diff --git a/xyz/openbmc_project/Certs.errors.yaml b/xyz/openbmc_project/Certs.errors.yaml
new file mode 100644
index 0000000..323eb8c
--- /dev/null
+++ b/xyz/openbmc_project/Certs.errors.yaml
@@ -0,0 +1,3 @@
+# xyz.openbmc_project.Certs.Error.InvalidCertificate
+ - name: InvalidCertificate
+   description: Invalid certificate file.
diff --git a/xyz/openbmc_project/Certs.metadata.yaml b/xyz/openbmc_project/Certs.metadata.yaml
new file mode 100644
index 0000000..1d36c14
--- /dev/null
+++ b/xyz/openbmc_project/Certs.metadata.yaml
@@ -0,0 +1,4 @@
+- name: InvalidCertificate
+  meta:
+    - str: "REASON = %s"
+      type: string
diff --git a/xyz/openbmc_project/Certs/Install.interface.yaml b/xyz/openbmc_project/Certs/Install.interface.yaml
index 8781834..5fe9c1d 100644
--- a/xyz/openbmc_project/Certs/Install.interface.yaml
+++ b/xyz/openbmc_project/Certs/Install.interface.yaml
@@ -4,6 +4,7 @@
     - name: Install
       description: >
           Install the certificate and restart the associated services.
+          NotAllowed error thrown if certificate already exists.
       parameters:
         - name: Path
           type: string
@@ -13,4 +14,5 @@
               certificate and private key).
       errors:
         - xyz.openbmc_project.Common.Error.InternalFailure
-        - xyz.openbmc_project.Certs.Install.Error.InvalidCertificate
+        - xyz.openbmc_project.Common.Error.NotAllowed
+        - xyz.openbmc_project.Certs.Error.InvalidCertificate
diff --git a/xyz/openbmc_project/Certs/Replace.interface.yaml b/xyz/openbmc_project/Certs/Replace.interface.yaml
new file mode 100644
index 0000000..8f13b11
--- /dev/null
+++ b/xyz/openbmc_project/Certs/Replace.interface.yaml
@@ -0,0 +1,16 @@
+description: >
+    Certificate interface to replace existing certificate.
+methods:
+    - name: Replace
+      description: >
+          Replace the certificate and restart the associated services.
+      parameters:
+        - name: Path
+          type: string
+          description: >
+              Path of file that contains both the certificate public and
+              private key. (Example: a .PEM file containing both signed
+              certificate and private key).
+      errors:
+        - xyz.openbmc_project.Common.Error.InternalFailure
+        - xyz.openbmc_project.Certs.Error.InvalidCertificate