Network Working Group P. M. Hallam-Baker Internet-Draft Venture Cryptography. Intended status: Informational 5 January 2020 Expires: 8 July 2020 Threshold Key Generation and Decryption in Ed25519 and Ed448 draft-hallambaker-threshold-00 Abstract Threshold cryptography schemes are described with application to the Ed25519, Ed448, X25519 and X448 Elliptic Curves. Threshold key generation allows generation of keypairs to be divided between two or more parties with verifiable security guaranties. Threshold decryption allows elliptic curve key agreement to be divided between two or more parties such that all the parties must co-operate to complete a private key agreement operation. The same primitives may be applied to improve resistance to side channel attacks. https://mailarchive.ietf.org/arch/browse/cfrg/ (http://whatever)Discussion of this draft should take place on the CFRG mailing list (cfrg@irtf.org), which is archived at . 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 https://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 8 July 2020. Copyright Notice Copyright (c) 2020 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 (https://trustee.ietf.org/ Hallam-Baker Expires 8 July 2020 [Page 1]

Internet-Draft Threshold Key Generation and Decryption January 2020 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2.2. Defined Terms . . . . . . . . . . . . . . . . . . . . . . 3 2.3. Related Specifications . . . . . . . . . . . . . . . . . 5 2.4. Implementation Status . . . . . . . . . . . . . . . . . . 5 3. Principles . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Application to Diffie Hellman (not normative) . . . . . . 5 3.2. Threshold Decryption . . . . . . . . . . . . . . . . . . 7 3.2.1. Key Splitting . . . . . . . . . . . . . . . . . . . . 7 3.2.2. Decryption . . . . . . . . . . . . . . . . . . . . . 7 3.3. Threshold Key Generation . . . . . . . . . . . . . . . . 8 3.3.1. Device Provisioning . . . . . . . . . . . . . . . . . 8 3.3.2. Key Rollover . . . . . . . . . . . . . . . . . . . . 9 3.3.3. Host Activation . . . . . . . . . . . . . . . . . . . 10 3.3.4. Separation of Duties . . . . . . . . . . . . . . . . 10 3.4. Side Channel Resistance . . . . . . . . . . . . . . . . . 10 4. Application to Elliptic Curves . . . . . . . . . . . . . . . 11 4.1. Implementation for Ed25519 and Ed448 . . . . . . . . . . 11 4.1.1. Ed25519 . . . . . . . . . . . . . . . . . . . . . . . 12 4.1.2. Ed448 . . . . . . . . . . . . . . . . . . . . . . . . 12 4.2. Implementation for X25519 and X448 . . . . . . . . . . . 13 4.2.1. Point Encoding . . . . . . . . . . . . . . . . . . . 14 4.2.2. X25519 Point Encoding . . . . . . . . . . . . . . . . 14 4.2.3. X448 Point Encoding . . . . . . . . . . . . . . . . . 14 4.2.4. Point Addition . . . . . . . . . . . . . . . . . . . 14 4.2.5. Montgomery Ladder with Coordinate Recovery . . . . . 15 5. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 17 5.1. Threshold Key Generation . . . . . . . . . . . . . . . . 17 5.1.1. X25519 . . . . . . . . . . . . . . . . . . . . . . . 17 5.1.2. X448 . . . . . . . . . . . . . . . . . . . . . . . . 19 5.1.3. Ed25519 . . . . . . . . . . . . . . . . . . . . . . . 21 5.1.4. Ed448 . . . . . . . . . . . . . . . . . . . . . . . . 22 5.2. Threshold Decryption . . . . . . . . . . . . . . . . . . 24 5.2.1. Key Splitting X25519 . . . . . . . . . . . . . . . . 24 5.2.2. Decryption X25519 . . . . . . . . . . . . . . . . . . 25 5.2.3. Key Splitting X448 . . . . . . . . . . . . . . . . . 27 5.2.4. Decryption X448 . . . . . . . . . . . . . . . . . . . 29 6. Security Considerations . . . . . . . . . . . . . . . . . . . 31 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31 9. Normative References . . . . . . . . . . . . . . . . . . . . 31 Hallam-Baker Expires 8 July 2020 [Page 2]

Internet-Draft Threshold Key Generation and Decryption January 2020 10. Informative References . . . . . . . . . . . . . . . . . . . 32 1. Introduction Public key cryptography provides greater functionality than symmetric key cryptography by introducing separate keys for separate roles. Knowledge of the public encryption key does not provide the ability to decrypt. Knowledge of the public signature verification key does not provide the ability to sign. Threshold cryptography extends the scope of traditional public key cryptography with further separation of roles. A Threshold scheme is _interchangeable_ if the inputs and outputs of the threshold scheme are identical to those of a conventional scheme. A threshold scheme is _auditable_ if it to verify that the output made use of a particular contribution. This document describes a threshold decryption and an auditable threshold key generation scheme for the elliptic curve scheme described in [RFC7748] and [RFC8032]. Both schemes are interchangeable in their own right but require minor modifications to the underlying elliptic curve systems to encode the necessary information in the public (X25519/X448) or private key (Ed25519/ Ed448). Threshold key agreement and threshold signatures will be specified in future drafts. 2. Definitions This section presents the related specifications and standard, the terms that are used as terms of art within the documents and the terms used as requirements language. 2.1. Requirements Language 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]. 2.2. Defined Terms The following terms are used as terms of art in this document. Auditable Whether it is possible (or not) to verify that the output of a threshold scheme made use of a particular contribution. Hallam-Baker Expires 8 July 2020 [Page 3]

Internet-Draft Threshold Key Generation and Decryption January 2020 Interchangeable A Threshold scheme is _interchangeable_ if the inputs and outputs of the threshold scheme are identical to those of a conventional scheme Multi-Encryption The use of multiple decryption fields to allow a document encrypted under a session key to be decrypted by multiple parties under different decryption keys. Multi-Encryption allows a document to be shared with multiple recipients but does not allow the decryption role to be divided between multiple parties. Multi-Signatures The use of multiple independently verifiable digital signatures to authenticate a document. Multi-Signatures allow separation of the signing roles and thus achieve a threshold capability. But they are not true threshold signatures as the set of signers is visible to external parties. Onboarding The process by which an embedded device is provisioned for deployment in a local network. Threshold Key Generation An aggregate public, private key pair is constructed from a set of contributions such that the private key is a function of the private key of all the contributions. A Threshold Key Generation function is auditable if and only if the public component of the aggregate key can be computed from the public keys of the contributions alone. Threshold Decryption Threshold decryption divides the decryption role between two or more parties. Threshold Key Agreement A bilateral key agreement exchange in which one or both sides present multiple public keys and the key agreement value is a function of all of them. This approach allows a party to present multiple credentials in a single exchange, a de Threshold Signatures Threshold signatures divide the signature role between two or more parties in such a way that the parties and their roles is not visible to an external observer. Hallam-Baker Expires 8 July 2020 [Page 4]

Internet-Draft Threshold Key Generation and Decryption January 2020 2.3. Related Specifications This document extends the elliptic curve cryptography systems described in [RFC7748] and [RFC8032] to provide threshold capabilities. This work was originally motivated by the requirements of the Mathematical Mesh [draft-hallambaker-mesh-architecture] 2.4. Implementation Status The implementation status of the reference code base is described in the companion document [draft-hallambaker-mesh-developer]. 3. Principles The threshold cryptography mechanisms described in this specification are made possible by the fact that Diffie Hellman key agreement and elliptic curve variants thereof support properties we call the Key Combination Law and the Result Combination Law. Let {_X_, _x_}, {_Y_, _y_}, {_A_, _a_} be {public, private} key pairs and r [.] S represent the Diffie Hellman operation applying the private key r to the public key S. The Key Combination law states that we can define an operator [x] such that there is a keypair {_Z_, _z_} such that: _Z_ = _X_ [x] _Y_ and _z_ = (_x_ + _y_) mod _o_ (where _o_ is the order of the group) The Result Combination Law states that we can define an operator [+] such that: (_x_ [.] _A_) [+] (_y_ [.] _A_) = (_z_ [.] _A_) = (_a_ [.] _Z_) It will be noted that each of these laws is interchangeable. The output of the key combination law to a Diffie Hellman key pair is a Diffie Hellman key pair and the output of the result combination law is a Diffie Hellman result. This allows modular and recursive application of these principles. 3.1. Application to Diffie Hellman (not normative) Diffie Hellman in a modular field provides a concise demonstration of the key combination and result combination laws [RFC2631]. The realization of the threshold schemes in a modular field is outside the scope of this document. Hallam-Baker Expires 8 July 2020 [Page 5]

Internet-Draft Threshold Key Generation and Decryption January 2020 For the Diffie Hellman system in a modular field p, with exponent e: * r [.] S = S^(r) mod p * o = p-1 * _A_[x] _B_ = _A_[.] _B_ = _AB_mod _p_. _Proof:_ Let z = x + y By definition, X = e^(x) mod p, Y = e^(y) mod p, and Z = e^(z)mod p. Therefore, Z = e^(z) mod p = e^(x+y) mod p = (e^(x)e^(y)) mod p = e^(x)mod p.e^(y) mod p = X.Y Moreover, A = e^(a) mod p, Therefore, (A^(x) mod p).(A^(y) mod p) = (A^(x)A^(y)) mod p) = (A^(x+y)) mod p) = A^(z) mod p = e^(az) mod p = (e^(z))^(a) mod p = Z^(a) mod p Since e^(o) mod p = 1, the same result holds for z = (x + y) mod o since e^(x+y+no) = e^(x+y).e^(no) = e^(x+y).1 = e^(x+y). Hallam-Baker Expires 8 July 2020 [Page 6]

Internet-Draft Threshold Key Generation and Decryption January 2020 3.2. Threshold Decryption Threshold decryption allows a decryption key to be divided into two or more parts, allowing these roles to be assigned to different parties. This capability can be employed within a machine to divide use of a private key between an implementation running in the user mode and a process running in a privileged mode that is bound to the machine. Alternatively, threshold cryptography can be employed to The key combination law and result combination law provide a basis for threshold decryption. 3.2.1. Key Splitting We begin by creating a base key pair { X, x }. The public component X is used to perform encryption operations by means of a key agreement against an ephemeral key in the usual fashion. The private component x may be used for decryption in the normal fashion or to provide the source material for a key splitting operation. To split the base key into n shares { S_(1), s_(1) }, { S_(2), s_(2) }, ... { S_(n), s_(n) }, we begin by generating the first n-1 private keys in the normal fashion. It is not necessary to generate the corresponding public keys as these are not required. The private key of the final key share s_(n) is given by: _s_(n) = (x - s1 - s2 - ... - sn-1) mod o_ Thus, the base private key x is equal to the sum of the private key shares modulo the group order. 3.2.2. Decryption To encrypt a document, we first generate an ephemeral key pair { Y, y }. The key agreement value e^(xy) is calculated from the base public key X = e^(x) and the ephemeral private key y. A key derivation function is then used to obtain the session key to be used to encrypt the document under a symmetric cipher. To decrypt a document using the threshold key shares, each key share holder first performs a Diffie Hellman operation using their private key on the ephemeral public key. The key shares are then combined using the result combination law to obtain the key exchange value from which the session key is recovered. The key contribution c_(i) for the holder of the i^(th) key share is calculated as: Hallam-Baker Expires 8 July 2020 [Page 7]

Internet-Draft Threshold Key Generation and Decryption January 2020 c_(i) = Y^(si) The key agreement value is thus A = c_(1) . c_(2) . ... . c_(n) This value is equal to the encryption key agreement value according to the group law. 3.3. Threshold Key Generation The key combination law allows an aggregate private key to be derived from private key contributions provided by two or more parties such that the corresponding aggregate public key may be derived from the public keys corresponding to the contributions. The resulting key generation mechanism is thus, auditable and interoperable. 3.3.1. Device Provisioning Auditable Threshold Key Generation provides a simple and efficient means of securely provisioning keys to devices. This is encountered in the IoT space as a concern in 'onboarding' and in the provisioning of unique keys for use with cryptographic applications (e.g. SSH, S/ MIME, OpenPGP, etc.). Consider the case in which Alice purchases an IoT connected Device D which requires a unique device key pair _{ X , x }_ for its operation. The process of provisioning (aka 'onboarding') is performed using an administration device. Traditional key pair generation gives us three options: * Generate and install a key pair during manufacture. * Generate a new key pair during device provisioning. * Generate a key pair on the administration device and transfer it to the device being provisioned. The first approach has the obvious disadvantage that the manufacturer has knowledge of the private key. This represents a liability for both the user and the manufacturer. Less obvious is the fact that the second approach doesn't actually provide a solution unless the process of generating keys on the device is auditable. The third approach is auditable with respect to the device being provisioned but not with respect to the administration device being used for provisioning. If that device were to be compromised, so could every device it was used to provision. Hallam-Baker Expires 8 July 2020 [Page 8]

Internet-Draft Threshold Key Generation and Decryption January 2020 Threshold key generation allows the administration device and the joining device being provisioned to jointly provision a key pair as follows: * The joining device has public, private key pair_{ D, d }_. * A provisioning request is made for the device which includes the joining device public key _D_. * The administration device generates a key pair contribution _{ A, a }_. * The administration device private key is transmitted to the Device by means of a secure channel. * The joining device calculates the aggregate key pair _{ A [x] D, a+d }_. * The administration device authorizes the joining device to participate in the local network using the public key _A [x] D_. The Device key pair MAY be installed during manufacture or generated during provisioning or be derived from a combination of both using threshold key generation recursively. The provisioning request MAY be originated by the device or be generated by a purchasing system. Note that the provisioning protocol does not require either party to authenticate the aggregate key pair. The protocol provides security by ensuring that both parties obtain the correct results if and only if the values each communicated to the other were correct. Out of band authentication of the joining device public key _D_ is sufficient to prevent Man-in-the-Middle attack. This may be achieved by means of a QR code printed on the device itself that discloses or provides a means of obtaining _D._ [Note add serious warning that a party providing a private contribution MUST make sure they see the public key first. Otherwise a rogue key attack is possible. The Mesh protocols ensure that this is the case but other implementations may overlook this detail.] 3.3.2. Key Rollover Traditional key rollover protocols in PKIX and other PKIs following the Kohnfelder model, require a multi-step interaction between the key holder and the Certificate Authority. Hallam-Baker Expires 8 July 2020 [Page 9]

