Internet-Draft | Threshold Signatures in Elliptic Curves | September 2020 |
Hallam-Baker | Expires March 8, 2021 | [Page] |
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.¶
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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.¶
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:¶
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.¶
A single signature created using a single private key stored in an encrypted form whose use requires participation of multiple key decryption shares.¶
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¶
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 builds on the approach described in [draft-hallambaker-threshold] to define 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.¶
The approach used is based on that developed in FROST [Komlo]. This document describes the signature scheme itself. The techniques used to generate keys are described separately in [draft-hallambaker-threshold].¶
As in the base document, we first describe signature generation for the case that n = t using 'direct' coefficients, that is the secret scalar is the sum of the secret shares. We then show how the scheme is modified using Shamir secret sharing [Shamir79] and Lagrange coefficients for the case that n > t.¶
Threshold signatures have application in any situation where it is desired to have finer grain control of signing operations without this control structure being visible to external applications. It is of particular interest in situations where legacy applications do not support multi-signatures.¶
Hardware Security Modules (HSMs) prevent accidental disclosures of signature keys by binding private keys to a hardware device from which it cannot be extracted without substantial effort. This provides effective mitigation of the chief causes of key disclosure but requires the signer to rely on the trustworthiness of a device that represents a black box they have no means of auditing.¶
Threshold signatures allow the signer to take advantage of the key binding control provided by an HSM without trusting it. The HSM only contributes one of the key shares used to create the signature. The other is provided by the application code (or possibly an additional HSM).¶
Code signing is an important security control used to enable rapid detection of malware by demonstrating the source of authorized code distributions but places a critical reliance on the security of the signer's private key. Inadvertent disclosure of code signing keys is commonplace as they are typically stored in a form that allows them to be used in automatic build processes. Popular source code repositories are regularly scanned by attackers seeking to discover private signature keys and passwords embedded in scripts.¶
Threshold signatures allow the code signing operation to be divided between a developer key and an HSM held locally or by a signature service. The threshold shares required to create the signature can be mapped onto the process roles and personnel responsible for authorizing code release. This last concern might be of particular advantage in open source projects where the concentration of control embodied in a single code signing key has proved to be difficult to reconcile with community principles.¶
Redundancy is as desirable for trusted services as for any other service. But in the case that multiple hosts are tasked with compiling a data set and signing the result, there is a risk of different hosts obtaining a different view of the data set due to timing or other concerns. This presents the risk of the hosts signing inconsistent views of the data set.¶
Use of threshold signatures allows the criteria for agreeing on the data set to be signed to be mapped directly onto the requirement for creating a signature. So if there are three hosts and two must agree to create a signature, three signature shares are created and with a threshold of two.¶
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.¶
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].¶
The implementation status of the reference code base is described in the companion document [draft-hallambaker-mesh-developer].¶
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:¶
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¶
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 checks that:¶
S.B = R + k.A¶
This equality must hold for a valid signature since:¶
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¶
S_{1} = r + k_{1}.s mod L¶
S_{2} = r + k_{2}.s mod L¶
s = (S_{1} - S_{2})(k_{1} - k_{2})^{-1} mod L¶
The method of constructing r MUST ensure that it is unique and unguessable.¶
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:¶
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.¶
The method of constructing the values r_{i} MUST ensure that each is unique and unguessable both to external parties, the signers and the dealer. The deterministic method specified in [RFC8032] cannot be applied to generation of the values r_{i} as it allows the dealer to cause signers to reveal their key shares by requesting multiple signature contributions for the same message but with different values of k. In particular, requesting signature contributions for the same message:¶
With different Lagrange coefficients.¶
With a false value of R¶
To avoid these attacks, the value r_{i} is generated using a secure random number generator. This approach requires the signer to ensure that values are never reused requiring that the signing API maintain state between the first and second rounds of the algorithm.¶
While there are many approaches to deterministic generation of r_{i} that appear to be sound, closer inspection has demonstrated these to be vulnerable to rogue key and rogue contribution attacks.¶
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 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.¶
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.¶
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.¶
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.¶
The means by which threshold shares are created is described in [draft-hallambaker-threshold].¶
The dealer selects the signers who are to construct the signature. Each signer then computes the value R_{i}:¶
The dealer then completes the signature by:¶
The means by which threshold shares are created is described in [draft-hallambaker-threshold].¶
The dealer selects the signers who are to construct the signature. Each signer then computes the value R_{i}:¶
Transmit the value R_{i} to the dealer¶
The dealer then completes the signature by:¶
The signers are Alice and Bob's Threshold Signature Service '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 composite Signature Key A = A_{a} + A_{b}¶
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 first generates her value r and multiplies it by the base point to obtain the value R_{a}:¶
Alice: r_a = 39481530989500570659081556772521617337911026343586730199076077 22114057563970 R_a = 97 4A 15 AC C8 05 21 91 87 B7 1F C1 2F AB C4 00 6E 52 85 09 54 04 00 5E 30 62 99 F5 62 A5 2F 7E¶
Alice passes her value R_{A} to Bob along with the other parameters required to calculate i. Bob then calculates his value R_{A} and multiplies it by the base point to obtain the value R_{b}:¶
Bob: r_b = 53794602633571448603419732447329315217287976777312087913574775 08723470105822 R_b = 8E 48 01 72 91 74 D0 C9 36 44 EA 7B 8A D2 50 0D F2 4B F9 2C 53 B7 AF 2D 91 06 81 79 3D 28 43 DD¶
Bob can now calculate the composite value R = R_{a} + R_{b} and thus the value k.¶
R = 55 1A 67 0A D2 8C F6 74 FD A7 0E BE 4D AA A8 8F E6 AB 98 61 7B 7D 04 D4 F2 05 0A 71 D3 81 59 AF k = 2421363997835522134020668429031063921379440688711932467881614021 504091928861¶
Bob calculates his signature scalar contribution and returns the value to Alice:¶
Bob: S_b = 16218971061984715973738168270294546698092320250831576357174059 25399123547360¶
Alice can now calculate her signature scalar contribution and thus the signature scalar S.¶
Alice: S_a = 48633098250297226038262755114982589781022109179929876390922466 96122249236384 S = 6485206931228194201200092338527713647911442943076145274809652621 521372783744¶
Alice checks to see that the signature verifies:¶
S.B = R + kA = X: 31970754396382072160768904923868016501704809130751339430468771 226924902729588 Y: 42652466254796906167986746499263519020730974689311471235758253 40736275086949¶
The signers are Alice and Bob's Threshold Signature Service 'Bob'. Each creates a key pair:¶
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 composite Signature Key A = A_{a} + A_{b}¶
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 first generates her value r and multiplies it by the base point to obtain the value R_{a}:¶
Alice: r_a = 12579897228229965435145658294327712989795531501893232106499892 89681302839491331449888721535664675983310277410977228241931824542 50578091 R_a = 25 81 8E 72 12 06 2B 9F C2 90 9F 9E 16 55 51 16 A1 53 24 FD C3 29 82 A5 3E DC 0E 69 5C C7 FF 7E 71 60 4E 21 CB 71 59 84 68 9B 1F 59 E1 56 F0 FD 4A 80 67 9C 99 2D C9 A8 80¶
Alice passes her value R_{A} to Bob along with the other parameters required to calculate i. Bob then calculates his value R_{A} and multiplies it by the base point to obtain the value R_{b}:¶
Bob: r_b = 94475541305183315492441638684696640787219219352824613903602377 05284653346212377761617015793185322655995130417461462487958500972 3713102 R_b = AF 91 A2 24 B1 B3 5E F4 F3 2C A5 06 45 A7 90 23 28 68 4A 3E E8 8D D7 B2 40 12 55 85 20 BF 53 82 86 97 9A 3D 84 40 D5 A3 4E E6 A6 D2 2E 80 1A DC 40 32 04 DF AB 09 E1 FE 80¶
Bob can now calculate the composite value R = R_{a} + R_{b} and thus the value k.¶
R = FB 04 F2 8A 1D 26 6D AF 0F C5 27 3C 19 4C 9F 30 D7 B8 CB B3 73 F2 EC 7C 0C 41 ED D5 AB B6 50 A9 C9 AD 45 50 34 A6 24 83 50 91 0A 9A 56 EC 59 D9 15 34 81 7A 5D 40 96 F4 80 k = 1114611014063501116613867141529424596251655496753131787628559383 37364120817354370936515245042197321114567413248741160748378478790 681471¶
Bob calculates his signature scalar contribution and returns the value to Alice:¶
Bob: S_b = 16039476917135265334527230911158039502139318338070597455056095 64681863169906962950761847223946073430255036087562611717818559618 1028696¶
Alice can now calculate her signature scalar contribution and thus the signature scalar S.¶
Alice: S_a = 21026356483062916567544814937806998370021341112531790514607516 15676503362950349198935476068933071440579133782644266606317107429 0101794 S = 3706583340019818190207204584896503787216065945060238796966361180 35836653285731214969732329287914487083416987020687832413566704711 30490¶
Alice checks to see that the signature verifies:¶
S.B = R + kA = X: 13430030555801715651108391534497698605996566164478554520490263 44715275527428 Y: 23294518891760930618220409397109930286557491844791521429604956 720890326572862¶
The administrator creates the composite key pair¶
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 = 1642892179747526362125411177719513794207741359956661956935408266 531254199801¶
The key share values for the participants are¶
xa = 1 ya = 4806511901195328949259285312942500680391961950481568516300719654 965058168672 xb = 2 yb = 6449404080942855311384696490662014474599703310438230473236127921 496312368473 xc = 3 yc = 8552906833581194595369211053385340279503283110149848241695852497 42112317285¶
Alice and Carol are selected to sign the message "This is another test"¶
The Lagrange coefficients are:¶
la = 3618502788666131106986593281521497120428558179689953803000975469 142727125496 lc = 3618502788666131106986593281521497120428558179689953803000975469 142727125494¶
Alice and Carol select their values ra, rc¶
ra = 4582163519135697954212356121831281867610872330474418182918654552 719487887500 Ra = 2A E0 F9 8B D2 85 C5 61 5D 14 D8 BF EA E9 96 D3 1A E2 0B 37 8A 7C 5C FE 90 6C DF 36 70 D0 BC 90 rc = 3679566825305648240874136350616349919194440179398151235437522804 353602423864 Rc = 3D 0B 8B D5 B4 5A 93 B0 3E B0 24 CE 42 AD F7 FF 4D 43 6C 09 1C 07 0A F9 76 9C F5 D2 59 E2 B7 9A¶
The composite value R = R_{a} + R_{c}¶
R = B7 93 54 14 DD AC A5 94 00 3D 09 EB 60 E6 91 18 9A AF 60 07 DA CA E5 83 7E 1F 3E 6A C4 49 DF A3¶
The value k is¶
k = 40993397091938598651697285820684701229880737187373552994300226559 88467436177¶
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 = 7209767851792993423888927969413751020587942925722352774451079482 447587253008; sc = 3190857446987071377218132728852230106453394024182461390916182844 271670966852¶
The signature contributions can now be calulated:¶
Sa = 3328989745507326292266821261476737658921447882205957270253608502 484638974498 Sc = 5932746180467179638638850117242322706693329974834825884925701595 858986229620¶
The dealer calculates the composite value S = S_{a} + S_{b}¶
S = 2024730348642243716932484815676066124757661497660875549177359160 058170953129¶
The dealer checks to see that the signature verifies:¶
S.B = R + kA = X: 13666591543765071964834317735790194069963726165121686955487832 105142108752977 Y: 20775966697642032279966691396030726941085308409030793184460185 676304105925271¶
The administrator creates the composite 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 = 5089046065641972153127358795828409601581098276054157542072680505 39683337837216003977228732536078674802149039736292653681850024283 019712 a1 = 1550999901841386151901998425672690051264689056286441960494229439 42535003917136508299604076339687566636344266479521172051858028847 271091¶
The key share values for the participants are¶
