diff --git a/protocol/common.go b/protocol/common.go
new file mode 100644
index 0000000..ba641d1
--- /dev/null
+++ b/protocol/common.go
@@ -0,0 +1,25 @@
+package protocol
+
+import (
+	"fmt"
+
+	"github.com/iden3/iden3comm/v2"
+)
+
+// GoalCode represents the goal code of targeted protocol message
+type GoalCode string
+
+const (
+	// ProposalRequest is the goal code for proposal request
+	ProposalRequest GoalCode = "iden3comm.credentials.v1-1.proposal-request"
+)
+
+// GetProtocolMessageTypeByGoalCode returns the protocol message type by goal code
+func GetProtocolMessageTypeByGoalCode(goalCode GoalCode) (iden3comm.ProtocolMessage, error) {
+	switch goalCode {
+	case ProposalRequest:
+		return CredentialProposalRequestMessageType, nil
+	default:
+		return "", fmt.Errorf("unknown goal code %s", goalCode)
+	}
+}
diff --git a/protocol/transparent_payment.go b/protocol/transparent_payment.go
index f7d755e..9597657 100644
--- a/protocol/transparent_payment.go
+++ b/protocol/transparent_payment.go
@@ -28,7 +28,7 @@ type TransparentPaymentInstructionMessage struct {
 
 // TransparentPaymentInstructionMessageBody represents the body of the transparent payment instruction message
 type TransparentPaymentInstructionMessageBody struct {
-	GoalCode    string                 `json:"goal_code"`
+	GoalCode    GoalCode               `json:"goal_code"`
 	DID         string                 `json:"did,omitempty"`
 	Credentials []CredentialSchemaInfo `json:"credentials"`
 	PaymentData TransparentPaymentData `json:"paymentData"`