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CH08: Edits to intro and other prev edition content

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David A. Harding 2023-04-08 11:16:43 -10:00
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chapters/signatures.adoc
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@ -1,33 +1,32 @@
[[c_signatures]]
== Digital Signatures
[[sighashes]]
=== Signature Hashes (Sighashes)
((("transactions", "digital signatures and", id="Tdigsig06")))So far, we
have not delved into any detail about "digital signatures." In this
section we look at how digital signatures work and how they can present
proof of ownership of a private key without revealing that private key.
chapter we look at how digital signatures work and how they can present
proof of control of a private key without revealing that private key.
FIXME
((("digital signatures", "algorithm used")))((("Elliptic Curve Digital
Signature Algorithm (ECDSA)")))Two signature algorithms are currently
used in Bitcoin, the _schnorr signature algorithm_ and the _Elliptic
Curve Digital Signature Algorithm_ (_ECDSA_).
These algorithms are used for digital signatures based on elliptic
curve private/public key pairs, as described in <<elliptic_curve>>.
They are used by the script functions +OP_CHECKSIG+,
+OP_CHECKSIGVERIFY+, +OP_CHECKMULTISIG+, and +OP_CHECKMULTISIGVERIFY+.
Any time one of those is executed in a script, a signature must be
They are used for spending segwit v0 P2WPKH outputs, segwit v1 P2TR
keypath spending, and by the script functions +OP_CHECKSIG+,
+OP_CHECKSIGVERIFY+, +OP_CHECKMULTISIG+, +OP_CHECKMULTISIGVERIFY+, and
+OP_CHECKSIGADD+.
Any time one of those is executed, a signature must be
provided.
((("digital signatures", "purposes of")))A digital signature serves
three purposes in bitcoin (see the following sidebar). First, the
signature proves that the owner of the private key, who is by
three purposes in Bitcoin (see the following sidebar). First, the
signature proves that the controller of a private key, who is by
implication the owner of the funds, has _authorized_ the spending of
those funds. Secondly, the proof of authorization is _undeniable_
(nonrepudiation). Thirdly, the signature proves that the transaction (or
specific parts of the transaction) have not and _cannot be modified_ by
anyone after it has been signed.
(nonrepudiation). Thirdly, the signature proves that specific parts of
the transaction have not and _cannot be modified_ by
anyone besides the signer.
Note that each transaction input is signed independently. This is
critical, as neither the signatures nor the inputs have to belong to or
@ -37,7 +36,7 @@ privacy.
[NOTE]
====
Each transaction input and any signature it may contain is _completely_
Each transaction input and any signatures it may contain is _completely_
independent of any other input or signature. Multiple parties can
collaborate to construct transactions and sign only one input each.
====
@ -56,7 +55,7 @@ to believe that the message was created by a known sender
_Source: https://en.wikipedia.org/wiki/Digital_signature_
****
==== How Digital Signatures Work
=== How Digital Signatures Work
((("digital signatures", "how they work")))A digital signature is a
_mathematical scheme_ that consists of two parts. The first part is an
@ -65,7 +64,7 @@ key), from a message (the transaction). The second part is an algorithm
that allows anyone to verify the signature, given also the message and a
public key.
===== Creating a digital signature
==== Creating a digital signature
In Bitcoin's implementation of digital signature algorithms, the "message" being
signed is the transaction, or more accurately a hash of a specific
@ -82,7 +81,8 @@ where:
* _F_~_sig_~ is the signing algorithm
* _Sig_ is the resulting signature
More details on the mathematics of schnorr and ECDSA signatures can be found in <<signature_math>>.
More details on the mathematics of schnorr and ECDSA signatures can be found in <<schnorr_signatures>>
and <<ecdsa_signatures>>.
In both schnorr and ECDSA signatures, the function _F_~_sig_~ produces a signature +Sig+ that is composed of
two values. There are differences between the two values in the
@ -99,13 +99,13 @@ simpler serialization format is used.
