Adds a runtime method for fetching drand beacon randomness without ne…

…cessarily mixing in additional entropy. Expose beacon randomness through new EVM precompile.

actors/evm/src/interpreter/precompiles/fvm.rs

@@ -220,3 +220,23 @@ pub(super) fn call_actor_shared<RT: Runtime>(
 
     Ok(output)
 }
+
+/// Params:
+///
+/// | Param            | Value                     |
+/// |------------------|---------------------------|
+/// | randomness_epoch | U256 - low i64             |
+///
+/// Errors if unable to fetch randomness
+pub(super) fn get_randomness<RT: Runtime>(
+    system: &mut System<RT>,
+    input: &[u8],
+    _: PrecompileContext,
+) -> PrecompileResult {
+    let mut input_params = ValueReader::new(input);
+
+    let randomness_epoch = input_params.read_value()?;
+
+    let randomness = system.rt.get_beacon_randomness(randomness_epoch);
+    randomness.map(|r| r.to_vec()).map_err(|_| PrecompileError::InvalidInput)
+}

actors/evm/src/interpreter/precompiles/mod.rs

@@ -13,7 +13,7 @@ mod evm;
 mod fvm;
 
 use evm::{blake2f, ec_add, ec_mul, ec_pairing, ec_recover, identity, modexp, ripemd160, sha256};
-use fvm::{call_actor, call_actor_id, lookup_delegated_address, resolve_address};
+use fvm::{call_actor, call_actor_id, get_randomness, lookup_delegated_address, resolve_address};
 
 type PrecompileFn<RT> = fn(&mut System<RT>, &[u8], PrecompileContext) -> PrecompileResult;
 pub type PrecompileResult = Result<Vec<u8>, PrecompileError>;
@@ -45,7 +45,7 @@ impl<RT: Runtime> Precompiles<RT> {
         Some(resolve_address::<RT>),          // 0xfe00..01
         Some(lookup_delegated_address::<RT>), // 0xfe00..02
         Some(call_actor::<RT>),               // 0xfe00..03
-        None,                                 // 0xfe00..04 DISABLED
+        Some(get_randomness::<RT>),           // 0xfe00..04
         Some(call_actor_id::<RT>),            // 0xfe00..05
     ]);
 

runtime/src/runtime/fvm.rs

@@ -255,14 +255,7 @@ where
         rand_epoch: ChainEpoch,
         entropy: &[u8],
     ) -> Result<[u8; RANDOMNESS_LENGTH], ActorError> {
-        let digest = fvm::rand::get_beacon_randomness(rand_epoch).map_err(|e| {
-            match e {
-                ErrorNumber::LimitExceeded => {
-                    actor_error!(illegal_argument; "randomness lookback exceeded: {}", e)
-                }
-                e => actor_error!(assertion_failed; "get beacon randomness failed with an unexpected error: {}", e),
-            }
-        })?;
+        let digest = self.get_beacon_randomness(rand_epoch)?;
         Ok(draw_randomness(
             fvm::crypto::hash_blake2b,
             &digest,
@@ -272,6 +265,20 @@ where
         ))
     }
 
+    fn get_beacon_randomness(
+        &self,
+        rand_epoch: ChainEpoch,
+    ) -> Result<[u8; RANDOMNESS_LENGTH], ActorError> {
+        fvm::rand::get_beacon_randomness(rand_epoch).map_err(|e| {
+            match e {
+                ErrorNumber::LimitExceeded => {
+                    actor_error!(illegal_argument; "randomness lookback exceeded: {}", e)
+                }
+                e => actor_error!(assertion_failed; "get beacon randomness failed with an unexpected error: {}", e),
+            }
+        })
+    }
+
     fn get_state_root(&self) -> Result<Cid, ActorError> {
         Ok(fvm::sself::root()?)
     }

runtime/src/runtime/mod.rs

@@ -117,6 +117,14 @@ pub trait Runtime: Primitives + RuntimePolicy {
         entropy: &[u8],
     ) -> Result<[u8; RANDOMNESS_LENGTH], ActorError>;
 
