Another Support for Multiple Hash Algorithms in Cryptographically Generated Addresses (CGAs)
draft-zhou-6man-mhash-cga-00

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.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on August 31, 2012.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 1.1. Terminology
• 2. Mhash-method Extension
• 3. Hash Algorithm Identity Parameter
• 4. CGA Generation Procedure
• 5. CGA Verification Procedure
• 6. IANA Considerations
• 7. Security Considerations
• 8. References
• Authors' Addresses

1. Introduction

Cryptographically Generated Addresses (CGAs) defined in [RFC 3972] is a method of binding a public key to an IPv6 address, with the aim of providing address ownership in many internet protocols. But RFC3972 only defined a unique hash algorithm, SHA-1, used to hashing a public key into part of the neitwork IP address. To provide a support for multiple hash algorishms, a method of reusing the security parameter bits in the address is secified [RFC 4982] . This method can only support three hash algorithm at most, and at the same time limiting security parameter to a few values. In this document, a support for multiple hash algorithms is provided without limiting security parameter or downgrading the security level of CGAs.

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].

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 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 defiend to denote the hash algorithm adopted when caculating 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

       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 teh 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 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

Veirfy 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 extenstion 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 extenstion filed is 0, do exactly as sepcified 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.

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 method in this document have the hash algorithm identifier as part of the interface identifier and ultimatly part of IPv6 address, downgrading attack can be avoided as analysed in RFC 4982.

WIth 3 bits off the interface identifier used for output of HASH1, the security level will be downgraded to O(2^56) from O(2^59), but with the increment by 3 of the check bits in the output of HASH2, the whole security level is kept the same, i.e.,O( 2^(16*sec+59)).

8. References

Authors' Addresses