Diagram update and minor improvements

zips/zip-0227.rst

@@ -161,7 +161,8 @@ Define $\mathsf{IssueAuthSig.DerivePublic} \;{\small ⦂}\; (\mathsf{isk} \;{\sm
 * Return $\bot$ if the $\textit{PubKey}$ algorithm invocation fails, otherwise return $\mathsf{ik}$.
 
 where the $\textit{PubKey}$ algorithm is defined in BIP 340 [#bip-0340]_, and the output of the algorithm is in big-endian order as defined in BIP 340.
-The encoding of $\mathsf{ik}$ begins with a version byte that is $\mathtt{0x00}$. This enables future ZIPs to specify alternative signature schemes.
+The encoding of $\mathsf{ik}$ begins with a byte indicating the signature scheme, which MUST be $\mathtt{0x00}$ indicating BIP 340. 
+This enables future ZIPs to specify alternative signature schemes.
 
 It is possible for the $\textit{PubKey}$ algorithm to fail with very low probability, which means that $\mathsf{IssueAuthSig.DerivePublic}$ could return $\bot$ with very low probability.
 If this happens, discard the keys and repeat with a different $\mathsf{isk}$.
@@ -179,9 +180,8 @@ Define $\mathsf{IssueAuthSig.Sign} \;{\small ⦂}\; (\mathsf{isk} \;{\small ⦂}
 
 where the $\mathsf{Sign}$ algorithm is defined in BIP 340 and $a$ denotes the auxiliary data used in BIP 340 [#bip-0340]_.
 Note that $\mathsf{IssueAuthSig.Sign}$ could return $\bot$ with very low probability.
-Note that the initial $\mathtt{0x00}$ byte in the signature is a version byte, enabling future ZIPs to
-specify alternative issuance authorization signature schemes.
-
+The encoding of the signature begins with a byte indicating the signature scheme, which MUST be $\mathtt{0x00}$ indicating BIP 340. 
+This enables future ZIPs to specify alternative signature schemes.
 
 Define $\mathsf{IssueAuthSig.Validate} \;{\small ⦂}\; (\mathsf{ik} \;{\small ⦂}\; \mathsf{IssueAuthSig.Public}) \times (M \;{\small ⦂}\; \mathsf{IssueAuthSig.Message}) \times (\text{σ} \;{\small ⦂}\; \mathsf{IssueAuthSig.Signature}) \to \mathbb{B}$ as: