In the anonymous token RSA blind signature client, stop using RSA public exponent in key derivation to adhere to the latest version of the protocol.

Startblock:
   cl-status Boq PrivateMembershipServer contains cl/553200683
   cl-status Boq PrivateMembershipServer contains cl/552898843
PiperOrigin-RevId: 555993058

quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client.cc

diff --git a/quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client.cc b/quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client.cc
index 3738654..cbc7fb0 100644
--- a/quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client.cc
+++ b/quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client.cc
@@ -137,6 +137,7 @@
       // Empty public metadata is a valid value.
       public_metadata = input.public_metadata();
     }
+    const bool use_rsa_public_exponent = false;
     // Owned by BoringSSL.
     ANON_TOKENS_ASSIGN_OR_RETURN(
         const EVP_MD* sig_hash,
@@ -150,7 +151,7 @@
         auto rsa_bssa_blinder,
         RsaBlinder::New(rsa_public_key_proto.n(), rsa_public_key_proto.e(),
                         sig_hash, mgf1_hash, public_key_.salt_length(),
-                        /*use_rsa_public_exponent=*/true, public_metadata));
+                        use_rsa_public_exponent, public_metadata));
     ANON_TOKENS_ASSIGN_OR_RETURN(const std::string blinded_message,
                                  rsa_bssa_blinder->Blind(masked_message));
 
@@ -168,6 +169,7 @@
     blinded_token->set_key_version(public_key_.key_version());
     blinded_token->set_serialized_token(blinded_message);
     blinded_token->set_public_metadata(input.public_metadata());
+    blinded_token->set_do_not_use_rsa_public_exponent(!use_rsa_public_exponent);
     blinding_info_map_[blinded_message] = std::move(blinding_info);
   }
 
@@ -235,6 +237,12 @@
         anonymous_token.public_metadata()) {
       return absl::InvalidArgumentError(
           "Response public metadata does not match input.");
+    } else if (public_key_.public_metadata_support() &&
+               !anonymous_token.do_not_use_rsa_public_exponent()) {
+      // Bool do_not_use_rsa_public_exponent does not matter for the non-public
+      // metadata version.
+      return absl::InvalidArgumentError(
+          "Setting do_not_use_rsa_public_exponent to false is deprecated.");
     }
 
     // Unblind the blinded anonymous token to obtain the final anonymous token

quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client_test.cc

diff --git a/quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client_test.cc b/quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client_test.cc
index 5602cba..55fbbf3 100644
--- a/quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client_test.cc
+++ b/quiche/blind_sign_auth/anonymous_tokens/cpp/client/anonymous_tokens_rsa_bssa_client_test.cc
@@ -94,6 +94,7 @@
     const AnonymousTokensSignRequest& request, const RSAPrivateKey& private_key,
     bool enable_public_metadata = false) {
   AnonymousTokensSignResponse response;
+  const bool use_rsa_public_exponent = false;
   for (const auto& request_token : request.blinded_tokens()) {
     auto* response_token = response.add_anonymous_tokens();
     response_token->set_use_case(request_token.use_case());
@@ -101,13 +102,15 @@
     response_token->set_public_metadata(request_token.public_metadata());
     response_token->set_serialized_blinded_message(
         request_token.serialized_token());
+    response_token->set_do_not_use_rsa_public_exponent(
+        !use_rsa_public_exponent);
     std::optional<std::string> public_metadata = std::nullopt;
     if (enable_public_metadata) {
       public_metadata = request_token.public_metadata();
     }
     ANON_TOKENS_ASSIGN_OR_RETURN(
         std::unique_ptr<RsaBlindSigner> blind_signer,
-        RsaBlindSigner::New(private_key, /*use_rsa_public_exponent=*/true,
+        RsaBlindSigner::New(private_key, use_rsa_public_exponent,
                             public_metadata));
     ANON_TOKENS_ASSIGN_OR_RETURN(
         *response_token->mutable_serialized_token(),