Network Working Group P. M. Hallam-Baker
Internet-Draft Venture Cryptography.
Intended status: Informational 5 January 2020
Expires: 8 July 2020
Threshold Signatures Using Ed25519 and Ed448
draft-hallambaker-threshold-sigs-00
Abstract
A Threshold signature scheme is described. The signatures created
are computationally indistinguishable from those produced using the
Ed25519 and Ed448 curves as specified in RFC8032 except in that they
are non-deterministic. Threshold signatures are a form of digital
signature whose creation requires two or more parties to interact but
does not disclose the number or identities of the parties involved.
Status of This Memo
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Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2.2. Defined Terms . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Related Specifications . . . . . . . . . . . . . . . . . 4
2.4. Implementation Status . . . . . . . . . . . . . . . . . . 4
3. Threshold Signature Construction . . . . . . . . . . . . . . 4
3.1. Threshold signature . . . . . . . . . . . . . . . . . . . 6
3.2. Ed2519 Signature . . . . . . . . . . . . . . . . . . . . 8
3.3. Ed448 Signature . . . . . . . . . . . . . . . . . . . . . 9
3.4. Security Analysis . . . . . . . . . . . . . . . . . . . . 10
3.4.1. Replay Attack . . . . . . . . . . . . . . . . . . . . 10
3.4.2. Malicious Contribution Attack . . . . . . . . . . . . 11
3.4.3. Rogue Key Attack . . . . . . . . . . . . . . . . . . 11
4. Unanimous Signature . . . . . . . . . . . . . . . . . . . . . 12
4.1. Using threshold key generation . . . . . . . . . . . . . 12
4.2. Using key splitting . . . . . . . . . . . . . . . . . . . 13
5. Quorate Signature . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Key Generation . . . . . . . . . . . . . . . . . . . . . 13
5.2. Calculating the secret scalar value . . . . . . . . . . . 14
6. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Unanimous Threshold Signature Ed25519 . . . . . . . . . . 15
6.2. Unanimous Threshold Signature Ed448 . . . . . . . . . . . 17
6.3. Quorate Threshold Signature Ed25519 . . . . . . . . . . . 20
6.4. Quorate Threshold Signature Ed448 . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
10. Normative References . . . . . . . . . . . . . . . . . . . . 26
11. Informative References . . . . . . . . . . . . . . . . . . . 26
1. Introduction
Threshold encryption and key generation provide compelling advantages
over single private key approaches because splitting the private key
permits the use of that key to be divided between two or more roles.
All existing digital signatures allow the signer role to be divided
between multiple parties by attaching multiple signatures to the
signed document. This approach, known as multi-signatures is
distinguished from a threshold signature scheme in that the identity
and roles of the individual signers is exposed. In a threshold
signature scheme, the creation of a single signature requires the
participation of multiple signers and the signature itself does not
reveal the means by which it was constructed.
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Rather than considering multi-signatures or threshold signatures to
be inherently superior, it is more useful to regard both as two
points on a continuum of choices:
Multi-signatures Multiple digital signatures on the same document.
Multi-signatures are simple to create and provide the verifier
with more information but require the acceptance criteria to be
specified independently of the signature itself. This requires
that the application logic or PKI provide some means of describing
the criteria to be applied.
Multi-party key release A single signature created using a single
private key stored in an encrypted form whose use requires
participation of multiple key decryption shares.
Threshold signatures A single signature created using multiple
signature key shares. Signature creation may be subject to
complex criteria such as requiring an (n,t) quorum of signers but
these criteria are fixed at the time the signature is created
Aggregate Signatures A single signature created using multiple
signature key shares such that validation of the aggregate
signature serves to validate the participation of each of the
individual signers.
This document describes a scheme that creates threshold signatures
that are computationally indistinguishable from those produced
according to the algorithm specified in [RFC8032]. The scheme does
not support the creation of aggregate signatures.
Two versions of the algorithm are presented. Both versions allow a
creation of a signature to be divided between a set of n signers such
that a minimum of t signers are required to create a signature. The
first version of the algorithm (unanimous signature) requires that
every member of the set of signers participate in the signing process
(i.e. n=t). The second version (quorate signature) allows a
signature to be created by a subset of the authorized signers.
The unanimous signature scheme allows key shares to be generated by
either dividing a master key or using threshold key generation to
construct the master key from two or more key contributions as
described in [draft-hallambaker-threshold].
The quorate signature scheme requires that the signature key shares
be derived from a master signature key using Shamir secret sharing.
The process of key share generation is thus fundamentally one of
dividing a master key.
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If required, division of the signing key administration roles may be
achieved by using threshold key generation to construct the master
key and performing the key splitting process separately for each
contribution.
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
See [draft-hallambaker-threshold].
2.3. Related Specifications
This document extends the approach described in
[draft-hallambaker-threshold] to support threshold signatures. The
use of Lagrange polynomials to resolve Shamir Secret Shares is
currently described in the Uniform Data Fingerprint specification
[draft-hallambaker-mesh-udf]. It is expected that these three
documents will eventually form the basis for two CFRG proposals.
2.4. Implementation Status
The implementation status of the reference code base is described in
the companion document [draft-hallambaker-mesh-developer].
