LibTLS: Implement the DHE_RSA key exchange algorithm

This adds two methods, handle_dhe_rsa_server_key_exchange and
build_dhe_rsa_pre_master_secret, to TLSv12 and a struct,
server_diffie_hellman_params, to Context, which are used to implement
the DHE_RSA key exchange algorithm. This grants us the benefits of
forward secrecy and access to sites which support DHE_RSA.

It is worth noting that the signature of the server provided
Diffie-Hellman parameters is not currently validated. This will need to
be addressed to prevent man-in-the-middle attacks.

Userland/Libraries/LibTLS/HandshakeClient.cpp

@@ -7,6 +7,8 @@
 #include <AK/Debug.h>
 #include <AK/Random.h>
 #include <LibCrypto/ASN1/DER.h>
+#include <LibCrypto/BigInt/UnsignedBigInteger.h>
+#include <LibCrypto/NumberTheory/ModularFunctions.h>
 #include <LibCrypto/PK/Code/EMSA_PSS.h>
 #include <LibTLS/TLSv12.h>
 
@@ -224,6 +226,43 @@ void TLSv12::build_rsa_pre_master_secret(PacketBuilder& builder)
     builder.append(outbuf);
 }
 
+void TLSv12::build_dhe_rsa_pre_master_secret(PacketBuilder& builder)
+{
+    auto& dh = m_context.server_diffie_hellman_params;
+    auto dh_p = Crypto::UnsignedBigInteger::import_data(dh.p.data(), dh.p.size());
+    auto dh_g = Crypto::UnsignedBigInteger::import_data(dh.g.data(), dh.g.size());
+    auto dh_Ys = Crypto::UnsignedBigInteger::import_data(dh.Ys.data(), dh.Ys.size());
+    auto dh_key_size = dh.p.size();
+
+    auto dh_random = Crypto::NumberTheory::random_number(0, dh_p);
+    auto dh_Yc = Crypto::NumberTheory::ModularPower(dh_g, dh_random, dh_p);
+    auto dh_Yc_bytes = ByteBuffer::create_uninitialized(dh_key_size);
+    dh_Yc.export_data(dh_Yc_bytes);
+
+    auto premaster_key = Crypto::NumberTheory::ModularPower(dh_Ys, dh_random, dh_p);
+    m_context.premaster_key = ByteBuffer::create_uninitialized(dh_key_size);
+    premaster_key.export_data(m_context.premaster_key, true);
+
+    dh.p.clear();
+    dh.g.clear();
+    dh.Ys.clear();
+
+    if constexpr (TLS_DEBUG) {
+        dbgln("dh_random: {}", dh_random.to_base(16));
+        dbgln("dh_Yc: {:hex-dump}", (ReadonlyBytes)dh_Yc_bytes);
+        dbgln("premaster key: {:hex-dump}", (ReadonlyBytes)m_context.premaster_key);
+    }
+
+    if (!compute_master_secret_from_pre_master_secret(48)) {
+        dbgln("oh noes we could not derive a master key :(");
+        return;
+    }
+
+    builder.append_u24(dh_key_size + 2);
+    builder.append((u16)dh_key_size);
+    builder.append(dh_Yc_bytes);
+}
+
 ByteBuffer TLSv12::build_certificate()
 {
     PacketBuilder builder { MessageType::Handshake, m_context.options.version };
@@ -296,8 +335,7 @@ ByteBuffer TLSv12::build_client_key_exchange()
         TODO();
         break;
     case KeyExchangeAlgorithm::DHE_RSA:
-        dbgln("Client key exchange for DHE_RSA is not implemented");
-        TODO();
+        build_dhe_rsa_pre_master_secret(builder);
         break;
     case KeyExchangeAlgorithm::DH_anon:
         dbgln("Client key exchange for DH_anon is not implemented");

Userland/Libraries/LibTLS/HandshakeServer.cpp

@@ -199,7 +199,7 @@ ByteBuffer TLSv12::build_server_key_exchange()
     return {};
 }
 
