DESCRIPTION

This man page focuses on setting up CoAP to use encryption.

When the libcoap library was built, it will have been compiled using a specific underlying TLS implementation type (e.g. OpenSSL, GnuTLS, TinyDTLS or noTLS). When the libcoap library is linked into an application, it is possible that the application needs to dynamically determine whether DTLS or TLS is supported, what type of TLS implementation libcoap was compiled with, as well as detect what is the version of the currently loaded TLS library.

NOTE: If OpenSSL is being used, then the minimum supported OpenSSL library version is 1.1.0.

NOTE: If GnuTLS is being used, then the minimum GnuTLS library version is 3.3.0.

NOTE: If GnuTLS is going to interoperate with TinyDTLS, then a minimum revision of GnuTLS 3.5.5 which supports CCM algorithms is required by TinyDTLS as TinyDTLS currently only supports CCM.

Network traffic can be un-encrypted or encrypted with libcoap if there is an underlying TLS library.

If TLS is going to be used for encrypting the network traffic, then the TLS information for Pre-Shared Keys (PSK) or Public Key Infrastructure (PKI) needs to be configured before any network traffic starts to flow. For Servers, this has to be done before the Endpoint is created, for Clients, this is done during the Client Session set up.

For Servers, all the encryption information is held internally by the TLS Context level and the CoAP Context level as the Server is listening for new incoming traffic based on the Endpoint definition. The TLS and CoAP session will not get built until the new traffic starts, which is done by the libcoap library, with the session having a reference count of 1.

For Clients, all the encryption information can be held at the TLS Context and CoAP Context levels, or usually at the TLS Session and CoAP Session levels. If defined at the Context level, then when Sessions are created, they will inherit the Context definitions, unless they have separately been defined for the Session level, in which case the Session version will get used. Typically the information will be configured at the Session level for Clients.

In principle the set-up sequence for CoAP Servers looks like

coap_new_context()
coap_context_set_pki_root_cas() - if the root CAs need to be updated and PKI
coap_context_set_pki() and/or coap_context_set_psk() - if encryption is required
coap_new_endpoint()

Multiple endpoints can be set up per Context, each listening for a new traffic flow with different TCP/UDP protocols, TLS protocols, port numbers etc. When a new traffic flow is started, then the CoAP library will create and start a new server session.

In principle the set-up sequence for CoAP Clients looks like

coap_new_context()
coap_context_set_pki_root_cas() - if the root CAs need to be updated and PKI
coap_new_client_session(), coap_new_client_session_pki() or coap_new_client_session_psk()

Multiple client sessions are supported per Context.

Due to the nature of TLS, there are Callbacks that are invoked as the TLS session negotiates encryption algorithms, encryption keys etc. Where possible, the CoAP layer handles all this automatically based on different configuration options passed in by the coap_*_pki() functions.

For PSK setup, the required information needs to be provided in the setup calls with no application Callbacks required. Both the Client and Server have to provide a PSK. The Server must have a Hint defined and the Client must have an Identity defined.

For PKI setup, if the libcoap PKI configuration options do not handle a specific requirement as defined by the available options, then an application defined Callback can called to do the additional specific checks.

The information passed to this Application Callback will be the TLS session (as well the configuration information), but the structures containing this information will be different as they will be based on the underlying TLS library type. coap_get_tls_library_version() is provided to help here.

Libcoap will add in the defined Certificate, Private Key and CA Certificate into the TLS environment. The CA Certificate is also added in to the list of valid CAs for Certificate checking.

The internal Callbacks (and optionally the Application Callback) will then check the required information as defined in the coap_dtls_pki_t described below.

typedef struct coap_dtls_pki_t {
  uint8_t version;            /* COAP_DTLS_PKI_SETUP_VERSION */

  /* Options to enable different TLS functionality in libcoap */
  uint8_t verify_peer_cert;         /* 1 if peer cert is to be verified */
  uint8_t require_peer_cert;        /* 1 if peer cert is required */
  uint8_t allow_self_signed;        /* 1 if self signed certs are allowed */
  uint8_t allow_expired_certs;      /* 1 if expired certs are allowed */
  uint8_t cert_chain_validation;    /* 1 if to check cert_chain_verify_depth */
  uint8_t cert_chain_verify_depth;  /* recommended depth is 3 */
  uint8_t check_cert_revocation;    /* 1 if revocation checks wanted */
  uint8_t allow_no_crl;             /* 1 ignore if CRL not there */
  uint8_t allow_expired_crl;        /* 1 if expired crl is allowed */
  uint8_t reserved[6];              /* Reserved - must be set to 0 for
                                       future compatibility */

  /** CN check call-back function
   * If not NULL, is called when the TLS connection has passed the configured
   * TLS options above for the application to verify if the CN is valid.
   */
  coap_dtls_cn_callback_t validate_cn_call_back;
  void *cn_call_back_arg;  /* Passed in to the CN call-back function */

  /** SNI check call-back function
   * If not NULL, called if the SNI is not previously seen and prior to sending
   * a certificate set back to the client so that the appropriate certificate set
   * can be used based on the requesting SNI.
   */
  coap_dtls_sni_callback_t validate_sni_call_back;
  void *sni_call_back_arg;  /* Passed in to the sni call-back function */

  /** Additional Security call-back handler that is invoked when libcoap has
   * done the standerd, defined validation checks at the TLS level,
   * If not NULL, called from within the TLS Client Hello connection
   * setup.
   */
  coap_dtls_security_setup_t additional_tls_setup_call_back;

  char* client_sni;       /* If not NULL, SNI to use in client TLS setup.
                             Owned by the client app and must remain valid
                             during the call to coap_new_client_session_pki() */

  coap_dtls_key_t pki_key; /* PKI key definition */
} coap_dtls_pki_t;

More detailed explanation of the coap_dtls_pki_t structure follows.

WARNING: All the parameter definitions that are pointers to other locations, these locations must remain valid during the lifetime of all the underlying TLS sessions that are, or will get created based on this PKI definition.

The first parameter in each subsection enables/disables the functionality, the remaining parameter(s) control control what happens when the functionality is enabled.

SECTION: coap_dtls_pki_t Version

#define COAP_DTLS_PKI_SETUP_VERSION 1

version is set to COAP_DTLS_PKI_SETUP_VERSION. This will then allow support for different versions of the coap_dtls_pki_t structure in the future.

SECTION: Peer Certificate Checking

verify_peer_cert Set to 1 to check that the peer’s certificate is valid if provided, else 0.

require_peer_cert Set to 1 to enforce that the peer provides a certificate, else 0. If the Server, this initates a request for the peer certificate.

allow_self_signed Set to 1 to allow the peer (or any certificate in the certificate chain) to be a self-signed certificate, else 0.

allow_expired_certs Set to 1 to allow certificates that have either expired, or are not yet valid to be allowed, else 0.

SECTION: Certificate Chain Validation

cert_chain_validation Set to 1 to check that the certificate chain is valid, else 0.

cert_chain_verify_depth Set to the chain depth that is to be checked. This is the number of intermediate CAs in the chain. If set to 0, then there can be no intermediate CA in the chain.

SECTION: Certificate Revocation

check_cert_revocation Set to 1 to check whether any certificate in the chain has been revoked, else 0.

allow_no_crl Set to 1 to not check any certificate that does not have a CRL.

allow_expired_crl Set to 1 to allow an certificate that has an expired CRL definition to be valid, else 0.

SECTION: Reserved

reserved Must be set to 0. Future functionality updates will make use of these reserved definitions.

SECTION: Common Name (CN) Callback

/**
 * CN Validation call-back that can be set up by coap_context_set_pki().
 * Invoked when libcoap has done the validation checks at the TLS level,
 * but the application needs to check that the CN is allowed.
 * CN is the SubjectAltName in the cert, if not present, then the leftmost
 * Common Name (CN) component of the subject name
 *
 * @param cn  The determined CN from the certificate
 * @param asn1_public_cert  The ASN.1 encoded (DER) X.509 certificate
 * @param asn1_length  The ASN.1 length
 * @param session  The coap session associated with the certificate update
 * @param depth  Depth in cert chain.  If 0, then client cert, else a CA
 * @param validated  TLS can find no issues if 1
 * @param arg  The same as was passed into coap_context_set_pki()
 *             in setup_data->cn_call_back_arg
 *
 * @return 1 if accepted, else 0 if to be rejected
 */
typedef int (*coap_dtls_cn_callback_t)(const char *cn,
             const uint8_t *asn1_public_cert,
             size_t asn1_length,
             coap_session_t *session,
             unsigned depth,
             int validated,
             void *arg);

validate_cn_call_back points to an application provided CN callback checking function or NULL. The application can make use of this CN information to decide, for example, that the CN is valid coming from a particular peer. The Callback returns 1 on success, 0 if the TLS connection is to be aborted.

cn_call_back_arg points to a user defined set of data that will get passed in to the validate_cn_call_back() function and can be used by that function. An example would be a set of CNs that are allowed.

