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CH08: Describe schnorr signatures, multisignatures, t-signatures

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@ -272,9 +272,526 @@ use protocols that have been extensively reviewed to understand the
influence of the alternative flags.
====
[[schnorr_signatures]]
=== Schnorr signatures
In 1989, Claus Schnorr published a paper describing the signature
algorithm that's become eponymous with him. The algorithm isn't
specific to the Elliptic Curve Cryptography (ECC) that Bitcoin and many
other applications use, although it is perhaps most strongly associated
with ECC today. Schnorr signatures have a number of nice properties:
Provable security::
A mathematical proof of the security of schnorr signatures depends on
only the difficulty of solving the Discrete Logarithm Problem (DLP),
particularly for Elliptic Curves (EC) for Bitcoin, and the ability of
a hash function (like the SHA256 function used in Bitcoin) to produce
unpredictable values, called the Random Oracle Model (ROM). Other
signature algorithms have additional dependencies or require much
larger public keys or signatures for equivalent security to
ECC-Schnorr (when the threat is defined as classical computers; other
algorithms may provide more efficient security against quantum
computers).
Linearity::
Schnorr signatures have a property that mathematicians call
_linearity_, which applies to functions with two particular
properties. The first property is that summing together two or more
variables and then running a function on that sum will produce the
same value as running the function on each of the variables
independently and then summing together the results, e.g.
+f(x + y + z) = f(x) + f(y) + f(z)+; this property is called
_additivity_. The second property is that multiplying a variable and
then running a function on that product will produce the same value as
running the function on the variable and then multiplying it by the
same amount, e.g. +f(a * x) = a * f(x)+; this property is called
_homogeneity of degree 1_.
+
In cryptographic operations, some of the functions may be private (such
as functions involving private keys or secret nonces), so being able
to get the same result whether performing an operation inside or
outside of a function makes it easy for multiple parties to coordinate
and cooperate without sharing their secrets. We'll see some of the
specific benefits of linearity in schnorr signature in
<<schnorr_multisignatures>> and <<schnorr_threshold_signatures>>.
Batch Verification::
When used in a certain way (which Bitcoin does), one consequence of
schnorr's linearity is that it's relatively straightforward to verify
more than one schnorr signature at the same time in less time than it
would take to verify each signature independently. The more
signatures that are verified in a batch, the greater the speed up.
For the typical number of signatures in a block, it's possible to
batch verify them in about half the amount of time it would take to
verify each signature independently.
Later in this chapter, we'll describe the schnorr signature algorithm
exactly as it's used in Bitcoin, but we're going to start with a
simplified version of it and work our way towards the actual protocol in
stages.
Alice starts by chooses a large random number (+x+), which we call her
_private key_. She also knows a public point on Bitcoin's Elliptic
Curve (EC) called the Generator (+G+) (see <<public_key_derivation>>). Alice uses EC
multiplication to multiply +G+ by her private key +x+, in which case +x+
is called a _scalar_ because it scales up +G+. The result is +xG+,
which we call Alice's _public key_. Alice gives her public key to Bob.
Even though Bob also knows +G+, the Discrete Logarithm Problem (DLP)
prevents Bob from being able to divide +xG+ by +G+ to derive Alice's
private key.
At some later time, Bob wants Alice to identify herself by proving
that she knows the scalar +x+ for the public key (+xG+) that Bob
received earlier. Alice can't give Bob +x+ directly because that would
allow him to identify as her to other people, so she needs to prove
her knowledge of +x+ without revealing +x+ to Bob, called a
_zero-knowledge proof_. For that, we begin the schnorr identity
process:
1. Alice chooses another large random number (+k+), which we call the
_private nonce_. Again she uses it as a scalar, multiplying it by +G+
to produce +kG+, which we call the _public nonce_. She gives the
public nonce to Bob.
2. Bob chooses a large random number of his own, +e+, which we call the
_challenge scalar_. We say "challenge" because it's used to challenge
Alice to prove that she knows the private key (+x+) for the public key
(+xG+) she previously gave Bob; we say "scalar" because it will later
be used to multiply an EC point.
3. Alice now has the numbers (scalars) +x+, +k+, and +e+. She combines
them together to produce a final scalar +s+ using the formula:
+s = k + ex+. She gives +s+ to Bob.
