Allow to get key from SPAKE2 after the first round trip.

crypto/p224_spake.cc

Index: crypto/p224_spake.cc
diff --git a/crypto/p224_spake.cc b/crypto/p224_spake.cc
index 31109a43503fe821d525bb055c9aab570292c6ff..f20b10c5ec6f0f78cf14f0e55fa52eea8cf8155f 100644
--- a/crypto/p224_spake.cc
+++ b/crypto/p224_spake.cc
@@ -240,8 +240,16 @@ const std::string& P224EncryptedKeyExchange::error() const {
   return error_;
 }
 
-const std::string& P224EncryptedKeyExchange::GetKey() {
+const std::string& P224EncryptedKeyExchange::GetKey() const {
   DCHECK_EQ(state_, kStateDone);
+  return GetUnverifiedKey();
+}
+
+const std::string& P224EncryptedKeyExchange::GetUnverifiedKey() const {
+  // Key is already final when state is kStateSendHash. Subsequent states are
+  // used only for verification of the key. Some users may combine verification
+  // with sending verifiable data instead of |expected_authenticator_|.
+  DCHECK_GE(state_, kStateSendHash);
   return key_;
 }