Enhancing Security in EAP-AKA' with Hybrid Post-Quantum Cryptography

Enhancing Security in EAP-AKA' with Hybrid Post-Quantum Cryptography Nokia

Munich Germany aritra.banerjee@nokia.com

Nokia

Bangalore Karnataka India kondtir@gmail.com

Security EMU PQC EAP-AKA' Forward Secrecy for the Extensible Authentication Protocol Method for Authentication and Key Agreement (EAP-AKA' FS) is specified in , providing updates to with an optional extension that offers ephemeral key exchange using the traditional Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) key agreement algorithm for achieving perfect forward secrecy (PFS). However, it is susceptible to future threats from Cryptographically Relevant Quantum Computers, which could potentially compromise a traditional ephemeral public key. If the adversary has also obtained knowledge of the long-term key and ephemeral public key, it could compromise session keys generated as part of the authentication run in EAP-AKA'. This draft aims to enhance the security of EAP-AKA' FS protocol by leveraging PQ/T Hybrid algorithms to make it quantum-safe. About This Document Status information for this document may be found at . Discussion of this document takes place on the emu Working Group mailing list (), which is archived at . Subscribe at .

Introduction Forward Secrecy for the Extensible Authentication Protocol Method for Authentication and Key Agreement (EAP-AKA' FS) defined in updates the improved Extensible Authentication Protocol Method for 3GPP Mobile Network Authentication and Key Agreement (EAP-AKA') specified in , with an optional extension providing ephemeral key exchange. This prevents an attacker who has gained access to the long term key from obtaining session keys established in the past, assuming these have been properly deleted. EAP-AKA' FS mitigates passive attacks (e.g., large scale pervasive monitoring) against future sessions. Nevertheless, EAP-AKA' FS uses traditional algorithms public-key algorithms (e.g., ECDH) which will be broken by a Cryptographically Relevant Quantum Computer (CRQC) using Shor's algorithm. The presence of a CRQC would render state-of-the-art, traditional public-key algorithms deployed today obsolete and insecure, since the assumptions about the intractability of the mathematical problems for these algorithms that offer confident levels of security today no longer apply in the presence of a CRQC. A CRQC could recover the SHARED_SECRET from the ECDHE public keys (Section 6.3 of ). If the adversary has also obtained knowledge of the long-term key, it could then compute CK', IK', and the SHARED_SECRET, and any derived output keys. This means that the CRQC would disable the forward security capability provided by . The migration to PQC is unique in the history of modern digital cryptography in that neither the traditional algorithms nor the post-quantum algorithms are fully trusted to protect data for the required data lifetimes. The post-quantum algorithms face uncertainty about the underlying mathematics, compliance issues, unknown vulnerabilities, hardware and software implementations that have not had sufficient maturing time to rule out classical cryptanalytic attacks and implementation bugs. During the transition from traditional to post-quantum algorithms, there is a desire or a requirement for protocols that use both algorithm types. This specification defines HPKE for use with EAP-AKA' FS. HPKE offers a variant of public-key encryption of arbitrary-sized plaintexts for a recipient public key. HPKE works for any combination of an asymmetric key encapsulation mechanism (KEM), key derivation function (KDF), and authenticated encryption with additional data (AEAD) function. HPKE can be extended to support hybrid post-quantum Key Encapsulation Mechanisms (KEMs) as defined in .

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here.

Terminology This document makes use of the terms defined in . For the purposes of this document, it is helpful to be able to divide cryptographic algorithms into two classes: "Traditional Algorithm": An asymmetric cryptographic algorithm based on integer factorisation, finite field discrete logarithms or elliptic curve discrete logarithms. In the context of JOSE, examples of traditional key exchange algorithms include Elliptic Curve Diffie-Hellman Ephemeral Static . In the context of COSE, examples of traditional key exchange algorithms include Ephemeral-Static (ES) DH and Static-Static (SS) DH . "Post-Quantum Algorithm": An asymmetric cryptographic algorithm that is believed to be secure against attacks using quantum computers as well as classical computers. Examples of PQC key exchange algorithms include Kyber. "Hybrid" key exchange, in this context, means the use of two key exchange algorithms based on different cryptographic assumptions, e.g., one traditional algorithm and one Post-Quantum algorithm, with the purpose of the final shared secret key being secure as long as at least one of the component key exchange algorithms remains unbroken. It is referred to as PQ/T Hybrid Scheme in . PQ/T Hybrid Key Encapsulation Mechanism: A Key Encapsulation mechanism (KEM) made up of two or more component KEM algorithms where at least one is a post-quantum algorithm and at least one is a traditional algorithm.

