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CH07: Switch from M-of-N to K-of-N

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Explanation for change is in changed text, but briefly: k-of-n is
clearer when pronounced.
This commit is contained in:
David A. Harding 2023-03-28 06:57:35 -10:00
parent 7417842837
commit ec534165ba
1 changed files with 21 additions and 14 deletions
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35
chapters/authorization-authentication.adoc
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@ -324,33 +324,40 @@ image::../images/mbc2_0606.png["Tx_Script_P2PubKeyHash_2"]
scripts")))((("transactions", "advanced", id="Tadv07")))((("scripting",
"multisignature scripts", id="Smulti07")))((("multisignature
scripts")))Multisignature scripts set a condition where N public keys
are recorded in the script and at least M of those must provide
signatures to spend the funds. This is also known as an M-of-N scheme,
where N is the total number of keys and M is the threshold of signatures
are recorded in the script and at least K of those must provide
signatures to spend the funds. This is also known as a K-of-N scheme,
where N is the total number of keys and K is the threshold of signatures
required for validation. For example, a 2-of-3 multisignature is one
where three public keys are listed as potential signers and at least two
of those must be used to create signatures for a valid transaction to
spend the funds.
At this time, _standard_ multisignature scripts are limited to at most 3
[TIP]
====
Some Bitcoin documentation, including earlier editions of this book,
uses the term M-of-N for traditional multisignature. However, it's hard
to tell "M" and "N" apart when they're spoken, so we use the alternative
K-of-N. Both phrases refer to the same type of signature scheme.
====
At this time, _standard_ multisignature scriptPubKeys are limited to at most 3
listed public keys, meaning you can do anything from a 1-of-1 to a
3-of-3 multisignature or any combination within that range. The
limitation to 3 listed keys might be lifted by the time this book is
published, so check the +IsStandard()+ function to see what is currently
3-of-3 multisignature or any combination within that range.
You may want to check the +IsStandard()+ function to see what is currently
accepted by the network. Note that the limit of 3 keys applies only to
standard (also known as "bare") multisignature scripts, not to
multisignature scripts wrapped in a Pay-to-Script-Hash (P2SH) script.
P2SH multisignature scripts are limited to 15 keys, allowing for up to
15-of-15 multisignature. We will learn about P2SH in <<p2sh>>.
The general form of a scriptPubKey setting an M-of-N multisignature
The general form of a scriptPubKey setting an K-of-N multisignature
condition is:
----
M <Public Key 1> <Public Key 2> ... <Public Key N> N OP_CHECKMULTISIG
K <Public Key 1> <Public Key 2> ... <Public Key N> N OP_CHECKMULTISIG
----
where N is the total number of listed public keys and M is the threshold
where N is the total number of listed public keys and K is the threshold
of required signatures to spend the output.
A scriptPubKey setting a 2-of-3 multisignature condition looks like
@ -388,7 +395,7 @@ the three public keys set as an encumbrance.
((("scripting", "multisignature scripts", "OP_CHECKMULTISIG
bug")))((("OP_CHECKMULTISIG bug workaround")))There is a bug in
++OP_CHECKMULTISIG++'s execution that requires a slight workaround. When
+OP_CHECKMULTISIG+ executes, it should consume M+N+2 items on the stack as
+OP_CHECKMULTISIG+ executes, it should consume K+N+2 items on the stack as
parameters. However, due to the bug, +OP_CHECKMULTISIG+ will pop an extra
value or one value more than expected.
@ -402,10 +409,10 @@ example:
First, +OP_CHECKMULTISIG+ pops the top item, which is +N+ (in this example
"3"). Then it pops +N+ items, which are the public keys that can sign.
In this example, public keys A, B, and C. Then, it pops one item, which
is +M+, the quorum (how many signatures are needed). Here M = 2. At this
point, +OP_CHECKMULTISIG+ should pop the final +M+ items, which are the
is +K+, the quorum (how many signatures are needed). Here M = 2. At this
point, +OP_CHECKMULTISIG+ should pop the final +K+ items, which are the
signatures, and see if they are valid. However, unfortunately, a bug in
the implementation causes +OP_CHECKMULTISIG+ to pop one more item (M+1
the implementation causes +OP_CHECKMULTISIG+ to pop one more item (K+1
total) than it should. The extra item is disregarded when checking the
signatures so it has no direct effect on +OP_CHECKMULTISIG+ itself.
However, an extra value must be present because if it is not present,