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docs: fix up key derivation chart #2968

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Sep 5, 2023
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docs: fix up key derivation chart #2968

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9 changes: 6 additions & 3 deletions docs/protocol/src/protocol/addresses_keys.md
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Expand Up @@ -47,11 +47,14 @@ flowchart BT
subgraph SK[Spending Key]
direction LR;
ask;
seed;
spend_key_bytes;
end;

seed --> ask;
seed --> nk;
BIP44(BIP44 Seed Phrase) --> spend_key_bytes;
BIP39(Legacy Raw BIP39) --> spend_key_bytes;

spend_key_bytes --> ask;
spend_key_bytes --> nk;

ask --> ak;

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14 changes: 9 additions & 5 deletions docs/protocol/src/protocol/addresses_keys/spend_key.md
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Expand Up @@ -9,8 +9,10 @@ PBKDF2 with:
* `mnemonic` concatenated with an index used as the salt, i.e. the
default spend authority is derived using the salt `mnemonic0`

TODO: describe new BIP44 derivation and change raw BIP39 derivation to be described as "legacy".

The root key material for a particular spend authority is the 32-byte
`seed` derived as above from the seed phrase. The `seed` value is used to derive
`spend_key_bytes` derived as above from the seed phrase. The `spend_key_bytes` value is used to derive
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I changed this to eliminate references to seed; I think it's confusing since although it's the "seed" of the Penumbra-specific part of the key heirarchy, it's not necessarily the seed in the sense of the seed phrase (i.e., the ultimate root of capability).


* $\mathsf{ask} \in \mathbb F_r$, the *spend authorization key*, and
* $\mathsf{nk} \in \mathbb F_q$, the *nullifier key*,
Expand All @@ -20,12 +22,14 @@ personalization `label`, key `key`, and input `input`. Define
`from_le_bytes(bytes)` as the function that interprets its input bytes as an
integer in little-endian order. Then
```
ask = from_le_bytes(prf_expand("Penumbra_ExpndSd", seed, 0)) mod r
nk = from_le_bytes(prf_expand("Penumbra_ExpndSd", seed, 0)) mod q
ask = from_le_bytes(prf_expand("Penumbra_ExpndSd", spend_key_bytes, 0)) mod r
nk = from_le_bytes(prf_expand("Penumbra_ExpndSd", spend_key_bytes, 0)) mod q
```

The *spending key* consists of `seed` and `ask`. When using a hardware wallet
or similar custody mechanism, the spending key remains on the device.
The *spending key* consists of `spend_key_bytes` and `ask`. (Since `ask` is
derived from `spend_key_bytes`, only the `spend_key_bytes` need to be stored,
but the `ask` is considered part of the spending key). When using a hardware
wallet or similar custody mechanism, the spending key remains on the device.

The spend authorization key $\mathsf{ask}$ is used as a `decaf377-rdsa` signing
key.[^1] The corresponding verification key is the *spend verification key*
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