Background on EAP-AKA' with perfect forward secrecy In EAP-AKA', The authentication vector (AV) contains a random part RAND, an authenticator part AUTN used for authenticating the network to the USIM, an expected result part XRES, a 128-bit session key for integrity check IK, and a 128-bit session key for encryption CK. As described in the draft , the server has the EAP identity of the peer. The server asks the AD to run AKA algorithm to generate RAND, AUTN, XRES, CK and IK. Further it also derives CK’ and IK’ keys which are tied to a particular network name. The server now generates the ephemeral key pair and sends the public key of that key pair and the first EAP method message to the peer. In this EAP message, AT_PUB_ECDHE (carries public key) and the AT_KDF_FS(carries other FS related parameters). Both of these might be ignored of USIM doesn’t support the Forward Secrecy extension. The peer checks if it wants to have a Forward extension in EAP AKA'. If yes, then it will eventually respond with AT_PUB_ECDHE and MAC. If not, it will ignore AT_PUB_ECDHE. If the peer wants to participate in FS extension, it will then generate its ECDH key pair, calculate a shared key based on its private key and server public key. The server will receive the RES from peer and AT_PUB_ECDHE. The shared key will be generated both in the peer and the server with key pairs exchanged, and later master key is also generated.

Hybrid Enhancements by Design We suggest the following changes and enhancements:

A new attribute, AT_PUB_HYBRID, is defined to carry the public key, which is the concatenation of traditional and PQC KEM public keys from the EAP server. The AT_PUB_HYBRID attribute will carry the encapsulated key, which is formed by concatenating the encapsulated key (enc) from the traditional KEM algorithm and the ciphertext (ct) from the PQC KEM Encapsulation function from the EAP peer.
The AT_KDF_FS attribute is updated to indicate the PQ/T Hybrid KEM in HPKE and HKDF for generating the Hybrid Master Key MK_HYBRID.
Multiple AT_KDF_FS attributes is included in the EAP-Request to handle the EAP peer not supporting PQ/T Hybrid KEM in HPKE.
The Hybrid key derivation function will be included first in the EAP-Request to indicate a higher priority than the traditional key derivation function.

Protocol Construction This section defines the construction for hybrid key exchange in EAP-AKA' FS. Hybrid key exchange refers to using multiple key exchange algorithms simultaneously and combining the result with the goal of providing security even if all but one of the component algorithms is broken. It is motivated by transition to post-quantum cryptography.

Key Steps in protocol construction We outline the following key steps in the protocol:

Server generates the PQC KEM Public key(pq_PK), private key (pq_SK) pair and the ECDH public key (trad_PK), private key (trad_SK) pair. The server will generate and send the EAP AKA' Authentication Vector (AV).
The server will store the expected response XRES, the ECDH private key trad_SK and the PQC KEM private key pq_SK. The server will forward the EAP AKA' AV to peer along with pq_PK and trad_PK.
The USIM will validate the AUTH received, also verifies the MAC. After the verification is successful and if the peer also supports the Forward secrecy, peer will invoke Encapsulate using concat(pq_PK, trad_PK) as defined in section 5.4 of .

"ct" is the concatenation of the ciphertext from PQC KEM and encapsulated key from ECDH whereas "ss" is hybrid shared secret key. Hybrid shared key ss is generated by the peer using the Encapsulate() ().

The peer will send the Authentication response RES and ct to the server.
The server will verify the RES with XRES. The server will use the ct, PQC KEM private key pq_SK and ECDH private key trad_SK to generate shared secret.

The generated ss from Decapsulate is the hybrid shared secret key derived from PQC KEM and traditional ECDH. The peer and the server first generate the MK_HYBRID and subsequently generate MSK, EMSK as shown below: where, pkR is concatenation of PQC KEM and traditional public keys of the receiver, ct is concatenation of the ciphertext from the PQC KEM and encapsulated key from ECDH; the Encapsulate() function is perfomed by the peer only.