xa = 1 ya = 1188755535266306625909428662341065645193477127185714050455733450 96780329869733755128245543000689113548562425088538838819386785551 341466 xb = 2 yb = 9226586263686755514381175682937443605740627817538608602462373924 41693298253306777116538640486843042215963978589683485664710585389 62778 xc = 3 yc = 6565617174710444769668064742464230759546484363220076700367413339 15583297809276002950621850966794948946303706293978583135553315265 84090¶
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 = 1496695989301362163168579893078939576055478053582077987842188159 81572439136632201150848887159722470726697626221876540830488497692 099942 Ra = 8F C2 62 01 43 39 35 2F 15 1C 97 BD D6 29 92 ED C2 DF E7 56 0B 49 37 E4 F7 BA 7A 5D BA 45 1C 39 EE 23 EF FE F3 CB 5B ED 33 AB A8 D9 97 0D B4 9C 70 C1 02 A2 71 08 20 F8 80 rc = 1315259132332389080097729070288895479376412145635234566561260348 85686598945854770737828828936179895865317203017520659128509983064 478834 Rc = B0 37 9E BC 5A 50 0E DC 19 56 7D 1A 4E 6F 92 63 33 C2 82 5C 77 45 2E A7 79 54 7A A2 FB FE F4 F8 90 CE D1 5C 04 A0 7C EB 87 D2 BA E5 57 CF 3F 45 FA FD 27 7E 10 29 91 AC 80¶
The composite value R = R_{a} + R_{c}¶
R = 02 F1 FC 76 4F 34 A6 74 39 CB AF D3 96 DF C2 6F F8 D2 8C C7 28 C8 36 F1 DE 9A E7 CE 9A DE C6 79 90 9E 2F 2A E8 14 E0 14 CD 06 8B 30 F1 61 EA C0 89 11 7C 85 33 F2 8A 08 00¶
The value k is¶
k = 24349522678580469525531601305349874786441698191777297108871400571 64322563268580245413433284674218785382878420998275959014579479614 7935¶
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 = 1783133302899459938864142993511598467790215690778571075683600176 45170494804600632692368314501033670322843637632808258229080178327 012199; sc = 1488815952003494987889906282596799797906779183557291315685354830 99366839071075785568664662743352628515995108394392733147996090096 357734¶
The signature contributions can now be calulated:¶
Sa = 1813098642235159385577750965787902646262662182590690423179327447 87548599871305490942337029804015073122558685157994421699467930005 403407 Sc = 1786732187196626949822274127967658987330014322486999403584441819 61287645876306473101042347188778074681315014827243713964951294010 546032¶
The dealer calculates the composite value S = S_{a} + S_{b}¶
S = 1782734018692769109026715574035550297708573103359394676060043769 53690241786072378327183621701100771840563406276146473359645468156 299660¶
The dealer checks to see that the signature verifies:¶
S.B = R + kA = X: 22269164075823155982206235662550890548495096320747189740776580 698080912420099 Y: 22192519940337997518518077627365944525840303920321484782585911 563432903433924¶
All the security considerations of [RFC7748], [RFC8032] and [draft-hallambaker-threshold] apply and are hereby incorporated by reference.¶
The rogue key attack described in [draft-hallambaker-threshold] is of particular concern to generation of threshold signatures.¶
If A and B are public keys, the intrinsic degree of trust in the composite keypair A + B is that of the lesser of A and B.¶
As in any Schnorr signature scheme, compromise of the value r results in compromise of the private key. The base signature specification [RFC8032] describes a deterministic construction of r that ensures confidentiality and uniqueness for a given value of k.¶
As described above, this approach is not applicable to the generation of values of r_{i} to compute threshold signature contributions. Accordingly the requirements of [RFC4086] regarding requirements for randomness MUST be observed.¶
Implementations MUST NOT use a deterministic generation of the value r_{i} for any threshold contribution except for calculating the final contribution when all the other parameters required to calculate k are known.¶
Implementation of the general two stage signing algorithm requires that signers track generation and use of the values r_{i} to avoid reuse for different values of R_{i}. Implementations MUST ensure that exhaustion of this resource by one party does not cause other parties to be denied service.¶
Signatures generated in strict conformance with [RFC8032] are guaranteed to be unique such that signing the same document with the same key will always result in the same signature value.¶
The signature modes described in this document are computationally indistinguishable from those created in accordance with [RFC8032] but are not unique.¶
Implementations MUST not use threshold signatures in applications where signature values are used in place of cryptographic digests as unique content identifiers.¶
This document requires no IANA actions.¶
[TBS]¶