Internet-Draft Threshold Key Generation and Decryption January 2020 Threshold key generation allows a Certificate Authority to implement key rollover with a single communication: Consider the case in which the service host has a base key pair { B , b }. A CA that has knowledge of the Host public key B may generate a certificate for the time period _t_ with a fresh key as follows: * Generate a key pair contribution { A_(t), a_(t) }. * Generate and sign an end entity certificate C_(t) for the key B [x] A_(t). * Transmit C_(t), a_(t) to the host by means of a secure channel 3.3.3. Host Activation Modern Internet service architectures frequently make use of short lived, ephemeral hosts running on virtualized machines. Provisioning cryptographic material in such environments is a significant challenge and especially so when the underlying hardware is a shared resource. The key rollover approach described above provides a means of provisioning short lived credentials to ephemeral hosts that potentially avoids the need to build sensitive keys into the service image or configuration. 3.3.4. Separation of Duties Threshold key generation provides a means of separating administration of cryptographic keys between individuals. This allows two or more administrators to control access to a private key without having the ability to use it themselves. This approach is of particular utility when used in combination with threshold decryption. Alice and Bob can be granted the ability to create key contributions allowing a user to decrypt information without having the ability to decrypt themselves. 3.4. Side Channel Resistance Side-channel attacks, present a major concern in the implementation of public key cryptosystems. Of particular concern are the timing attacks identified by Paul Kocher [Kocher96] and related attacks in the power and emissions ranges. Performing repeated observations of the use of the same private key material provides an attacker with considerably greater opportunity to extract the private key material. Hallam-Baker Expires 8 July 2020 [Page 10]

Internet-Draft Threshold Key Generation and Decryption January 2020 A simple but effective means of defeating such attacks is to split the private key value into two or more random shares for every private key operation and use the result combination law to recover the result. The implementation of this approach is identical to that for Threshold Decryption except that instead of giving the key shares to different parties, they are kept by the party performing the private key operation. While this approach doubles the number of private key operations required, the operations MAY be performed in parallel. Thus avoiding impact on the user experience. 4. Application to Elliptic Curves For elliptic curve cryptosystems, the operators [x] and [.] are point addition. Implementing a robust Key Co-Generation for the Elliptic Curve Cryptography schemes described in [RFC7748] and [RFC8032] requires some additional considerations to be addressed. * The secret scalar used in the EdDSA algorithm is calculated from the private key using a digest function. It is therefore necessary to specify the Key Co-Generation mechanism by reference to operations on the secret scalar values rather than operations on the private keys. * The Montgomery Ladder traditionally used to perform X25519 and X448 point multiplication does not require implementation of a function to add two arbitrary points. While the steps required to create such a function are fully constrained by [RFC7748], the means of performing point addition is not. 4.1. Implementation for Ed25519 and Ed448 [RFC8032] provides all the cryptographic operations required to perform threshold operations and corresponding public keys. The secret scalars used in [RFC8032] private key operations are derived from a private key value using a cryptographic digest function. This encoding allows the inputs to a private key combination to be described but not the output. Contrawise, the encoding allows the inputs to a private key splitting operation to be described but not the output Hallam-Baker Expires 8 July 2020 [Page 11]

Internet-Draft Threshold Key Generation and Decryption January 2020 It is therefore necessary to provide an alternative representation for the Ed25519 and Ed448 private keys. Moreover, the signature algorithm requires both a secret scalar and a prefix value as inputs. Since threshold signatures are out of scope for this document and [RFC8032] does not specify a key agreement mechanism, it suffices to specify the data formats required to encode private key values generated by means of threshold key generation. 4.1.1. Ed25519 Let the inputs to the threshold key generation scheme be a set of 32 byte private key values _P_(1), P2 ... Pn. For each private key value i in turn:_ 0. Hash the 32-byte private key using SHA-512, storing the digest in a 64-octet large buffer, denoted_h_(i)_. Let n_(i) be the first 32 octets of h_(i) and m_(i) be the remaining 32 octets of h_(i). 1. Prune n_(i): The lowest three bits of the first octet are cleared, the highest bit of the last octet is cleared, and the second highest bit of the last octet is set. 2. Interpret the buffer as the little-endian integer, forming a secret scalar s_(i). The private key values are calculated as follows: The aggregate secret scalar value _s_(a) = s1 + s2 + ... sn mod L, where L is the order of the curve._ The aggregate prefix value is calculated by either * Some function TBS on the values m_(1), m_(2), ... m_(n), or * Taking the SHA256 digest of s_(a). The second approach is the simplest and the most robust. It does however mean that the prefix is a function of the secret scalar rather than both being functions of the same seed. 4.1.2. Ed448 Let the inputs to the threshold key generation scheme be a set of 57 byte private key values _P_(1), P2 ... Pn. For each private key value i in turn:_ Hallam-Baker Expires 8 July 2020 [Page 12]

Internet-Draft Threshold Key Generation and Decryption January 2020 0. Hash the 57-byte private key using SHAKE256(x, 114), storing the digest in a 114-octet large buffer, denoted_h_(i)_. Let n_(i) be the first 57 octets of h_(i) and m_(i) be the remaining 57 octets of h_(i). 1. Prune n_(i): The two least significant bits of the first octet are cleared, all eight bits the last octet are cleared, and the highest bit of the second to last octet is set. 2. Interpret the buffer as the little-endian integer, forming a secret scalar s_(i). The private key values are calculated as follows: The aggregate secret scalar value _s_(a) = s1 + s2 + ... sn mod L, where L is the order of the curve._ The aggregate prefix value is calculated by either * Some function TBS on the values m_(1), m_(2), ... m_(n), or * Taking the SHAKE256(x, 57) digest of s_(a). The second approach is the simplest and the most robust. It does however mean that the prefix is a function of the secret scalar rather than both being functions of the same seed. 4.2. Implementation for X25519 and X448 [RFC7748] defines all the cryptographic operations required to perform threshold key generation and threshold decryption but does not describe how to implement them. The Montgomery curve described in [RFC7748] allows for efficient scalar multiplication using arithmetic operations on a single coordinate. Point addition requires both coordinate values. It is thus necessary to provide an extended representation for point encoding and provide an algorithm for recovering both coordinates from a scalar multiplication operation and an algorithm for point addition. The notation of [RFC7748] is followed using {u, v} to represent the coordinates on the Montgomery curve and {x, y} for coordinates on the corresponding Edwards curve. Hallam-Baker Expires 8 July 2020 [Page 13]

