ovpnhmac.hpp

//    OpenVPN -- An application to securely tunnel IP networks
//               over a single port, with support for SSL/TLS-based
//               session authentication and key exchange,
//               packet encryption, packet authentication, and
//               packet compression.
//
//    Copyright (C) 2012-2022 OpenVPN Inc.
//
//    This program is free software: you can redistribute it and/or modify
//    it under the terms of the GNU Affero General Public License Version 3
//    as published by the Free Software Foundation.
//
//    This program is distributed in the hope that it will be useful,
//    but WITHOUT ANY WARRANTY; without even the implied warranty of
//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//    GNU Affero General Public License for more details.
//
//    You should have received a copy of the GNU Affero General Public License
//    along with this program in the COPYING file.
//    If not, see <http://www.gnu.org/licenses/>.

// OpenVPN HMAC classes

#ifndef OPENVPN_CRYPTO_OVPNHMAC_H
#define OPENVPN_CRYPTO_OVPNHMAC_H

#include <string>

#include <openvpn/common/size.hpp>
#include <openvpn/common/exception.hpp>
#include <openvpn/common/rc.hpp>
#include <openvpn/common/memneq.hpp>
#include <openvpn/crypto/static_key.hpp>
#include <openvpn/crypto/cryptoalgs.hpp>

namespace openvpn {

// OpenVPN protocol HMAC usage for HMAC/CBC integrity checking and tls-auth

template <typename CRYPTO_API>
class OvpnHMAC
{
  public:
    OPENVPN_SIMPLE_EXCEPTION(ovpn_hmac_context_digest_size);
    OPENVPN_SIMPLE_EXCEPTION(ovpn_hmac_context_bad_sizing);

  public:
    OvpnHMAC()
    {
    }

    OvpnHMAC(const CryptoAlgs::Type digest, const StaticKey &key)
    {
        init(digest, key);
    }

    bool defined() const
    {
        return ctx.is_initialized();
    }

    // size of out buffer to pass to hmac
    size_t output_size() const
    {
        return ctx.size();
    }

    void init(const CryptoAlgs::Type digest, const StaticKey &key)
    {
        const CryptoAlgs::Alg &alg = CryptoAlgs::get(digest);

        // check that key is large enough
        if (key.size() < alg.size())
            throw ovpn_hmac_context_digest_size();

        // initialize HMAC context with digest type and key
        ctx.init(digest, key.data(), alg.size());
    }

    void hmac(unsigned char *out,
              const size_t out_size,
              const unsigned char *in,
              const size_t in_size)
    {
        ctx.reset();
        ctx.update(in, in_size);
        ctx.final(out);
    }

    // Special HMAC for OpenVPN control packets

    void ovpn_hmac_gen(unsigned char *data,
                       const size_t data_size,
                       const size_t l1,
                       const size_t l2,
                       const size_t l3)
    {
        if (ovpn_hmac_pre(data, data_size, l1, l2, l3))
            ctx.final(data + l1);
        else
            throw ovpn_hmac_context_bad_sizing();
    }

    // verify the HMAC generated by ovpn_hmac_gen, return true if verified
    bool ovpn_hmac_cmp(const unsigned char *data,
                       const size_t data_size,
                       const size_t l1,
                       const size_t l2,
                       const size_t l3)
    {
        unsigned char local_hmac[CRYPTO_API::HMACContext::MAX_HMAC_SIZE];
        if (ovpn_hmac_pre(data, data_size, l1, l2, l3))
        {
            ctx.final(local_hmac);
            return !crypto::memneq(data + l1, local_hmac, l2);
        }
        else
            return false;
    }

  private:
    // Convoluting OpenVPN control channel packets for HMAC:
    // <-- L1  -->   <-L2>   <L3>
    // [OP]  [PSID]  [HMAC]  [PID] [...]  -> canonical order
    //
    // [HMAC] [PID] [OP] [PSID] [...]     -> HMAC order

    bool ovpn_hmac_pre(const unsigned char *data,
                       const size_t data_size,
                       const size_t l1,
                       const size_t l2,
                       const size_t l3)
    {
        const size_t lsum = l1 + l2 + l3;
        if (lsum > data_size || l2 != ctx.size())
            return false;
        ctx.reset();
        ctx.update(data + l1 + l2, l3);
        ctx.update(data, l1);
        ctx.update(data + lsum, data_size - lsum);
        return true;
    }

    typename CRYPTO_API::HMACContext ctx;
};

