Internet DRAFT - draft-nir-tls-keyshare
draft-nir-tls-keyshare
TLS Working Group Y. Nir
Internet-Draft Check Point
Intended status: Standards Track March 26, 2012
Expires: September 27, 2012
A Method for Sharing Record Protocol Keys with a Middlebox in TLS
draft-nir-tls-keyshare-02
Abstract
This document contains a straw man proposal for a method for sharing
symmetric session keys between a TLS client and a middlebox, so that
the middlebox can decrypt the TLS-protected traffic.
This method is an alternative to the middlebox becoming a proxy.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used in This Document . . . . . . . . . . . . 3
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The tls_keyshare Extension . . . . . . . . . . . . . . . . 6
2.2. The KeyShareInfo Record . . . . . . . . . . . . . . . . . 6
2.2.1. The KeyShareInfo Discovery Subtype . . . . . . . . . . 6
2.2.2. The KeyShareInfo Rejection Subtype . . . . . . . . . . 6
2.2.3. The KeyShareInfo Keys Subtype . . . . . . . . . . . . 6
3. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Client Processing . . . . . . . . . . . . . . . . . . . . 7
3.2. Server Processing . . . . . . . . . . . . . . . . . . . . 7
3.3. Middlebox Processing . . . . . . . . . . . . . . . . . . . 8
4. Middlebox Discovery . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
TLS ([TLS]) is used in a wide variety of protocols. The most common
use is for protecting HTTP, as described in [HTTPS]. Middleboxes
such as firewalls scan protocols for attacks. For HTTP common
attacks to scan for are cross-site scripting and transfer of files
containing malware.
TLS provides authentication and privacy against eavesdropping, but it
hides the traffic not only from mallicious intercepters. It also
hides the traffic from the middlebox, and prevents it from doing its
job. Our goal is to allow the middlebox to inspect the traffic,
without allowing others to do the same.
The requirements can be summed up in the following points:
o The middlebox should be able to decrypt all TLS traffic, and
optionally (the client's option) also modify it.
o The protocol must not make it easier for other entities to decrypt
the traffic.
o The client should be able to opt out of TLS decryption, but opting
out may mean that the connection is blocked.
o The server should be able to opt out of TLS decryption, but opting
out may mean that the connection is blocked.
Two proposals have been offered to achieve these goals. One is
having the middlebox be a proxy, acting as server to the client, and
as a client to the server. This option is implemented in several
commercial products. [proxy_server_ext] describes an extension to TLS
for improving that mechanism, and also contains a good description in
the introduction.
This document describes an alternative mechanism, where the client
sends the keys to the middlebox in the TLS record stream. This
requires more changes to clients and servers, but has the advantage
that it does not break many of TLS guarantees.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. Protocol Overview
A supporting client will send a new extension in the ClientHello
message. This new extension is called tls_keyshare. A server that
supports this extension will send the extension in the ServerHello if
it has received that extension in the ClientHello. Note that sending
this extension only acknowledges understanding the protocol, not
agreement to decryption. The extension contains a sequence of SHA-
256 hashes of middlebox certificates. The client sends the hashes of
the certificates of middleboxes that it knows are on-path to the
server. See Section 4 for a discussion of middlebox discovery. The
server sends a subset of the same hashes, only those for which it
agrees to decryption.
This document defines a new record type called KeyshareInfo. This is
a new content type rather than a new handshake message so that it
doesn't figure in hash calculation of the hash message. A middlebox
inserts a KeyShareInfo record into the server-to-client stream
immediately after receiving the ClientHello message, if its hash was
not present in the client's tls_keyshare extension. It contains two
pieces of information:
o A certificate of the middlebox. The public key in the certificate
MUST be of the RSA type. The certificate should contain enough
information for the client to recognize the middlebox.
o A signature using the private key associated with the certificate
over the concatenation of the ClientHello and ServerHello
messages.
The middlebox inserts a KeyShareInfo record with a certificate into
the client-to-server stream without an alert, immediately following a
ServerHello message that does not contain the middlebox hash. The
server will reply with either a fatal UNAUTHORIZED_MIDDLEBOX alert,
or a fatal RETRY_MIDDLEBOX alert, depending on policy.
In cases where the client and server negotiate either a ciphersuite
that the middlebox does not support, or an extension that it doesn't
support, the middlebox inserts a different kind of KeyShareInfo
record into the stream, that identifies the unsupported ciphersuite
or extension. Both kinds of KeyShareInfo records are followed by a
fatal alert. The client is expected to add the hashes and remove the
unsupported ciphersuites and extensions, before attempting a new TLS
connection.
The client inserts a third type of KeyShareInfo record into the
client-to-server stream immediately following the ChangeCipherSpec
record (before the Finished handshake record). This KeyShareInfo
record is constructed differently, and contains an RSA encrypted
record of the write keys for both client and server. The client may
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send several records if there is more than one middlebox.
The diagram below outlines discovery.
Client Middlebox Server
------ --------- ------
ClientHello(tls_keyshare=0)
-------->
KeyShareInfo(cert,sig)
KeyShareInfo(reject cipher:0x0044)
alert(MIDDLEBOX_PRESENT)
<--------
ClientHello(tls_keyshare=cert_hash)
-------->
ServerHello(cert_hash)
(Certificate)
(ServerKeyExchange)
ServerHelloDone
<--------
(Certificate)
ClientKeyExchange
(CertificateVerify)
ChangeCipherSpec
KeyShareInfo(keys)
Finished -------->
ChangeCipherSpec
Finished
<--------
The diagram below outlines the protocol in a case where the server
refuses decryption.