4. Bob now knows the scalars +s+ and +e+, but not +x+ or +k+. However,
Bob does know +xG+ and +kG+, and he can compute for himself +sG+ and
+exG+. That means he can check the equality of a scaled-up version of
the operation Alice performed: +sG == kG + exG+. If that is equal,
then Bob can be sure that Alice knew +x+ when she generated +s+.
.Schnorr identity protocol with integers instead of vectors
====
It might be easier to understand the interactive schnorr identity
protocol if you oversimplify by substituting each of the values above
(including +G+) with simple integers instead of vectors like EC points.
For example, we'll use the prime numbers starting with 3:
Setup: Alice chooses +x=3+ as her private key. She multiplies it by the
generator +G=5+ to get her public key +xG=15+. She gives Bob +15+.
1. Alice chooses the private nonce +k=7+ and generates the public nonce
+kG=35+. She gives Bob +35+.
2. Bob chooses +e=11+ and gives it to Alice.
3. Alice generates +s = 40 = 7 + 11*3+. She gives Bob +40+.
4. Bob derives +sG = 200 = 40 * 5+ and +exG = 165 = 11 * 15+. He then
verifies that +200 == 35 + 165+. Note that this is the same operation
that Alice performed but all of the values have been scaled up by +5+
(the value of +G+).
Of course, this is an oversimplified example. When working with simple
integers, we can divide products by the generator +G+ to get the
underlying scalar, which isn't secure. This is why a critical property
of the Elliptic Curve Cryptography (ECC) used in Bitcoin is that
multiplication is easy but division is impractical. Also, with numbers
this small, finding underlying values (or valid substitutes) through
brute force is easy; the numbers used in Bitcoin are much larger.
====
Let's discuss some of the features of the interactive schnorr
identity protocol that make it secure:
- The nonce (+k+). In step 1, Alice chooses a number that Bob doesn't
know and can't guess and gives him the scaled form of that number,
+kG+. At that point, Bob also already has her public key (+xG+),
which is the scaled form of +x+. That means when Bob is working on
the final equation (+sG = kG + exG+), there are two independent
variables that Bob doesn't know (+xG+ and +kG+). It's possible to use
simple algebra to solve an equation with one unknown variable but not
two independent unknown variables, so the presence of Alice's nonce
prevents Bob from being able to derive her private key. It's critical
to note that this protection depends on nonces being unguessable in
any way. If there's anything predictable about Alice's nonce, Bob may
be able to leverage that into figuring out Alice's private key. See
<<nonce_warning>> for more details.
- The challenge scalar (+e+). Bob waits to receive Alice's public nonce
and then proceeds in step 2 to giver her a number (the challenge
scalar) that Alice didn't previously know and couldn't have guessed.
It's critical that Bob only give her the challenge scalar after she
commits to her public nonce. Consider what could happen if someone
who didn't know +x+ wanted to impersonate Alice, and Bob accidentally
gave them the challenge scalar +e+ before they told him the public
nonce +kG+. This allows them to change parameters on both sides of
the equation that Bob will use for verification, +sG == kG + exG+,
specifically they can change both +sG+ and +kG+. Think about a
simplified form of that expression: x = y + a. If you can change both
+x+ and +y+, you can cancel out +a+ using +x = (x - a) + a+. Any
value you choose for +x+ will now satisfy the equation. For the
actual equation they simply choose a random number for +s+, generate
+sG+, and then use EC subtraction to select a +kG+ that equals +kG =
sG - exG+. They give Bob their calculated +kG+ and later their random
+sG+, and Bob thinks that's valid because +sG == (sG - exG) + exG+.
This explains why the order of operations in the protocol is
essential: Bob must only give Alice the challenge scalar after Alice
has committed to her public nonce.
The interactive identity protocol described matches part of Claus
Schnorr's original description, but it lacks two essential features we
need for the decentralized Bitcoin network. The first of these is that
it relies on Bob waiting for Alice to commit to her public nonce and
then Bob giving her a random challenge scalar. In Bitcoin, the spender
of every transaction needs to be authenticated by thousands of Bitcoin
full nodes--including future nodes that haven't been started yet but
whose operators will one day want to ensure the bitcoins they receive
came from a chain of transfers where every transaction was valid. Any
Bitcoin node that is unable to communicate with Alice, today or in the
future, will be unable to authenticate her transaction and will be in
disagreement with every other node that did authenticate it. That's not
acceptable for a consensus system like Bitcoin. For Bitcoin to work, we
need a protocol that doesn't require interaction between Alice and each
node that wants to authenticate her.
