Split blob key verification into separate function

Signed-off-by: litt3 <[email protected]>

api/clients/v2/disperser_client.go

@@ -213,24 +213,44 @@ func (c *disperserClient) DisperseBlob(
 		return nil, [32]byte{}, err
 	}
 
+	if verifyReceivedBlobKey(blobHeader, reply) != nil {
+		return nil, [32]byte{}, fmt.Errorf("verify received blob key: %w", err)
+	}
+
+	return &blobStatus, corev2.BlobKey(reply.GetBlobKey()), nil
+}
+
+// verifyReceivedBlobKey computes the BlobKey from the BlobHeader which was sent to the disperser, and compares it with
+// the BlobKey which was returned by the disperser in the DisperseBlobReply
+//
+// A successful verification guarantees that the disperser didn't make any modifications to the BlobHeader that it
+// received from this client.
+//
+// This function returns nil if the verification succeeds, and otherwise returns an error describing the failure
+func verifyReceivedBlobKey(
+	// the blob header which was constructed locally and sent to the disperser
+	blobHeader *corev2.BlobHeader,
+	// the reply received back from the disperser
+	disperserReply *disperser_rpc.DisperseBlobReply) error {
+
 	actualBlobKey, err := blobHeader.BlobKey()
 	if err != nil {
 		// this shouldn't be possible, since the blob key has already been used when signing dispersal
-		return nil, [32]byte{}, fmt.Errorf("computing blob key: %w", err)
+		return fmt.Errorf("computing blob key: %w", err)
 	}
 
-	blobKeyFromDisperser, err := corev2.BytesToBlobKey(reply.GetBlobKey())
+	blobKeyFromDisperser, err := corev2.BytesToBlobKey(disperserReply.GetBlobKey())
 	if err != nil {
-		return nil, [32]byte{}, fmt.Errorf("converting returned bytes to blob key: %w", err)
+		return fmt.Errorf("converting returned bytes to blob key: %w", err)
 	}
 
 	if actualBlobKey != blobKeyFromDisperser {
-		return nil, [32]byte{}, fmt.Errorf(
+		return fmt.Errorf(
 			"blob key returned by disperser (%v) doesn't match blob which was dispersed (%v)",
 			blobKeyFromDisperser, actualBlobKey)
 	}
 
-	return &blobStatus, corev2.BlobKey(reply.GetBlobKey()), nil
+	return nil
 }
 
 // GetBlobStatus returns the status of a blob with the given blob key.