Export reverse byte order function (#10040)

* Reverse byte order

* Update BUILD.bazel

* Go imports

Co-authored-by: prylabs-bulldozer[bot] <58059840+prylabs-bulldozer[bot]@users.noreply.github.com>

runtime/interop/BUILD.bazel

@@ -17,6 +17,7 @@ go_library(
         "//container/trie:go_default_library",
         "//crypto/bls:go_default_library",
         "//crypto/hash:go_default_library",
+        "//encoding/bytesutil:go_default_library",
         "//proto/prysm/v1alpha1:go_default_library",
         "//time:go_default_library",
         "@com_github_pkg_errors//:go_default_library",

runtime/interop/generate_keys.go

@@ -5,6 +5,8 @@ import (
 	"math/big"
 	"sync"
 
+	"github.com/prysmaticlabs/prysm/encoding/bytesutil"
+
 	"github.com/pkg/errors"
 	"github.com/prysmaticlabs/prysm/async"
 	"github.com/prysmaticlabs/prysm/crypto/bls"
@@ -51,7 +53,7 @@ func deterministicallyGenerateKeys(startIndex, numKeys uint64) ([]bls.SecretKey,
 		binary.LittleEndian.PutUint32(enc, uint32(i))
 		hash := hash.Hash(enc)
 		// Reverse byte order to big endian for use with big ints.
-		b := reverseByteOrder(hash[:])
+		b := bytesutil.ReverseByteOrder(hash[:])
 		num := new(big.Int)
 		num = num.SetBytes(b)
 		order := new(big.Int)
@@ -76,12 +78,3 @@ func deterministicallyGenerateKeys(startIndex, numKeys uint64) ([]bls.SecretKey,
 	}
 	return privKeys, pubKeys, nil
 }
-
-// Switch the endianness of a byte slice by reversing its order.
-func reverseByteOrder(input []byte) []byte {
-	b := input
-	for i := 0; i < len(b)/2; i++ {
-		b[i], b[len(b)-i-1] = b[len(b)-i-1], b[i]
-	}
-	return b
-}