A simple technique, known as the Fiat-Shamir transform after its
discoverers, can turn the schnorr interactive identity protocol
into a non-interactive digital signature scheme. Recall the importance
of steps 1 and 2--including that they be performed in order. Alice must
commit to an unpredictable nonce; Bob must give Alice an unpredictable
challenge scalar only after he has received her commitment. Recall also
the properties of secure cryptographic hash functions we've used
elsewhere in this book: it will always produce the same output when
given the same input but it will produce a value indistinguishable from
random data when given a different input.
This allows Alice to choose her private nonce, derive her public nonce,
and then hash the public nonce to get the challenge scalar. Because
Alice can't predict the output of the hash function (the challenge), and
because it's always the same for the same input (the nonce), this
ensures that Alice gets a random challenge even though she chooses the nonce
and hashes it herself. We no longer need interaction from Bob. She can
simply publish her public nonce +kG+ and the scalar +s+, and each of the
thousands of full nodes (past and future) can hash +kG+ to produce +e+,
use that to produce +exG+, and then verify +sG == kG + exG+. Written
explicitly, the verification equation becomes +sG == kG + hash(kG) * xG+.
We need one other thing to finish converting the interactive schnorr
identity protocol into a digital signature protocol useful for
Bitcoin. We don't just want Alice to prove that she knows her private
key; we also want to give her the to ability to commit to a message. Specifically,
we want her to commit to the data related to the Bitcoin transaction she
wants to send. With the Fiat-Shamir transform in place, we already
have a commitment, so we can simply have it additionally commit to the
message. Instead of +hash(kG)+, we now also commit to to the message
+m+ using +hash(kG || m)+, where +||+ stands for concatenation.
We've now defined a version of the schnorr signature protocol, but
there's one more thing we need to do to address a Bitcoin-specific
concern. In BIP32 key derivation, as described in
<<public_child_key_derivation>>, the algorithm for unhardened derivation
takes a public key and adds to it a non-secret value to produce a
derived public key. That means it's also possible to add that
non-secret value to a valid signature for one key to produce a signature
for a related key that's valid but which wasn't authorized by someone
possessing the private key. To protect BIP32 unhardened derivation and
also support several protocols people wanted to build on top of schnorr
signatures, Bitcoin's version of schnorr signatures, called _BIP340
schnorr signatures for secp256k1_, also commits to the public key being
used in addition to the public nonce and the message. That makes the
full commitment +hash(kG || xG || m)+.
Now that we've described each part of the BIP340 schnorr signature
algorithm and explained what it does for us, we can define the protocol.
Setup: Alice chooses a large random number (+x+) as her private key
(either directly or by using a protocol like BIP32 to deterministically
generate a private key from a large random seed value). She uses the
parameters defined in secp256k1 (see <<elliptic_curve>>) to multiply the
generator +G+ by her scalar +x+, producing +xG+ (her public key). She
gives her public key to everyone who will later authenticate her Bitcoin
transactions (e.g. by having +xG+ included in a transaction output). When
she's ready to spend, she begins generating her signature:
1. Alice chooses a large random private nonce +k+ and derives the public
nonce +kG+.
2. She chooses her message +m+ (e.g. transaction data) and generates the
challenge scalar +e = hash(kG || xG || m)+.
3. She produces the scalar +s = k + ex+. The two values +kG+ and +s+
are her signature. She gives this signature to everyone who wants to
verify that signature; she also needs to ensure everyone receives her
message +m+. In Bitcoin, this is done by including her signature in
the witness of her spending transaction and then relaying that
transaction to full nodes.
4. The verifiers (e.g. full nodes) use +s+ to derive +sG+ and then
verify that +sG == kG + hash(kG || xG || m)*xG+. If the equation is
valid, Alice proved that she knows her private key +x+ (without
revealing it) and committed to the message +m+ (containing the
transaction data).