3. Threshold Signature Construction
The threshold signatures created according to the algorithms
described in this document are compatible with but not identical to
the signatures created according to the scheme described in
[RFC8032]. In particular:
* The signature verification algorithm is unchanged.
* The unanimous threshold scheme produces values of R and S that are
deterministic but different from the values that would be obtained
by using the aggregate private key to sign the same document.
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* The deterministic quorate threshold scheme produces values of R
and S that are deterministic for a given set of signers but will
change for a different set of signers or if the aggregate private
key was used to sign the same document.
* ?The non-deterministic quorate threshold scheme produces values of
R and S that will be different each time the document is signed.
Recall that a digital signature as specified by [RFC8032] consists of
a pair of values S, R calculated as follows:
R = r.B
S = r + k.s mod L
Where B is the base point of the elliptic curve.
r is an unique, unpredictable integer value such that 0 r L
k is the result of applying a message digest function determined
by the curve (Ed25519, Ed448) to a set of parameters known to the
verifier which include the values R, A and PH(M).
A is the public key of the signer, A = s.B
PH(M) is the prehash function of the message value.
s is the secret scalar value
L is the order of the elliptic curve group.
To verify the signature, the verifier calculates:
S.B = R + k.A
Therefore:
S.B = (r + k.s).B = r.B +k.(s.B) = R + k.A
The value r plays a critical role in the signature scheme as it
serves to prevent disclosure of the secret scalar. If the value r is
known, s can be calculated as s = (S-r).k^(-1) mod L. It is
therefore essential that the value r be unguessable.
Furthermore, if the same value of r is used to sign two different
documents, this results two signatures with the same value R and
different values of k and S. Thus
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S_(1) = r + k_(1).s mod L
S_(2) = r + k2.s mod L
s = (S_(1) - S_(2))(k_(1) - k_(2))^(-1) mod L
The method of constructing r specified in [RFC8032] ensures that the
value r is unique and unguessable provided that the private key from
which the secret scalar value is derived is kept secret.
The use of a deterministic means of generating the value r permits
auditing of hardware and software implementations to determine if the
correct means of constructing r is used by importing a known private
key and verifying that the expected values of R are produced when a
set of documents is signed.
The threshold schemes described in this document have been designed
to preserve the ability to audit the means of constructing the values
of r for each individual key share even though the sum of these
values and hence the value R will vary according to the number of
signature shares used.
3.1. Threshold signature
A threshold signature R, S is constructed by summing a set of
signature contributions from two or more signers. Each signer _i_
provides a contribution as follows:
A_(i) = s_(i).B
R_(i) = r_(i).B
S_(i) = r_(i) + k.s_(i) mod L
Where s_(i) and r_(i) are the secret scalar and unguessable value for
the individual signer.
The contributions of signers {1, 2, ... n} are then combined as
follows:
R = R_(1) + R_(2) + ... + R_(n)
S = S_(1) + S_(2) + ... + S_(n)
A = s.B
Where s = (s_(1) + s_(2) + ... + s_(n)) mod L
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The threshold signature is verified in the same manner as before:
S.B = R + k.A
Substituting for S.B we get:
= (S_(1) + S_(2) + ... + S_(n)).B
= S_(1).B + S_(2).B + ... + S_(n).B
= (r_(1) + k.s_(1)).B + (r_(2) + k.s_(2)).B + ... + (r_(n) +
k.s_(n)).B
= (r_(1).B + k.s_(1).B) + (r_(2).B + k.s_(2).B) + ... + (r_(n).B +
k.s_(n).B)
= (R1 + k.A1) + (R1 + k.A1) + ... + (Rn + k.An)
Substituting for R + k.A we get:
= R_(1) + R_(2) + ... + R_(n) + k.(A_(1) + A_(2) + ... + A_(n))
= R_(1) + R_(2) + ... + R_(n) + k.A_(1) + k.A_(2) + ... + k.A_(n)
= (R_(1) + k.A_(1)) + (R_(1) + k.A_(1)) + ... + (R_(n) + k.A_(n))
As expected, the operation of threshold signature makes use of the
same approach as threshold key generation and threshold decryption as
described in [draft-hallambaker-threshold]. As with threshold
decryption it is not necessary for each key share holder to have a
public key corresponding to their key share. All that is required is
that the sum of the secret scalar values used in calculation of the
signature modulo the group order be the value of the aggregate secret
scalar corresponding to the aggregate secret key.
While verification of [RFC8032] signatures is unchanged, the use of
threshold signatures requires a different approach to signing. In
particular, the fact that the value k is bound to the value R means
that the participants in the threshold signature scheme must agree on
the value R before the value k can be calculated. Since k is
required to calculate the signature contributions S_(i) can be
calculated, it is thus necessary to calculate the values R_(i) and
S_(i) in separate phases. The process of using a threshold signature
to sign a document thus has the following stages orchestrated by a
coordinator as follows:
0. The coordinator determines the values F, C and PH(M) as specified
in [RFC8032] and transmits them to the signers {1, 2, ... n}.
