Internet DRAFT - draft-zhou-6man-mhash-cga
draft-zhou-6man-mhash-cga
6man S. Zhou, Ed.
Internet-Draft R. Zhang
Intended status: Standards Track Z. Xie
Expires: March 17, 2013 ZTE Corporation
September 13, 2012
Another Support for Multiple Hash Algorithms in Cryptographically
Generated Addresses (CGAs)
draft-zhou-6man-mhash-cga-02
Abstract
This document provides a support for multiple hash algorithms in
Cryptographically Generated Addresses (CGAs) defined in RFC 3972.
Status of this Memo
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 March 17, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Mhash-method Extension . . . . . . . . . . . . . . . . . . . . 4
3. Hash Algorithm Identity Parameter . . . . . . . . . . . . . . . 4
4. CGA Generation Procedure . . . . . . . . . . . . . . . . . . . 5
5. CGA Verification Procedure . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
Cryptographically Generated Addresses (CGAs) defined in [RFC3972] is
a method of binding a public key to an IPv6 address, with the aim of
providing address ownership in many internet protocols. In the
generation and verification of a CGA address, a cryptographically
secure hash function, SHA-1 in the case of [RFC3972], is used to hash
a public key into part of the network IP address.
As pointed out in Section 4 of [RFC4982], it is wise to enable
multiple hash functions support in CGAs so that once the current hash
function does not satisfy future requirements ,e.g., potential future
applications of the CGAs may need a more cryptographically secure
hash algorithm than SHA-1, the transition to an alternative hash
function is as easy as possible.
To provide a sense of hash algorithms agility , a method of reusing
the security parameter bits in the address is specified[RFC4982].
Security parameter , sec, defined in [RFC3972], is a 3 bit value from
0 to 7 used in the hash extension technique (Section 7.2 in [RFC3972]
), to compensate the truncated SHA-1 output length because of
insufficient bits space in a CGA address. According to the method
specified in RFC4982, the security parameter is also used to
represent hash algorithm identity:
000 means sec=0 and SHA-1
001 means sec=1 and SHA-1
010 means sec=2 and SHA-1
Then security parameter is limited to 0,1,2 . They may be sufficient
for now, but higher security parameter value may also be required
with computers becoming faster, as pointed out in Section 7.2 , RFC
3972.
Even with limited security parameter value, the method in RFC 4982
can only support three hash algorithms at most. That is besides
SHA-1, we have a second choice of an alternative hash algorithm
number one with sec=0,1,2 and a third choice of another alternative
hash algorithm number two with sec can only be two values from
{0,1,2}.
Taking the above two factors into consideration, at some time in the
future we will be faced with a painful choice, high security
parameter or a more secure hash algorithm? And we may be also
challenged with pain of high cost of upgrading because of the massive
number of IPv6 nodes that may be using CGA addresses.
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In this document, a support for multiple hash algorithms without
limiting security parameter or downgrading the security level of CGAs
is provided. The proposed solution follows the idea of encoding the
hash algorithm identity in the CGA addresses to prevent from
downgrading attacks, the detailed description of downgrading attack
can be found in Section 4.1, [RFC4982].
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 [RFC2119].
2. Mhash-method Extension
To accomodate RFC 4982, an extension field "Mhash-method" is defined.
The format is illustrated in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Type | Extension Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mhash-method|
+-+-+-+-+-+-+-+
Extension Type: TBA. (16-bit unsigned integer).
Extension Data Length: 1. (16-bit unsigned integer. Length of the
multiple-hash-method field of this option, in octets.)
Mhash-method: 1 octet length field. If Mhash-method equal 0, it
means the method of denoting hash algorithm specified in RFC 4982 is
adopted, if Mhash-method equal 1, it means the method specified in
this document is adopted.
3. Hash Algorithm Identity Parameter
A hash algorithm identity parameter (hid) in CGA is defined to denote
the hash algorithm adopted when calculating HASH1 and HASH2. The
hash algorithm identity parameter is a three-bit unsigned integer,
and it is encoded in the 3rd-5th bits of the interface identifier.
This can be written as follows:
hid = (interface identifier & 0x1c00000000000000) >> 58
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0 1 2 3 4 5 6 7 8 9 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sec | hid |0 0| Leftmost 56 bits of HASH1 output |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4. CGA Generation Procedure
Generate a CGA as defined in RFC 3972 except some modification to
steps 2,3,5,6 and 9 as shown in the following:
1. Set the modifier to a random or pseudo-random 128-bit value.
2. Concatenate from left to right the modifier, 9 zero octets, the
encoded public key, and any optional extension fields. Execute
the adopted hash algorithm ( denoted by value of hid) on the
concatenation. Take the 112( or 115 in case sec=7 ) leftmost
bits of the hash value. The result is Hash2.
3. Compare the 16*Sec+3 leftmost bits of Hash2 with zero. If they
are all zero, continue with step 4. Otherwise, increment the
modifier by one and go back to step 2.
4. Set the 8-bit collision count to zero.
5. Concatenate from left to right the final modifier value, the
subnet prefix, the collision count, the encoded public key, and
any optional extension fields. Execute the adopted hash
algorithm on the concatenation. Take the 56 leftmost bits of the
hash value. The result is Hash1.