+    /// Returns a (pseudo)random byte array drawing from the latest
+    /// beacon from a given epoch.
+    /// This randomness is not tied to any fork of the chain, and is unbiasable.
+    fn get_beacon_randomness(
+        &self,
+        rand_epoch: ChainEpoch,
+    ) -> Result<[u8; RANDOMNESS_LENGTH], ActorError>;
+
     /// Initializes the state object.
     /// This is only valid when the state has not yet been initialized.
     /// NOTE: we should also limit this to being invoked during the constructor method

runtime/src/test_utils.rs

@@ -200,6 +200,7 @@ pub struct Expectations {
     pub expect_verify_consensus_fault: Option<ExpectVerifyConsensusFault>,
     pub expect_get_randomness_tickets: VecDeque<ExpectRandomness>,
     pub expect_get_randomness_beacon: VecDeque<ExpectRandomness>,
+    pub expect_get_beacon_randomness: VecDeque<ExpectGetBeacon>,
     pub expect_batch_verify_seals: Option<ExpectBatchVerifySeals>,
     pub expect_aggregate_verify_seals: Option<ExpectAggregateVerifySeals>,
     pub expect_replica_verify: VecDeque<ExpectReplicaVerify>,
@@ -285,6 +286,11 @@ impl Expectations {
             "expect_get_randomness_beacon {:?}, not received",
             this.expect_get_randomness_beacon
         );
+        assert!(
+            this.expect_get_beacon_randomness.is_empty(),
+            "expect_get_beacon_randomness {:?}, not received",
+            this.expect_get_beacon_randomness
+        );
         assert!(
             this.expect_batch_verify_seals.is_none(),
             "expect_batch_verify_seals {:?}, not received",
@@ -422,6 +428,12 @@ pub struct ExpectRandomness {
     out: [u8; RANDOMNESS_LENGTH],
 }
 
+#[derive(Clone, Debug)]
+pub struct ExpectGetBeacon {
+    epoch: ChainEpoch,
+    out: [u8; RANDOMNESS_LENGTH],
+}
+
 #[derive(Debug)]
 pub struct ExpectBatchVerifySeals {
     input: Vec<SealVerifyInfo>,
@@ -748,6 +760,12 @@ impl MockRuntime {
         self.expectations.borrow_mut().expect_get_randomness_beacon.push_back(a);
     }
 
+    #[allow(dead_code)]
+    pub fn expect_get_beacon_randomness(&self, epoch: ChainEpoch, out: [u8; RANDOMNESS_LENGTH]) {
+        let a = ExpectGetBeacon { epoch, out };
+        self.expectations.borrow_mut().expect_get_beacon_randomness.push_back(a);
+    }
+
     #[allow(dead_code)]
     pub fn expect_batch_verify_seals(
         &self,
@@ -1070,6 +1088,26 @@ impl Runtime for MockRuntime {
         Ok(expected.out)
     }
 
+    fn get_beacon_randomness(
+        &self,
+        epoch: ChainEpoch,
+    ) -> Result<[u8; RANDOMNESS_LENGTH], ActorError> {
+        let expected = self
+            .expectations
+            .borrow_mut()
+            .expect_get_beacon_randomness
+            .pop_front()
+            .expect("unexpected call to get_randomness_from_beacon");
+
+        assert!(epoch <= *self.epoch.borrow(), "attempt to get randomness from future");
+        assert_eq!(
+            expected.epoch, epoch,
+            "unexpected epoch, expected: {:?}, actual: {:?}",
+            expected.epoch, epoch
+        );
+        Ok(expected.out)
+    }
+
     fn create<T: Serialize>(&self, obj: &T) -> Result<(), ActorError> {
         if self.state.borrow().is_some() {
             return Err(actor_error!(illegal_state; "state already constructed"));

test_vm/src/messaging.rs

@@ -549,6 +549,13 @@ impl<'invocation> Runtime for InvocationCtx<'invocation> {
         Ok(TEST_VM_RAND_ARRAY)
     }
 
+    fn get_beacon_randomness(
+        &self,
+        _rand_epoch: ChainEpoch,
+    ) -> Result<[u8; RANDOMNESS_LENGTH], ActorError> {
+        Ok(TEST_VM_RAND_ARRAY)
+    }
+
     fn get_state_root(&self) -> Result<Cid, ActorError> {
         Ok(self.v.actor(&self.to()).unwrap().state)
     }