Extensions to EAP-AKA' FS

AT_PUB_HYBRID The format of the AT_PUB_HYBRID attribute is shown below. The fields are as follows: AT_PUB_HYBID: This is set to TBA1 BY IANA. Reserved: A 1-byte field reserved for future use. Including this field ensures that the fixed header (Type, Reserved, Length) is 4 bytes long, maintaining 4-byte alignment for the following Value field. The Reserved field MUST be set to 0 on transmission and ignored on receipt. Length: A 2-byte unsigned integer indicating the total length of the attribute in bytes, including the Type, Reserved, Length, and Value fields, as well as any padding. The length is expressed in multiples of 4 bytes. This differs from the attribute format used in EAP-AKA , where the Length field is 1 byte. The modification is necessary because PQ/T Hybrid KEM public keys, such as X25519 and ML-KEM-768 (e.g., X-Wing), would exceed the 1024-byte limit imposed by the original EAP-AKA format. Value: Because the length of the attribute must be a multiple of 4 bytes, the sender pads the Value field with zero bytes when necessary. To retain the security of the keys, the sender SHALL generate a fresh value for each run of the protocol.

Security Considerations ML-KEM is IND-CCA2 secure based on multiple analyses. The ML-KEM variant and its underlying components should be selected consistently with the desired security level. For further clarity on the sizes and security levels of ML-KEM variants, please refer to the tables in Sections 12 and 13 of . The security of the ML-KEM algorithm depends on a high-quality pseudo-random number generator. For further discussion on random number generation, see . In general, good cryptographic practice dictates that a given ML-KEM key pair should be used in only one EAP session. This practice mitigates the risk that compromising one EAP session will not compromise the security of another EAP session and is essential for maintaining forward security. For security properties of traditional ECDHE for EAP-AKA FS, see section 7 of . The overall Hybrid scheme needs to be IND-CCA2 robust; i.e., atleast one of the schemes should be IND-CCA2 secure.

IANA Considerations Two new values (TBA2) and (TBA3) in the skippable range need to be assigned by IANA for AT_PUB_HYBRID () in the "Attribute Types" registry under the "EAP-AKA and EAP-SIM Parameters" group. IANA is requested to update the registry "EAP-AKA' AT_KDF_FS Key Derivation Function Values" with the Hybrid key derivation function entry:

