New Cryptographic Algorithms for HIP
draft-moskowitz-hip-new-crypto-02

Abstract

This document provides new cryptographic algorithms to be used with HIP. The Edwards Elliptic Curve and the Keccak sponge functions are the main focus. The HIP parameters and processing instructions impacted by these algorithms are defined.

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Table of Contents

• 1. Introduction
• 2. Terms and Definitions
• 2.1. Requirements Terminology
• 2.2. Definitions
• 3. HIP Parameter values for new Crytpo
• 3.1. Elliptic Curves for Diffie-Hellman
• 3.1.1. DIFFIE_HELLMAN
• 3.2. Edward Digital Signature Algorithm
• 3.2.1. HOST_ID
• 3.2.2. HIT_SUITE_LIST
• 3.3. Hashing with the Keccak Function
• 3.3.1. The Keccak Permutation
• 3.3.2. RHASH
• 3.3.3. HIP_MAC and HIP_MAC2
• 3.4. HIP Cipher
• 3.4.1. HIP_CIPHER
• 4. Generating a HIT from an HI
• 5. HIP KEYMAT Generation
• 6. Using Keccak for a Pseudorandom Function
• 7. IANA Considerations
• 8. Security Considerations
• 8.1. Keymat vulnerabilities
• 9. Acknowledgments
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

This document adds new cryptographic algorithms for HIPv2. This includes:

• New elliptic curves for ECDH.
• The Edwards Elliptic Curve Digital Signature Algorithm (EdDSA) used in Host Identities (HI) and for Base Exchange (BEX) signatures.
• Hashes used in Host Identity Tag (HIT) generation, and wherever else hashes are needed.
• Keyed hashes used for KEYMAT generation and packet MACing operations.
• AEAD and stream ciphers to use in HIP and HIP enabled secure communication protocols.

The hashes and encryption are all built on the Keccak sponge function.

These additions reflect selection of advances in the field of cryptography that would best benefit HIP, particularly in constrained devices and communications.

2. Terms and Definitions

2.1. Requirements Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. `

2.2. Definitions

Keccak (KECCAK Message Authentication Code):

The family of all sponge functions with a KECCAK-f permutation as the underlying function and multi-rate padding as the padding rule.

KMAC (KECCAK Message Authentication Code):

A PRF and keyed hash function based on KECCAK.

cSHAKE (The customizable SHAKE function):

Extends the SHAKE scheme to allow users to customize their use of the function.

SHAKE (Secure Hash Algorithm KECCAK):

A secure hash that allows for an arbitrary output length.

PRF (Pseudorandom Function):

A function that can be used to generate output from a random seed such that the output is computationally indistinguishable from truly random output.

capacity:

In the sponge construction, the width of the underlying function minus the rate.

rate:

In the sponge construction, the number of input bits processed per invocation of the underlying function.

XOF (eXtendable-Output Function):

A function on bit strings (also called messages) in which the output can be extended to any desired length.

3. HIP Parameter values for new Crytpo

HIP parameters carry information that is necessary for establishing and maintaining a HIP association. For example, the device's public keys as well as the signaling for negotiating ciphers and payload handling are encapsulated in HIP parameters. Additional information, meaningful for end hosts or middleboxes, may also be included in HIP parameters. The specification of the HIP parameters and their mapping to HIP packets and packet types is flexible to allow HIP extensions to define new parameters and new protocol behavior.

3.1. Elliptic Curves for Diffie-Hellman

Elliptic curves Curve25519 and Curve448 RFC 7748) are specified here for use in the HIP Diffie-Hellman exchange.

Curve25519 and Curve448 are already defined in Section 5.2.1 of [I-D.ietf-hip-dex], using the HIP-DEX CKDF. Here they are defined for using the new KMAC NIST SP 800-185 derived KDF in Section 5.

3.1.1. DIFFIE_HELLMAN

The DIFFIE_HELLMAN parameter may be included in selected HIP packets based on the DH Group ID selected. The DIFFIE_HELLMAN parameter is defined in Section 5.2.7 of [RFC7401].

The following Elliptic Curves are defined here:

Group                              KDF              Value

Curve25519 [RFC7748]               KKDF             13
Curve448   [RFC7748]               KKDF             14
      

A new KDF for KEYMAT, Section 6.5 of [RFC7401] and Section 6.3 of [I-D.ietf-hip-dex] using Keccak is defined in Section 5.

