diff --git a/ssl/custom_extensions.cc b/ssl/custom_extensions.cc
new file mode 100644
index 0000000..f438f73
--- /dev/null
+++ b/ssl/custom_extensions.cc
@@ -0,0 +1,267 @@
+/* Copyright (c) 2014, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include <openssl/ssl.h>
+
+#include <assert.h>
+#include <string.h>
+
+#include <openssl/bytestring.h>
+#include <openssl/err.h>
+#include <openssl/mem.h>
+#include <openssl/stack.h>
+
+#include "internal.h"
+
+
+void SSL_CUSTOM_EXTENSION_free(SSL_CUSTOM_EXTENSION *custom_extension) {
+  OPENSSL_free(custom_extension);
+}
+
+static const SSL_CUSTOM_EXTENSION *custom_ext_find(
+    STACK_OF(SSL_CUSTOM_EXTENSION) *stack,
+    unsigned *out_index, uint16_t value) {
+  for (size_t i = 0; i < sk_SSL_CUSTOM_EXTENSION_num(stack); i++) {
+    const SSL_CUSTOM_EXTENSION *ext = sk_SSL_CUSTOM_EXTENSION_value(stack, i);
+    if (ext->value == value) {
+      if (out_index != NULL) {
+        *out_index = i;
+      }
+      return ext;
+    }
+  }
+
+  return NULL;
+}
+
+/* default_add_callback is used as the |add_callback| when the user doesn't
+ * provide one. For servers, it does nothing while, for clients, it causes an
+ * empty extension to be included. */
+static int default_add_callback(SSL *ssl, unsigned extension_value,
+                                const uint8_t **out, size_t *out_len,
+                                int *out_alert_value, void *add_arg) {
+  if (ssl->server) {
+    return 0;
+  }
+  *out_len = 0;
+  return 1;
+}
+
+static int custom_ext_add_hello(SSL_HANDSHAKE *hs, CBB *extensions) {
+  SSL *const ssl = hs->ssl;
+  STACK_OF(SSL_CUSTOM_EXTENSION) *stack = ssl->ctx->client_custom_extensions;
+  if (ssl->server) {
+    stack = ssl->ctx->server_custom_extensions;
+  }
+
+  if (stack == NULL) {
+    return 1;
+  }
+
+  if (ssl->cert->enable_early_data) {
+    /* TODO(svaldez): Support Custom Extensions with 0-RTT. For now the caller
+     * is expected not to configure both together.
+     * https://crbug.com/boringssl/173. */
+    OPENSSL_PUT_ERROR(SSL, SSL_R_CUSTOM_EXTENSION_ERROR);
+    return 0;
+  }
+
+  for (size_t i = 0; i < sk_SSL_CUSTOM_EXTENSION_num(stack); i++) {
+    const SSL_CUSTOM_EXTENSION *ext = sk_SSL_CUSTOM_EXTENSION_value(stack, i);
+
+    if (ssl->server &&
+        !(hs->custom_extensions.received & (1u << i))) {
+      /* Servers cannot echo extensions that the client didn't send. */
+      continue;
+    }
+
+    const uint8_t *contents;
+    size_t contents_len;
+    int alert = SSL_AD_DECODE_ERROR;
+    CBB contents_cbb;
+
+    switch (ext->add_callback(ssl, ext->value, &contents, &contents_len, &alert,
+                              ext->add_arg)) {
+      case 1:
+        if (!CBB_add_u16(extensions, ext->value) ||
+            !CBB_add_u16_length_prefixed(extensions, &contents_cbb) ||
+            !CBB_add_bytes(&contents_cbb, contents, contents_len) ||
+            !CBB_flush(extensions)) {
+          OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
+          ERR_add_error_dataf("extension %u", (unsigned) ext->value);
+          if (ext->free_callback && 0 < contents_len) {
+            ext->free_callback(ssl, ext->value, contents, ext->add_arg);
+          }
+          return 0;
+        }
+
+        if (ext->free_callback && 0 < contents_len) {
+          ext->free_callback(ssl, ext->value, contents, ext->add_arg);
+        }
+
+        if (!ssl->server) {
+          assert((hs->custom_extensions.sent & (1u << i)) == 0);
+          hs->custom_extensions.sent |= (1u << i);
+        }
+        break;
+
+      case 0:
+        break;
+
+      default:
+        ssl3_send_alert(ssl, SSL3_AL_FATAL, alert);
+        OPENSSL_PUT_ERROR(SSL, SSL_R_CUSTOM_EXTENSION_ERROR);
+        ERR_add_error_dataf("extension %u", (unsigned) ext->value);
+        return 0;
+    }
+  }
+
+  return 1;
+}
+
+int custom_ext_add_clienthello(SSL_HANDSHAKE *hs, CBB *extensions) {
+  return custom_ext_add_hello(hs, extensions);
+}
+
+int custom_ext_parse_serverhello(SSL_HANDSHAKE *hs, int *out_alert,
+                                 uint16_t value, const CBS *extension) {
+  SSL *const ssl = hs->ssl;
+  unsigned index;
+  const SSL_CUSTOM_EXTENSION *ext =
+      custom_ext_find(ssl->ctx->client_custom_extensions, &index, value);
+
+  if (/* Unknown extensions are not allowed in a ServerHello. */
+      ext == NULL ||
+      /* Also, if we didn't send the extension, that's also unacceptable. */