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1. Each signer generates a random value r_(i) such that 1 r_(i) L,
calculates the value R_(i) = r_(i).B and returns R to the
coordinator.
2. The coordinator calculates the value R = R_(1) + R_(2) + ... +
R_(n) and transmits R and A to the signers {1, 2, ... n}.
3. Each signer uses the suppled data to determine the value k and
hence S_(i) = r_(i) + k.s_(i) mod L and transmits it to the
coordinator.
4. The coordinator calculates the value S = S_(1) + S_(2) + ... +
S_(n) and verifies that the resulting signature R, S verifies
according to the mechanism specified in [RFC8032]. If the
signature is correct, the coordinator publishes it. Otherwise,
the coordinator MAY identify the signer(s) that provided
incorrect contributions by verifying the values R_(i) and S_(i)
for each.
For clarity, the coordinator role is presented here as being
implemented by a single party.
3.2. Ed2519 Signature
The process for creating an Ed25519 signature contribution is as
follows
0. Determine the value of the secret scalar value s_(i) according
to the means used to construct the secret shares.
1. Generate a random integer r_(i) such that 1 r_(i) L
2. Compute the point R_(i) = r_(i)B. For efficiency, do this by
first reducing r_(i) modulo L, the group order of B. Let the
string R_(i) be the encoding of this point.
3. Transmit the value R_(i) to the coordinator
4. Note that the construction of prefix is chosen so as to
guarantee that the use of a different message value M or secret
scalar value s_(i) will result in a different value of prefix
and thus ensure a different choice of the value r_(i).
5. At some later point, the coordinator MAY complete the signature
by returning the values F, C, A and R as specified in [RFC8032].
The signer MAY then complete the signature contribution as
follows:
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6. Compute SHA512(dom2(F, C) || R || A || PH(M)), and interpret the
64-octet digest as a little-endian integer k.
7. Compute S_(i) = (r_(i) + k * s_(i)) mod L. For efficiency,
again reduce k modulo L first.
8. Return the values R_(i), S_(i) to the coordinator.
9. The coordinator assembles the signature values R = S = S_(1) +
S_(2) + ... + S_(n)
10. The coordinator verifies that the signature R, S is valid.
3.3. Ed448 Signature
The process for creating an Ed448 signature contribution is as
follows
0. Determine the value of the secret scalar value s_(i) according to
the means used to construct the secret shares.
1. Generate a random integer r_(i) such that 1 r_(i) L
2. Compute the point R_(i) = r_(i)B. For efficiency, do this by
first reducing r_(i) modulo L, the group order of B. Let the
string R_(i) be the encoding of this point.
Transmit the value R_(i) to the coordinator
At some later point, the coordinator MAY complete the signature by
returning the values F, C, A and R as specified in [RFC8032]. The
signer MAY then complete the signature contribution as follows:
0. Compute SHAKE256(dom4(F, C) || R || A || PH(M), 114), and
interpret the 114-octet digest as a little-endian integer k.
1. Compute S_(i) = (r_(i) + k * s_(i)) mod L. For efficiency, again
reduce k modulo L first.
2. Return the values R_(i), S_(i) to the coordinator.
3. The coordinator assembles the signature values R = S = S_(1) +
S_(2) + ... + S_(n)
4. The coordinator verifies that the signature R, S is valid.
Note that the process of generating the signature contribution is
deterministic for a given secret scalar value but the signature
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computed MAY be deterministic or non-deterministic according to the
desired signature properties.
3.4. Security Analysis
We consider a successful breach of the threshold signature scheme to
be any attack that allows the attacker to create a valid signature
for any message without the participation of the required threshold
of signers.
Potential breaches include:
* Disclosure of the signature key or signature key share.
* Modification of signature data relating to message M to allow
creation of a signature for message M'.
* Ability of one of the signers to choose the value of the aggregate
public key.
* Access control attacks inducing a signer to create a signature
contribution that was not properly authenticated or authorized.
We regard attacks on the access control channel to be out of scope
for the threshold signature algorithm, though they are certainly a
concern for any system in which a threshold signature algorithm is
employed.
We do not consider the ability of a signer to cause creation of an
invalid signature to represent a breach.
3.4.1. Replay Attack
The most serious concern in the implementation of any Schnorr type
signature scheme is the need to ensure that the value r_(i) is never
revealed to any other party and is never used to create signatures
for two different values of k.s_(i).
Ensuring this does not occur imposes significant design constraints
as creating a correct signature contribution requires that the signer
use the same value of r_(i) to construct its value or R_(i) and
S_(i).
For example, a HSM device may be required to perform multiple
signature operations simultaneously. Since the storage capabilities
of an HSM device are typically constrained, it is tempting to attempt
to avoid the need to track the value of r_(i) within the device
itself using an appropriately authenticated and encrypted opaque
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state token. Such mechanisms provide the HSM with the value of r_(i)
but do not and cannot provide protection against a replay attack in
which the same state token is presented with a request to sign
different values of k.
3.4.2. Malicious Contribution Attack
In a malicious contribution attack, one or more parties present a
signature contribution that does not meet the criteria R_(i) =
r_(i).B and S_(i) = r_(i) + ks_(i).
Such an attack is not considered to be a breach as it merely causes
the signature process to fail.
3.4.3. Rogue Key Attack
A threshold signature scheme that allows the participants to 'bring
their own key' may be vulnerable to a rogue key attack in which a
signer is able to select the value of the aggregate public signature
key by selecting a malicious public signature key value.
The scheme described in this document is a threshold signature scheme
and does not support this feature. Consequently, this attack is not
relevant. It is described here for illustrative purposes only.
This particular attack only applies when the individual signers
create their own signature shares. It is not a concern when the
signature shares are created by splitting a master signature private
key.
Consider the case where the aggregate public key signature is
calculated from the sum of public signature key share values
presented by the signers:
A = A_(1) + A_(2) + ... + A_(n)
If the public key values are presented in turn, the last signer
presenting their key share can force the selection of any value of A
that they choose by selecting A_(n) = A_(m) - (A_(1) + A_(2) + ... +
A_(n-1))
The attacker can thus gain control of the aggregate signature key by
choosing A_(m) = s_(m).B where s_(m) is a secret scalar known only to
the attacker. But does so at the cost of not knowing the value s_(n)
and so the signer cannot participate in the signature protocol.
This attack allows the attacker and the attacker alone to create
signatures which are validated under the aggregate signature key.
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The attack is a consequence of the mistaken assumption that a
signature created under the signature key A_(1) + A_(2) + ... + A_(n)
provides evidence of the individual participation of the
corresponding key holders without separate validation of the
aggregate key.
Enabling the use of threshold signature techniques by ad-hoc groups
of signers using their existing signature keys as signature key
shares presents serious technical challenges that are outside the
scope of this specification.
4. Unanimous Signature
A unanimous threshold signature is a signature created by a set of
two or more signers that requires the participation of every member
of the set. The key shares used by the signers MAY be generated by
either:
* Each key share holder generating their own unique key share and
combining them.
* Generating a master signature key and dividing it.
Both approaches are described in [draft-hallambaker-threshold].
4.1. Using threshold key generation
To create a threshold signature key share using threshold key
generation, each signer generates a pair {A_(i), s_(i)} and transmits
the public component to the coordinator.
The coordinator calculates the aggregate public key A = A_(1) + A_(2)
+ ... + A_(n)
= s_(1).B + s_(2).B + ... + s_(n).B
= ((s_(1) + s_(2) + ... + s_(n)) mod L).B
= s.B
Where s = s_(1) + s_(2) + ... + s_(n)) mod L
The coordinator MUST require and verify proof of possession of the
private key component s_(i) to defeat rogue key attacks of the type
described in Section 3.4.3.
For example, the coordinator MAY require each signer to create a test
signature for their individual signature key share or to participate
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in creating a test signature under the aggregate signature key. Such
a test signature MAY be a Certificate Signing Request as specified in
[RFC2314].
Relying parties MUST NOT accept signatures purporting to be aggregate
signatures under ad-hoc collections of public keys as proof of the
involvement of the signers.
4.2. Using key splitting
To divide a keypair {A, p} into n parts, we begin by generating n-1
key pairs according to the method described in [RFC8032].
To calculate the final secret scalar share s_(n) we begin by
calculating the secret scalar value s corresponding to the master
private key p and the key shares s_(1), s_(2), ..., s_(n-1). The
final secret scalar is given by:
s_(n) = s - s_(1), s_(2), ..., s_(n-1)
Since the final secret scalar is not derived from a private key by
means of a digest, it is not possible to use [RFC8032] format to
store keys. The format described in [draft-hallambaker-threshold] is
used instead.
5. Quorate Signature
Constructing the set of secret scalar key shares using the Shamir
secret sharing scheme allows a signature scheme in which the
threshold of signers t required to create a signature is smaller than
the number of key shares n.
5.1. Key Generation
The key generation process begins with the generation of a master
signature key {A, p}. The private key p is used to calculate the
secret scalar value s according to the procedure specified in
Section 4.1 above.
The secret scalar is then divided as follows.
First, we construct a polynomial of degree t in the modular field L,
where L is the order of the curve sub-group:
f(x) = a_(0) + a_(1).x + a_(2).x^(2) + ... a_(t).x^(t) mod L
where a_(0) = s
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a_(1) ... a_(t) are randomly chosen integers in the range 1 a_(i)
L
The values of the key shares are the values _f_(x_(1)), _f_(x_(2)),
... ,_f_(x_(n)). That is
p_(i) = _f_(x_(i))
where p_(1) ... p_(n) are the private key shares
x_(1) ... x_(n) are distinct integers in the range 1 x_(i) L
The means of constructing distinct values x_(1) ... x_(n) is left to
the implementation. It is not necessary for these values to be
secret.
As with the final secret scalar value above, it is not possible to
use [RFC8032] format to store keys. The format described in
[draft-hallambaker-threshold] is used instead.
5.2. Calculating the secret scalar value
The value of the secret scalar value used to create a signature
depends on the specific choice of signature shares used to construct
it. Since the numbering of the key shares is arbitrary, we choose
the set of signers {1, 2, ... , t}.
Lagrange polynomials are used to compute a set of coeficients l_(1),
l_(2), ... l_(t) such that
s = a_(0) = l_(1)p_(1) + l_(2)p_(2) + ... + l_(t)p_(t)
The value of the secret scalars s_(1) + s_(2) + ... + s_(n) are thus
given by
s_(i) = l_(i)p_(i)
The coordinator identifies the signers to participate in the
signature event and uses the set of x coordinates corresponding to
their key shares to calculate the corresponding Lagrange coefficient
li for each as described in [draft-hallambaker-mesh-udf].
[Note: The discussion of Shamir Secret Sharing should probably be
moved from its current location in [draft-hallambaker-mesh-udf] to
either this document or the base document
[draft-hallambaker-threshold]. Which document is better suited will
depend on whether it is decided to implement (n,k) threshold
signatures and/or key escrow of elliptic curve scalar secrets.]
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6. Test Vectors
6.1. Unanimous Threshold Signature Ed25519
The signers are Alice and Bob. Each creates a key pair:
ED25519Alice's Key (ED25519)
UDF: ZAAA-GTSI-GXED-255X-XALI-CEXS-XKEY
Scalar: 56271244081186130980636545017945156580516101894352492
459594967614223399428880
Encoded Private
33 40 0E 22 D8 67 17 F4 8A 9F 6A 46 61 B4 0E AD
8C D0 DD C3 79 CD 85 BD 95 5C 90 B9 6C CB 8C 23
X: 11116793672970427161790264469280294507189044728140547954071022
7976454124042406369344932655633664630560242213431409139866940
284702002648469365756492647970
Y: 61655404171611396573021808119108664749574235125343680206454285
6299141386615046548323087409388548650272224487089895079970526
0143544115364878870129761259200
Encoded Public
E2 AB 8F 37 62 C8 7B F9 E9 BC 59 0C 2E 99 A5 58
0C C3 19 D5 CD DA 53 DF 3E C1 F0 C0 FE D3 55 5E
ED25519Bob's Key (ED25519)
UDF: ZAAA-GTSI-G2ED-255X-XBOB-XSXK-EY
Scalar: 54940772670153459146152925564198105262971485730889818
986727608573229799020168
Encoded Private
68 9A 68 92 8A 06 17 84 35 3C B7 08 F8 56 00 3F
BA 31 8C 42 B0 42 FE 2D 18 F2 7F AB CD 10 49 F1
X: 14271495069349838216379540196263140964032393512903842206168182
5518850827098876289800868735522232908519794251130907125878675
6343411484065706313568410880015
Y: 28094328948004112428189466223757440886388684291254605355859923
6240968229706795825282419594219442074647093851302547452470435
9438513477629978601366725015573
Encoded Public
32 E5 8D 5E 66 B2 F9 E9 14 79 08 71 96 3B 9A 75
A2 31 59 4B 8E ED 18 EF BD FF 11 D4 47 2A 8C F4
The Aggregate Signature Key A = A_(a) + A_(b)
Hallam-Baker Expires 8 July 2020 [Page 15]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
Aggregate Key = Alice + Bob ()
UDF: TBS
Scalar: 26569330913556569171916721364983482306308422345436973
56293312113171384684213
Encoded Private
B5 CE 0E B3 9C CF 18 99 CF 8D 4C BB AE 81 79 1F
CE 13 AA 3E 63 59 5B AC 8D 2C EB A4 55 C5 DF 05
X: 67872685043898469012456949773240814121645904736114813455820339
8688906486811443744733724675994181258029547346985079901494367
752381127781166234556148580090
Y: 36481740058369645484420180976004932062085375941522344052907594
0118552792158551197107484892204562290802810655253510302448455
4128548992118101415797909250954
Encoded Public
29 65 63 86 4F FB 10 8D BA 7A 0A 68 04 6D 00 DA
9B 1D C3 A4 AF BA 95 B4 5D 27 B4 35 00 2F DF 32
To sign the text "This is a test", Alice and Bob first generate their
values r which they multiply by the base point to obtain the value
R_(i):
Alice:
r_a = 63604859197442869293148706887422539627692219221403898700132668
2121229168067
R_a = System.Byte[]
Bob:
r_a = 90588289213224300609691011367428583037027596393781713986313426
4309321564224
R_a = System.Byte[]
The aggregate value R = R_(a) + R_(b)
R =
FB 4A B1 B4 53 C4 90 54 C4 F3 FD 00 87 AE EF 3D
33 CE 2B 5B F2 88 DB 13 FD F7 28 67 26 EE 07 1F
The value k is calculated
k = 4033579620745134235581641900982768938545463892767267607223155591
591374371726
Alice and Bob both calculate their signature scalar contribution:
Hallam-Baker Expires 8 July 2020 [Page 16]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
Alice:
S_a = 66555407144639732988508254705363964829787697756754631094914457
8662199168944
Bob:
S_b = 14755481297036888929325321941188765612781723011853930571004511
25046339648114
The coordinator calculates the aggregate value S = S_(a) + S_(b)
S = 2141102201150086222817614741172516209576049278752939368049595703
708538817058
The coordinator checks to see that the signature verifies:
S.B = R + kA =
X: 20168784599297644264752251999842912626695351875278949808545814
