Update to EIP 2333's BLS v4 support

eth2deposit/key_handling/key_derivation/tree.py

@@ -48,13 +48,18 @@ def _HKDF_mod_r(*, IKM: bytes, key_info: bytes=b'') -> int:
     Ref: https://github.com/ethereum/EIPs/blob/master/EIPS/eip-2333.md#hkdf_mod_r
     """
     L = 48  # `ceil((3 * ceil(log2(r))) / 16)`, where `r` is the order of the BLS 12-381 curve
-    okm = HKDF(
-        salt=b'BLS-SIG-KEYGEN-SALT-',
-        IKM=IKM + b'\x00',  # add postfix `I2OSP(0, 1)`
-        L=L,
-        info=key_info + L.to_bytes(2, 'big'),
-    )
-    return int.from_bytes(okm, byteorder='big') % bls_curve_order
+    salt = b'BLS-SIG-KEYGEN-SALT-'
+    SK = 0
+    while SK == 0:
+        salt = SHA256(salt)
+        okm = HKDF(
+            salt=salt,
+            IKM=IKM + b'\x00',  # add postfix `I2OSP(0, 1)`
+            L=L,
+            info=key_info + L.to_bytes(2, 'big'),
+        )
+        SK = int.from_bytes(okm, byteorder='big') % bls_curve_order
+    return SK
 
 
 def derive_child_SK(*, parent_SK: int, index: int) -> int:

tests/test_key_handling/test_key_derivation/test_tree.py

@@ -16,7 +16,7 @@ test_vector_filefolder = os.path.join(os.getcwd(), 'tests', 'test_key_handling',
 with open(test_vector_filefolder, 'r') as f:
     test_vectors = json.load(f)['kdf_tests']
 
-
+@pytest.mark.skip(reason="py_ecc doesn't support BLS v4 yet")
 @pytest.mark.parametrize(
     'test',
     test_vectors
@@ -26,6 +26,7 @@ def test_hkdf_mod_r(test) -> None:
     assert bls.KeyGen(seed) == _HKDF_mod_r(IKM=seed)
 
 
+@pytest.mark.skip(reason="py_ecc doesn't support BLS v4 yet")
 @pytest.mark.parametrize(
     'seed',
     [b'\x00' * 32]

tests/test_key_handling/test_key_derivation/test_vectors/tree_kdf.json

@@ -2,27 +2,27 @@
     "kdf_tests": [
         {
             "seed": "c55257c360c07c72029aebc1b53c05ed0362ada38ead3e3e9efa3708e53495531f09a6987599d18264c1e1c92f2cf141630c7a3c4ab7c81b2f001698e7463b04",
-            "master_SK": 5399117110774477986698372024995405256382522670366369834617409486544348441851,
+            "master_SK": 6083874454709270928345386274498605044986640685124978867557563392430687146096,
             "child_index": 0,
-            "child_SK": 11812940737387919040225825939013910852517748782307378293770044673328955938106
+            "child_SK": 20397789859736650942317412262472558107875392172444076792671091975210932703118
         },
         {
             "seed": "3141592653589793238462643383279502884197169399375105820974944592",
-            "master_SK": 36167147331491996618072159372207345412841461318189449162487002442599770291484,
+            "master_SK": 29757020647961307431480504535336562678282505419141012933316116377660817309383,
             "child_index": 3141592653,
-            "child_SK": 41787458189896526028601807066547832426569899195138584349427756863968330588237
+            "child_SK": 25457201688850691947727629385191704516744796114925897962676248250929345014287
         },
         {
             "seed": "0099FF991111002299DD7744EE3355BBDD8844115566CC55663355668888CC00",
-            "master_SK": 13904094584487173309420026178174172335998687531503061311232927109397516192843,
+            "master_SK": 27580842291869792442942448775674722299803720648445448686099262467207037398656,
             "child_index": 4294967295,
-            "child_SK": 12482522899285304316694838079579801944734479969002030150864436005368716366140
+            "child_SK": 29358610794459428860402234341874281240803786294062035874021252734817515685787
         },
         {
             "seed": "d4e56740f876aef8c010b86a40d5f56745a118d0906a34e69aec8c0db1cb8fa3",
-            "master_SK": 44010626067374404458092393860968061149521094673473131545188652121635313364506,
+            "master_SK": 19022158461524446591288038168518313374041767046816487870552872741050760015818,
             "child_index": 42,
-            "child_SK": 4011524214304750350566588165922015929937602165683407445189263506512578573606
+            "child_SK": 31372231650479070279774297061823572166496564838472787488249775572789064611981
         }
     ]
 }