Add a “Random” group to the post quantum experiment.

The “Random” subgroup will alternate between offering CECPQ2 and CECPQ2b
at run-time for each connection. This will allow the tail of slow
clients to be better measured at the server. Installs in this subgroup
will not send the PQ signal extension and thus will not record
client-side metrics in the Net.SSL_Connection_Latency_PostQuantum
histogram.

Bug: 801302
Change-Id: I3ddbf65de87588e8de92b42dcf0e9a0522049a58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804049Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Auto-Submit: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696994}

net/socket/ssl_client_socket_impl.cc

@@ -22,6 +22,7 @@
 #include "base/metrics/field_trial_params.h"
 #include "base/metrics/histogram_functions.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/rand_util.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/stringprintf.h"
 #include "base/synchronization/lock.h"
@@ -345,6 +346,7 @@ class SSLClientSocketImpl::SSLContext {
                                       NID_X9_62_prime256v1, NID_secp384r1};
         SSL_CTX_set1_curves(ssl_ctx_.get(), kCurves, base::size(kCurves));
       }
+      // The "Random" group is configured on the |SSL*|, below.
 
       if (send_signal) {
         SSL_CTX_enable_pq_experiment_signal(ssl_ctx_.get());
@@ -908,6 +910,36 @@ int SSLClientSocketImpl::Init() {
 
   SSL_set_shed_handshake_config(ssl_.get(), 1);
 
+#if defined(ARCH_CPU_X86_64) || defined(ARCH_CPU_ARM64)
+  // CECPQ2b is only optimised for x86-64 and aarch64, and is too slow on
+  // other CPUs to even experiment with.
+  const std::string post_quantum_group = kPostQuantumGroup.Get();
+  // The Random group does per-connection randomisation and so is configured
+  // on the |SSL*|. Other groups are configured on the |SSL_CTX*|, above.
+  if (post_quantum_group == "Random") {
+    uint8_t coins;
+    base::RandBytes(&coins, sizeof(coins));
+    switch (coins % 3) {
+      case 0: {
+        static const int kCurves[] = {NID_CECPQ2, NID_X25519,
+                                      NID_X9_62_prime256v1, NID_secp384r1};
+        SSL_set1_curves(ssl_.get(), kCurves, base::size(kCurves));
+        break;
+      }
+
+      case 1: {
+        static const int kCurves[] = {NID_CECPQ2b, NID_X25519,
+                                      NID_X9_62_prime256v1, NID_secp384r1};
+        SSL_set1_curves(ssl_.get(), kCurves, base::size(kCurves));
+        break;
+      }
+
+      case 2:
+        break;
+    }
+  }
+#endif
+
   // TODO(https://crbug.com/775438), if |ssl_config_.privacy_mode| is enabled,
   // this should always continue with no client certificate.
   send_client_cert_ = context_->GetClientCertificate(