-ssize_t TLSv12::handle_server_key_exchange(ReadonlyBytes)
+ssize_t TLSv12::handle_server_key_exchange(ReadonlyBytes buffer)
 {
     switch (get_key_exchange_algorithm(m_context.cipher)) {
     case KeyExchangeAlgorithm::RSA:
@@ -214,8 +214,7 @@ ssize_t TLSv12::handle_server_key_exchange(ReadonlyBytes)
         TODO();
         break;
     case KeyExchangeAlgorithm::DHE_RSA:
-        dbgln("Server key exchange for DHE_RSA is not implemented");
-        TODO();
+        handle_dhe_rsa_server_key_exchange(buffer);
         break;
     case KeyExchangeAlgorithm::DH_anon:
         dbgln("Server key exchange for DH_anon is not implemented");
@@ -237,4 +236,29 @@ ssize_t TLSv12::handle_server_key_exchange(ReadonlyBytes)
     return 0;
 }
 
+ssize_t TLSv12::handle_dhe_rsa_server_key_exchange(ReadonlyBytes buffer)
+{
+    auto dh_p_length = AK::convert_between_host_and_network_endian(ByteReader::load16(buffer.offset_pointer(3)));
+    auto dh_p = buffer.slice(5, dh_p_length);
+    m_context.server_diffie_hellman_params.p = ByteBuffer::copy(dh_p.data(), dh_p.size());
+
+    auto dh_g_length = AK::convert_between_host_and_network_endian(ByteReader::load16(buffer.offset_pointer(5 + dh_p_length)));
+    auto dh_g = buffer.slice(7 + dh_p_length, dh_g_length);
+    m_context.server_diffie_hellman_params.g = ByteBuffer::copy(dh_g.data(), dh_g.size());
+
+    auto dh_Ys_length = AK::convert_between_host_and_network_endian(ByteReader::load16(buffer.offset_pointer(7 + dh_p_length + dh_g_length)));
+    auto dh_Ys = buffer.slice(9 + dh_p_length + dh_g_length, dh_Ys_length);
+    m_context.server_diffie_hellman_params.Ys = ByteBuffer::copy(dh_Ys.data(), dh_Ys.size());
+
+    if constexpr (TLS_DEBUG) {
+        dbgln("dh_p: {:hex-dump}", dh_p);
+        dbgln("dh_g: {:hex-dump}", dh_g);
+        dbgln("dh_Ys: {:hex-dump}", dh_Ys);
+    }
+
+    // FIXME: Validate signature of Diffie-Hellman parameters as defined in RFC 5246 section 7.4.3.
+
+    return 0;
+}
+
 }

Userland/Libraries/LibTLS/TLSv12.h

@@ -300,6 +300,12 @@ struct Context {
     size_t send_retries { 0 };
 
     time_t handshake_initiation_timestamp { 0 };
+
+    struct {
+        ByteBuffer p;
+        ByteBuffer g;
+        ByteBuffer Ys;
+    } server_diffie_hellman_params;
 };
 
 class TLSv12 : public Core::Socket {
@@ -397,6 +403,7 @@ private:
     ByteBuffer build_change_cipher_spec();
     ByteBuffer build_verify_request();
     void build_rsa_pre_master_secret(PacketBuilder&);
+    void build_dhe_rsa_pre_master_secret(PacketBuilder&);
 
     bool flush();
     void write_into_socket();
@@ -408,6 +415,7 @@ private:
     ssize_t handle_handshake_finished(ReadonlyBytes, WritePacketStage&);
     ssize_t handle_certificate(ReadonlyBytes);
     ssize_t handle_server_key_exchange(ReadonlyBytes);
+    ssize_t handle_dhe_rsa_server_key_exchange(ReadonlyBytes);
     ssize_t handle_server_hello_done(ReadonlyBytes);
     ssize_t handle_certificate_verify(ReadonlyBytes);
     ssize_t handle_handshake_payload(ReadonlyBytes);