has authenticated. The client MUST use the "ssh-
version" value which was most recently received from the server.
4.1. "ssh-version"
The "ssh-version" extension is encoded in SSH_MSG_EXT_INFO as
follows:
string "ssh-version"
string ssh-version-string
Figure 16
The extension value, "ssh-version-string", contains the same SSH
version string as sent at the start of SSH over TCP ([RFC4253],
Section 4.2), but stripping the prefix "SSH-2.0-". Examples inspired
by version strings used in practice:
GenericSoftware
Product_1.2.00
0.12 Library: Application 1.23p1
Figure 17
4.2. "no-flow-control"
The extension "no-flow-control" has no effect in SSH/QUIC. It SHOULD
NOT be sent in SSH/QUIC and MUST be ignored by both parties.
4.3. "delay-compression"
Semantics of the "delay-compression" extension are modified as per
Section 6.1.1.
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5. QUIC Session Setup
When the server has sent its SSH_QUIC_REPLY, and when the client has
received it, they each initialize the QUIC session [QUIC] [QUIC-TLS]
as follows:
* The QUIC protocol version is set to the first version advertised
in the client's SSH_QUIC_INIT which is also present in the
server's SSH_QUIC_REPLY.
* Session state is set as if a TLS handshake had just completed.
* The TLS cipher suite is set to the first TLS cipher suite
advertised in SSH_QUIC_INIT which is also present in
SSH_QUIC_REPLY.
* The QUIC Key Phase bit is set to 0.
* The shared secrets that would have been obtained from the TLS
handshake are instead generated from the SSH key exchange
(Section 5.1).
Clients and servers MUST immediately begin to use QUIC Short Header
Packets. Implementations MUST NOT send QUIC Long Header Packets,
since they could be confused with the SSH/QUIC key exchange.
5.1. Shared Secrets
QUIC-TLS [QUIC-TLS] uses a client secret and a server secret from
which it generates an AEAD key, an IV, and a header protection key
for each sending direction.
An SSH key exchange produces a shared secret K, represented as an SSH
multi-precision integer, and an exchange digest H, represented as
binary data [RFC4253]. An SSH key exchange is parameterized with a
hash function we call HASH. Note that HASH can be a different hash
function, producing a different hash length, than the hash function
used by the negotiated TLS cipher suite.
To compute the initial QUIC client and server secrets, the client and
server encode the following binary data, which we call "secret_data":
mpint K
string H
Figure 18
The client and server secrets are then calculated as follows:
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client_secret = HMAC-HASH("ssh/quic client", secret_data)
server_secret = HMAC-HASH("ssh/quic server", secret_data)
Figure 19
The HMAC construct is as specified in [RFC2104], instantiated using
the SSH key exchange hash function, HASH.
QUIC keys and IVs are derived from these secrets using the regular
QUIC-TLS key derivation process [QUIC-TLS]. Keys generated from
these secrets are considered 1-RTT keys.
Clients and servers MUST implement QUIC key updates using the regular
QUIC-TLS key update process [QUIC-TLS], respecting the QUIC-TLS
minimum key update frequencies.
6. Adaptation of SSH to QUIC Streams
6.1. SSH/QUIC Packet Format
Each side serializes its SSH packets for sending over QUIC as
follows:
uint32 n = payload-len, high bit set if compressed
byte[n] payload (compressed or uncompressed)
Figure 20
Since security is provided by QUIC-TLS [QUIC-TLS], MAC and random
padding are omitted at this stage.
The "payload-len" field has its high bit set if the "payload" field
is compressed. See Section 6.1.1.
The "payload" field contains the same packet information as the
"payload" field in the Binary Packet Protocol defined in [RFC4253].
6.1.1. Compression
Compression MAY be negotiated using the "delay-compression" extension
in [RFC8308]. If "delay-compression" was negotiated, then:
* If compression is enabled for the server-to-client direction, the
server MAY compress packets on any stream after it has sent
SSH_MSG_USERAUTH_SUCCESS.
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* If compression is enabled for the client-to-server direction, the
client MAY compress packets on any stream after it has received
SSH_MSG_USERAUTH_SUCCESS.