6. Form an interface identifier from Hash1 by writing the value of
Sec into the three leftmost bits, writing the value of hid into
the following three bits and by setting bits 6 and 7 (i.e., the
"u" and "g" bits) to zero.
7. Concatenate the 64-bit subnet prefix and the 64-bit interface
identifier to form a 128-bit IPv6 address with the subnet prefix
to the left and interface identifier to the right, as in a
standard IPv6 address .
8. Perform duplicate address detection if required. If an address
collision is detected, increment the collision count by one and
go back to step 5. However, after three collisions, stop and
report the error.
9. Form the CGA Parameters data structure by concatenating from left
to right the final modifier value, the subnet prefix, the final
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collision count value, the encoded public key, Mhash-method value
(equal 1 in this case) and any other optional extension fields.
5. CGA Verification Procedure
Verify a CGA as defined in RFC 3972 except some modification to steps
3,4,6 and 7 as shown in the following:
1. Check that the collision count in the CGA Parameters data
structure is 0, 1, or 2. The CGA verification fails if the
collision count is out of the valid range.
2. Check that the subnet prefix in the CGA Parameters data structure
is equal to the subnet prefix (i.e., the leftmost 64 bits) of the
address. The CGA verification fails if the prefix values differ.
3. If the Mhash-method value in the Mhash-method extension filed is
1, read the hash algorithm identity parameter hid from the 3rd-
5th bits of the 64-bit interface identifier of the address,
execute the hash algorithm denoted by hid on the CGA Parameters
data structure. Take the 56 leftmost bits of the hash value.
The result is Hash1. If the Mhash-method value in the Mhash-
method extension filed is 0, do exactly as specified in RFC 3972
and RFC4982.
4. Compare Hash1 with the interface identifier (i.e., the rightmost
56 bits) of the address. If the 56-bit values differ, the CGA
verification fails.
5. Read the security parameter Sec from the three leftmost bits of
the 64-bit interface identifier of the address. (Sec is an
unsigned 3-bit integer.)
6. Concatenate from left to right the modifier, 9 zero octets, the
public key, and any extension fields that follow the public key
in the CGA Parameters data structure. Execute the hash algorithm
denoted by hid on the concatenation. Take the 112 (or 115 in
case sec=7) leftmost bits of the SHA-1 hash value. The result is
Hash2.
7. Compare the 16*Sec+3 leftmost bits of Hash2 with zero. If any
one of them is not zero, the CGA verification fails. Otherwise,
the verification succeeds.
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6. IANA Considerations
This document defines one new CGA Extension Type [RFC4581] option,
which must be assigned by IANA:
Name: Mhash-method extension type;
Value: TBA.
Description: see Section 2.
The values of Mhash-method are also defined:
Name: Mhash-method extension value;
Value: 0 meaning RFC 4982, 1 meaning this document;
Description: see Section 2.
This document also defines a new parameter (hid) in CGA, the value of
which must be assigned by IANA. It may be assigned as follows:
Name | Value
-------------------+-------
SHA-1 | 000
SHA-244 | 001
SHA-256 | 010
SHA-384 | 011
SHA-512 | 100
TBD | 101
TBD | 110
TBD | 111
7. Security Considerations
The security of applications using CGAs relies on the adopted public
key schemes, which is out of the scope of this document, as well as
the adopted hash algorithms.
A high cryptographically secure hash algorithm is obviously required.
But no hash algorithms are guaranteed to be secure for ever, it is
wise to add algorithm agility into CGAs in case current hash
algorithm be successfully attacked.
This document suggests adding more flexible hash algorithm agility to
CGAs. The method in this document follows the idea of encoding the
hash algorithm identifier in the interface identifier to avoid
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downgrading attack, as analyzed in Section 4.1, RFC 4982.
Actually CGAs only adopt truncated forms of a hash algorithm which is
considered cryptographically secure in the sense of its regular form.
As specified in RFC 3972, the effective bits relating to the security
of CGAs are only the leftmost 59 bits (in the case of HASH1) , and
the left most 16*sec bits (in the case of HASH2) of the whole 160
bits SHA-1 output . It is roughly estimated that the overall
security of the hash algorithm is O( 2^(16*sec+59))(Section 7.2,
RFC3972).
In this document, 3 bits originally used for output of HASH1are taken
off the interface identifier to denote hash algorithm identity, while
3 more bits of output of HASH2 are checked, in a whole the whole
security level is kept roughly the same, i.e.,O( 2^(16*sec+59)) .
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[RFC4982] Bagnulo, M. and J. Arkko, "Support for Multiple Hash
Algorithms in Cryptographically Generated Addresses
(CGAs)", RFC 4982, July 2007.
Authors' Addresses
Sujing Zhou (editor)
ZTE Corporation
No.68 Zijinghua Rd. Yuhuatai District
Nanjing, Jiang Su 210012
R.R.China
Email: zhou.sujing@zte.com.cn
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Ruishan Zhang
ZTE Corporation
889 Bibo Rd, Zhangjiang Hi-Tech Park
Shanghai 201203
P.R.China
Email: zhang.ruishan@zte.com.cn
Zhenhua XIe
ZTE Corporation
No.68 Zijinghua Rd. Yuhuatai District
Nanjing, Jiang Su 210012
P.R.China
Phone: +86-25-52871287
Fax: +86-25-52871000
Email: xie.zhenhua@zte.com.cn
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