INTERNET-DRAFT S. Santesson (3xA Security)
Intended Status: Proposed Standard
Expires: October 22, 2010 April 20, 2010
Transport Layer Security (TLS) Cached Information Extension
<draft-ietf-tls-cached-info-08.txt>
Abstract
This document defines a Transport Layer Security (TLS) extension for
cached information. This extension allows the TLS client to inform a
server of cached information from previous TLS sessions, allowing the
server to omit sending cached static information to the client during
the TLS handshake protocol exchange.
Status of this Memo
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Copyright and License Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Cached Information Extension . . . . . . . . . . . . . . . . . 4
3. Extension Exchange . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Reconnaissance . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Cached Information . . . . . . . . . . . . . . . . . . . . 5
4. Data Substitution . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Data Substitution Syntax for certificate_chain . . . . . . 6
4.2. Data Substitution Syntax for trusted_cas . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Annex A - 64 bit FNV-1a digest . . . . . . . . . . . . . . . . . 10
A.1. Definition (Normative) . . . . . . . . . . . . . . . . . 10
A.2 Java code sample (Informative) . . . . . . . . . . . . . 11
A.3. C code sample (Informative) . . . . . . . . . . . . . . . 12
A.4. Digest samples (Informative) . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
TLS handshakes often include fairly static information such as server
certificate and a list of trusted Certification Authorities (CAs).
Static information such as a server certificate can be of
considerable size. This is the case in particular if the server
certificate is bundled with a complete certificate path, including
all intermediary certificates up to the trust anchor public key.
Significant benefits can be achieved in low bandwidth and high
latency networks, in particular if the communication channel also has
a relatively high rate of transmission errors, if a known and
previously cached server certificate path can be omitted from the TLS
handshake.
This specification defines the Cached Information TLS extension,
which may be used by a client and a server to exclude transmission of
known cached parameters from the TLS handshake.
1.1. Terminology
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 RFC 2119 [RFC2119].
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2. Cached Information Extension
A new extension type (cached_information(TBD)) is defined and used in
both the client hello and server hello messages. The extension type
is specified as follows.
enum {
cached_information(TBD), (65535)
} ExtensionType;
The "extension_data" field of this extension, when included in the
client hello, SHALL contain "CachedInformation" according to the
following structure:
enum {
certificate_chain(1), trusted_cas(2), (255)
} CachedInformationType;
struct {
CachedInformationType type;
opaque digest_value<0..8>;
} CachedObject;
struct {
CachedObject cached_info<1..2048>;
} CachedInformation;
The digest_value of a CachedObject MUST either be empty (0 bytes) or
contain a 64 bit FNV digest (8 bytes) as specified in Annex A. The 64
bit integer is represented as an 8 byte digest_value in big-endian
order (with most significant bits in the first byte and least
significant bits in the last byte).
When CachedInformationType identifies certificate_chain, then
digest_value MUST include a digest calculated over the
certificate_list element of a server side Certificate message,
excluding the three length bytes of the certificate_list vector.
When CachedInformationType identifies trusted_cas, then digest_value
MUST include a digest calculated over the certificate_authorities
element of a server side CertificateRequest message, excluding the
two length bytes of the certificate_authorities vector.
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Other specifications MAY define more CachedInformationType types.
3. Extension Exchange
3.1. Reconnaissance
A client MAY include an empty cached_information extension (with
empty extension_data field) in its (extended) client hello to query
whether the server supports cached information.
A server indicates that it supports cached information in handshakes
according to section 3.2. by including a cached_information extension
in its (extended) server hello.
3.2. Cached Information
Clients MAY specify cached information from previous handshakes by
including a "cached_information" extension in the (extended) client
hello, which contains at least one cached object (CachedObject) for
each present object type (CachedInformationType), as specified in
section 2. Clients MAY need the ability to cache different values
depending on other information in the Client Hello that modify what
values the server uses, in particular the Server Name Indication
[RFC4366] value. Clients sending a non-empty cached_information
extension MUST provide a 64 bit (8 byte) digest_value for each cached
object.
Servers that receive an extended client hello containing a
"cached_information" extension, MAY indicate that they support
caching of information objects by including an cached_information
extension in their (extended) server hello.
