diff --git a/docs/configuration.md b/docs/configuration.md
index 55b67bba83..82edb51418 100644
--- a/docs/configuration.md
+++ b/docs/configuration.md
@@ -129,12 +129,12 @@ db:
 
 ### Key Access Server (KAS)
 
-| Field              | Description                                                                     | Default |
-| ------------------ | ------------------------------------------------------------------------------- | ------- |
-| `enabled`          | Enable the Key Access Server                                                    | `true`  |
-| `keyring.*.kid`    | Which key id this is binding                                                    |         |
-| `keyring.*.alg`    | (Optional) Associated algorithm. (Allows reusing KID with different algorithms) |         |
-| `keyring.*.legacy` | When loading a TDF with no key identifier, this key may be used.                | `false` |
+| Field              | Description                                                                     | Default  |
+| ------------------ | ------------------------------------------------------------------------------- | -------- |
+| `enabled`          | Enable the Key Access Server                                                    | `true`   |
+| `keyring.*.kid`    | Which key id this is binding                                                    |          |
+| `keyring.*.alg`    | (Optional) Associated algorithm. (Allows reusing KID with different algorithms) |          |
+| `keyring.*.legacy` | Indicates this may be used for TDFs with no key ID; default if all unspecified. | inferred |
 
 Example:
 
@@ -179,4 +179,4 @@ services:
 | `clientsecret` | Client Credentials Secret | |
 | `tokenendpoint` | OAuth 2 Token Endpoint (Will be removed at a later time) | |
 | `rego.path` | Path to rego policy file | Leverages embedded rego policy |
-| `rego.query` | Rego query to execute in policy | `data.opentdf.entitlements.attributes` |
\ No newline at end of file
+| `rego.query` | Rego query to execute in policy | `data.opentdf.entitlements.attributes` |
diff --git a/service/kas/kas.go b/service/kas/kas.go
index 05dd2a8bb4..76deb2d903 100644
--- a/service/kas/kas.go
+++ b/service/kas/kas.go
@@ -60,6 +60,8 @@ func NewRegistration() serviceregistry.Registration {
 				}
 				deprecatedOrDefault(kasCfg.ECCertID, security.AlgorithmECP256R1)
 				deprecatedOrDefault(kasCfg.RSACertID, security.AlgorithmRSA2048)
+			default:
+				kasCfg.Keyring = append(kasCfg.Keyring, inferLegacyKeys(kasCfg.Keyring)...)
 			}
 
 			p := access.Provider{
@@ -86,3 +88,19 @@ func NewRegistration() serviceregistry.Registration {
 		},
 	}
 }
+
+// If there exists *any* legacy keys, returns empty list.
+// Otherwise, create a copy with legacy=true for all values
+func inferLegacyKeys(keys []access.CurrentKeyFor) []access.CurrentKeyFor {
+	for _, k := range keys {
+		if k.Legacy {
+			return nil
+		}
+	}
+	l := make([]access.CurrentKeyFor, len(keys))
+	for i, k := range keys {
+		l[i] = k
+		l[i].Legacy = true
+	}
+	return l
+}
diff --git a/service/kas/kidless_test.go b/service/kas/kidless_test.go
new file mode 100644
index 0000000000..02d0275576
--- /dev/null
+++ b/service/kas/kidless_test.go
@@ -0,0 +1,58 @@
+package kas
+
+import (
+	"testing"
+
+	"github.com/opentdf/platform/service/internal/security"
+	"github.com/opentdf/platform/service/kas/access"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestInferLegacyKeys_empty(t *testing.T) {
+	assert.Empty(t, inferLegacyKeys(nil))
+}
+
+func TestInferLegacyKeys_singles(t *testing.T) {
+	one := []access.CurrentKeyFor{
+		{
+			Algorithm: security.AlgorithmRSA2048,
+			KID:       "rsa",
+		},
+	}
+
+	oneLegacy := []access.CurrentKeyFor{
+		{
+			Algorithm: security.AlgorithmRSA2048,
+			KID:       "rsa",
+			Legacy:    true,
+		},
+	}
+
+	assert.Equal(t, oneLegacy, inferLegacyKeys(one))
+	assert.False(t, one[0].Legacy)
+	assert.True(t, oneLegacy[0].Legacy)
+}
+
+func TestInferLegacyKeys_Mixed(t *testing.T) {
+	in := []access.CurrentKeyFor{
+		{
+			Algorithm: security.AlgorithmRSA2048,
+			KID:       "a",
+		},
+		{
+			Algorithm: security.AlgorithmECP256R1,
+			KID:       "b",
+		},
+		{
+			Algorithm: security.AlgorithmECP256R1,
+			KID:       "c",
+			Legacy:    true,
+		},
+		{
+			Algorithm: security.AlgorithmECP256R1,
+			KID:       "d",
+		},
+	}
+
+	assert.Empty(t, inferLegacyKeys(in))
+}