Using cSHAKE in ORCHIDs

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Table of Contents

2. Terms, Notation and Definitions

3. Adding additional information to the ORCHID

ORCHIDv2 [RFC7343] is currently defined as consisting of three components:¶

ORCHID     :=  Prefix | OGA ID | Encode_96( Hash )

where:

Prefix          : A constant 28-bit-long bitstring value
                  (IANA IPv6 assigned).

OGA ID          : A 4-bit long identifier for the Hash_function
                  in use within the specific usage context.

Encode_96( )    : An extraction function in which output is obtained
                  by extracting the middle 96-bit-long bitstring
                  from the argument bitstring.

This addendum will be constructed as follows:¶

ORCHID     :=  Prefix | OGA ID | Info (n) | Hash (m)

where:

Prefix          : A constant 28-bit-long bitstring value
                  (IANA IPv6 assigned).

OGA ID          : A 4-bit long identifier for the Hash_function
                  in use within the specific usage context.

Info (n)        : n bits of information that define a use of the
                  ORCHID.  n can be zero, that is no additional
                  information.

Hash (m)        : An extraction function in which output is m bits.

n + m = 96 bits

The 96 bits currently allocated to the Encode_96 function can be divided in any manner between the additional information and the hash output. Care must be taken in determining the size of the hash portion, taking into account risks like pre-image attacks. Thus 64 bits as used in Hierarchical HITs may be as small as is acceptable.¶

4. ORCHID Decoding

With this addendum, the decoding of an ORCHID is determined by the Prefix and OGA ID. ORCHIDv2 [RFC7343] decoding is selected when the Prefix is: 2001:20::/28.¶

For Heirarchical HITs, the decoding is determined by the presence of the HHIT Prefix as specified in the HHIT document.¶

5. ORCHID Encoding

ORCHIDv2 has a number of inputs including a Context ID, some header bits, the hash algorithm, and the input bitstream, normally just the public key. The output is a 96 bit value.¶

This addendum adds a different encoding process to that currently used. The input to the hash function explicitly includes all the fixed header content plus the Context ID. The fixed header content consists of the Prefix, OGA ID, and the Additional Information. Secondly, the length of the resulting hash is set by the rules set by the Prefix/OGA ID. In the case of Hierarchical HITs, this is 64 bits.¶

To achieve the variable length output in a consistent manner, the cSHAKE hash is used. For this purpose, cSHAKE128 is appropriate. The the cSHAKE function call for this addendum is:¶

    cSHAKE128(Input, L, "", Context ID)

    Input      :=  Prefix | OGA ID | Additional Information | HOST_ID
    L          :=  Length in bits of hash portion of ORCHID

Hierarchical HIT uses the same context as all other HIPv2 HIT Suites as they are clearly separated by the distinct HIT Suite ID.¶

6. Initial use case for cSHAKE

The EdDSA/cSHAKE based HITs in New Cryptographic Algorithms for HIP [new-hip-crypto] is the first HIP Suite to use cSHAKE, thus using this addendum.¶

7. Initial use case for Additional Information

Hierarchical HITs [hierarchical-hit] (HHITs) is the first HIT construct that specifies the need of dividing the 96 bits available to ORCHID into its Hierarchy ID (HID) and HI Hash, thus using this addendum.¶

HHITs use a unique Context ID as well as a Prefix different from HIPv2 [RFC7401]. The different Prefix enables receivers to properly decode the HID out of the HIT and validate the HIT, given the HI.¶

8. Collision risks with Hierarchical HITs

The 64 bit hash size does have an increased risk of collisions over the 96 bit hash size used for the other HIT Suites. There is a 0.01% probability of a collision in a population of 66 million. The probability goes up to 1% for a population of 663 million. See Appendix A for the collision probability formula.¶

However, this risk of collision is within a single "Additional Information" value. Some registration process should be used to reject a collision, forcing the client to generate a new HI and thus HIT and reapplying to the registration process.¶

9. IANA Considerations

TBD.¶

Appendix A. Calculating Collision Probabilities

The accepted formula for calculating the probability of a collision is:¶

    p = 1 - e^{-k^2/(2n)}


    P   Collision Probability
    n   Total possible population
    k   Actual population

Authors' Addresses