238064903547119
Y: 17798208503807446200181400098695224792791795831502230059016274
558554804974562
6.2. Unanimous Threshold Signature Ed448
The signers are Alice and Bob. Each creates a key pair:
Hallam-Baker Expires 8 July 2020 [Page 17]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
ED448Alice's Key (ED448)
UDF: ZAAA-ITSI-GXED-44XA-LICE-XSXK-EY
Scalar: 63495803583658817688110446314786076976347236361354035
5597788771064742993095132758589292255654895141583596922516472
738879360490167934280
Encoded Private
A0 53 4C 93 3C 34 00 76 AE 5D B5 4A C2 71 5F 43
E1 D6 63 2C 5C 56 53 C8 98 A0 8F 03 FF F5 22 96
91 45 8C 2B CF E3 FD 7E 2A 9E 0B D6 F4 CC 66 61
43 62 72 7B 34 B4 79 92
X: 24743197509267833262111449556527285120868167712209919570838426
3466168536901525943558973091346360088759980994772668117646359
614426660579
Y: 21342899120576770537664462049685258390853729788303428349051130
8752175233505795318243164692156369495328007220135137156078814
081547431302
Encoded Public
0A 3B F3 27 E7 E1 67 63 2C 59 E2 1C D1 84 C7 83
E8 1E D1 68 9F 32 A1 16 99 00 5C DA 29 B9 6C 08
E4 15 57 7E E5 63 C2 32 08 23 41 68 5F 49 1F FF
BC 4D CD 3A 4E A6 85 49 00
ED448Bob's Key (ED448)
UDF: ZAAA-ITSI-G2ED-44XB-OBXS-XKEY
Scalar: 72649803773199751564998543891898904839718409312910780
0262041941160989643727331987658132182181970054245587322070535
846720571414845714224
Encoded Private
BC 53 B4 74 3E A7 A7 FA 9F 05 9A BC 8C 22 26 15
A1 4E BB 10 0E B5 59 6B DE 9C 1B E9 F2 3C 65 42
E7 B4 47 18 60 AC 18 A6 D2 78 B8 BC CE F5 F4 28
B2 3A FF 08 61 EF 6B 7C
X: 58235851934808640621920816872959059172692411187640950432203039
8116748997750134460231406698091317008063030408798536634284207
667468558264
Y: 34390767697909283892495761259186538632120422458392131201372282
6056455656591826216381185634080685718154852726725624178995827
091591132128
Encoded Public
93 63 5A 45 2D 4C 94 32 45 23 CD E2 A8 46 E4 78
A0 80 59 DA 36 CB 6B 0C 06 64 6F BE 51 AB C0 BF
1E DB A8 3F 2B 3B 80 0F BF 00 E6 78 DD E0 83 E9
AC 20 02 55 87 07 39 38 00
The Aggregate Signature Key A = A_(a) + A_(b)
Hallam-Baker Expires 8 July 2020 [Page 18]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
Aggregate Key = Alice + Bob ()
UDF: TBS
Scalar: 89488306051273634069773238262841883041784075539841550
3672228636597106090916876462340541507950185640860121886233669
49466515613996100051
Encoded Private
D3 29 DD AB F6 0D 99 8B 75 65 B8 06 36 C9 3A 2C
D4 08 C3 9B 7C F9 77 8C 68 29 0E 3D 5D C7 3E 00
92 8B DC AE 26 FB 16 39 CD 25 1B 23 4A 5A 05 61
1D 5C C4 70 0A C9 84 1F
X: 17985659098670117617173315763082238685735647626871251468000984
2080317111091696183607307614171726960576308774975742249260532
199160570999
Y: 31506323224859159594386181995639405170623657273945727288760063
1624406694682617334725040181287905351066763414658543828623841
509161975864
Encoded Public
9B 3E DF 49 55 40 9F 7B EA 0B AA 40 B7 3D 15 82
60 9F 7C 40 CF 67 DE 56 56 0D 03 87 63 3B 15 F2
45 33 FE 48 BD 2D A0 A2 8B CC 74 DA 94 0F 39 00
AC 39 CB 0A 9F A4 EB B0 00
To sign the text "This is a test", Alice and Bob first generate their
values r which they multiply by the base point to obtain the value
R_(i):
Alice:
r_a = 73759418290054794861373572858566288781619358129342401750774507
86228048858000588481919454774928817062768941685761418007593280530
2999804
R_a = System.Byte[]
Bob:
r_a = 46237963278513215322396148050034865586946889704122042348536832
25811783723521941968309643248450810747601167927994249458319388189
6018677
R_a = System.Byte[]
The aggregate value R = R_(a) + R_(b)
R =
A0 FF BC 4A 92 40 39 FE A3 E2 98 19 8A CB 60 54
65 E1 EF C5 3E E2 AA 54 8D 27 78 CB F8 34 DC E2
9C DF A0 D7 F1 D2 7F EA 33 A2 A7 07 76 29 A1 7C
EB D4 97 F1 A3 CE 0B 61 80
The value k is calculated
Hallam-Baker Expires 8 July 2020 [Page 19]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
k = 4253302316461679481082434282193340085469714728082631996749509171
36528863341161130189337827407821840447093029569242109238035123293
90659
Alice and Bob both calculate their signature scalar contribution:
Alice:
S_a = 98531765382438321188310203252700171214078910507928709174117939
03719918811956876581076180944672089155783210289818359855928311296
3570131
Bob:
S_b = 10826185029558642533243398893637670455024435365016467182849367
92853602994592020853268312315426021226313042112304897550405147626
03231952
The coordinator calculates the aggregate value S = S_(a) + S_(b)
S = 2508393460412302388341324021707574217591292398626386593223906852