3.2. Edward Digital Signature Algorithm

Edwards-Curve Digital Signature Algorithm (EdDSA) (RFC 8032) are specified here for use as Host Identities (HIs).

3.2.1. HOST_ID

The HOST_ID parameter specifies the public key algorithm, and for elliptic curves, a name. The HOST_ID parameter is defined in Section 5.2.19 of [RFC7401].

     Algorithm
     profiles         Values

     EdDSA            13 [RFC8032]       (RECOMMENDED)
    

For hosts that implement EdDSA as the algorithm, the following ECC curves are available:

     Algorithm    Curve            Values

     EdDSA        RESERVED         0
     EdDSA        EdDSA25519       1 [RFC8032]
     EdDSA        EdDSA25519ph     2 [RFC8032]
     EdDSA        EdDSA448         3 [RFC8032]
     EdDSA        EdDSA448ph       4 [RFC8032]
    

3.2.2. HIT_SUITE_LIST

The HIT_SUITE_LIST parameter contains a list of the supported HIT suite IDs of the Responder. Based on the HIT_SUITE_LIST, the Initiator can determine which source HIT Suite IDs are supported by the Responder. The HIT_SUITE_LIST parameter is defined in Section 5.2.10 of [RFC7401].

The following HIT Suite ID is defined, and the relationship between the four-bit ID value used in the OGA ID field and the eight-bit encoding within the HIT_SUITE_LIST ID field is clarified:

     HIT Suite       Four-bit ID    Eight-bit encoding
     RESERVED            0             0x00
     EdDSA/SHAKE128      5             0x50           (RECOMMENDED)
    

The following table provides more detail on the above HIT Suite combinations. The input for each generation algorithm is the encoding of the HI as defined in Section 4. The output is 96 bits long and is directly used in the ORCHID.

3.3. Hashing with the Keccak Function

The Keccak sponge function is the basis for the new SHA-3, standard NIST FIPS 202, and the customized XOF functions in NIST SP 800-185. These are used here as an alternative to all the hashing functions in HIP.

Hardware implementation of Keccak in VHDL is available from Keccak.

3.3.1. The Keccak Permutation

Keccak is described as a sponge function. The analogy to a sponge is that an arbitrary number of input bits are "absorbed" into the state of the function, after which an arbitrary number of output bits are "squeezed" out of its state.

• The Keccak function is defined to have a width of b bits. Where b is the capacity (c) + rate (r).
• The rate is the number of bits "fed" into the sponge at a time.
• The capacity is twice the desired hash "strength" and part of the sponge width.

b is one of the set {25, 50, 100, 200, 400, 800, 1600}. In FIPS 202, b=1600. Thus a hash strength of 128 bits can be delivered with c=256 and r=1344, or 168 byte segment input to the sponge.

Keccak can also provide a hash strength of 128 bit with b=800 (r=544 or 68 bytes) and b=400 (r=144 or 18 bytes). 256 bit strength can only be provided with b=1600 or 800.

FIPS 202 does not specify use of these smaller values for b which may be preferred in memory constrained devices, processing relatively short input strings. Future work will determine if the smaller values for b result in a significant performance/memory improvement to warrant their use.

3.3.2. RHASH

The RHASH is the general term used throughout [RFC7401] to refer to the hash used for a specific HIT suite. For this addendum SHAKE128 is used, even for HIs of EdDSA448.

Unless otherwise specified, L of SHAKE128 is 256, resulting in a similar output to SHA256. Any truncation used for, older, fixed output hashes is still used. This is to simplify code integration. One exception to this is in Section 4.

3.3.3. HIP_MAC and HIP_MAC2

The HIP_MAC and HIP_MAC2 parameters in [RFC7401] use HMAC [RFC2104]. This performs two hashes on a string with a key for a keyed hash the length of the underlying hash.