A cached_information extension provided in the server hello has the
following semantics:
o An empty cached_information extension indicates that the server
supports information caching but provides no information about
what information types it supports.
o A non-empty cached information extension indicates that the
server supports only those CachedInformationType types that are
identified by each present CachedObject.
o A CachedObject with an empty digest_value indicates that the
server supports caching of the specified object type
(CachedInformationType), but does not specify any digest values
it will accept.
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o A present non-empty digest_value indicates that the server will
honor caching of objects of the specified type that matches the
present digest value.
4. Data Substitution
Following a successful exchange of "cached_information" extensions,
the server may substitute data objects in the handshake exchange with
a matching digest_value representing a matching object type. received
from the client in its client hello.
The handshake protocol will proceed using the cached data as if it
was provided in the handshake protocol. The Finished message will
however be calculated over the actual data exchanged in the handshake
protocol. That is, the Finished message will be calculated over the
digest values of cached information objects and not over the cached
objects that were omitted from transmission.
Each CachedInformationType MUST specify how actual data is replaced
by a digest in a way that does not violate the defined syntax of
existing handshake messages. the data exchange syntax for
certificate_chain(1) and trusted_cas(2) are provided below.
The server MUST NOT provide more than one digest value as
substitution for the cached data.
4.1. Data Substitution Syntax for certificate_chain
When a digest for an object of type certificate_chain is provided in
the client hello, the server MAY substitute the cached data with a
matching digest value received from the client by expanding the
Certificate handshake message as follows.
Original handshake message syntax defined in RFC 5246 [RFC5246]:
opaque ASN.1Cert<1..2^24-1>;
struct {
ASN.1Cert certificate_list<0..2^24-1>;
} Certificate;
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Substitution syntax is defined by expanding the definition of the
opaque ASN.1Cert structure:
DigestInfo ASN.1Cert<1..2^24-1>;
struct {
opaque digest_value<0..8>;
} DigestInfo;
4.2. Data Substitution Syntax for trusted_cas
When a digest for an object of type trusted_cas is provided in the
client hello, the server MAY substitute the cached data with a
matching digest value received from the client by expanding the
CertificateRequest handshake message as follows.
Original handshake message syntax defined in RFC 5246 [RFC5246]:
opaque DistinguishedName<1..2^16-1>;
struct {
ClientCertificateType certificate_types<1..2^8-1>;
SignatureAndHashAlgorithm
supported_signature_algorithms<2^16-1>;
DistinguishedName certificate_authorities<0..2^16-1>;
} CertificateRequest
The substitution syntax is defined by expanding the definition of the
opaque DistinguishedName structure:
DigestInfo DistinguishedName<1..2^16-1>;
struct {
opaque digest_value<0..8>;
} DigestInfo;
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5. Security Considerations
The digest algorithm used in this specification is required to have
reasonable random properties in order to provide reasonably unique
identifiers. There is no requirement that this digest algorithm must
have strong collision resistance. A non unique digest may at most
lead to a failed TLS handshake followed by a new attempt without the
cached information extension. There are no identified security
threats that require the selected digest algorithm to have strong
collision resistance.
6. IANA Considerations
1) Create an entry, cached_information(TBD), in the existing registry
for ExtensionType (defined in RFC 5246 [RFC5246]).
2) Establish a registry for TLS CachedInformationType values. The
first entries in the registry are certificate_chain(1) and
trusted_cas(2). TLS CachedInformationType values in the inclusive
range 0-63 (decimal) are assigned via RFC 5226 [RFC5226] Standards
Action. Values from the inclusive range 64-223 (decimal) are
assigned via RFC 5226 Specification Required. Values from the
inclusive range 224-255 (decimal) are reserved for RFC 5226
Private Use.
7. Acknowledgements
The author acknowledge input from many members of the TLS working
group, Martin Rex for extensive review and input, Marsh Ray and Simon
Josefsson for coding and test vectors.
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8. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[RFC5226] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 5226, May 2008
[RFC5246] T. Dierks, E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008
[RFC4366] S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen, T.
Wright, "Transport Layer Security (TLS) Extensions", RFC
4366, April 2006
NOTE: RFC 4366 will be updated by RFC4366bis, currently in IESG
process.