SECTION: Subject Name Identifier (SNI) Callback

typedef struct coap_dtls_key_t {
  coap_pki_key_t key_type;          /* key format type */
  union {
    coap_pki_key_pem_t pem;         /* for PEM keys */
    coap_pki_key_asn1_t asn1;       /* for ASN.1 (DER) keys */
  } key;
} coap_dtls_key_t;

/**
 * SNI Validation call-back that can be set up by coap_context_set_pki().
 * Invoked if the SNI is not previously seen and prior to sending a certificate
 * set back to the client so that the appropriate certificate set can be used
 * based on the requesting SNI.
 *
 * @param sni  The requested SNI
 * @param arg  The same as was passed into coap_context_set_pki()
 *             in setup_data->sni_call_back_arg
 *
 * @return new set of certificates to use, or NULL if SNI is to be rejected.
 */
typedef coap_dtls_key_t *(*coap_dtls_sni_callback_t)(const char *sni,
             void* arg);

validate_sni_call_back points to an application provided SNI callback checking function or NULL. The application can make use of this SNI information to decide whether the SNI is valid, and what set of certificates to give to the client. Thus it is possible for the coap server to host multiple domains with different certificates allocated to each domain. The Callback returns a pointer to the certificates to use for this SNI, or NULL if the connection it to get rejected. libcoap remembers the association between the SNI and Certificate set and will only invoke this callback if the SNI is unknown.

sni_call_back_arg points to a user defined set of data that will get passed in to the validate_sni_call_back() function and can be used by that function. An example would be a set of SNIs that are allowed with their matching certificate sets.

SECTION: Application Additional Setup Callback

/**
 * Additional Security setup handler that can be set up by
 * coap_context_set_pki().
 * Invoked when libcoap has done the validation checks at the TLS level,
 * but the application needs to do some additional checks/changes/updates.
 *
 * @param session The security session definition - e.g. SSL * for OpenSSL.
 *                This will be dependent on the underlying TLS library
 *                - see coap_get_tls_library_version()
 * @param setup_data A structure containing setup data originally passed into
 *                   coap_context_set_pki() or coap_new_client_session_pki().
 * @return 1 if successful, else 0
 */
typedef int (*coap_dtls_security_setup_t)(void *context, void* session,
                                        struct coap_dtls_pki_t *setup_data);

additional_tls_setup_call_back points to an application provided callback function that will do additional checking/changes/updates after libcoap has done all of the configured TLS setup checking, or NULL to do no additional checking.

SECTION: Subject Name Indicator (SNI) Definition

client_sni points to the SNI name that will be added in as a TLS extension, or set NULL. This typically is the DNS name of the server that the client is trying to contact. This is only used by a client application and the server is then able to decide, based on the name in the SNI extension, whether, for example, a different certificate should be provided.

SECTION: Key Type Definition

typedef enum coap_pki_key_t {
  COAP_PKI_KEY_PEM,    /* PEM type informaiton */
  COAP_PKI_KEY_ASN1,   /* ASN1 type information */
} coap_pki_key_t;

key_type defines the format that the certificates / keys are provided in. This can be COAP_PKI_KEY_PEM or COAP_PKI_KEY_ASN1.

SECTION: PEM Key Definitions

typedef struct coap_pki_key_pem_t {
  const char *ca_file;       /* File location of Common CA in PEM format */
  const char *public_cert;   /* File location of Public Cert in PEM format */
  const char *private_key;   /* File location of Private Key in PEM format */
} coap_pki_key_pem_t;

key.pem.ca_file points to the CA File location on disk which will be in PEM format, or NULL. This file should only contain 1 CA (who signed the Public Certificate) as this is passed from the server to the client when requesting the client’s certificate. This certificate is also added into the valid root CAs list if not already present.

key.pem.public_cert points to the Public Certificate location on disk which will be in PEM format.

key.pem.private_key points to the Private Key location on disk which will be in PEM format. This file cannot be password protected.

SECTION: ASN1 Key Definitions

typedef struct coap_pki_key_asn1_t {
  const uint8_t *ca_cert;     /* ASN1 Common CA Certificate */
  const uint8_t *public_cert; /* ASN1 Public Certificate */
  const uint8_t *private_key; /* ASN1 Private Key */
  int ca_cert_len;            /* ASN1 CA Certificate length */
  int public_cert_len;        /* ASN1 Public Certificate length */
  int private_key_len;        /* ASN1 Private Key length */
  coap_asn1_privatekey_type_t private_key_type; /* Private Key Type
                                                   COAP_ASN1_PKEY_* */
} coap_pki_key_asn1_t;

typedef enum coap_asn1_privatekey_type_t {
  COAP_ASN1_PKEY_NONE,
  COAP_ASN1_PKEY_RSA,
  COAP_ASN1_PKEY_RSA2,
  COAP_ASN1_PKEY_DSA,
  COAP_ASN1_PKEY_DSA1,
  COAP_ASN1_PKEY_DSA2,
  COAP_ASN1_PKEY_DSA3,
  COAP_ASN1_PKEY_DSA4,
  COAP_ASN1_PKEY_DH,
  COAP_ASN1_PKEY_DHX,
  COAP_ASN1_PKEY_EC,
  COAP_ASN1_PKEY_HMAC,
  COAP_ASN1_PKEY_CMAC,
  COAP_ASN1_PKEY_TLS1_PRF,
  COAP_ASN1_PKEY_HKDF
} coap_asn1_privatekey_type_t;

key.asn1.ca_cert points to a DER encoded ASN.1 definition of the CA Certificate, or NULL. This certificate is passed from the server to the client when requesting the client’s certificate. This certificate is also added into the valid root CAs list if not already present.

key.asn1.public_cert points to a DER encoded ASN.1 definition of the Public Certificate.

key.asn1.private_key points to DER encoded ASN.1 definition of the Private Key.

key.asn1.ca_cert_len is the length of the DER encoded ASN.1 definition of the CA Certificate.

key.asn1.public_cert_len is the length of the DER encoded ASN.1 definition of the Public Certificate.

key.asn1.private_key_len is the length of the DER encoded ASN.1 definition of the Private Key.

key.asn1.private_key_type is the encoding type of the DER encoded ASN.1 definition of the Private Key. This will be one of the COAP_ASN1_PKEY_* definitions.

EXAMPLES

CoAP Server DTLS PKI Setup

#include <coap2/coap.h>

typedef struct valid_cns_t {
  int count;
  char **cn_list;
} valid_cns_t;

/**
 * CN Validation call-back that is set up by coap_context_set_pki().
 * Invoked when libcoap has done the validation checks at the TLS level,
 * but the application needs to check that the CN is allowed.
 * CN is the SubjectAltName in the cert, if not present, then the leftmost
 * Common Name (CN) component of the subject name
 *
 * @param cn  The determined CN from the certificate
 * @param asn1_public_cert  The ASN.1 encoded (DER) X.509 certificate
 * @param asn1_length  The ASN.1 length
 * @param session  The coap session associated with the certificate update
 * @param depth  Depth in cert chain.  If 0, then client cert, else a CA
 * @param validated  TLS can find no issues if 1
 * @param arg  The same as was passed into coap_context_set_pki()
 *             in setup_data->cn_call_back_arg
 *
 * @return 1 if accepted, else 0 if to be rejected
 */
static int
verify_cn_callback(const char *cn,
                   const uint8_t *asn1_public_cert,
                   size_t asn1_length,
                   coap_session_t *session,
                   unsigned depth,
                   int validated,
                   void *arg
) {
  valid_cns_t *valid_cn_list = ( valid_cns_t*)arg;
  int i;

