Add comment about group check caching and use new deserialization

blscurve/blst/bls_sig_min_pubkey_size_pop.nim

@@ -16,7 +16,14 @@
 #   approach, since the size of (PK_1, ..., PK_n, signature) is
 #   dominated by the public keys even for small n.
 
-# We expose the same API as nim-blscurve
+# We expose the same API as MIRACL
+#
+# Design:
+# - We check public keys and signatures at deserialization
+#   - non-zero
+#   - in the correct subgroup
+#   The primitives called assume that input are already subgroup-checked
+#   and so do not call "KeyValidate" again in verification procs.
 
 import
   # Status libraries
@@ -166,7 +173,7 @@ func fromBytes*(
       return false
     let pa = cast[ptr array[L, byte]](raw[0].unsafeAddr)
     obj.scalar.blst_scalar_from_bendian(pa[])
-  if obj.vec_is_zero():
+  if obj.vec_is_zero() or not obj.blst_sk_check():
     return false
   return true
 

blscurve/blst/blst_abi.nim

@@ -140,6 +140,7 @@ proc blst_bendian_from_scalar*(ret: var array[32, byte]; a: blst_scalar)
 proc blst_scalar_from_lendian*(ret: var blst_scalar; a: array[32, byte])
 proc blst_lendian_from_scalar*(ret: var array[32, byte]; a: blst_scalar)
 proc blst_scalar_fr_check*(a: blst_scalar): CTBool
+proc blst_sk_check*(a: blst_scalar): CTBool
 
 # BLS12-381-specific Fr operations (Modulo curve order)
 proc blst_fr_add*(ret: var blst_fr; a: blst_fr; b: blst_fr)