74134836172312654213972856976306382258260204195816912950241197071
52304
The coordinator checks to see that the signature verifies:
S.B = R + kA =
X: 87229317542997013818252634997777965098763712747235444768375375
25263195569863
Y: 48426719549368988040047207185774675871089544448034802040243984
177418779288237
6.3. Quorate Threshold Signature Ed25519
The administrator creates the aggregate key pair
Hallam-Baker Expires 8 July 2020 [Page 20]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
ED25519Aggregate Key (ED25519)
UDF: ZAAA-GTSI-GQED-255X-XAGG-REGA-TEXK-EY
Scalar: 39348647608109113656999806950437958090469802387424444
589375066079861075223816
Encoded Private
37 39 5E 7A 8B A5 A0 19 46 4B 58 22 EA 24 A5 71
45 2C 2A AC 7A 3E FB CA CE 3F D4 12 9A BA EB 70
X: 14198837758377867455716504277518729070915183249890461230792115
9904969716778427995951234766002164511738587575257530388758374
7824906047250057721855068523970
Y: 20211025649802071998810413948266748565975140520947927724517956
2067625505077751598018629551746824533726709810990193455662385
6152736116303441031851305458040
Encoded Public
6E 13 79 B4 39 DA 97 9C 5A 34 CE 79 CD 1B 50 DF
A0 76 AD 49 81 6D 52 59 A4 2C DB CE 44 FF 3E F5
Three key shares are required for Alice, Bob and Carol with a
threshold of two. The parameters of the Shamir Secret Sharing
polynomial are:
a0 = 3934864760810911365699980695043795809046980238742444458937506607
9861075223816
a1 = 6478235074936669232922546709062853526800747723284435893560379998
498854036401
The key share values for the participants are
xa = 1
ya = 2404849219052209606083234281242846172127851954429434846923740448
647203754283
xb = 2
yb = 1646078716656616625032594427262705458071483318333963134482169508
860603539695
xc = 3
yc = 8873082142610236439819545732825647440151146822384914220405985690
74003325107
Alice and Carol are selected to sign the message "This is another
test"
The Lagrange coefficients are:
Hallam-Baker Expires 8 July 2020 [Page 21]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
la = 3618502788666131106986593281521497120428558179689953803000975469
142727125496
lc = 3618502788666131106986593281521497120428558179689953803000975469
142727125494
Alice and Carol select their values ra, rc
ra = 5467990291185231529722642292282910677180928186673090898286725351
703148584114
Ra = System.Byte[]
rc = 3842295179874978910349644562407562715869337917437976620141371939
137906271828
Rc = System.Byte[]
The aggregate value R = R_(a) + R_(c)
R =
B1 23 76 E0 38 C5 15 2A B4 0F AB E2 FA 0C AC 81
D0 34 28 B7 13 14 75 A2 29 08 3C C0 62 3B 97 2D
The value k is
k = 24387631527158770388984723649123275410144206977564237560989138844
76357243053
The values R and k (or the document to be signed) and the Lagrange
coefficients are passed to Alice and Carol who use them to calculate
their secret scalar values:
sa = 7225776617244445516111444703385766378620336111334106073386586142
113532756919;
sc = 3174848681535619284995615994880214748421000838570708091980676184
605725462941
The signature contributions can now be calulated:
Sa = 5258509372798138258048339062398668871787395474915394506826958407
016143211994
Sc = 2369087102951219635858054121785113036716245524183189984191118268
317926650670
The coordinator calculates the aggregate value S = S_(a) + S_(b)
S = 3905908984170956799332066211407876676465246397186768850161257370
48615611675
The coordinator checks to see that the signature verifies:
Hallam-Baker Expires 8 July 2020 [Page 22]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
S.B = R + kA =
X: 44137199296357774791154193859950611817340233105447413062332736
902705770844448
Y: 29688538012678578878269116100526571773188243259702429043049505
096610475369173
6.4. Quorate Threshold Signature Ed448
The administrator creates the aggregate key pair
ED448Aggregate Key (ED448)
UDF: ZAAA-ITSI-GQED-44XA-GGRE-GATE-XKEY
Scalar: 50890460656419721531273587958284096015810982760541575
4207268050539683337837216003977228732536078674802149039736292
653681850024283019712
Encoded Private
78 22 7E 3B 89 95 80 5D 04 19 DC 27 F1 7F 9B E4
86 2B 0B DD 55 64 EE 04 19 49 4D DE B9 04 3B 9E
8B 7D DC EC EC 8F DD 1D E7 88 86 FD 11 FD 78 EF
1A 8B 84 8F 77 00 73 65
X: 44109173355278142669484438370724914685176368933547176239809629
7503768465595321590690311221269514682222687386378631457535068
446135118173
Y: 53219402718535721212460981200104434180077825188675868294070079
5084662920552823356888138706016038637934794839496624474125511
419755284720
Encoded Public
43 61 20 A0 B1 DF AA BD 6B 55 00 97 A3 BE CB B8
