Add Fork Digest Helper

beacon-chain/core/helpers/signing_root.go

@@ -4,6 +4,7 @@ import (
 	"github.com/prysmaticlabs/go-ssz"
 	p2ppb "github.com/prysmaticlabs/prysm/proto/beacon/p2p/v1"
 	pb "github.com/prysmaticlabs/prysm/proto/beacon/p2p/v1"
+	"github.com/prysmaticlabs/prysm/shared/bytesutil"
 	"github.com/prysmaticlabs/prysm/shared/params"
 )
 
@@ -99,3 +100,21 @@ func computeForkDataRoot(version []byte, root []byte) ([32]byte, error) {
 	}
 	return r, nil
 }
+
+// ComputeForkDigest returns the fork for the current version and genesis validator root
+//
+// Spec pseudocode definition:
+//	def compute_fork_digest(current_version: Version, genesis_validators_root: Root) -> ForkDigest:
+//    """
+//    Return the 4-byte fork digest for the ``current_version`` and ``genesis_validators_root``.
+//    This is a digest primarily used for domain separation on the p2p layer.
+//    4-bytes suffices for practical separation of forks/chains.
+//    """
+//    return ForkDigest(compute_fork_data_root(current_version, genesis_validators_root)[:4])
+func ComputeForkDigest(version []byte, genesisValidatorsRoot []byte) ([4]byte, error) {
+	dataRoot, err := computeForkDataRoot(version, genesisValidatorsRoot)
+	if err != nil {
+		return [4]byte{}, nil
+	}
+	return bytesutil.ToBytes4(dataRoot[:]), nil
+}

beacon-chain/core/helpers/signing_root_test.go

@@ -34,3 +34,24 @@ func TestComputeDomain_OK(t *testing.T) {
 		}
 	}
 }
+
+func TestComputeForkDigest_OK(t *testing.T) {
+	tests := []struct {
+		version []byte
+		root    [32]byte
+		result  [4]byte
+	}{
+		{version: []byte{'A', 'B', 'C', 'D'}, root: [32]byte{'i', 'o', 'p'}, result: [4]byte{0x69, 0x5c, 0x26, 0x47}},
+		{version: []byte{'i', 'm', 'n', 'a'}, root: [32]byte{'z', 'a', 'b'}, result: [4]byte{0x1c, 0x38, 0x84, 0x58}},
+		{version: []byte{'b', 'w', 'r', 't'}, root: [32]byte{'r', 'd', 'c'}, result: [4]byte{0x83, 0x34, 0x38, 0x88}},
+	}
+	for _, tt := range tests {
+		digest, err := ComputeForkDigest(tt.version, tt.root[:])
+		if err != nil {
+			t.Error(err)
+		}
+		if digest != tt.result {
+			t.Errorf("wanted domain version: %#x, got: %#x", digest, tt.result)
+		}
+	}
+}