==== Serialization of schnorr signatures
A schnorr signature consists of two values, +kG+ and +s+. The value
+kG+ is a point on Bitcoin's elliptic curve (called secp256k1) and so
would normally be represented by two 32-byte coordinates, e.g. +(x,y)+.
However, only the x coordinate is needed, so only that value is
included. When you see +kG+ below, note that it's only that point's x
coordinate.
The value +s+ is a scalar (a number meant to multiply other numbers). For
Bitcoin's secp256k1 curve, it can never be more than 32 bytes long.
Although both +kG+ and +s+ can sometimes be values that can be
represented with fewer than 32 bytes, it's improbable that they'd be
much smaller than 32 bytes, and so they're serialized as two 32 byte
values (i.e., values smaller than 32 bytes have leading zeroes).
They're serialized in the order of +kG+ and then +s+, producing exactly
64 bytes.
The taproot soft fork, also called v1 segwit, introduced schnorr signatures
to Bitcoin and is the only way they are used as of this writing. When
used with either taproot keypath or scriptpath spending, a 64-byte
schnorr signature is considered to use a default signature hash (sighash)
that is +SIGHASH_ALL+. If an alternative sighash is used, or if the
spender wants to waste space to explicitly specify +SIGHASH_ALL+, a
single additional byte is appended to the signature that specifies the
signature hash, making the signature 65 bytes.
As we'll see, either 64 or 65 bytes is considerably more efficient that
the serialization used for ECDSA signatures described in
<<serialization_of_signatures_der>>.
[[schnorr_multisignatures]]
==== Schnorr-based scriptless multisignatures
In the single-signature schnorr protocol described in <<schnorr_signatures>>, Alice
uses a signature (+kG+, +s+) to publicly prove her knowledge of her
private key, which in this case we'll call +y+. Imagine if Bob also has
a private key (+z+) and he's willing to work with Alice to prove that
together they know +x = y + z+ without either of them revealing their
private key to each other or anyone else. Let's go through the BIP340
schnorr signature protocol again.
[WARNING]
====
The simple protocol we are about to describe is not secure for the
reasons we will explain shortly. We use it only to demonstrate the
mechanics of schnorr multisignatures before describing related protocols
that are believed to be secure.
====
Alice and Bob need to derive the public key for +x+, which is +xG+.
Since it's possible to use Elliptic Curve (EC) operations to add two EC
points together, they start by Alice deriving +yG+ and Bob deriving
+zG+. Then then add them together to create +xG = yG + zG+. The point
+xG+ is their _aggregated public key_. To create a signature, they begin the
simple multisignature protocol:
1. They each individually choose a large random private nonce, +a+ for
Alice and +b+ for Bob. The also individually derive the corresponding
public nonce +aG+ and +bG+. Together, they produce an aggregated
public nonce +kG = aG + bG+.
2. They agree on the message to sign, +m+ (e.g. a transaction), and
each generate a copy of the challenge scalar: +e = hash(kG || xG || m)+.
3. Alice produces the scalar +q = a + ey+. Bob produces the scalar
+r = b + ez+. They add the scalars together to produce
+s = q + r+. Their signature is the two values +kG+ and +s+.
4. The verifiers check their public key and signature using the normal
equation: +sG == kG + hash(kG || xG || m)*xG+.
Alice and Bob have proven that they know the sum of their private keys without
either one of them revealing their private key to the other or anyone
else. The protocol can be extended to any number of participants, e.g.
a million people could prove they knew the sum of their million
different keys.
The protocol above has several security problems. Most notable is that one
party might learn the public keys of the other parties before committing
to their own public key. For example, Alice generates her public key
+yG+ honestly and shares it with Bob. Bob generates his public key
using +zG - yG+. When their two keys are combined (+yG + zG - yG+), the
positive and negative +yG+ terms cancel out so the public key only represents
the private key for +z+, i.e. Bob's private key. Now Bob can create a
valid signature without any assistance from Alice. This is called a
_key cancellation attack_.
There are various was to solve to the key cancellation attack. The
simplest scheme would be to require each participant commit to their
part of the public key before sharing anything about that key with all
of the other participants. For example, Alice and Bob each individually
hash their public keys and share their digests with each other. When
they both have the other's digest, they can share their keys. They
individually check that the other's key hashes to the previously
provided digest and then proceed with the protocol normally. This prevents
either one of them from choosing a public key that cancels out the keys
of the other participants. However, it's easy to fail to implement this
scheme correctly, such as using it in a naive way with unhardened
BIP32 public key derivation. Additionally, it adds an extra step for
communication between the participants, which may be undesirable in many
cases. More complex schemes have been proposed that address these
shortcomings.
In addition to the key cancellation attack, there are a number of
attacks possible against nonces. Recall that the purpose of the nonce
is prevent anyone from being able to use their knowledge of other values
in the signature verification equation to solve for your private key,
determining its value. To effectively accomplish that, you must use a
different nonce every time you sign a different message or change other
signature parameters. The different nonces must not be related in any
way. For a multisignature, every participant must follow these rules or
it could compromise the security of other participants. In addition,
cancellation and other attacks need to be prevented. Different
protocols that accomplish these aims make different tradeoffs, so
there's no single multisignature protocol to recommend in all cases.
Instead, we'll note three from the MuSig family of protocols:
MuSig::
Also called _MuSig1_, this protocol requires three rounds of
communication during the signing process, making it similar to the
process we described above. MuSig1's greatest advantage is its
simplicity.
MuSig2::
This only requires two rounds of communication and can sometimes allow
one of the rounds to be combined with key exchange. This can
significantly speed up signing for certain protocols, such as how
scriptless multisignatures are planned to be used in the Lightning
Network. MuSig2 is specified in BIP327 (the only scriptless
multisignature protocol that has a BIP as of this writing).
MuSig-DN::
DN stands for Deterministic Nonce, which eliminates as a concern a
problem known as the _repeated session attack_. It can't be combined
with key exchange and it's significantly more complex to implement
than MuSig or Musig2.
For most applications, MuSig2 is the best multisignature protocol
available at the time of writing.
[[schnorr_threshold_signatures]]
==== Schnorr-based scriptless threshold signatures
Scriptless multisignature protocols only work for n-of-n signing.
Everyone with a partial public key that becomes part of the aggregated
public key must contribute a partial signature and partial nonce to the
final signature. Sometimes, though, the participants want to allow a
subset of them to sign, such as k-of-n where k participants can sign for
a key constructed by n participants. That type of signature is called a
_threshold signature_.
We saw script-based threshold signatures in
<<multisig>>. But just as
scriptless multisignatures save space and increase privacy compared to
scripted multisigantures, _scriptless threshold signatures_ save space and
increase privacy compared to _scripted threshold signatures_. To anyone
not involved in the signing, a _scriptless threshold signatures_ looks
like any other signature which could've been created by a single-sig
user or through a scriptless multisignature protocol.
Various methods are known for generating scriptless threshold
signatures, with the simplest being a slight modification of how we
created scriptless multisignatures previously. This protocol also
depends on verifiable secret sharing (which itself depends on secure
secret sharing).
Basic secret sharing can work through simple splitting. Alice has a
secret number that she splits into three equal-length parts and shares
with Bob, Carol, and Dan. Those three can combine the partial numbers
they received (called _shares_) in the correct order to reconstruct
Alice's secret. A more sophisticated scheme would involve Alice adding
on some additional information to each share, called a correction code,
that allows any two of them to recover the number. This scheme is not
secure because each share gives its holder partial knowledge of Alice's
secret, making it easier for the participant to guess Alice's secret
than a non-participant who didn't have a share.
A secure secret sharing scheme prevents participants from learning
anything about the secret unless they combine the minimum threshold
number of shares. For example, Alice can choose a threshold (_k_) of
+2+ if she wants any two of Bob, Carol, and Dan to be able to
reconstruct her secret. The best known secure secret sharing algorithm
is _Shamir's Secret Sharing Scheme_, commonly abbreviated SSSS and named
after its discoverer, one of the same discovers of the Fiat-Shamir
transform we saw in <<schnorr_signatures>>.
In some cryptographic protocols, such as the scriptless threshold signature
schemes we're working towards, it's critical for Bob, Carol, and Dan to
know that Alice followed her side of the protocol correctly. They need
know that the shares she create all derive from the same secret, that
she used the threshold value she claims, and that she gave each one of
them a different share. A protocol that can accomplish all of that,
and still be a secure secret sharing scheme, is a _verifiable secret
sharing scheme_.