// OvpnHMAC wrapper API using dynamic polymorphism

class OvpnHMACInstance : public RC<thread_unsafe_refcount>
{
  public:
    typedef RCPtr<OvpnHMACInstance> Ptr;

    virtual void init(const StaticKey &key) = 0;

    virtual size_t output_size() const = 0;

    virtual void ovpn_hmac_gen(unsigned char *data,
                               const size_t data_size,
                               const size_t l1,
                               const size_t l2,
                               const size_t l3)
        = 0;

    virtual bool ovpn_hmac_cmp(const unsigned char *data,
                               const size_t data_size,
                               const size_t l1,
                               const size_t l2,
                               const size_t l3)
        = 0;
};

class OvpnHMACContext : public RC<thread_unsafe_refcount>
{
  public:
    typedef RCPtr<OvpnHMACContext> Ptr;

    virtual size_t size() const = 0;

    virtual OvpnHMACInstance::Ptr new_obj() = 0;
};

class OvpnHMACFactory : public RC<thread_unsafe_refcount>
{
  public:
    typedef RCPtr<OvpnHMACFactory> Ptr;

    virtual OvpnHMACContext::Ptr new_obj(const CryptoAlgs::Type digest_type) = 0;
};

// OvpnHMAC wrapper implementation using dynamic polymorphism

template <typename CRYPTO_API>
class CryptoOvpnHMACInstance : public OvpnHMACInstance
{
  public:
    CryptoOvpnHMACInstance(const CryptoAlgs::Type digest_arg)
        : digest(digest_arg)
    {
    }

    virtual void init(const StaticKey &key)
    {
        ovpn_hmac.init(digest, key);
    }

    virtual size_t output_size() const
    {
        return ovpn_hmac.output_size();
    }

    virtual void ovpn_hmac_gen(unsigned char *data,
                               const size_t data_size,
                               const size_t l1,
                               const size_t l2,
                               const size_t l3)
    {
        ovpn_hmac.ovpn_hmac_gen(data, data_size, l1, l2, l3);
    }

    virtual bool ovpn_hmac_cmp(const unsigned char *data,
                               const size_t data_size,
                               const size_t l1,
                               const size_t l2,
                               const size_t l3)
    {
        return ovpn_hmac.ovpn_hmac_cmp(data, data_size, l1, l2, l3);
    }

  private:
    typename CryptoAlgs::Type digest;
    OvpnHMAC<CRYPTO_API> ovpn_hmac;
};

template <typename CRYPTO_API>
class CryptoOvpnHMACContext : public OvpnHMACContext
{
  public:
    CryptoOvpnHMACContext(const CryptoAlgs::Type digest_type)
        : digest(digest_type)
    {
    }

    virtual size_t size() const
    {
        return CryptoAlgs::size(digest);
    }

    virtual OvpnHMACInstance::Ptr new_obj()
    {
        return new CryptoOvpnHMACInstance<CRYPTO_API>(digest);
    }

  private:
    CryptoAlgs::Type digest;
};

template <typename CRYPTO_API>
class CryptoOvpnHMACFactory : public OvpnHMACFactory
{
  public:
    virtual OvpnHMACContext::Ptr new_obj(const CryptoAlgs::Type digest_type)
    {
        return new CryptoOvpnHMACContext<CRYPTO_API>(digest_type);
    }
};

} // namespace openvpn

#endif