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Annex A - 64 bit FNV-1a digest
A.1. Definition (Normative)
FNV-1 digest algorithm is a non-cryptographic hash function created
by Glenn Fowler, Landon Curt Noll, and Phong Vo. The FNV digest
algorithms and sample FNV source code have been released into the
public domain. FNV-1 has two defined variants, FNV-1 and FNV-1a. The
algorithm specified in this annex specifies the FNV-1a variant.
The FNV-1a digest is generated as follows:
digest = FNV_offset_basis
for each octet_of_data to be digested {
digest = digest XOR octet_of_data
digest = digest * FNV_prime }
return digest
In the above pseudocode, all variables are unsigned integers. All
variables, except for octet_of_data, have the same number of bits as
the FNV digest (64 Bits). The variable, octet_of_data, is an 8 bit
unsigned integer. Specifically for a 64 bit FNV-1a digest the
following applies:
o All variables, except for octet_of_data, are 64-bit unsigned
integers.
o The variable, octet_of_data, is an 8 bit unsigned integer.
o The FNV_offset_basis is the 64-bit FNV offset basis value:
14695981039346656037.
o The FNV_prime is the 64-bit FNV prime value: 1099511628211.
o The multiply function (indicated by the '*' symbol) returns the
lower 64-bits of the product.
o The XOR is an 8-bit operation that modifies only the lower 8-bits
of the digest value.
o The digest value returned is an 64-bit unsigned integer.
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A.2 Java code sample (Informative)
/**
* Java code sample, implementing 64 bit FNV-1a
* By Stefan Santesson
*/
import java.math.BigInteger;
public class FNV {
static public BigInteger getFNV1a64Digest (String inpString) {
BigInteger m = new BigInteger("2").pow(64);
BigInteger fnvPrime = new BigInteger("1099511628211");
BigInteger fnvOffsetBasis = new BigInteger
("14695981039346656037");
BigInteger digest = fnvOffsetBasis;
for (int i = 0; i < inpString.length(); i++) {
digest = digest.xor(BigInteger.valueOf(
(int) inpString.charAt(i)));
digest = digest.multiply(fnvPrime).mod(m);
}
return (digest);
}
}
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A.3. C code sample (Informative)
fnv1a64.h:
#ifndef FNV1A64_H
#define FNV1A64_H
#include <string.h> /* For size_t */
#include <stdint.h> /* For uint64_t */
extern uint64_t fnv1a64 (const uint8_t *buffer, size_t len);
#endif
fnv1a64.c:
/* fnv1a.c -- Implementation of the FNV-1A non-cryptographic
* hash function.
* By Simon Josefsson <simon@josefsson.org> on 2010-03-30.
*/
#include "fnv1a64.h"
#define FNV1A64_OFFSET_BASIS 14695981039346656037ULL
#define FNV1A64_PRIME 1099511628211ULL
uint64_t
fnv1a64 (const uint8_t *buffer, size_t len)
{
uint64_t hash;
size_t i;
hash = FNV1A64_OFFSET_BASIS;
for (i = 0; i < len; i++)
{
hash = hash ^ buffer[i];
hash = hash * FNV1A64_PRIME;
}
return hash;
}
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A.4. Digest samples (Informative)
Digest samples for 64 bit FNV-1a
For input data:
null ("")
0 bytes
Digest is: CB F2 9C E4 84 22 23 25
For input data:
hex: 61 ("a")
1 byte
Digest is: AF 63 DC 4C 86 01 EC 8C
For input data:
hex: FF 00 00 01
4 bytes
Digest is: 69 61 19 64 91 CC 68 2D
For input data:
hex: 68 74 74 70 3A 2F 2F 65 6E 2E 77 69 6B 69 70 65
64 69 61 2E 6F 72 67 2F 77 69 6B 69 2F 46 6F 77
6C 65 72 5F 4E 6F 6C 6C 5F 56 6F 5F 68 61 73 68
("http://en.wikipedia.org/wiki/Fowler_Noll_Vo_hash")
48 bytes
Digest is: D9 B9 57 FB 7F E7 94 C5
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Authors' Addresses
Stefan Santesson
3xA Security AB
Bjornstorp 744
247 98 Genarp
Sweden
EMail: sts@aaa-sec.com
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