Due to multiple streams in SSH/QUIC, the packet SSH_MSG_NEWCOMPRESS
is not an effective mechanism to signal the start of compression and
MUST NOT be sent. It is replaced by the high bit in "payload-len".
6.2. Use of QUIC Streams
To avoid an unnecessary layer of flow control which has performance
and complexity impacts in SSH over TCP, SSH/QUIC uses QUIC streams
for SSH channels and dispenses with flow control on the level of SSH
channels. This simplifies future SSH/QUIC implementations which
might not implement SSH over TCP.
Conducting SSH channels over QUIC streams requires modifications of
the SSH Connection Protocol [RFC4254]. The following sections
describe these modifications.
6.3. Packet Sequence Numbers
In SSH over TCP, every SSH packet has an implicit sequence number
which is unique for the direction of sending (to server vs. to
client). The packet type SSH_MSG_UNIMPLEMENTED makes reference to
this sequence number.
In SSH/QUIC, sequence numbers are separate for each sending
direction, as well as each QUIC stream. This requires modification
of SSH_MSG_UNIMPLEMENTED. This packet type is changed as follows:
byte SSH_MSG_UNIMPLEMENTED
uint64 QUIC stream ID on which the packet was received
uint32 packet sequence number in stream, first packet = 0
Figure 21
6.4. Channel IDs
SSH over TCP uses 32-bit channel IDs which can be reused in the same
session and do not have to be used sequentially. Conflicts in
channel IDs are avoided by identifying each channel with two separate
channel IDs: one designated by the sender and one by the recipient.
[RFC4254]
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QUIC streams use 62-bit channel IDs which cannot be reused and MUST
be used sequentially. Both sides use the same stream ID. Conflicts
in stream IDs are avoided by using the least significant bit to
indicate whether the stream was opened by the client or by the
server. [QUIC]
SSH/QUIC uses QUIC stream IDs. This requires modification of SSH
channel-related packets. See Section 6.8.
6.5. Disconnection
The SSH packet type SSH_MSG_DISCONNECT is replaced by sending the
QUIC frame CONNECTION_CLOSE of type 0x1d. The "Error Code" field in
CONNECTION_CLOSE contains the value that would have been sent in the
"reason code" in SSH_MSG_DISCONNECT. The "Reason Phrase" field in
CONNECTION_CLOSE contains the value that would have been sent in
"description" in SSH_MSG_DISCONNECT. The "language tag" field of
SSH_MSG_DISCONNECT is not sent.
6.6. Prohibited SSH Packets
In SSH/QUIC, the following SSH packet types MUST NOT be sent:
SSH_MSG_DISCONNECT 1
SSH_MSG_NEWCOMPRESS 8
SSH_MSG_KEXINIT 20
SSH_MSG_NEWKEYS 21
key exchange packets 30-49
SSH_MSG_CHANNEL_WINDOW_ADJUST 93
SSH_MSG_CHANNEL_CLOSE 97
Figure 22
If they receive packets of these types, clients and servers MAY
disconnect with SSH_DISCONNECT_PROTOCOL_ERROR (Section 6.5).
Alternately, the receiver MAY send SSH_MSG_UNIMPLEMENTED
(Section 6.3).
6.7. Global SSH Packets
In SSH/QUIC, the following SSH packet types MUST be sent on QUIC
stream 0. With the exception of SSH_MSG_UNIMPLEMENTED (Section 6.3),
these packets use the same encoded formats as in SSH over TCP:
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SSH_MSG_IGNORE 2
SSH_MSG_UNIMPLEMENTED 3 (Changed format!)
SSH_MSG_DEBUG 4
SSH_MSG_SERVICE_REQUEST 5
SSH_MSG_SERVICE_ACCEPT 6
SSH_MSG_EXT_INFO 7
SSH_MSG_USERAUTH_REQUEST 50
SSH_MSG_USERAUTH_FAILURE 51
SSH_MSG_USERAUTH_SUCCESS 52
SSH_MSG_USERAUTH_BANNER 53
SSH_MSG_USERAUTH_INFO_REQUEST 60
SSH_MSG_USERAUTH_INFO_RESPONSE 61
SSH_MSG_GLOBAL_REQUEST 80
SSH_MSG_REQUEST_SUCCESS 81
SSH_MSG_REQUEST_FAILURE 82
Figure 23
6.8. SSH Channel Packets
All SSH/QUIC channels MUST be opened as bidirectional QUIC streams.