Internet-Draft Threshold Key Generation and Decryption January 2020 4.2.1. Point Encoding The relationship between the u and v coordinates is specified by the Montgomery Curve formula itself: _v^(2)_ = _u^(3) + Au2 + u_ An algorithm for extracting a square root of a number in a finite field is specified in . [RFC8032] Since _v^(2)_ has a positive (_v_) and a negative solution (_-v_), it follows that _v^(2)_ mod p will have the solutions _v_, _p-v_. Furthermore, since _p_ is odd, if _v_ is odd, _p-v_ must be even and vice versa. It is thus sufficient to record whether _v_ is odd or even to enable recovery of the _v_ coordinate from _u_. 4.2.2. X25519 Point Encoding The extended point encoding allowing the v coordinate to be recovered is as given in [draft-ietf-lwig-curve-representations] [TBS] 4.2.3. X448 Point Encoding The extended point encoding allowing the v coordinate to be recovered is as given in [draft-ietf-lwig-curve-representations] [TBS] 4.2.4. Point Addition The point addition formula for the Montgomery curve is defined as follows: Let P_(1) = {u_(1), v_(1)}, P_(2) = {u_(2), v_(2)}, P_(3) = {u_(3), v_(3)} = P_(1) + P_(2) By definition: u_(3) = B(v_(2) - v_(1))^(2) / (u_(2) - u_(1))^(2) - A - u_(1) - u_(2) = B((u_(2)v_(1) - u_(1)v_(2))^(2) ) / u_(1)u_(2) (u_(2) - u_(1))^(2) v_(3) = ((2u_(1) + u_(2) + A)(v_(2) - v_(1)) / (u_(2) - u_(1))) - B (v_(2) - v_(1))^(3) / (u_(2) -u_(1))^(3) - v_(1) Hallam-Baker Expires 8 July 2020 [Page 14]

Internet-Draft Threshold Key Generation and Decryption January 2020 For curves X25519 and X448, B = 1 and so: u_(3) = ((v_(2) - v_(1)).(u_(2) - u_(1))^(-1))^(2) - A - u_(1) - u_(2) v_(3) = ((2u_(1) + u_(2) + A)(v_(2) - v_(1)).(u_(2) - u_(1))^(-1)) - ((v_(2) - v_(1)).(u_(2) -u_(1))^(-1))^(3) - v_(1) This may be implemented using the following code: B = v2 - v1 C = u2 - u1 CINV = C^(p - 2) D = B * CINV DD = D * D DDD = DD * D u3 = DD - A - u1 - u2 v3 = ((u1 + u1 + u2 + A) * B * CINV) - DDD - v1 Performing point addition thus requires that we have sufficient knowledge of the values v_(1), v_(2). At minimum whether one is odd and the other even or if both are the same. 4.2.5. Montgomery Ladder with Coordinate Recovery As originally described, the Montgomery Ladder only provides the u coordinate as output. L?pez and Dahab [Lopez99] provided a formula for recovery of the v coordinate of the result for curves over binary fields. This result was then extended by Okeya and Sakurai [Okeya01] to prime field Montgomery curves such as X25519 and X448. The realization of this result described by Costello and Smith [Costello17] is applied here. The scalar multiplication function specified in [RFC7748] takes as input the scalar value k and the coordinate u_(1) of the point P_(1) = {u_(1), v_(1)} to be multiplied. The return value in this case is u_(2) where P_(2) = {u_(2), v_(2)} = k.P_(1). To recover the coordinate v_(2) we require the values x_2, z_2, x_3, z_3 calculated in the penultimate step: Hallam-Baker Expires 8 July 2020 [Page 15]

Internet-Draft Threshold Key Generation and Decryption January 2020 x_1 = u x_2 = 1 z_2 = 0 x_3 = u z_3 = 1 swap = 0 For t = bits-1 down to 0: k_t = (k >> t) & 1 swap ^= k_t // Conditional swap as specified in RFC 7748 (x_2, x_3) = cswap(swap, x_2, x_3) (z_2, z_3) = cswap(swap, z_2, z_3) swap = k_t A = x_2 + z_2 AA = A^2 B = x_2 - z_2 BB = B^2 E = AA - BB C = x_3 + z_3 D = x_3 - z_3 DA = D * A CB = C * B x_3 = (DA + CB)^2 z_3 = x_1 * (DA - CB)^2 x_2 = AA * BB z_2 = E * (AA + a24 * E) (x_2, x_3) = cswap(swap, x_2, x_3) (z_2, z_3) = cswap(swap, z_2, z_3) Return x_2, z_2, x_3, z_3 The values x_2, z_2 give the projective form of the u coordinate of the point P_(2) = {u_(2), v_(2)} = k.P_(1) and the values x_3, z_3 give the projective form of the u coordinate of the point P_(3) = {u_(3), v_(3)} = (k+1).P_(1) = P_(1) + k.P_(1) = P_(1) + P_(2). Given the coordinates {u_(1), v_(1)} of the point P1 and the u coordinates of the points P_(2), P_(1) + P_(2), the coordinate v_(2) MAY be recovered by trial and error as follows: v_test = SQRT (u3 + Au2 + u) u_test = ADD_X (u, v, u_2, v_test) if (u_test == u_3) return u_test else return u_test +p Hallam-Baker Expires 8 July 2020 [Page 16]

Internet-Draft Threshold Key Generation and Decryption January 2020 Alternatively, the following MAY be used to recover {u_(2), v_(2)} without the need to extract the square root and using a single modular exponentiation operation to convert from the projective coordinates used in the calculation. As with the Montgomery ladder algorithm above, the expression has been modified to be consistent with the approach used in [RFC7748] but any correct formula may be used. x_p = u y_p = v B = x_p * z_2 //v1 C = x_2 + B //v2 D = X_2 - B //v3 DD = D^2 //v3 E = DD. X_3 //v3 F = 2 * A * z_2 //v1 G = C + F //v2 H = x_p * x_2 //v4 I = H + z_2 //v4 J = G * I //v2 K = F * z_2 //v1 L = J - K //v2 M = L * z_3 //v2 yy_2 = M - E //Y' N = 2 * y_p //v1 O = N * z_2 //v1 P = O * z_3 //v1 xx_2 = P * x_q //X' zz_2 = P * z_ q //Z' ZINV = (zz_2^(p - 2)) u2 = xx_2 * ZINV v2 = yy_2 * ZINV return u2, v2 5. Test Vectors 5.1. Threshold Key Generation 5.1.1. X25519 The key parameters of the first key contribution are: Hallam-Baker Expires 8 July 2020 [Page 17]

