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LibTLS: Implement the DHE_RSA key exchange algorithm

Browse source 
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.
This commit is contained in:
Samuel Bowman 2021-08-12 15:25:47 -04:00 • committed by Ali Mohammad Pur
parent 020bfc9d93
commit b288016bbc
3 changed files with 75 additions and 5 deletions
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42
Userland/Libraries/LibTLS/HandshakeClient.cpp
View file

@ -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");