  /* Check that the CN is valid */
  for (i = 0; i < valid_cn_list->count; i++) {
    if (!strcasecmp(cn, valid_cn_list->cn_list[i])) {
      return 1;
    }
  }
  return 0;
}

typedef struct sni_def_t {
  char* sni;
  coap_dtls_key_t key;
} sni_def_t;

typedef struct valid_snis_t {
  int count;
  sni_def_t *sni_list;
} valid_snis_t;

/**
 * SNI Validation call-back that is set up by coap_context_set_pki().
 * Invoked if the SNI is not previously seen and prior to sending a certificate
 * set back to the client so that the appropriate certificate set can be used
 * based on the requesting SNI.
 *
 * @param sni  The requested SNI
 * @param arg  The same as was passed into coap_context_set_pki()
 *             in setup_data->sni_call_back_arg
 *
 * @return new set of certificates to use, or NULL if SNI is to be rejected.
 */
static coap_dtls_key_t *
verify_sni_callback(const char *sni,
                    void *arg
) {
  valid_snis_t *valid_sni_list = (valid_snis_t *)arg;
  int i;

  /* Check that the SNI is valid */
  for (i = 0; i < valid_sni_list->count; i++) {
    if (!strcasecmp(sni, valid_sni_list->sni_list[i].sni)) {
      return &valid_sni_list->sni_list[i].key;
    }
  }
  return NULL;
}

/*
 * Set up PKI encryption information
 */
static coap_context_t *
setup_server_context_pki (const char *public_cert_file,
                          const char *private_key_file,
                          const char *ca_file,
                          valid_cns_t *valid_cn_list,
                          valid_snis_t *valid_sni_list
) {
  coap_endpoint_t *endpoint;
  coap_address_t listen_addr;
  coap_dtls_pki_t dtls_pki;
  coap_context_t *context;

  /* See coap_tls_library(3) */
  if (!coap_dtls_is_supported())
    return NULL;

  /* See coap_context(3) */
  context = coap_new_context(NULL);
  if (!context)
    return NULL;

  memset (&dtls_pki, 0, sizeof (dtls_pki));

  dtls_pki.version                 = COAP_DTLS_PKI_SETUP_VERSION;
  dtls_pki.verify_peer_cert        = 1;
  dtls_pki.require_peer_cert       = 1;
  dtls_pki.allow_self_signed       = 1;
  dtls_pki.allow_expired_certs     = 1;
  dtls_pki.cert_chain_validation   = 1;
  dtls_pki.cert_chain_verify_depth = 1;
  dtls_pki.check_cert_revocation   = 1;
  dtls_pki.allow_no_crl            = 1;
  dtls_pki.allow_expired_crl       = 1;
  dtls_pki.validate_cn_call_back   = verify_cn_callback;
  dtls_pki.cn_call_back_arg        = valid_cn_list;
  dtls_pki.validate_sni_call_back  = verify_sni_callback;
  dtls_pki.sni_call_back_arg       = valid_sni_list;
  dtls_pki.additional_tls_setup_call_back = NULL;
  dtls_pki.sni                     = NULL;
  dtls_pki.pki_key.key_type        = COAP_PKI_KEY_PEM;
  dtls_pki.pki_key.key.pem.ca_file = ca_file;
  dtls_pki.pki_key.key.pem.public_cert = public_cert_file;
  dtls_pki.pki_key.key.pem.private_key = private_key_file;

  /* See coap_context(3) */
  if (coap_context_set_pki(context, &dtls_pki)) {
    coap_free_context(context);
    return NULL;
  }

  coap_address_init(&listen_addr);
  listen_addr.addr.sa.sa_family = AF_INET;
  listen_addr.addr.sin.sin_port = htons (5684);

  /* See coap_context(3) */
  endpoint = coap_new_endpoint(context, &listen_addr, COAP_PROTO_DTLS);
  if (!endpoint) {
    coap_free_context(context);
    return NULL;
  }

  /* See coap_resource(3) */
  init_resources(context);

  return context;
}

SEE ALSO

coap_context(3), coap_resource(3), coap_session(3) and coap_tls_library(3).

FURTHER INFORMATION

See "RFC7252: The Constrained Application Protocol (CoAP)" for further information.

BUGS

Please report bugs on the mailing list for libcoap: libcoap-developers@lists.sourceforge.net

AUTHORS

The libcoap project <libcoap-developers@lists.sourceforge.net>