Internet-Draft Threshold Key Generation and Decryption January 2020 X25519Key1 (X25519) UDF: ZAAA-CTKG-X255-XXKE-YX Scalar: 56751742936444772792970879017152360515706108153669948 486190735258502824077920 Encoded Private 60 2A E2 12 AC 8E C8 86 A1 79 51 7E 79 90 5E C2 9B AD 10 01 B9 2D 51 33 65 DB F4 9E 23 59 78 7D U: 25222393324990721517739552691612440154338285166262054281502859 684220669343438 V: 15622452724514925334849257786951944861130311422605147559630230 860481236780294 Encoded Public CE 36 B9 F1 56 BD 92 5C F4 B6 F5 E1 E0 BA CA 6A 9B 7C 37 7D F8 DC 39 CC 12 2E A6 8F 64 5E C3 37 00 The key parameters of the second key contribution are: X25519Key2 (X25519) UDF: ZAAA-CTKG-X255-XXKE-Y2 Scalar: 30800688691513612134093999707357841640579640775881469 593062950189697563564400 Encoded Private 70 19 5B 38 A4 46 21 79 31 AC 48 83 60 C9 BD F8 E1 EE 04 53 67 F2 B5 D8 9E 42 53 66 6F 92 18 44 U: 35108630063567318397224393939085269372284744000330218923799041 589332061533992 V: 13827314478911339710714490558315610168380330915483870499348836 357802235649136 Encoded Public 28 37 F5 39 16 C6 10 C6 8A AC 75 E9 20 EF 67 6D C2 6C AF 2C E4 F6 4F C9 E9 30 6C BD C9 C7 9E 4D 00 The aggregate private key is: Scalar_A = (Scalar_1 + Scalar_2) mod L = 70836469997123835938663996799427126600035261699252680723027418877 4936630452 Encoded Aggrgate Private Key: B4 54 B7 EF 13 30 0D DF C6 CB FE 5D 6A A3 A8 C0 7C 9C 15 54 20 20 07 0C 04 1E 48 05 93 EB 90 01 The aggregate public key is: Hallam-Baker Expires 8 July 2020 [Page 18]

Internet-Draft Threshold Key Generation and Decryption January 2020 Point_A = Point_1 + Point_2 U: 416454936139914218771704724014904891682742086807613599097165979348 46285027335 V: 473400233126764321363639652645349333601103100790621508110841442520 99552212729 Encoded Public 07 98 75 38 67 9C 66 21 A3 0A D1 06 CF F5 81 04 94 C0 52 C9 9C FD AE 4E 13 3B 43 9D 9A 83 12 5C Note that in this case, the unsigned representation of the key is used as the aggregate key is intended for unsigned CurveX key agreement. If the result is intended for use as a key contribution, the signed representation is required. 5.1.2. X448 The key parameters of the first key contribution are: X448Key1 (X448) UDF: ZAAA-ETKG-X44X-KEYX Scalar: 68165415229434843487640754974827937311214322558126978 8055715553507401814865302008262214951100710804646043741434925 630887320553400661768 Encoded Private 08 77 91 25 66 19 C6 1A 03 C7 60 9A 8C C8 10 9D DE F5 20 E1 A7 7F 3E 83 56 57 FE A6 C9 97 79 FB DC 85 55 6F CE 17 79 70 CA 3E B5 D1 6A B0 50 6A 60 F6 BF 3A 88 E5 15 F0 U: 60697849609835675975297341597995979787516605306209816088918249 8293453953345846660594020472424536065173283947670623780408505 23120715561 V: 60813500494147049417364978264586877278456315581444885337361828 5076034450004202627339591608123302429557097118744860203117206 220854848663 Encoded Public 29 C7 E7 1A ED 85 B5 66 F4 CA 8F 4D 07 72 EC 4B 15 42 FA 95 4D A3 25 F6 D2 BF C0 5E 11 C4 27 D3 A1 43 D8 74 B6 4C C8 22 7D 64 56 58 A4 8C C6 5D DA F2 AA 75 DE DE 60 15 80 The key parameters of the second key contribution are: Hallam-Baker Expires 8 July 2020 [Page 19]

Internet-Draft Threshold Key Generation and Decryption January 2020 X448Key2 (X448) UDF: ZAAA-ETKG-X44X-KEY2 Scalar: 67824881411761849798195083121628378835623370171088982 6937962011129206719268741815680700006802689991287015918654801 310197484516725932432 Encoded Private 90 C1 CE 67 A2 88 20 95 B9 A8 8A E7 5A 12 73 C6 4C E3 B0 0E 3A A4 1A 72 03 39 FC 9B 47 D9 6A E0 A2 81 63 57 77 EB 97 E5 CE 05 2C CB EE D7 64 F6 51 C1 42 E7 FE D9 E2 EE U: 32395912186842981800922536415382601434069282464793284039370624 1565981551756628007189667971676177150776689230729229736979561 639842244556 V: 18056267944998342850921302138832444826477211000541459460301605 4105989977716699286334066341414636204710801397092415122728296 636211077711 Encoded Public CC 67 05 A8 AE D3 8C 6E 17 F8 7F 66 77 14 7F 32 D3 F6 12 1C E2 80 A9 BF A9 AA 41 FC 88 EF E3 F9 38 C7 1C AA 1A 14 54 EC F0 4D 6D 20 ED 4F 63 24 F2 A0 68 F5 1C 09 1A 72 80 The aggregate private key is: Scalar_A = (Scalar_1 + Scalar_2) mod L = 87935198894654874397041717160555226349504546089353009501069716070 586506403266723929544670861554164189887604126085304951388779109 045747 Encoded Aggrgate Private Key: F3 55 F6 DD 05 50 99 B7 68 84 84 A1 C5 89 8A 79 3A 5B F6 27 DE 24 31 97 F5 94 73 D9 14 71 E4 DB 7F 07 B9 C6 45 03 11 56 99 44 E1 9C 59 88 B5 60 B2 B7 02 22 87 BF F8 1E The aggregate public key is: Hallam-Baker Expires 8 July 2020 [Page 20]

Internet-Draft Threshold Key Generation and Decryption January 2020 Point_A = Point_1 + Point_2 U: 611634634479536677984900819195998637894371405676964036939736481366 88134799342739585406562158256601376457049422599663606975867088547 575 V: 547531628982729065710146631050685048629332114514125362339393102647 61134803271330580187933395652539791547319114595107754138802418952 4364 Encoded Public F7 2E 68 4B 64 DC 2E 24 61 B9 28 14 2E 1D D9 41 6A 29 4F A2 5F F1 AF 07 24 6C 9B 8A 9E C0 E5 58 Note that in this case, the unsigned representation of the key is used as the aggregate key is intended for unsigned CurveX key agreement. If the result is intended for use as a key contribution, the signed representation is required. 5.1.3. Ed25519 The key parameters of the first key contribution are: ED25519Key1 (ED25519) UDF: ZAAA-GTKG-ED25-5XXK-EYX Scalar: 39507802390720856312219571924476007168388547774368948 368537778683821975155688 Encoded Private 1C C7 DE DF 19 7B 39 5F 82 98 26 62 AA DE 6C 66 04 C3 E3 A2 C8 3D 18 58 06 2C 3E EC 7C D4 B4 F2 X: 42353721841561159243771574200946096579404715276724838688117248 3158919506245796568293273125470706294881250827210288815928170 449575540044968280671060652600 Y: 14453248808291445687399372639220007070442564445118267751942208 0837579501036794314829741330844154033441251810135221340268136 7161993702790547954006637246092 Encoded Public 6D F1 94 33 33 CC 66 4D 93 89 E2 FB 38 61 21 D5 C5 6B 29 0F 5C 12 A8 4D 99 06 31 2D 35 32 22 A5 The key parameters of the second key contribution are: Hallam-Baker Expires 8 July 2020 [Page 21]

Internet-Draft Threshold Key Generation and Decryption January 2020 ED25519Key2 (ED25519) UDF: ZAAA-GTKG-ED25-5XXK-EY2 Scalar: 39507802390720856312219571924476007168388547774368948 368537778683821975155688 Encoded Private 1C C7 DE DF 19 7B 39 5F 82 98 26 62 AA DE 6C 66 04 C3 E3 A2 C8 3D 18 58 06 2C 3E EC 7C D4 B4 F2 X: 42353721841561159243771574200946096579404715276724838688117248 3158919506245796568293273125470706294881250827210288815928170 449575540044968280671060652600 Y: 14453248808291445687399372639220007070442564445118267751942208 0837579501036794314829741330844154033441251810135221340268136 7161993702790547954006637246092 Encoded Public 6D F1 94 33 33 CC 66 4D 93 89 E2 FB 38 61 21 D5 C5 6B 29 0F 5C 12 A8 4D 99 06 31 2D 35 32 22 A5 The aggregate private key is: Scalar_A = (Scalar_1 + Scalar_2) mod L = 66455490081190904847072782185220719282059319549388206770560479847 89407801486 Encoded Aggrgate Private Key: 8E 20 46 06 EE 61 70 82 FA 37 43 E2 5A 68 E7 3C 73 4A 36 B7 AC A4 DF 68 A7 95 5C 8E 58 3F B1 0E The aggregate public key is: Point_A = Point_1 + Point_2 X: -78285292761951767745666894197721180606214882184104422609189932681 69896558088044165355551471001743951239695738047106458517545601916 4578961762059345997440 Y: 260549350612676062448188625658154114443427558625932490186172625180 16179787773477706605100876536271693949174654809503102876480720244 0555166017893772852160 Encoded Public 8E 89 98 D0 2D 7F 76 C3 A7 FF B3 1D 2B 41 7E E9 51 6B 51 B5 F2 84 8D 17 6F 59 9B 5B 6F 01 CF 73 5.1.4. Ed448 The key parameters of the first key contribution are: Hallam-Baker Expires 8 July 2020 [Page 22]

Internet-Draft Threshold Key Generation and Decryption January 2020 ED448Key1 (ED448) UDF: ZAAA-ITKG-ED44-XKEY-X Scalar: 53741526827163371875813321995189037002816220481405056 0575131176561199901538761967283817989566517114321868559709090 857214825841130713784 Encoded Private E5 0F 73 50 27 0A 2F 7D FD D0 96 E5 03 D3 35 2C 99 CB 71 7C 0B D9 49 E0 40 5E C7 FB D1 F5 05 18 18 6B 04 81 8B 4D 81 DC 33 CE DF 81 D5 EA 90 43 D9 E5 D0 A7 F1 EF 9C F3 X: 46070586985722000292864744471871508558334313374829024264732526 0256877451971853613261831326120959789917482766653217856979552 299464523257 Y: 60551781313893332009337150573641968782237423331690407655569563 6098829567124403075067563581124781122484845054468415282443786 254887063867 Encoded Public B5 B9 3F B5 B2 5B 82 E1 08 F5 6C 79 80 A1 68 5A 5C BB 2A FD 27 B2 9A F8 DF 91 CD CA 60 B8 75 3F 62 96 40 38 78 96 77 0C 21 40 E6 D4 0B 05 8F 24 D6 FD 65 61 A2 6C C1 86 80 The key parameters of the second key contribution are: ED448Key2 (ED448) UDF: ZAAA-ITKG-ED44-XKEY-2 Scalar: 53741526827163371875813321995189037002816220481405056 0575131176561199901538761967283817989566517114321868559709090 857214825841130713784 Encoded Private E5 0F 73 50 27 0A 2F 7D FD D0 96 E5 03 D3 35 2C 99 CB 71 7C 0B D9 49 E0 40 5E C7 FB D1 F5 05 18 18 6B 04 81 8B 4D 81 DC 33 CE DF 81 D5 EA 90 43 D9 E5 D0 A7 F1 EF 9C F3 X: 46070586985722000292864744471871508558334313374829024264732526 0256877451971853613261831326120959789917482766653217856979552 299464523257 Y: 60551781313893332009337150573641968782237423331690407655569563 6098829567124403075067563581124781122484845054468415282443786 254887063867 Encoded Public B5 B9 3F B5 B2 5B 82 E1 08 F5 6C 79 80 A1 68 5A 5C BB 2A FD 27 B2 9A F8 DF 91 CD CA 60 B8 75 3F 62 96 40 38 78 96 77 0C 21 40 E6 D4 0B 05 8F 24 D6 FD 65 61 A2 6C C1 86 80 The aggregate private key is: Hallam-Baker Expires 8 July 2020 [Page 23]

Internet-Draft Threshold Key Generation and Decryption January 2020 Scalar_A = (Scalar_1 + Scalar_2) mod L = 16628213117375882432961168004377507211427270876895354579839960414 666978326982600598665720267457234882718565087272340290578290296 3178673 Encoded Aggrgate Private Key: B1 0C A6 9F 18 5C CE 75 68 CF A3 94 FD 1C 20 DC 08 4A 1A 8D EA 8F ED 45 17 68 B6 9F 55 03 DA 18 5F A8 2E F3 98 92 24 C7 C2 05 8E 86 9E BD 4E A2 6F 38 45 74 67 F6 90 3A The aggregate public key is: Point_A = Point_1 + Point_2 X: 577530061094566245645474620282168197911998758313406086914794815409 17246422939462333536659252558650386613682198540830437043888246526 5810 Y: 505836808606768081321267696834164575314792405646755177389264860926 82650383803094471233632502605998926567195957732072433352975006488 1824 Encoded Public 99 67 9B 7C 5E 1C 7E 51 CD 39 A2 41 83 62 73 3E 60 93 17 A9 20 0E E3 BA 25 B3 B5 23 A3 A7 84 2E 9D 67 6F D7 0B 33 02 1D EB 76 83 F8 77 D8 48 F8 8B A3 72 E8 A9 6F 20 18 80 5.2. Threshold Decryption 5.2.1. Key Splitting X25519 The encryption key pair is Hallam-Baker Expires 8 July 2020 [Page 24]