This means QUIC stream IDs where the least significant bits are 10 or
11 MUST NOT be used in SSH/QUIC. Implementations that receive such
stream IDs MUST disconnect with SSH_DISCONNECT_PROTOCOL_ERROR
(Section 6.5)
A client MUST NOT open a non-zero QUIC stream before the server has
sent SSH_MSG_USERAUTH_SUCCESS. If a client does so, the server MUST
disconnect with SSH_DISCONNECT_PROTOCOL_ERROR.
A server MUST NOT open a non-zero QUIC stream before it has sent
SSH_MSG_USERAUTH_SUCCESS. However, a client MUST be prepared for the
possibility that, due to network delays, a stream opened by the
server can be received by the client before SSH_MSG_USERAUTH_SUCCESS.
Therefore, if the client receives a server-initiated stream before
SSH_MSG_USERAUTH_SUCCESS, it MUST assume that the server has also
sent SSH_MSG_USERAUTH_SUCCESS. If the client then receives packets
on QUIC stream 0 which invalidate this assumption, the client MUST
disconnect with SSH_DISCONNECT_PROTOCOL_ERROR.
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The initiator of any non-zero QUIC stream MUST send
SSH_MSG_CHANNEL_OPEN as the first packet. If the receiver refuses
the channel, it replies with SSH_MSG_CHANNEL_OPEN_FAILURE. Both
sides then MUST close the QUIC stream as per Section 6.9. In this
case, even though a QUIC stream was opened, an SSH channel was not.
Therefore, other SSH_MSG_CHANNEL_xxxx packets MUST NOT be sent. This
includes SSH_MSG_CHANNEL_EOF.
If the receiver accepts the channel, it replies with
SSH_MSG_CHANNEL_OPEN_CONFIRMATION. Both sides then send SSH packets
of types SSH_MSG_CHANNEL_xxxx. In SSH/QUIC, these packets have the
following formats:
byte SSH_MSG_CHANNEL_OPEN
string channel type in US-ASCII only
uint32 maximum packet size
.... channel-type-specific data follows
Figure 24
byte SSH_MSG_CHANNEL_OPEN_CONFIRMATION
uint32 maximum packet size
.... channel-type-specific data follows
Figure 25
byte SSH_MSG_CHANNEL_OPEN_FAILURE
uint32 reason code
string description in UTF-8
string language tag
Figure 26
byte SSH_MSG_CHANNEL_DATA
string data
Figure 27
byte SSH_MSG_CHANNEL_EXTENDED_DATA
uint32 data_type_code
string data
Figure 28
byte SSH_MSG_CHANNEL_EOF
Figure 29
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byte SSH_MSG_CHANNEL_REQUEST
string request type in US-ASCII characters only
boolean want reply
.... type-specific data follows
Figure 30
byte SSH_MSG_CHANNEL_SUCCESS
Figure 31
byte SSH_MSG_CHANNEL_FAILURE
Figure 32
6.9. Closing a Channel
The SSH packet type SSH_MSG_CHANNEL_CLOSE is replaced by QUIC stream
state transitions [QUIC]. Each side considers a channel closed when
the QUIC stream is both in a terminal sending state, and a terminal
receiving state. This means:
* The QUIC sending stream state has become "Data Recvd" or "Reset
Recvd".
* The QUIC receiving stream state has become "Data Read" or "Reset
Read".
The SSH packet type SSH_MSG_CHANNEL_EOF continues to be used. This
packet often does NOT correspond with the end of the stream in its
direction. As in SSH over TCP, SSH channel requests MAY be sent
after SSH_MSG_CHANNEL_EOF, and MUST be handled gracefully by
receivers. A common example is the request "exit-status", which is
sent by a server to communicate a process exit code to the SSH
client, and is commonly sent after the end of output.
7. Acknowledgements
Paul Ebermann for first review and the encouragement to use QUIC
streams.
Ilari Liusvaara for "server-name-indication" and value 1200 for
SSH_QUIC_INIT padding target.
Benjamin Kaduk for idea of additional cross-protocol protection in
the calculation of H.
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Stephane Bortzmeyer for the PRECIS Framework to handle the
obfuscation keyword.
Yuki Goto for the QUIC Transport Parameters.
8. IANA Considerations
This document requests no changes to IANA registries.
9. Security Considerations
Clients and servers MUST insert into SSH_QUIC_INIT and SSH_QUIC_REPLY
at least the minimum amount of cryptographically random data as
specified in the section Random Elements. Compromising on this
requirement reduces the security of any session created on the basis
of such an SSH_QUIC_INIT or SSH_QUIC_REPLY.
10. References
10.1. Normative References
[QUIC] Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", 2020, <https://tools.ietf.org/html/
draft-ietf-quic-transport-29>.
[QUIC-TLS] Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
2020,
<https://tools.ietf.org/html/draft-ietf-quic-tls-29>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
January 2006, <https://www.rfc-editor.org/info/rfc4251>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.
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[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009,
<https://www.rfc-editor.org/info/rfc5656>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8264] Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
Preparation, Enforcement, and Comparison of
Internationalized Strings in Application Protocols",
RFC 8264, DOI 10.17487/RFC8264, October 2017,
<https://www.rfc-editor.org/info/rfc8264>.
[RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 8265,
DOI 10.17487/RFC8265, October 2017,
<https://www.rfc-editor.org/info/rfc8265>.
[RFC8308] Bider, D., "Extension Negotiation in the Secure Shell
(SSH) Protocol", RFC 8308, DOI 10.17487/RFC8308, March
2018, <https://www.rfc-editor.org/info/rfc8308>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8731] Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure
Shell (SSH) Key Exchange Method Using Curve25519 and
Curve448", RFC 8731, DOI 10.17487/RFC8731, February 2020,
<https://www.rfc-editor.org/info/rfc8731>.
10.2. Informative References
[IANA-SSH] IANA, "Secure Shell (SSH) Protocol Parameters",
<https://www.iana.org/assignments/ssh-parameters/>.
[RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250,
DOI 10.17487/RFC4250, January 2006,
<https://www.rfc-editor.org/info/rfc4250>.
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[RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
January 2006, <https://www.rfc-editor.org/info/rfc4252>.
[RFC4254] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Connection Protocol", RFC 4254, DOI 10.17487/RFC4254,
January 2006, <https://www.rfc-editor.org/info/rfc4254>.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May
2006, <https://www.rfc-editor.org/info/rfc4462>.
[RFC8268] Baushke, M., "More Modular Exponentiation (MODP) Diffie-
Hellman (DH) Key Exchange (KEX) Groups for Secure Shell
(SSH)", RFC 8268, DOI 10.17487/RFC8268, December 2017,
<https://www.rfc-editor.org/info/rfc8268>.
Appendix A. Generating Random Lengths
The SSH/QUIC extensibility mechanism calls for generating random
lengths such that values in the shorter end of the range are
significantly more probable, but long lengths are still selected.
The following C example shows a simple two-step process to prefer
shorter lengths:
int RandomIntBetweenZeroAnd(int maxValueInclusive);
int RandomLen_PreferShort(int minLen, int maxLen)
{
int const SPAN_THRESHOLD = 7;
int lenSpan = maxLen - minLen;
if (lenSpan <= 0)
return minLen;
if (lenSpan > SPAN_THRESHOLD)
if (0 != RandomIntBetweenZeroAnd(3))
return minLen + RandomIntBetweenZeroAnd(SPAN_THRESHOLD);
return minLen + RandomIntBetweenZeroAnd(lenSpan);
}
Figure 33
Author's Address
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denis bider
Bitvise Limited
4105 Lombardy Ct
Colleyville, TX 76034
United States
Email: ietf-draft@denisbider.com
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