Here, KMAC from NIST SP 800-185 is used. This is a single pass using the underlying cSHAKE function. The function call is:

     KMAC128(Key, Input String, 256, "")
    

3.4. HIP Cipher

HIP encrypted parameters use the HIP_CIPHER, Section 5.2.8 of [RFC7401]. The Keccak Keyak cipher, [I-D.ietf-hip-dex], is recommended. Keyak is a candidate in the NIST Lightweight Cryptography competition and is consistent with the overall approach in this addendum to use Keccak functions for simplicity in design and implementation.

3.4.1. HIP_CIPHER

The HIP_CIPHER parameter values for Keyak are:

hip_cipher
     Suite ID           Value

     RIVER KEYAK        6     (Keyak)
     LAKE KEYAK         7     (Keyak)
      

For use as the HIP Cipher, the TAG generated in Keyak is length 0. The Keyak SUV is the key plus IV specified for the encrypted parameter. River Keyak MAY be used for [Keyak_Cipher], in place of AES-CTR.

Lake Keyak can provide 256 bits of security by following the recommendations for the Keyak cipher.

4. Generating a HIT from an HI

The EdDSA/cSHAKE based HITs vary slightly the ORCHID generation method described in section 3.2 of [RFC7401]. The XOF functionality of cSHAKE produces an output of L bits. This replaces the Encode_96 function in the ORCHID generation.

For identities that are EdDSA public keys, the ORCHID input consists of the public key encoding as specified for the Host Identity field of the HOST_ID parameter (see Section 3.2.1). Since L is less than 128, cSHAKE128 is used for all EdDSA curve sizes:

     cSHAKE128(EdDSA public key, 96, "", Context ID)
    

5. HIP KEYMAT Generation

The KMAC function provides a new, more efficient, key derivation function over HKDF [RFC5869]. This will be referred to as KKDF.

The choice of KMAC128 or KMAC256 is based on the strength of the output key material. For 256 bits of strength equivalent to HMAC-SHA256, use KMAC256. Per NIST SP 800-56Cr1, Section 4.1, Option 3:

     OKM = KMAC[128|256](salt | info, IKM, L, S)
    

L is the derived key bit length. Since 4 HIP keys are "drawn" from this output, the length is 4 * HIP_key_size. Per ASIACRYPT 2017, pp. 606-637 each of these derived keys will have the same strength as the Diffie-Hellman shared secret.

S is the byte string 01001011 || 01000100 || 01000110, which represents the sequence of characters “K”, “D”, and “F” in 8-bit ASCII.

Salt and info are derived as defined in [RFC7401] or [I-D.ietf-hip-dex]. There are special security considerations for IKM per [RFC7748]. The two HIs MUST be used in constructing IKM as follows:

     IKM = Diffie-Hellman secret | HI-R |  HI-I
    

These are separately DER encoded.

6. Using Keccak for a Pseudorandom Function

Appendix B of NIST SP 800-185 defines how to use SHAKE, cSHAKE, or KMAC as a PRF.

7. IANA Considerations

IANA will need to make the following changes to the "Host Identity Protocol (HIP) Parameters" registries:

Diffie Hellman:

This document defines the new Curve25519 and Curve448 for the Diffie-Hellman exchange (see Section 3.1.1).

Host ID:

This document defines the new EdDSA Host ID (see Section 3.2.1).

HIT Suite ID:

This document defines the new HIT Suite of EdDSA/cSHAKE (see Section 3.2.2).

HIP Cipher:

This document defines the new Keyak ciphers for HIP encrypted parameters (see Section 3.4.1).

8. Security Considerations

8.1. Keymat vulnerabilities

[RFC7748] warns about using Curve25519 and Curve448 in Diffie-Hellman for key derivation:

Designers using these curves should be aware that for each public key, there are several publicly computable public keys that are equivalent to it, i.e., they produce the same shared secrets. Thus using a public key as an identifier and knowledge of a shared secret as proof of ownership (without including the public keys in the key derivation) might lead to subtle vulnerabilities.

This applies to [I-D.ietf-hip-dex], but may have broader consequences. Thus the two Host IDs are included with the Diffie-Hellman secret.

9. Acknowledgments

Quynh Dang of NIST gave considerable guidance on using Keccak and the NIST supporting documents. Joan Deamen of the Keccak team was especially helpful in many aspects of using Keccak, particularly with the KEYMAT section and the strength of the derived keys.

10. References

10.1. Normative References

10.2. Informative References

Authors' Addresses