Internet-Draft Threshold Key Generation and Decryption January 2020 X25519KeyA (X25519) UDF: ZAAA-CTHD-X255-XXKE-YA Scalar: 36212799908425711450656372795692477094724455418915704 216848228525958587810064 Encoded Private 10 01 D5 D1 E2 D3 DB 42 9E 40 5F D9 DB AE E8 09 DE 43 C3 E6 D1 4F 3A 31 92 BF 19 8A E9 B7 0F 50 U: 14523539712308371644546850739155588238080554014514563739095172 886049239361031 V: 56685060472089790044070522288405984326906734250304251487683593 932889808473139 Encoded Public 07 66 84 48 25 85 F6 4A 3A EE DF B7 69 1B 57 51 EC 18 BE AF 08 BA 0D FE BE F8 74 4E 3C 08 1C 20 To create n key shares we first create n-1 key pairs in the normal fashion. Since these key pairs are only used for decryption operations, it is not necessary to calculate the public components: X25519Key1 (X25519) UDF: ZAAA-CTHD-X255-XXKE-YX Scalar: 32951726132685026729149224926255648061071804906258082 061427666995947179849152 Encoded Private C0 B5 33 D4 F3 D0 16 4F 96 DF C3 AD 97 93 02 EF B4 25 E2 46 A3 69 1D 22 9B 5B A2 78 1C 04 DA 48 The secret scalar of the final key share is the secret scalar of the base key minus the sum of the secret scalars of the other shares modulo the group order: Scalar_2 = (Scalar_A - Scalar_1) mod L = 403147584512037825404691865456117698808221309075461782425833707 7336679400315 This is encoded as a binary integer in little endian format: 7B 43 64 61 E9 28 4D 79 AB 9C 6E CC 9F 79 14 3D 92 69 A5 2D 75 B9 57 53 2D 1B BC 02 06 BC E9 08 5.2.2. Decryption X25519 The means of encryption is unchanged. We begin by generating an ephemeral key pair: Hallam-Baker Expires 8 July 2020 [Page 25]

Internet-Draft Threshold Key Generation and Decryption January 2020 X25519KeyE (X25519) UDF: ZAAA-CTHD-X255-XXKE-YE Scalar: 41955577142906312619127105554814681129195921605852142 704362465226652441661496 Encoded Private 38 50 3C 88 22 4F 61 D7 9A 2E 1D 71 F0 31 74 44 A2 3B 2B 35 21 21 CA 19 4B 11 EB F0 DF 03 C2 5C U: 10080018124246254127076649374753145019412450363156572968151721 892767560820008 V: 43683938787921854603630290352714276342923724280578266457509078 671566344321831 Encoded Public 28 E5 5E 1D DD 1D 93 71 24 53 0A 83 B3 68 0D 28 8F 37 AC 53 B6 65 97 7E C1 54 44 41 8C 16 49 16 The key agreement result is given by multiplying the public key of the encryption pair by the secret scalar of the ephemeral key to obtain the u-coordinate of the result. U: 247351751388894803426442650867524924086144759194834658830326105266 22202018180 The u-coordinate is encoded in the usual fashion (i.e. without specifying the sign of v). 84 39 A5 21 13 F9 13 F0 7F F4 44 C0 DF 5D 44 DD DD F4 9B 87 4C DD E1 AB 64 00 8F A2 ED 9C AF 36 The first decryption contribution is generated from the secret scalar of the first key share and the public key of the ephemeral. The outputs from the Montgomery Ladder are: x_2 57800249527850149046770413207257250301842844049677844025524059085 132359257003 z_2 37229326806761131733056994095424883574786241198535734197174081138 402379671391 x_3 30722194817314627970562030033494699359853137448471883846088158083 361967945513 z_3 29143359268139878301695995826542801325089258636824690596939399658 126254126746 The coordinates of the corresponding point are: u 2625200443692459084967263034650122583671912028244890150161521677645 8728744244 v 2340339249609928967870268630489687123941624857494487121340604194885 7707717709 Hallam-Baker Expires 8 July 2020 [Page 26]

Internet-Draft Threshold Key Generation and Decryption January 2020 The encoding of this point specifies the u coordinate and the sign (oddness) of the v coordinate: 28 E5 5E 1D DD 1D 93 71 24 53 0A 83 B3 68 0D 28 8F 37 AC 53 B6 65 97 7E C1 54 44 41 8C 16 49 16 The second decryption contribution is generated from the secret scalar of the second key share and the public key of the ephemeral in the same way: u 2568180775076864300893967221119748767931055928591855851227298301978 9028635830 v 5237624535641756510077423429806596028526835148653096601777403098805 4910628425 28 E5 5E 1D DD 1D 93 71 24 53 0A 83 B3 68 0D 28 8F 37 AC 53 B6 65 97 7E C1 54 44 41 8C 16 49 16 To obtain the key agreement value, we add the two decryption contributions: u 5363809193902384353244842537457427150937755976201184630020143767288 0976727749 v 2576238777948215852102595446870010694371604288066653261024661407554 3602367190 This returns the same u coordinate value as before, allowing us to obtain the encoding of the key agreement value and decrypt the message. 5.2.3. Key Splitting X448 The encryption key pair is Hallam-Baker Expires 8 July 2020 [Page 27]

Internet-Draft Threshold Key Generation and Decryption January 2020 X448KeyA (X448) UDF: ZAAA-ETHD-X44X-KEYA Scalar: 70596789123829480733485730386174565339013185647363028 6777277621057939099785091228353248522408450794128800398810019 569879502484967206280 Encoded Private 88 2D AF 58 10 66 9E 1E F9 F2 C5 76 A2 00 86 F5 B0 B9 C6 B9 E6 34 12 57 64 E3 63 B7 99 48 01 77 9B A3 49 2D 7C B8 80 D7 63 44 6B C9 CB 83 F0 01 B6 55 E0 92 1C 2A A6 F8 U: 54256629638851994806054576189463839532492460394052748417730874 4299533502601001906894660938607827805200569088593927035891085 28218439174 V: 12640494198304757803713993624351573936804262085795518571061045 0631383333635306581037675004961525698205648857075020359124084 524068583614 Encoded Public 06 FE 38 7A 1B 1E 99 D4 89 00 07 B9 88 6F 97 01 BD 88 BB 9D A9 31 30 CC 47 E6 2F 9C 44 35 AF A4 To create n key shares we first create n-1 key pairs in the normal fashion. Since these key pairs are only used for decryption operations, it is not necessary to calculate the public components: X448Key1 (X448) UDF: ZAAA-ETHD-X44X-KEYX Scalar: 63066265672668423343291438147840057172035337936373473 3594300758463732521976388313294665447253881782852832499090049 354258188417511652528 Encoded Private B0 FC CE 55 87 AA A5 36 D2 5B E5 F2 5C 1B F7 9A 5A 3D 97 D8 BB C0 81 84 98 3B 7C 29 C3 02 FC AE 91 1B EA 67 68 C5 5E 87 7A ED 16 1F CB D0 20 9D C0 D6 62 BD 0F 35 20 DE The secret scalar of the final key share is the secret scalar of the base key minus the sum of the secret scalars of the other shares modulo the group order: Hallam-Baker Expires 8 July 2020 [Page 28]