((("digital signatures", "verifying")))To verify the signature, one must
have the signature, the serialized transaction, some data about the
output being spend, and the public key (that corresponds to the private key used to create the
signature). Essentially, verification of a signature means "Only the
owner of the private key that generated this public key could have
output being spent, and the public key that corresponds to the private key used to create the
signature. Essentially, verification of a signature means "Only the
controller of the private key that generated this public key could have
produced this signature on this transaction."
The signature verification algorithm takes the message (a hash of the
transaction or parts of it), the signer's public key and the signature,
The signature verification algorithm takes the message (a hash of
parts of the transaction and related data), the signer's public key and the signature,
and returns TRUE if the signature is valid for
this message and public key.
@ -114,7 +114,7 @@ this message and public key.
((("digital signatures", "signature hash
types")))((("commitment")))Digital signatures are applied to messages,
which in the case of bitcoin, are the transactions themselves. The
which in the case of Bitcoin, are the transactions themselves. The
signature implies a _commitment_ by the signer to specific transaction
data. In the simplest form, the signature applies to almost the entire
transaction, thereby committing all the inputs, outputs, and other
@ -135,7 +135,7 @@ transaction.
Remember, each input may contain one or more signatures. As
a result, an input may have signatures
with different +SIGHASH+ flags that commit different parts of the
transaction. Note also that bitcoin transactions
transaction. Note also that Bitcoin transactions
may contain inputs from different "owners," who may sign only one input
in a partially constructed transaction, collaborating with
others to gather all the necessary signatures to make a valid
@ -194,7 +194,7 @@ can't be modified once signed.
====
In
<<serialization_of_signatures_der>>, we saw that the last part of the
<<serialization_of_signatures_der>>, we will see that the last part of the
DER-encoded signature was +01+, which is the +SIGHASH_ALL+ flag for ECDSA signatures. This
locks the transaction data, so Alice's signature is committing to the state
of all inputs and outputs. This is the most common signature form.
@ -212,13 +212,21 @@ it by adding an input of their own, as a donation. They sign their own
input with +ALL|ANYONECANPAY+. Unless enough inputs are gathered to
reach the value of the output, the transaction is invalid. Each donation
is a "pledge," which cannot be collected by the fundraiser until the
entire goal amount is raised.
entire goal amount is raised. Unfortunately, this protocol can be
circumvented by the fundraiser adding an input of their own (or from
someone who lends them funds), allowing them to collect the donations
even if they haven't reached the specified value.
+NONE+ :: This construction can be used to create a "bearer check" or
"blank check" of a specific amount. It commits to the input, but allows
the output locking script to be changed. Anyone can write their own
the output script to be changed. Anyone can write their own
Bitcoin address into the output scriptPubKey. However, the output value
itself cannot be changed.
itself cannot be changed. By itself, this allows any miner to change
the output destination and claim the funds for themselves, but if other
required signatures in the transaction use +SIGHASH_ALL+ or another type
that commits to the output, it allows those spenders to change the
destination without allowing any third-parties (like miners) to modify
the outputs.
+NONE|ANYONECANPAY+ :: This construction can be used to build a "dust
collector." Users who have tiny UTXOs in their wallets can't spend these
@ -240,7 +248,7 @@ could be copied and used to spend the other output to the same
destination.
A signature using +SIGHASH_ANYPREVOUTANYSCRIPT+ would not
commit to the outpoint, the amount, the witness program, or that
commit to the outpoint, the amount, the witness program, or the
tapleaf_hash used, allowing it to be used to spend any previous output
which the signature could satisfy. For example, if Alice received two
outputs for different amounts and different witness programs (e.g. one
@ -396,7 +404,7 @@ you _must_ use RFC 6979 or a similarly deterministic-random algorithm to
ensure you generate a different _k_ for each transaction.((("",
startref="Tdigsig06")))
==== Segregated Witness' New Signing Algorithm
=== Segregated Witness's New Signing Algorithm
Segregated Witness modified the semantics of the four signature
verification functions from legacy Bitcoin Script (+CHECKSIG+, +CHECKSIGVERIFY+, +CHECKMULTISIG+,