Update keygen in accordance with latest spec

For details on these changes, see urbit/fora-posts#9.

lib/keygen.hoon

@@ -28,6 +28,14 @@
   %+  to-seed:bip39  nom
   (trip (fall pass ''))
 ::
+++  derive-network-seed
+  |=  [mngs=@ rev=@ud]
+  ^-  @ux
+  =+  (seed:ds 64^mngs (weld "network" (a-co:co rev)))
+  ?:  =(0 rev)  -
+  ::  hash again to prevent length extension attacks
+  (sha-256l:sha 32 -)
+::
 ++  full-wallet-from-ticket
   ::  who:    username
   ::  ticket: password
@@ -62,7 +70,7 @@
     %+  to-seed:bip39
       seed:management
     (trip (fall pass ''))
-  =+  sed=(seed:ds 64^mad (weld "network" (a-co:co rev)))
+  =+  sed=(derive-network-seed mad rev)
   [rev sed (urbit:ds sed)]
 ::
 ++  ds                                                  ::  derive from raw seed

tests/lib/keygen.hoon

@@ -334,4 +334,15 @@
           ::        ralmed-dinfeb-fopbyr-sanbet-sovmyl-dozsut-
           ::        mogsyx-mapwyc-sorrup-ricnec-marnys-lignex
         [6 `'froot loops']
+::
+++  test-derive-network-seed-rev-0
+  %+  expect-eq
+    !>  (seed:ds 64^0x5eed "network0")
+    !>  (derive-network-seed 0x5eed 0)
+::
+++  test-derive-network-seed-rev-up
+  %+  expect-eq
+    !>  |
+    !>  .=  (seed:ds 64^0x5eed "network1")
+        (derive-network-seed 0x5eed 1)
 --