Client Middlebox Server
------ --------- ------
ClientHello(tls_keyshare=cert_hash)
-------->
ServerHello(keyshare=0)
(Certificate)
(ServerKeyExchange)
ServerHelloDone
<--------
KeyShareInfo(cert,sig)
-------->
alert(UNAUTHORIZED_MIDDLEBOX)
<--------
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2.1. The tls_keyshare Extension
The tls_keyshare extension is a ClientHello and ServerHello extension
as defined in section 2.3 of [TLS-EXT]. The extension_type field is
TBA by IANA. The format is to be added.
2.2. The KeyShareInfo Record
The format of the KeyShareInfo record is to be added. The content
type is TBA by IANA.
2.2.1. The KeyShareInfo Discovery Subtype
The KeyShareInfo Discovery record gives client or server information
about the middlebox. Format is TBA.
2.2.2. The KeyShareInfo Rejection Subtype
The KeyShareInfo Rejection record gives client a list of unsupported
ciphersuites and extensions. Format is TBA.
2.2.3. The KeyShareInfo Keys Subtype
The KeyShareInfo Keys record is send by the client to the middlebox
and includes the session keys. Format is TBA.
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3. Processing
3.1. Client Processing
If the client policy prohibits decryption, the client SHOULD send the
tls_keyshare extension without hashes. Note that the middlebox might
still try to proxy the connection, but that is in conflict with this
specification, and is outside the scope of this document.
If there are some middleboxes that are by policy acceptable to the
client, their certificates are known in advance, and the client
believes that they are on-path to the server, then the client MUST
send the SHA-256 hashes of their certificates in the tls_keyshare
extension.
If a KeyShareInfo Discovery record is received with an unknown
certificate, it MAY be ignored, or the user MAY be prompted to
authorize the decryption, and optionally change the configuration to
allow future decryption by this certificate. There will certainly be
controversy about this, but the configuration must happen an some
point.
If policy dictates that the particular middlebox referenced in the
KeyShareInfo record is not allowed to decrypt, then such a record
MUST be ignored. In that case the connection fails. If the
middlebox is acceptable, then the client retries the connection, this
time adding the SHA-256 hash of the certificate to the tls_keyshare
extension. This is the discovery mechanism.
For all the middleboxes that are not ignored, the client MUST send a
KeyShareInfo record with the symmetric keys immediately following the
ChangeCipherSpec record before any protected record is sent.
If a KeyShareInfo Rejection record is received, the client SHOULD
retry the handshake, this time without the flagged ciphersuites and
extensions. If it is not acceptable to run the connection without
these ciphersuites or extensions, the client should log the event or
inform the user.
If the server sends a RETRY_MIDDLEBOX alert, the client should retry
the handshake. If it sends an UNAUTHORIZED_MIDDLEBOX alert, then the
client should log the event or alert the user.
3.2. Server Processing
The server SHOULD send the tls_keyshare extension even if policy
dictates that the decryption is prohibited. If policy allows all
middleboxex to decrypt, it makes sense to simply copy the client's
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tls_keyshare extension.
If some of the middlebox hashes included in the client's tls_keyshare
extension are recognized as those of acceptable middleboxes, then
only those are copied to the server's tls_keyshare extension. When
the middlebox sends a KeyShareInfo Discovery record, the server may
decide whether that is acceptable or not, and accordingly send the
RETRY_MIDDLEBOX or UNAUTHORIZED_MIDDLEBOX alerts. In any case, every
time the server does not copy all hashes from the client's
tls_keyshare, the connection is probably going to end in an alert.
3.3. Middlebox Processing
The middlebox MUST send a KeyShareInfo Discovery record to the client
if the client has indicated support for this extension, and has not
included the middlebox hash in the extension. The discovery record
is followed by a MIDDLEBOX_PRESENT alert, breaking the connection.
Similarly, if the hash is missing from the server's tls_keyshare
extension, then the middlebox injects a KeyShareInfo Discovery record
into the client-to-server stream. The server will usually then send
an Alert record.
If the ServerHello specifies a ciphersuite that the middlebox does
not support, or if it includes a TLS extension that might prevent the
middlebox from processing, then the middlebox MAY send a KeyShareInfo
Reject record with all unacceptable ciphersuites and extension
numbers, followed by a MIDDLEBOX_PRESENT alert.
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4. Middlebox Discovery
Discovering that the middlebox is present has already been described
in Section 3.1. The client that is not aware of the presence of the
middlebox receives a KeyShareInfo Discovery record followed by a
MIDDLEBOX_PRESENT alert message.
Discovering that a middlebox in no longer on the path is trickier,
because the superfluous KeyShareInfo Keys records do not lead to any
observable effects for the client. We suggest that the client keep a
list of discovered middleboxes, and periodically clear entries from
the list, requiring a repeated discovery. System events such as a
change to host IP address, a reboot or the computer entering sleep
mode MAY be used as triggers for clearing the list.
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5. Security Considerations
To be added
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6. IANA Considerations
To be added.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[TLS-EXT] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
7.2. Informative References
[HTTPS] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[proxy_server_ext]
McGrew, D. and P. Gladstone, "TLS Proxy Server Extension",
draft-mcgrew-tls-proxy-server-00 (work in progress),
July 2011.
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Author's Address
Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim st.
Tel Aviv 67897
Israel
Email: ynir@checkpoint.com
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