diff --git a/internal/proto/credential.proto b/internal/proto/credential.proto
index 6edbf83b..53506e88 100644
--- a/internal/proto/credential.proto
+++ b/internal/proto/credential.proto
@@ -104,7 +104,7 @@ message SharedCredential {
 
   // Signature algorithm version. Used to determine which algorithm to use to
   // verify incoming signatures.
-  string signature_version = 16;
+  string signature_version = 16 [deprecated = true];
 
   // The HMAC of the plaintext identity token included (in encrypted form) in an
   // unsigned, extended salt, V1 advertisement.
diff --git a/internal/proto/local_credential.proto b/internal/proto/local_credential.proto
index 60203210..ea23278e 100644
--- a/internal/proto/local_credential.proto
+++ b/internal/proto/local_credential.proto
@@ -83,7 +83,7 @@ message LocalCredential {
 
   // Signature algorithm version. Used to determine which algorithm to use to
   // sign data.
-  string signature_version = 12;
+  string signature_version = 12 [deprecated = true];
 
   // Credential version used to infer the expected credential material.
   int64 credential_version = 13;