Check kzg commitment for beacon-api propose block (#14702)

Co-authored-by: Preston Van Loon <pvanloon@offchainlabs.com>

testing/util/deneb.go

@@ -1,10 +1,14 @@
 package util
 
 import (
+	"bytes"
+	"crypto/sha256"
 	"encoding/binary"
 	"math/big"
 	"testing"
 
+	"github.com/consensys/gnark-crypto/ecc/bls12-381/fr"
+	GoKZG "github.com/crate-crypto/go-kzg-4844"
 	"github.com/ethereum/go-ethereum/common"
 	gethTypes "github.com/ethereum/go-ethereum/core/types"
 	"github.com/prysmaticlabs/prysm/v5/beacon-chain/core/signing"
@@ -19,6 +23,7 @@ import (
 	ethpb "github.com/prysmaticlabs/prysm/v5/proto/prysm/v1alpha1"
 	"github.com/prysmaticlabs/prysm/v5/testing/require"
 	"github.com/prysmaticlabs/prysm/v5/time/slots"
+	"github.com/sirupsen/logrus"
 )
 
 type DenebBlockGeneratorOption func(*denebBlockGenerator)
@@ -197,3 +202,36 @@ func ExtendBlocksPlusBlobs(t *testing.T, blks []blocks.ROBlock, size int) ([]blo
 
 	return blks, blobs
 }
+
+func deterministicRandomness(seed int64) [32]byte {
+	// Converts an int64 to a byte slice
+	buf := new(bytes.Buffer)
+	err := binary.Write(buf, binary.BigEndian, seed)
+	if err != nil {
+		logrus.WithError(err).Error("Failed to write int64 to bytes buffer")
+		return [32]byte{}
+	}
+	bytes := buf.Bytes()
+
+	return sha256.Sum256(bytes)
+}
+
+// Returns a serialized random field element in big-endian
+func GetRandFieldElement(seed int64) [32]byte {
+	bytes := deterministicRandomness(seed)
+	var r fr.Element
+	r.SetBytes(bytes[:])
+
+	return GoKZG.SerializeScalar(r)
+}
+
+// Returns a random blob using the passed seed as entropy
+func GetRandBlob(seed int64) GoKZG.Blob {
+	var blob GoKZG.Blob
+	bytesPerBlob := GoKZG.ScalarsPerBlob * GoKZG.SerializedScalarSize
+	for i := 0; i < bytesPerBlob; i += GoKZG.SerializedScalarSize {
+		fieldElementBytes := GetRandFieldElement(seed + int64(i))
+		copy(blob[i:i+GoKZG.SerializedScalarSize], fieldElementBytes[:])
+	}
+	return blob
+}