Network Working Group E. Rescorla
Internet-Draft Network Resonance
Expires: August 29, 2006 February 25, 2006
Transport Layer Security (TLS) Partial Encryption Mode
draft-rescorla-tls-partial-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
This document describes an extension to TLS to allow partial
encryption of record bodies. This allows the beginning of the record
body to be in the clear, thus facilitating debugging and header
compression.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . . 3
3. Negotiating the Partial Encryption Extension . . . . . . . . . 3
4. Record Processing . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Record Transmission . . . . . . . . . . . . . . . . . . . . 4
4.2. Record Reception . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7
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1. Introduction
Encryption in Transport Layer Security (TLS) [2] is currently an all-
or-nothing proposition. The choices are a cipher suite that has
encryption or one of the NULL cipher suites which offer no
encryption. This has disadvantages in settings where the application
layer itself has some data (such as a header) that it wishes to have
in the clear (e.g., for debugging purposes) and some data (such as a
payload) that it wishes to have encrypted. This document describes
an extension to TLS that allows for the initial portion of the record
to remain uncompressed and unencrypted.
2. 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 [1].
3. Negotiating the Partial Encryption Extension
The client requests support for the partial encryption feature by
sending the "partial_encryption" extension in its ClientHello. The
"extension_data" field contains a PartialEncryption field:
struct {
uint16 InitialClearBytes;
} PartialEncryption
The InitialClearBytes value contains the number of bytes which will
be in the clear for each application_data record. This value will
obtain for the entire life of this association.
The server indicates support for the partial encryption feature
sending a "partial_encryption" extension with an empty
"extension_data" field. This indicates its acceptance of the
extension and of the the number of bytes to be sent in the clear. If
the server does not support the extension or does not accept the
InitialClearBytes value, it MUST ignore the extension. The first
application_data record in the new association (after the
change_cipher_spec message) MUST use the new encryption mode as
described below.
4. Record Processing
The partial encryption extension only matters for records of type
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"application_data". All other records should be processed via the
usual TLS/DTLS rules.
4.1. Record Transmission
When the partial encryption extension is in effect, the
TLSCiphertext.fragment struct becomes:
select (CipherSpec.cipher_type) {
opaque plaintext_bytes[InitialClearBytes]; // New field
case stream: GenericStreamCipher;
case block: GenericBlockCipher;
} fragment;
The first InitialClearBytes bytes of the TLSPlaintext.fragment are
inserted in the TLSCiphertext.plaintext_bytes value. The rest are
passed through compression and encryption to form the
GenericStreamCipher or GenericBlockCipher values. If the
TLSPlaintext.fragment is less than InitialClearBytes then the entire
plaintext is left un-encrypted. The same processing applies to DTLS
[4].
The TLS MAC remains unchanged and is applied to both the
plaintext_bytes and the TLSCompressed.fragment. Where length is
computed as InitialClearBytes + TLSCompressed.length.
4.2. Record Reception
Record reception is relatively simple. The receiver knows whether
the partial_plaintext extension is in effect and simply treats the
first InitialClearBytes of what would otherwise be the ciphertext as
plaintext. After those bytes are removed, the rest of the record can
be processed as usual.
5. Security Considerations
There are two security concerns introduced by these extensions. The
first involves the security of the negotiation and the second the
security of the transport protocol. Because the negotiation is
protected by the TLS/DTLS handshake, attackers can neither force the
use of these extensions nor block them while allowing the negotiation
to succeed.
The second concern is the security if the data. Obviously, no
confidentiality is provided for any data in the initial plaintext.
However, because the length of the initial plaintext is fixed in the
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negotiation and the MAC covers the total length, an active attacker
cannot convince the receiver to accept values which are encrypted as
if they were plaintext or vice versa.
One concern that applies solely to DTLS is that an active attacker
might manipulate MTU values to attempt to force the sender to split
data across multiple records and thus have some application layer
data which would otherwise be encrypted sent in the clear. DTLS
itself does not do any fragmentation and applications which use this
extension MUST NOT fragment the data that they send to DTLS in such a
way that sensitive data could be transmitted unencrypted.
6. IANA Considerations
This document defines an extension to TLS, in accordance with [3]:
enum { partial_encryption (??) } ExtensionType;
[[ NOTE: These values need to be assigned by IANA ]]
7. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1",
draft-ietf-tls-rfc2246-bis-13 (work in progress), June 2005.
[3] Blake-Wilson, S., "Transport Layer Security (TLS) Extensions",
draft-ietf-tls-rfc3546bis-02 (work in progress), October 2005.
[4] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", draft-rescorla-dtls-05 (work in progress), June 2005.
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Author's Address
Eric Rescorla
Network Resonance
2483 E. Bayshore Rd., #212
Palo Alto, CA 94303
USA
Email: ekr@networkresonance.com
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