diff --git a/relayer/application_relayer.go b/relayer/application_relayer.go
index 682f4772..8daecf58 100644
--- a/relayer/application_relayer.go
+++ b/relayer/application_relayer.go
@@ -209,6 +209,7 @@ func (r *ApplicationRelayer) ProcessMessage(handler messages.MessageHandler) (co
 		// TODO: do we actually want to pass the pointer here or adapt the interface?
 		signedMessage, err = r.signatureAggregator.AggregateSignaturesAppRequest(
 			unsignedMessage,
+			make([]byte, 0),
 			r.signingSubnetID,
 			r.warpQuorum.QuorumNumerator,
 		)
diff --git a/signature-aggregator/aggregator/aggregator.go b/signature-aggregator/aggregator/aggregator.go
index 9c01176c..7077732e 100644
--- a/signature-aggregator/aggregator/aggregator.go
+++ b/signature-aggregator/aggregator/aggregator.go
@@ -72,6 +72,7 @@ func NewSignatureAggregator(
 
 func (s *SignatureAggregator) AggregateSignaturesAppRequest(
 	unsignedMessage *avalancheWarp.UnsignedMessage,
+	justification []byte,
 	inputSigningSubnet ids.ID,
 	quorumPercentage uint64,
 ) (*avalancheWarp.Message, error) {
diff --git a/signature-aggregator/api/api.go b/signature-aggregator/api/api.go
index 0f48f23c..d5e4684a 100644
--- a/signature-aggregator/api/api.go
+++ b/signature-aggregator/api/api.go
@@ -25,8 +25,12 @@ const (
 // Defines a request interface for signature aggregation for a raw unsigned message.
 // Currently a copy of the `ManualWarpMessageRequest` struct in relay_message.go
 type AggregateSignaturesByRawMsgRequest struct {
-	// Required. hex-encoded message, optionally prefixed with "0x".
-	UnsignedMessage string `json:"unsigned-message"`
+	// Required: either Message or Justification must be provided
+
+	// hex-encoded message, optionally prefixed with "0x".
+	Message string `json:"message"`
+	// hex-encoded justification, optionally prefixed with "0x".
+	Justification string `json:"justification"`
 	// Optional hex or cb58 encoded signing subnet ID. If omitted will default to the subnetID of the source BlockChain
 	SigningSubnetID string `json:"signing-subnet-id"`
 	// Optional. Integer from 0 to 100 representing the percentage of the quorum that is required to sign the message
@@ -66,6 +70,15 @@ func writeJsonError(
 	}
 }
 
+func isEmptyOrZeroes(bytes []byte) bool {
+	for _, b := range bytes {
+		if b != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 func signatureAggregationAPIHandler(logger logging.Logger, aggregator *aggregator.SignatureAggregator) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		var req AggregateSignaturesByRawMsgRequest
@@ -78,25 +91,48 @@ func signatureAggregationAPIHandler(logger logging.Logger, aggregator *aggregato
 		}
 		var decodedMessage []byte
 		decodedMessage, err = hex.DecodeString(
-			strings.TrimPrefix(req.UnsignedMessage, "0x"),
+			strings.TrimPrefix(req.Message, "0x"),
 		)
 		if err != nil {
 			msg := "Could not decode message"
 			logger.Warn(
 				msg,
-				zap.String("msg", req.UnsignedMessage),
+				zap.String("msg", req.Message),
 				zap.Error(err),
 			)
 			writeJsonError(logger, w, msg)
 			return
 		}
-		unsignedMessage, err := types.UnpackWarpMessage(decodedMessage)
+		message, err := types.UnpackWarpMessage(decodedMessage)
 		if err != nil {
 			msg := "Error unpacking warp message"
 			logger.Warn(msg, zap.Error(err))
 			writeJsonError(logger, w, msg)
 			return
 		}
+
+		justification, err := hex.DecodeString(
+			strings.TrimPrefix(req.Justification, "0x"),
+		)
+		if err != nil {
+			msg := "Could not decode justification"
+			logger.Warn(
+				msg,
+				zap.String("justification", req.Justification),
+				zap.Error(err),
+			)
+			writeJsonError(logger, w, msg)
+			return
+		}
+
+		if isEmptyOrZeroes(message.Bytes()) || isEmptyOrZeroes(justification) {
+			writeJsonError(
+				logger,
+				w,
+				"Must provide either message or justification",
+			)
+		}
+
 		quorumNum := req.QuorumNum
 		if quorumNum == 0 {
 			quorumNum = defaultQuorumNum
@@ -123,7 +159,8 @@ func signatureAggregationAPIHandler(logger logging.Logger, aggregator *aggregato
 		}
 
 		signedMessage, err := aggregator.AggregateSignaturesAppRequest(
-			unsignedMessage,
+			message,
+			justification,
 			signingSubnetID,
 			quorumNum,
 		)
diff --git a/tests/signature_aggregator_api.go b/tests/signature_aggregator_api.go
index 205e3d1d..cd8ed9a9 100644
--- a/tests/signature_aggregator_api.go
+++ b/tests/signature_aggregator_api.go
@@ -56,7 +56,7 @@ func SignatureAggregatorAPI(network interfaces.LocalNetwork) {
 	time.Sleep(5 * time.Second)
 
 	reqBody := api.AggregateSignaturesByRawMsgRequest{
-		UnsignedMessage: "0x" + hex.EncodeToString(warpMessage.Bytes()),
+		Message: "0x" + hex.EncodeToString(warpMessage.Bytes()),
 	}
 
 	client := http.Client{