References Normative References Hybrid Public Key Encryption This document describes a scheme for hybrid public key encryption (HPKE). This scheme provides a variant of public key encryption of arbitrary-sized plaintexts for a recipient public key. It also includes three authenticated variants, including one that authenticates possession of a pre-shared key and two optional ones that authenticate possession of a key encapsulation mechanism (KEM) private key. HPKE works for any combination of an asymmetric KEM, key derivation function (KDF), and authenticated encryption with additional data (AEAD) encryption function. Some authenticated variants may not be supported by all KEMs. We provide instantiations of the scheme using widely used and efficient primitives, such as Elliptic Curve Diffie-Hellman (ECDH) key agreement, HMAC-based key derivation function (HKDF), and SHA2. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. Improved Extensible Authentication Protocol Method for 3GPP Mobile Network Authentication and Key Agreement (EAP-AKA') The 3GPP mobile network Authentication and Key Agreement (AKA) is an authentication mechanism for devices wishing to access mobile networks. RFC 4187 (EAP-AKA) made the use of this mechanism possible within the Extensible Authentication Protocol (EAP) framework. RFC 5448 (EAP-AKA') was an improved version of EAP-AKA. This document is the most recent specification of EAP-AKA', including, for instance, details about and references related to operating EAP-AKA' in 5G networks. EAP-AKA' differs from EAP-AKA by providing a key derivation function that binds the keys derived within the method to the name of the access network. The key derivation function has been defined in the 3rd Generation Partnership Project (3GPP). EAP-AKA' allows its use in EAP in an interoperable manner. EAP-AKA' also updates the algorithm used in hash functions, as it employs SHA-256 / HMAC-SHA-256 instead of SHA-1 / HMAC-SHA-1, which is used in EAP-AKA. This version of the EAP-AKA' specification defines the protocol behavior for both 4G and 5G deployments, whereas the previous version defined protocol behavior for 4G deployments only. While EAP-AKA' as defined in RFC 5448 is not obsolete, this document defines the most recent and fully backwards-compatible specification of EAP-AKA'. This document updates both RFCs 4187 and 5448. Forward Secrecy Extension to the Improved Extensible Authentication Protocol Method for Authentication and Key Agreement (EAP-AKA' FS) This document updates RFC 9048, "Improved Extensible Authentication Protocol Method for 3GPP Mobile Network Authentication and Key Agreement (EAP-AKA')", and its predecessor RFC 5448 with an optional extension providing ephemeral key exchange. The extension EAP-AKA' Forward Secrecy (EAP-AKA' FS), when negotiated, provides forward secrecy for the session keys generated as a part of the authentication run in EAP-AKA'. This prevents an attacker who has gained access to the long-term key from obtaining session keys established in the past. In addition, EAP-AKA' FS mitigates passive attacks (e.g., large-scale pervasive monitoring) against future sessions. This forces attackers to use active attacks instead. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Extensible Authentication Protocol Method for 3rd Generation Authentication and Key Agreement (EAP-AKA) This document specifies an Extensible Authentication Protocol (EAP) mechanism for authentication and session key distribution that uses the Authentication and Key Agreement (AKA) mechanism. AKA is used in the 3rd generation mobile networks Universal Mobile Telecommunications System (UMTS) and CDMA2000. AKA is based on symmetric keys, and typically runs in a Subscriber Identity Module, which is a UMTS Subscriber Identity Module, USIM, or a (Removable) User Identity Module, (R)UIM, similar to a smart card. EAP-AKA includes optional identity privacy support, optional result indications, and an optional fast re-authentication procedure. This memo provides information for the Internet community. Informative References FIPS-203: Module-Lattice-based Key-Encapsulation Mechanism Standard Terminology for Post-Quantum Traditional Hybrid Schemes UK National Cyber Security Centre UK National Cyber Security Centre Naval Postgraduate School One aspect of the transition to post-quantum algorithms in cryptographic protocols is the development of hybrid schemes that incorporate both post-quantum and traditional asymmetric algorithms. This document defines terminology for such schemes. It is intended to be used as a reference and, hopefully, to ensure consistency and clarity across different protocols, standards, and organisations. Post-Quantum and Post-Quantum/Traditional Hybrid Algorithms for HPKE Your Organization Here Updating key exchange and public-key encryption protocols to resist attack by quantum computers is a high priority given the possibility of "harvest now, decrypt later" attacks. Hybrid Public Key Encryption (HPKE) is a widely-used public key encryption scheme based on combining a Key Encapsulation Mechanism (KEM), a Key Derivation Function (KDF), and an Authenticated Encryption with Associated Data (AEAD) scheme. In this document, we define KEM algorithms for HPKE based on both post-quantum KEMs and hybrid constructions of post- quantum KEMs with traditional KEMs, as well as a KDF based on SHA-3 that is suitable for use with these KEMs. When used with these algorithms, HPKE is resilient with respect to attack by a quantum computer. Hybrid PQ/T Key Encapsulation Mechanisms SandboxAQ This document defines generic techniques to achive hybrid post- quantum/traditional (PQ/T) key encapsulation mechanisms (KEMs) from post-quantum and traditional component algorithms that meet specified security properties. It then uses those generic techniques to construct several concrete instances of hybrid KEMs. Fundamental Elliptic Curve Cryptography Algorithms This note describes the fundamental algorithms of Elliptic Curve Cryptography (ECC) as they were defined in some seminal references from 1994 and earlier. These descriptions may be useful for implementing the fundamental algorithms without using any of the specialized methods that were developed in following years. Only elliptic curves defined over fields of characteristic greater than three are in scope; these curves are those used in Suite B. This document is not an Internet Standards Track specification; it is published for informational purposes. CFRG Elliptic Curve Diffie-Hellman (ECDH) and Signatures in JSON Object Signing and Encryption (JOSE) This document defines how to use the Diffie-Hellman algorithms "X25519" and "X448" as well as the signature algorithms "Ed25519" and "Ed448" from the IRTF CFRG elliptic curves work in JSON Object Signing and Encryption (JOSE). CBOR Object Signing and Encryption (COSE): Structures and Process Concise Binary Object Representation (CBOR) is a data format designed for small code size and small message size. There is a need to be able to define basic security services for this data format. This document defines the CBOR Object Signing and Encryption (COSE) protocol. This specification describes how to create and process signatures, message authentication codes, and encryption using CBOR for serialization. This specification additionally describes how to represent cryptographic keys using CBOR. This document, along with RFC 9053, obsoletes RFC 8152. Post-Quantum Cryptography for Engineers Nokia Nokia Nokia DigiCert Entrust Limited The advent of a cryptographically relevant quantum computer (CRQC) would render state-of-the-art, traditional public-key algorithms deployed today obsolete, as the mathematical assumptions underpinning their security would no longer hold. To address this, protocols and infrastructure must transition to post-quantum algorithms, which are designed to resist both traditional and quantum attacks. This document explains why engineers need to be aware of and understand post-quantum cryptography (PQC), detailing the impact of CRQCs on existing systems and the challenges involved in transitioning to post-quantum algorithms. Unlike previous cryptographic updates, this shift may require significant protocol redesign due to the unique properties of post-quantum algorithms. Randomness Requirements for Security Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space. Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.

Acknowledgements This draft leverages text from .