Another Support for Multiple Hash Algorithms in Cryptographically Generated Addresses (CGAs)
draft-zhou-6man-mhash-cga-01

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 October 09, 2012.

Copyright Notice

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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 [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 cryptographicaly secure hash function, SHA-1 in the case of [RFC3972], is used to hash a public key into part of the neitwork 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 cryptographicaly secure hash algorithm than SHA-1, the transition to an alternative hash function is as easy as possible.

To provide a sense of hash algorishms agility , a method of reusing the security parameter bits in the address is secified [RFC4982]. Security parameter , sec, defined in RFC 3972, 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 insuffient 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.

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 decription of downgrading attack can be found in Section 4.1, RFC 4982.

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 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 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 cryptographicaly 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 agilibility to CGAs. The method in this document follows the idea of encoding the hash algorithm identifier in the interface identifier to avoid downgrading attack, as analysed in Section 4.1, RFC 4982.

Actually CGAs only adopt truncated forms of a hash algorithm which is considered cryptographicaly 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 algoritm 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. References

Authors' Addresses