Renaming `idk` to `imk` throughout ZIP 227 (#41)

This is a consistent renaming of the issuance derivation key to call it the issuance master key.

zip-0227.rst

@@ -83,7 +83,7 @@ Specification: Issuance Keys and Issuance Authorization Signature Scheme
 
 The ZSA Protocol adds the following three keys to the key components [#protocol-addressesandkeys]_:
 
-1. The issuance derivation key, denoted as :math:`\mathsf{idk}`, as the name suggests, is the key that is used to derive the other two keys.
+1. The issuance master key, denoted as :math:`\mathsf{imk}`, is the key that is used to derive the other two keys.
 
 2. The issuance authorizing key is the key that is used to sign the issuance transaction, and is denoted as :math:`\mathsf{isk}`. This key is used to authorize the issuance of a specific Asset Identifier, and is only used by the issuer.
 
@@ -108,35 +108,35 @@ Specifically, we instantiate :math:`\mathsf{IssueAuthSig}` as :math:`\mathsf{Red
 Issuance Key Derivation
 -----------------------
 
-The issuance derivation key is generated by choosing a bit sequence uniformly at random from :math:`\mathbb{B}^{\mathbb{Y}[32]}`, like the Orchard spending key [#protocol-orchardkeycomponents]_.
+The issuance master key is generated by choosing a bit sequence uniformly at random from :math:`\mathbb{B}^{\mathbb{Y}[32]}`, like the Orchard spending key [#protocol-orchardkeycomponents]_.
 
-Issuance derivation key generation for hierarchical deterministic wallets
-`````````````````````````````````````````````````````````````````````````
+Issuance master key generation for hierarchical deterministic wallets
+`````````````````````````````````````````````````````````````````````
 
-The issuance derivation key is generated using the Orchard master key derivation procedure defined in ZIP 32 [#zip-0032-orchard-master]_. We reuse the functions defined there in what follows in this section.
+The issuance master key is generated using the Orchard master key derivation procedure defined in ZIP 32 [#zip-0032-orchard-master]_. We reuse the functions defined there in what follows in this section.
 
 Let :math:`S` be a seed byte sequence of a chosen length, which MUST be at least 32 and at most 252 bytes.
 We define the master extended issuance key :math:`m_{\mathsf{Issuance}} := \mathsf{MasterKeyGen}(\texttt{"ZIP32ZSAIssue_V1"}, S)`.
 
-As in ZIP 32 for Orchard [#zip-0032-orchard-child-key-derivation]_, we only use hardened child key derivation for the issuance derivation key. 
+As in ZIP 32 for Orchard [#zip-0032-orchard-child-key-derivation]_, we only use hardened child key derivation for the issuance master key. 
 We reuse the :math:`\mathsf{CDKsk}` function for Orchard child key derivation from ZIP 32.
 
-We use the notation of ZIP 32 [#zip-0032-orchard-key-path]_ for shielded HD paths, and define the issuance derivation key path as :math:`m_\mathsf{Issuance} / purpose' / coin\_type' / account'`. We fix the path levels as follows:
+We use the notation of ZIP 32 [#zip-0032-orchard-key-path]_ for shielded HD paths, and define the issuance master key path as :math:`m_\mathsf{Issuance} / purpose' / coin\_type' / account'`. We fix the path levels as follows:
 
 - :math:`purpose`: a constant set to :math:`227` (i.e. :math:`\texttt{0xe3}`). :math:`purpose'` is thus :math:`227'` (or :math:`\texttt{0x800000e3}`) following the BIP 43 recommendation.
 - :math:`coin\_type`: Defined as in ZIP 32 [#zip-0032-key-path-levels]_.
 - :math:`account`: fixed to index :math:`0`.
 
-From the generated :math:`(\mathsf{sk}, \mathsf{c})`, we set the issuance derivation key to be :math:`\mathsf{idk} := \mathsf{sk}`.
+From the generated :math:`(\mathsf{sk}, \mathsf{c})`, we set the issuance master key to be :math:`\mathsf{imk} := \mathsf{sk}`.
 
 Derivation of issuance authorizing key and issuance validating key
 ``````````````````````````````````````````````````````````````````
 
-The issuance authorizing key and issuance validating key are derived from the issuance derivation key in an analogous manner to the derivation of the Orchard spend authorizing key and Orchard spend validating key from the Orchard spending key [#protocol-orchardkeycomponents]_, as described below.
+The issuance authorizing key and issuance validating key are derived from the issuance master key in an analogous manner to the derivation of the Orchard spend authorizing key and Orchard spend validating key from the Orchard spending key [#protocol-orchardkeycomponents]_, as described below.
 
-- The issuance authorizing key is derived from the issuance derivation key, :math:`\mathsf{idk}`, as a private signature key. The function :math:`\mathsf{PRF^{expand}_{idk}}` is as defined in the Zcash protocol specification [#protocol-abstractprfs]_:
+- The issuance authorizing key is derived from the issuance master key, :math:`\mathsf{imk}`, as a private signature key. The function :math:`\mathsf{PRF^{expand}_{imk}}` is as defined in the Zcash protocol specification [#protocol-abstractprfs]_:
 
-.. math:: \mathsf{isk} := \mathsf{ToScalar}^{\mathsf{Orchard}}(︀ \mathsf{PRF^{expand}_{idk}}([\mathtt{0x0a}]) )
+.. math:: \mathsf{isk} := \mathsf{ToScalar}^{\mathsf{Orchard}}(︀ \mathsf{PRF^{expand}_{imk}}([\mathtt{0x0a}]) )
 
 - The issuance validating key is derived from the issuance authorizing key, :math:`\mathsf{isk}`, as a public verification key: