diff --git a/proto/aes_cmac.proto b/proto/aes_cmac.proto
index d1de8e3..b5dbd94 100644
--- a/proto/aes_cmac.proto
+++ b/proto/aes_cmac.proto
@@ -29,7 +29,7 @@ message AesCmacParams {
 // key_type: type.googleapis.com/google.crypto.tink.AesCmacKey
 message AesCmacKey {
   uint32 version = 1;
-  bytes key_value = 2;
+  bytes key_value = 2;  // Placeholder for ctype.
   AesCmacParams params = 3;
 }
 
diff --git a/proto/aes_cmac_prf.proto b/proto/aes_cmac_prf.proto
index dd8dcae..94fe509 100644
--- a/proto/aes_cmac_prf.proto
+++ b/proto/aes_cmac_prf.proto
@@ -25,7 +25,7 @@ option go_package = "github.com/tink-crypto/tink-go/v2/proto/aes_cmac_prf_go_pro
 // key_type: type.googleapis.com/google.crypto.tink.AesCmacPrfKey
 message AesCmacPrfKey {
   uint32 version = 1;
-  bytes key_value = 2;
+  bytes key_value = 2;  // Placeholder for ctype.
 }
 
 message AesCmacPrfKeyFormat {
diff --git a/proto/aes_ctr.proto b/proto/aes_ctr.proto
index a873097..66c0ca1 100644
--- a/proto/aes_ctr.proto
+++ b/proto/aes_ctr.proto
@@ -35,5 +35,5 @@ message AesCtrKeyFormat {
 message AesCtrKey {
   uint32 version = 1;
   AesCtrParams params = 2;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
diff --git a/proto/aes_ctr_hmac_streaming.proto b/proto/aes_ctr_hmac_streaming.proto
index 599e267..7927bde 100644
--- a/proto/aes_ctr_hmac_streaming.proto
+++ b/proto/aes_ctr_hmac_streaming.proto
@@ -42,5 +42,5 @@ message AesCtrHmacStreamingKeyFormat {
 message AesCtrHmacStreamingKey {
   uint32 version = 1;
   AesCtrHmacStreamingParams params = 2;
-  bytes key_value = 3;  // the main key, aka. "ikm", input key material
+  bytes key_value = 3;  // Placeholder for ctype.  // the main key, aka. "ikm", input key material
 }
diff --git a/proto/aes_gcm.proto b/proto/aes_gcm.proto
index 1908c4a..3724b0a 100644
--- a/proto/aes_gcm.proto
+++ b/proto/aes_gcm.proto
@@ -63,5 +63,5 @@ message AesGcmKeyFormat {
 
 message AesGcmKey {
   uint32 version = 1;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
diff --git a/proto/aes_gcm_hkdf_streaming.proto b/proto/aes_gcm_hkdf_streaming.proto
index a436abb..c5e4023 100644
--- a/proto/aes_gcm_hkdf_streaming.proto
+++ b/proto/aes_gcm_hkdf_streaming.proto
@@ -42,5 +42,5 @@ message AesGcmHkdfStreamingKeyFormat {
 message AesGcmHkdfStreamingKey {
   uint32 version = 1;
   AesGcmHkdfStreamingParams params = 2;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
diff --git a/proto/aes_gcm_siv.proto b/proto/aes_gcm_siv.proto
index c663aee..fee87d7 100644
--- a/proto/aes_gcm_siv.proto
+++ b/proto/aes_gcm_siv.proto
@@ -32,5 +32,5 @@ message AesGcmSivKeyFormat {
 // key_type: type.googleapis.com/google.crypto.tink.AesGcmSivKey
 message AesGcmSivKey {
   uint32 version = 1;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
diff --git a/proto/aes_siv.proto b/proto/aes_siv.proto
index 0f605ea..2c5e013 100644
--- a/proto/aes_siv.proto
+++ b/proto/aes_siv.proto
@@ -40,5 +40,5 @@ message AesSivKeyFormat {
 message AesSivKey {
   uint32 version = 1;
   // First half is AES-CTR key, second is AES-SIV.
-  bytes key_value = 2;
+  bytes key_value = 2;  // Placeholder for ctype.
 }
diff --git a/proto/chacha20_poly1305.proto b/proto/chacha20_poly1305.proto
index 86f8545..8f26821 100644
--- a/proto/chacha20_poly1305.proto
+++ b/proto/chacha20_poly1305.proto
@@ -29,5 +29,5 @@ message ChaCha20Poly1305KeyFormat {}
 // at https://tools.ietf.org/html/rfc7539#section-2.8.
 message ChaCha20Poly1305Key {
   uint32 version = 1;
-  bytes key_value = 2;
+  bytes key_value = 2;  // Placeholder for ctype.
 }
diff --git a/proto/ecdsa.proto b/proto/ecdsa.proto
index 89dfebb..e89783e 100644
--- a/proto/ecdsa.proto
+++ b/proto/ecdsa.proto
@@ -74,7 +74,7 @@ message EcdsaPrivateKey {
   EcdsaPublicKey public_key = 2;
   // Unsigned big integer in bigendian representation.
   // Required.
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
 
 message EcdsaKeyFormat {
diff --git a/proto/ecies_aead_hkdf.proto b/proto/ecies_aead_hkdf.proto
index 488b2f0..fa7eca5 100644
--- a/proto/ecies_aead_hkdf.proto
+++ b/proto/ecies_aead_hkdf.proto
@@ -105,7 +105,7 @@ message EciesAeadHkdfPrivateKey {
   EciesAeadHkdfPublicKey public_key = 2;
 
   // Required.
-  bytes key_value = 3;  // Big integer in bigendian representation.
+  bytes key_value = 3;  // Placeholder for ctype.  // Big integer in bigendian representation.
 }
 
 message EciesAeadHkdfKeyFormat {
diff --git a/proto/ed25519.proto b/proto/ed25519.proto
index 763db39..8d6234a 100644
--- a/proto/ed25519.proto
+++ b/proto/ed25519.proto
@@ -36,7 +36,7 @@ message Ed25519PublicKey {
   // The public key is 32 bytes, encoded according to
   // https://tools.ietf.org/html/rfc8032#section-5.1.2.
   // Required.
-  bytes key_value = 2;
+  bytes key_value = 2;  // Placeholder for ctype.
 }
 
 // key_type: type.googleapis.com/google.crypto.tink.Ed25519PrivateKey
@@ -46,7 +46,7 @@ message Ed25519PrivateKey {
   // The private key is 32 bytes of cryptographically secure random data.
   // See https://tools.ietf.org/html/rfc8032#section-5.1.5.
   // Required.
-  bytes key_value = 2;
+  bytes key_value = 2;  // Placeholder for ctype.
   // The corresponding public key.
   Ed25519PublicKey public_key = 3;
 }
diff --git a/proto/hkdf_prf.proto b/proto/hkdf_prf.proto
index e2c4f4e..3b98d06 100644
--- a/proto/hkdf_prf.proto
+++ b/proto/hkdf_prf.proto
@@ -38,7 +38,7 @@ message HkdfPrfParams {
 message HkdfPrfKey {
   uint32 version = 1;
   HkdfPrfParams params = 2;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
 
 message HkdfPrfKeyFormat {
diff --git a/proto/hmac.proto b/proto/hmac.proto
index 8f0f9e5..bd268f0 100644
--- a/proto/hmac.proto
+++ b/proto/hmac.proto
@@ -33,7 +33,7 @@ message HmacParams {
 message HmacKey {
   uint32 version = 1;
   HmacParams params = 2;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
 
 message HmacKeyFormat {
diff --git a/proto/hmac_prf.proto b/proto/hmac_prf.proto
index 5338951..159d3db 100644
--- a/proto/hmac_prf.proto
+++ b/proto/hmac_prf.proto
@@ -32,7 +32,7 @@ message HmacPrfParams {
 message HmacPrfKey {
   uint32 version = 1;
   HmacPrfParams params = 2;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
 
 message HmacPrfKeyFormat {
diff --git a/proto/jwt_ecdsa.proto b/proto/jwt_ecdsa.proto
index 25b6980..e8f1be2 100644
--- a/proto/jwt_ecdsa.proto
+++ b/proto/jwt_ecdsa.proto
@@ -52,7 +52,7 @@ message JwtEcdsaPrivateKey {
   uint32 version = 1;
   JwtEcdsaPublicKey public_key = 2;
   // Unsigned big integer in bigendian representation.
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }
 
 message JwtEcdsaKeyFormat {
diff --git a/proto/jwt_hmac.proto b/proto/jwt_hmac.proto
index 0053002..c629323 100644
--- a/proto/jwt_hmac.proto
+++ b/proto/jwt_hmac.proto
@@ -34,7 +34,7 @@ enum JwtHmacAlgorithm {
 message JwtHmacKey {
   uint32 version = 1;
   JwtHmacAlgorithm algorithm = 2;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 
   // Optional, custom kid header value to be used with "RAW" keys.
   // "TINK" keys with this value set will be rejected.
diff --git a/proto/xchacha20_poly1305.proto b/proto/xchacha20_poly1305.proto
index 530e573..d080b77 100644
--- a/proto/xchacha20_poly1305.proto
+++ b/proto/xchacha20_poly1305.proto
@@ -29,5 +29,5 @@ message XChaCha20Poly1305KeyFormat {
 // key_type: type.googleapis.com/google.crypto.tink.XChaCha20Poly1305Key
 message XChaCha20Poly1305Key {
   uint32 version = 1;
-  bytes key_value = 3;
+  bytes key_value = 3;  // Placeholder for ctype.
 }