+      !(hs->custom_extensions.sent & (1u << index))) {
+    OPENSSL_PUT_ERROR(SSL, SSL_R_UNEXPECTED_EXTENSION);
+    ERR_add_error_dataf("extension %u", (unsigned)value);
+    *out_alert = SSL_AD_UNSUPPORTED_EXTENSION;
+    return 0;
+  }
+
+  if (ext->parse_callback != NULL &&
+      !ext->parse_callback(ssl, value, CBS_data(extension), CBS_len(extension),
+                           out_alert, ext->parse_arg)) {
+    OPENSSL_PUT_ERROR(SSL, SSL_R_CUSTOM_EXTENSION_ERROR);
+    ERR_add_error_dataf("extension %u", (unsigned)ext->value);
+    return 0;
+  }
+
+  return 1;
+}
+
+int custom_ext_parse_clienthello(SSL_HANDSHAKE *hs, int *out_alert,
+                                 uint16_t value, const CBS *extension) {
+  SSL *const ssl = hs->ssl;
+  unsigned index;
+  const SSL_CUSTOM_EXTENSION *ext =
+      custom_ext_find(ssl->ctx->server_custom_extensions, &index, value);
+
+  if (ext == NULL) {
+    return 1;
+  }
+
+  assert((hs->custom_extensions.received & (1u << index)) == 0);
+  hs->custom_extensions.received |= (1u << index);
+
+  if (ext->parse_callback &&
+      !ext->parse_callback(ssl, value, CBS_data(extension), CBS_len(extension),
+                           out_alert, ext->parse_arg)) {
+    OPENSSL_PUT_ERROR(SSL, SSL_R_CUSTOM_EXTENSION_ERROR);
+    ERR_add_error_dataf("extension %u", (unsigned)ext->value);
+    return 0;
+  }
+
+  return 1;
+}
+
+int custom_ext_add_serverhello(SSL_HANDSHAKE *hs, CBB *extensions) {
+  return custom_ext_add_hello(hs, extensions);
+}
+
+/* MAX_NUM_CUSTOM_EXTENSIONS is the maximum number of custom extensions that
+ * can be set on an |SSL_CTX|. It's determined by the size of the bitset used
+ * to track when an extension has been sent. */
+#define MAX_NUM_CUSTOM_EXTENSIONS \
+  (sizeof(((SSL_HANDSHAKE *)NULL)->custom_extensions.sent) * 8)
+
+static int custom_ext_append(STACK_OF(SSL_CUSTOM_EXTENSION) **stack,
+                             unsigned extension_value,
+                             SSL_custom_ext_add_cb add_cb,
+                             SSL_custom_ext_free_cb free_cb, void *add_arg,
+                             SSL_custom_ext_parse_cb parse_cb,
+                             void *parse_arg) {
+  if (add_cb == NULL ||
+      0xffff < extension_value ||
+      SSL_extension_supported(extension_value) ||
+      /* Specifying a free callback without an add callback is nonsensical
+       * and an error. */
+      (*stack != NULL &&
+       (MAX_NUM_CUSTOM_EXTENSIONS <= sk_SSL_CUSTOM_EXTENSION_num(*stack) ||
+        custom_ext_find(*stack, NULL, extension_value) != NULL))) {
+    return 0;
+  }
+
+  SSL_CUSTOM_EXTENSION *ext =
+      (SSL_CUSTOM_EXTENSION *)OPENSSL_malloc(sizeof(SSL_CUSTOM_EXTENSION));
+  if (ext == NULL) {
+    return 0;
+  }
+  ext->add_callback = add_cb;
+  ext->add_arg = add_arg;
+  ext->free_callback = free_cb;
+  ext->parse_callback = parse_cb;
+  ext->parse_arg = parse_arg;
+  ext->value = extension_value;
+
+  if (*stack == NULL) {
+    *stack = sk_SSL_CUSTOM_EXTENSION_new_null();
+    if (*stack == NULL) {
+      SSL_CUSTOM_EXTENSION_free(ext);
+      return 0;
+    }
+  }
+
+  if (!sk_SSL_CUSTOM_EXTENSION_push(*stack, ext)) {
+    SSL_CUSTOM_EXTENSION_free(ext);
+    if (sk_SSL_CUSTOM_EXTENSION_num(*stack) == 0) {
+      sk_SSL_CUSTOM_EXTENSION_free(*stack);
+      *stack = NULL;
+    }
+    return 0;
+  }
+
+  return 1;
+}
+
+int SSL_CTX_add_client_custom_ext(SSL_CTX *ctx, unsigned extension_value,
+                                  SSL_custom_ext_add_cb add_cb,
+                                  SSL_custom_ext_free_cb free_cb, void *add_arg,
+                                  SSL_custom_ext_parse_cb parse_cb,
+                                  void *parse_arg) {
+  return custom_ext_append(&ctx->client_custom_extensions, extension_value,
+                           add_cb ? add_cb : default_add_callback, free_cb,
+                           add_arg, parse_cb, parse_arg);
+}
+
+int SSL_CTX_add_server_custom_ext(SSL_CTX *ctx, unsigned extension_value,
+                                  SSL_custom_ext_add_cb add_cb,
+                                  SSL_custom_ext_free_cb free_cb, void *add_arg,
+                                  SSL_custom_ext_parse_cb parse_cb,
+                                  void *parse_arg) {
+  return custom_ext_append(&ctx->server_custom_extensions, extension_value,
+                           add_cb ? add_cb : default_add_callback, free_cb,
+                           add_arg, parse_cb, parse_arg);
+}