ovpnhmac.hpp

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//    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- OpenVPN Inc.
//
//    SPDX-License-Identifier: MPL-2.0 OR AGPL-3.0-only WITH openvpn3-openssl-exception
//
 
// 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() = default;
 
    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);
        }
        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:
    using Ptr = RCPtr<OvpnHMACInstance>;
 
    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:
    using Ptr = RCPtr<OvpnHMACContext>;
 
    virtual size_t size() const = 0;
 
    virtual OvpnHMACInstance::Ptr new_obj() = 0;
};
 
class OvpnHMACFactory : public RC<thread_unsafe_refcount>
{
  public:
    using Ptr = RCPtr<OvpnHMACFactory>;
 
    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)
    {
    }
 
    void init(const StaticKey &key) override
    {
        ovpn_hmac.init(digest, key);
    }
 
    size_t output_size() const override
    {
        return ovpn_hmac.output_size();
    }
 
    void ovpn_hmac_gen(unsigned char *data,
                       const size_t data_size,
                       const size_t l1,
                       const size_t l2,
                       const size_t l3) override
    {
        ovpn_hmac.ovpn_hmac_gen(data, data_size, l1, l2, l3);
    }
 
    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) override
    {
        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)
    {
    }
 
    size_t size() const override
    {
        return CryptoAlgs::size(digest);
    }
 
    OvpnHMACInstance::Ptr new_obj() override
    {
        return new CryptoOvpnHMACInstance<CRYPTO_API>(digest);
    }
 
  private:
    CryptoAlgs::Type digest;
};
 
template <typename CRYPTO_API>
class CryptoOvpnHMACFactory : public OvpnHMACFactory
{
  public:
    OvpnHMACContext::Ptr new_obj(const CryptoAlgs::Type digest_type) override
    {
        return new CryptoOvpnHMACContext<CRYPTO_API>(digest_type);
    }
};
 
} // namespace openvpn
 
#endif