Internet-Draft Threshold Key Generation and Decryption January 2020 Scalar_2 = (Scalar_A - Scalar_1) mod L = 646627804476669823064550215361382899916128546345584148789705309 5564959403070244163454368262048594523846084193642728800427461 1461310355 This is encoded as a binary integer in little endian format: 93 47 14 FF 94 BE F6 5C 77 63 44 89 DD CA 83 A6 1A 79 82 CA 9E F6 6B 7D 98 23 59 77 60 4B FD 25 2D BF 33 95 E4 7D DF 5E DE 31 82 E8 96 54 11 9F 76 2C 43 50 2C 5F C6 16 5.2.4. Decryption X448 The means of encryption is unchanged. We begin by generating an ephemeral key pair: X448KeyE (X448) UDF: ZAAA-ETHD-X44X-KEYE Scalar: 40831502887772840901106715270468009328116701340228919 4447950742749557088789408677311466089336893170031425082958041 776608657845012501716 Encoded Private D4 94 79 EE 56 3A 43 D5 FC EB 88 3E F0 63 EF 2F B0 92 B2 9D FD E1 43 8F 67 70 2A FC 2A AB A3 8B 40 5A C6 D8 DE 8E B8 81 BF AD 17 BA 14 7F A4 B0 D4 B1 9F CE D3 0D D0 8F U: 41902542857582644710501442087876846551351583947506685319975417 2921931242764163121977613761818608927787788470856012050834001 655292441835 V: 40254626888669687592165362896510117701831215997629302922912638 4107109948063151002535904960734463095918076287222011626898597 213849340021 Encoded Public EB 34 D3 9E 92 3E 82 CC E6 EC 77 9F 3D 11 83 3C B6 5B 5C 04 E8 1F D6 E1 07 C0 62 FE F8 F6 34 BB The key agreement result is given by multiplying the public key of the encryption pair by the secret scalar of the ephemeral key to obtain the u-coordinate of the result. U: 414519841929159382730636919036875317796178034011999213235983537052 30510678702415242441193107876747178183775523375063128358893667955 0233 The u-coordinate is encoded in the usual fashion (i.e. without specifying the sign of v). Hallam-Baker Expires 8 July 2020 [Page 29]

Internet-Draft Threshold Key Generation and Decryption January 2020 19 ED 3F 7A 63 6D AA 9A 3E 05 29 DE CC BA C7 F1 E0 A7 FA C0 C4 70 E0 E1 A5 FC DA 0A B0 52 EC 8A The first decryption contribution is generated from the secret scalar of the first key share and the public key of the ephemeral. The outputs from the Montgomery Ladder are: x_2 60087238657789265874539701675840521614326868580285788286550399232 20277081132468800878653228200859196207538481852097024468118288584 83595 z_2 12573140552649037921890899942919440571502561130496393232716617155 24490660805576517881339517005970098784771472127066810694797758409 67645 x_3 40216911160845555507626938485596306260547743077930604703891475599 53207238052152872831803734397389643529057507149471429452955111471 57394 z_3 48671808823760924633626118221453591105199403417491562559814414359 34079336265430685974504655698846599309734305554546571306740770389 79747 The coordinates of the corresponding point are: u 5310627084956226133549480379012439149584638171147187426442235629260 04186474991811830611467926523417739685244466041108245014409383321 437 v 6480384951984610654865132490606294355962228129695701220316681167173 89634554460437237795204236689985354028508124817314496063921770833 81 The encoding of this point specifies the u coordinate and the sign (oddness) of the v coordinate: EB 34 D3 9E 92 3E 82 CC E6 EC 77 9F 3D 11 83 3C B6 5B 5C 04 E8 1F D6 E1 07 C0 62 FE F8 F6 34 BB The second decryption contribution is generated from the secret scalar of the second key share and the public key of the ephemeral in the same way: u 2432029307606011854651950642127064534858415441240032460817128643691 02601495764607628214871657677482439232238254450281582409532827123 057 v 3304237495909049135999182737314299324454426462198935317955136317282 40167193898154033571033048069157608137872491595181800632292085611 537 Hallam-Baker Expires 8 July 2020 [Page 30]

Internet-Draft Threshold Key Generation and Decryption January 2020 EB 34 D3 9E 92 3E 82 CC E6 EC 77 9F 3D 11 83 3C B6 5B 5C 04 E8 1F D6 E1 07 C0 62 FE F8 F6 34 BB To obtain the key agreement value, we add the two decryption contributions: u 2259776336573351712090323684006287979378142231675451855263136351631 84627019682834233607456823018602790573389381890060345691088913511 436 v 3356804465434915738990773252496451168197759413718456519751139752920 22788339143143140400339843420207702195408750068698590142923542805 226 This returns the same u coordinate value as before, allowing us to obtain the encoding of the key agreement value and decrypt the message. 6. Security Considerations This section TBS One concern to be particularly careful of is that when adding two public keys, a rogue key attack is possible. The security of the private keys is the maximum of the two inputs but the trustworthiness of the public keys is the minimum. 7. IANA Considerations This document requires no IANA actions (yet). It will be necessary to define additional code points for the signed version of the X25519 and X448 public key and the threshold decryption final private keys. 8. Acknowledgements Rene Struik, Tony Arcieri, Scott Fluhrer, Scott Fluhrer, Dan Brown, Mike Hamburg 9. Normative References [draft-ietf-lwig-curve-representations] Struik, R., "Alternative Elliptic Curve Representations", Work in Progress, Internet-Draft, draft-ietf-lwig-curve- representations-08, 24 July 2019, <https://tools.ietf.org/html/draft-ietf-lwig-curve- representations-08>. Hallam-Baker Expires 8 July 2020 [Page 31]

Internet-Draft Threshold Key Generation and Decryption January 2020 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>. [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016, <https://www.rfc-editor.org/rfc/rfc7748>. [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017, <https://www.rfc-editor.org/rfc/rfc8032>. 10. Informative References [Costello17] Costello, C. and B. Smith, "Montgomery curves and their arithmetic", 2017. [draft-hallambaker-mesh-architecture] Hallam-Baker, P., "Mathematical Mesh 3.0 Part I: Architecture Guide", Work in Progress, Internet-Draft, draft-hallambaker-mesh-architecture-11, 23 October 2019, <https://tools.ietf.org/html/draft-hallambaker-mesh- architecture-11>. [draft-hallambaker-mesh-developer] Hallam-Baker, P., "Mathematical Mesh: Reference Implementation", Work in Progress, Internet-Draft, draft- hallambaker-mesh-developer-09, 23 October 2019, <https://tools.ietf.org/html/draft-hallambaker-mesh- developer-09>. [Kocher96] Kocher, P. C., "Timing attacks on implementations of Diffie-Hellman, RSA, DSS, and other systems.", 1996. [Lopez99] L?opez, J. and R. Dahab, "Fast multiplication on elliptic curves over GF(2m) without precomputation.", 1999. [Okeya01] Okeya, K. and K. Sakurai, "Efficient elliptic curve cryptosystems from a scalar multiplication algorithm with recovery of the y-coordinate on a Montgomeryform elliptic curve.", 2001. [RFC2631] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631, DOI 10.17487/RFC2631, June 1999, <https://www.rfc-editor.org/rfc/rfc2631>. Hallam-Baker Expires 8 July 2020 [Page 32]