add getDER() to folly::AsyncTransportCertificate

Summary: Some implementations of this cert will need to access a DER representation of the Cert. So adding this to the interface.

Reviewed By: mingtaoy

Differential Revision: D64149926

fbshipit-source-id: f471ef71e042111aba67277cd3b0101e34f92169

folly/io/async/AsyncTransportCertificate.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <string>
+#include <folly/Optional.h>
 
 namespace folly {
 
@@ -37,5 +38,16 @@ class AsyncTransportCertificate {
    * implementations of AsyncTransport.
    */
   virtual std::string getIdentity() const = 0;
+
+  /**
+   * Returns the DER representation of this certificate, if available.
+   *
+   * NOTE: Not every AsyncTransportCertificate implementation will
+   * have a DER representation. Whenever possible, applications should
+   * prefer to structure logic around the _identity_ that the
+   * certificate conveys (with `getIdentity()`), rather than
+   * certificate itself.
+   */
+  virtual std::optional<std::string> getDER() const = 0;
 };
 } // namespace folly

folly/io/async/ssl/OpenSSLTransportCertificate.h

@@ -36,6 +36,27 @@ class OpenSSLTransportCertificate : virtual public AsyncTransportCertificate {
    */
   virtual folly::ssl::X509UniquePtr getX509() const = 0;
 
+  virtual std::optional<std::string> getDER() const override {
+    auto x509 = getX509();
+    if (!x509) {
+      return std::nullopt;
+    }
+
+    int len = i2d_X509(x509.get(), nullptr);
+    if (len < 0) {
+      return std::nullopt;
+    }
+
+    std::string der(len, '\0');
+    auto derPtr = reinterpret_cast<unsigned char*>(der.data());
+
+    if (i2d_X509(x509.get(), &derPtr) < 0) {
+      return std::nullopt;
+    }
+
+    return der;
+  }
+
   static ssl::X509UniquePtr tryExtractX509(
       const AsyncTransportCertificate* cert) {
     auto opensslCert = dynamic_cast<const OpenSSLTransportCertificate*>(cert);