To see how multisignatures and verifiable secret sharing works for
Alice, Bob, and Carol, imagine they each wish to receive funds that can
be spent by any two of them. They collaborate as described in
<<schnorr_multisignatures>> to produce a regular multisignature public
key to accept the funds (n-of-n). Then each participant derives two
secret shares from their private key--one for each of two the other
participants. The shares allow any two of them to reconstruct the
originating partial private key for the multisignature. Each participant
distributes one of their secret shares to the other two participants,
resulting in each participant storing their own partial private key and
one share for every other participant. Subsequently, each participant
verifies the authenticity and uniqueness of the shares they received
compared to the shares given to the other participants.
Later on, when (for example) Alice and Bob want to generate a scriptless
threshold signature without Carol's involvement, they exchange the two
shares they possess for Carol. This enables them to reconstruct Carol's
partial private key. Alice and Bob also have their private keys,
allowing them to create a scriptless multisignature with all three
necessary keys.
In other words, the scriptless threshold signature scheme described
above is the same as a scriptless multisignature scheme except that
a threshold number of participants have the ability to reconstruct the
partial private keys of any other participants who are unable or
unwilling to sign.
This does point to a few things to be aware about when considering a
scriptless threshold signature protocol:
1. No accountability: because Alice and Bob reconstruct Carol's partial
private key, there can be no fundamental difference between a scriptless
multisignature produced by a process that involved Carol and one that
didn't. Even if Alice, Bob, or Carol claim that they didn't sign,
there's no absolutely guaranteed way for them to prove that they didn't
help produce the signature. If it's important to know which members of
the group signed, you will need to use scripted threshold signatures.
2. Manipulation attacks: imagine that Bob tells Alice that Carol is
unavailable, so they work together to reconstruct Carol's partial
private key. Then Bob tells Carol that Alice is unavailable, so they
work together to reconstruct Alice's partial private key. Now Bob has
his own partial private key plus the keys of Alice and Carol, allowing
him to spend the funds himself without their involvement. This attack can
be addressed if all of the participants agree to only communicate using a
scheme that allows any one of them to see all of the other's messages,
e.g. if Bob tells Alice that Carol is unavailable, Carol is able to see
that message before she begins working with Bob. Other solutions,
possibly more robust solutions, to this problem were being researched at
the time of writing.
No scriptless threshold signature protocol has been proposed as a BIP
yet, although significant research into the subject has been performed
by multiple Bitcoin contributors and we expect peer-reviewed solutions
will become available after the publication of this book.
[[ecdsa_math]]
==== ECDSA Math
Unfortunately for the future development of Bitcoin and many other
applications, Schnorr patented the algorithm and effectively prevented
its use for almost two decades.
((("Elliptic Curve Digital Signature Algorithm (ECDSA)")))As mentioned
previously, signatures are created by a mathematical function _F_~_sig_~
that produces a signature composed of two values. In ECDSA, those two

11
preface.asciidoc
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@ -138,7 +138,7 @@ link:$$https://linkedin.com/company/aantonop$$[]
Many thanks to all my patrons who support my work through monthly donations. You can follow my Patreon page here:
link:$$https://patreon.com/aantonop$$[]
=== Acknowledgments
=== Acknowledgments for the first and second editions
((("acknowledgments", id="acknowledge0")))This book represents the efforts and contributions of many people. I am grateful for all the help I received from friends, colleagues, and even complete strangers, who joined me in this effort to write the definitive technical book on cryptocurrencies and bitcoin.
@ -162,6 +162,15 @@ I owe my love of words and books to my mother, Theresa, who raised me in a house
Thank you all for supporting me throughout this journey.
=== Acknowledgements for the third edition
The introduction to the non-interactive schnorr signature protocol by
first describing the interactive schnorr identity protocol in
<<schnorr_signatures>> was heavily influenced by the introduction to the
subject in Borrommean Ring Signatures (2015) by Gregory Maxwell and
Andrew Poelstra. I am deeply indebted to each of them for all of their
freely-provided assistance over the past decade.
[[github_contrib]]
==== Early Release Draft (GitHub Contributions)