09 57 20 88 16 69 E4 B9 E1 7E 9C 13 C0 41 5B CB
4D 3E E4 99 2E 2D 48 89 1C C0 FB 26 58 C2 DD 5C
C1 DC 17 82 D7 A0 43 EE 80
Three key shares are required for Alice, Bob and Carol with a
threshold of two. The parameters of the Shamir Secret Sharing
polynomial are:
a0 = 0
a1 = 3230310441090132544881502506497842088200690521964525360879407271
94765745753141655224661941966620396823169163035586912417623145586
91569
The key share values for the participants are
Hallam-Baker Expires 8 July 2020 [Page 23]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
xa = 1
ya = 3230310441090132544881502506497842088200690521964525360879407271
94765745753141655224661941966620396823169163035586912417623145586
91569
xb = 2
yb = 6460620882180265089763005012995684176401381043929050721758814543
89531491506283310449323883933240793646338326071173824835246291173
83138
xc = 3
yc = 9690931323270397634644507519493526264602071565893576082638221815
84297237259424965673985825899861190469507489106760737252869436760
74707
Alice and Carol are selected to sign the message "This is another
test"
The Lagrange coefficients are:
la = 9085484053695086131866547598600056679420517008591475753518627489
75730019807697928580978776458461879816551468545458311523868779298
24891
lc = 9085484053695086131866547598600056679420517008591475753518627489
75730019807697928580978776458461879816551468545458311523868779298
24889
Alice and Carol select their values ra, rc
ra = 9916409373832855445680225409665819204694890272466243758088723611
70412660541692764122871086733859409013013307204388508168153245726
71888
Ra = System.Byte[]
rc = 1073609511703983208477950323599584281909008734279687416788909134
22950735978683478421395579947183492087041733558644230420650276766
461770
Rc = System.Byte[]
The aggregate value R = R_(a) + R_(c)
R =
30 A1 B3 17 69 AB 99 0E 57 5E 0B A5 2E 51 72 C0
BA 31 35 39 51 7C 83 B8 22 01 9F 85 45 93 9B 54
19 A2 FA FD 34 7A CA 88 7D 05 52 A6 21 5C E0 B7
69 62 F4 94 6C 15 C3 11 80
The value k is
Hallam-Baker Expires 8 July 2020 [Page 24]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
k = 10235373755332719753132453070965244233375545860969817073088585850
34984308611997081872546544930901042673608404328479261067005777756
07973
The values R and k (or the document to be signed) and the Lagrange
coefficients are passed to Alice and Carol who use them to calculate
their secret scalar values:
sa = 1393094971533028494918880135834681981172155279153826379483773839
76787863843741041141797168940839247505130521309883868015030349767
862243;
sc = 4240018392059887314544293838853293547119481225644687712199516581
83581401177985445743985863508531284581797723992077942897434060917
87536
The signature contributions can now be calulated:
Sa = 1176422144911387757819650694682546452875900733977789124175969611
96064995455877990887534924392638074098909532952322658866770310075
667839
Sc = 1261695317878032144403764439936251221873899581791569343175257655
85463382113907822175563223482227224266776195506309879565242141085
378620
The coordinator calculates the aggregate value S = S_(a) + S_(b)
S = 6210206520504026758501056148987863388656969140510633166475017698
63823736082462273469023925831729224023754347495408761272386953013
96680
The coordinator checks to see that the signature verifies:
S.B = R + kA =
X: 41458922531347602467333936260245029252955857925674064823925020
498684283758735
Y: 46892836594134893138380416498249507660693894634845371281694628
683354340901899
7. Security Considerations
TBS.
8. IANA Considerations
This document requires no IANA actions.
9. Acknowledgements
Hallam-Baker Expires 8 July 2020 [Page 25]
Internet-Draft Threshold Signatures Ed25519 & Ed448 January 2020
10. Normative References
[draft-hallambaker-mesh-udf]
Hallam-Baker, P., "Mathematical Mesh 3.0 Part II: Uniform
Data Fingerprint.", Work in Progress, Internet-Draft,
draft-hallambaker-mesh-udf-07, 18 October 2019,
.
[draft-hallambaker-threshold]
"[Reference Not Found!]".
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
Signature Algorithm (EdDSA)", RFC 8032,
DOI 10.17487/RFC8032, January 2017,
.
11. Informative References
[draft-hallambaker-mesh-developer]
Hallam-Baker, P., "Mathematical Mesh: Reference
Implementation", Work in Progress, Internet-Draft, draft-
hallambaker-mesh-developer-09, 23 October 2019,
.
[RFC2314] Kaliski, B., "PKCS #10: Certification Request Syntax
Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998,
.
Hallam-Baker Expires 8 July 2020 [Page 26]