| TLS Working Group | Y. Nir |
| Internet-Draft | Check Point |
| Intended status: Standards Track | August 25, 2011 |
| Expires: February 26, 2012 |
A Method for Sharing Record Protocol Keys with a Middlebox in TLS
draft-nir-tls-keyshare-00
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.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 26, 2012.
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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:
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.
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].
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 been received in the ClientHello. Note that sending this extension only acknowledges understanding the protocol, not agreement to decryption. The extension may contain a sequence of SHA-256 hashes of known middlebox certificates.
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 following the ServerHello message. It contains two pieces of information:
The middlebox inserts a KeyShareInfo record into the client-to-server stream immediately preceding the ClientKeyExchange handshake record with a similar construction, except that this time the signature is over all the handshake records from ClientHello to ServerHelloDone. This is sent only if the server indicated support.
The client inserts a 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 send several records if there is more than one middlebox.
Client Middlebox Server
------ --------- ------
ClientHello(*) -------->
ServerHello(*)
KeyShareInfo(cert,sig)
(Certificate)
ServerKeyExchange
<-------- ServerHelloDone
KeyShareInfo(cert,sig)
ClientKeyExchange
(CertificateVerify)
ChangeCipherSpec
KeyShareInfo(keys) - removed by middlebox
Finished -------->
ChangeCipherSpec
Finished
<--------
(*) The ClientHello and ServerHello include the tls_keyshare
extension to indicate support
The diagram below outlines the protocol structure.
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.
The format of the KeyShareInfo record is to be added. The content type is TBA by IANA.
If the client policy prohibits decryption, the client SHOULD NOT send the tls_keyshare extension. Note that the middlebox might still try to proxy the connection, but that is outside the scope of this document.
If there are some middleboxes that are by policy acceptable to the client and their certificates are known in advance, the client MAY send the SHA-256 hashes of their certificates in the tls_keyshare extension.
If a KeyShareInfo record is received with an unknown hash, it MUST be ignored.
If a KeyShareInfo 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.
If policy dictates that the particular middlebox referenced in the KeyShareInfo record is not allowed to decrypt, then such a record is ignored.
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.
The server SHOULD send the tls_keyshare extension even if policy dictates that the decryption is prohibited. If a middlebox is detected, either by its KeyShareInfo record or by the client's KeyShareInfo record, the server MAY terminate the connection with a DECRYPTION_PROHIBITED alert.
The server MAY also terminate the connection with a DECRYPTION_PROHIBITED alert if it is just the particular middlebox that is not authorized.
The middlebox MUST send a KeyShareInfo record to the client if the client has indicated support for this extension. The middlebox MUST send a KeyShareInfo record to the server if the server has indicated support for this extension. The middlebox MUST use the full certificate format if the hash was not included in the extension, or the hash format if it was.
To be added
To be added.
| [1] | Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. |
| [2] | Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and T. Wright, "Transport Layer Security (TLS) Extensions", RFC 4366, April 2006. |
| [3] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [1] | Rescorla, E, "HTTP Over TLS", RFC 2818, May 2000. |
| [2] | McGrew, D and P Gladstone, "TLS Proxy Server Extension", Internet-Draft draft-mcgrew-tls-proxy-server-00, July 2011. |