libcoap  4.3.0-develop-0585fb4
coap_encryption(3)
coap_encryption

NAME

coap_encryption, coap_dtls_cpsk_t, coap_dtls_spsk_t, coap_dtls_pki_t — Work with CoAP TLS/DTLS

SYNOPSIS

#include <coap3/coap.h>

struct coap_dtls_cpsk_t;

struct coap_dtls_spsk_t;

struct coap_dtls_pki_t;

For specific (D)TLS library support, link with -lcoap-3-notls, -lcoap-3-gnutls, -lcoap-3-openssl, -lcoap-3-mbedtls or -lcoap-3-tinydtls. Otherwise, link with -lcoap-3 to get the default (D)TLS library support.

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, Mbed TLS, 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 Mbed TLS is being used, then the minimum Mbed TLS library version is 2.7.10.

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.

NOTE: For Raw Public Key support, GnuTLS library version must be 3.6.6 or later. TinyDTLS only supports TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8, curve secp256r1 and hash SHA-256. There currently is no OpenSSL or Mbed TLS RPK support (respective library limitations).

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), Public Key Infrastructure (PKI) or Raw Public Key (RPK) 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 will be held internally by the TLS Context and/or TLS Session level and internally by the CoAP Session level.

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_psk2() - 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_psk2()

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_context_set_pki(), coap_new_client_session_pki(), coap_context_set_psk2() and coap_new_client_session_psk2() functions.

PSK CLIENT INFORMATION

For Client PSK setup, the required information needs to be provided in the setup calls with optional application callbacks defined to update the Identity and PSK. Initially, the Client has to provide an Identity and a Pre-Shared Key.

Libcoap will put the Identity and Pre-Shared Key as appropriate into the TLS environment.

SECTION: PSK Client: coap_dtls_cpsk_t

typedef struct coap_dtls_cpsk_t {
  uint8_t version; /** Set to COAP_DTLS_CPSK_SETUP_VERSION
                       to support the version of the struct */

  /* Options to enable different TLS functionality in libcoap */
  uint8_t reserved[7];             /* Reserved - must be set to 0 for
                                      future compatibility */

  /** Identity Hint check callback function.
   * If not NULL, is called when the Identity Hint (TLS1.2 or earlier) is
   * provided by the server.
   * The appropriate Identity and Pre-Shared Key to use can then be returned.
   */
  coap_dtls_ih_callback_t validate_ih_call_back;
  void *ih_call_back_arg;  /* Passed in to the Identity Hint callback
                              function */

  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().
                           Note: Not supported by TinyDTLS. */

  coap_dtls_cpsk_info_t psk_info;  /* Client PSK definition */
} coap_dtls_cpsk_t;

More detailed explanation of the coap_dtls_cpsk_t structure follows.

WARNING: For 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 PSK definition.

SECTION: PSK Client: coap_dtls_cpsk_t: Version

#define COAP_DTLS_CPSK_SETUP_VERSION 1 /**< Latest CPSK setup version */

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

SECTION: PSK Client: coap_dtls_cpsk_t: Reserved

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

SECTION: PSK Client: coap_dtls_cpsk_t: Identity Hint Callback

/**
 * Identity Hint Validation callback that can be set up by
 * coap_new_client_session_psk2().
 * Invoked when libcoap has done the validation checks at the TLS level,
 * but the application needs to check that the Identity Hint is allowed, and
 * needs to use the appropriate PSK information for the (D)TLS session.
 * Note: Identity Hint is not supported in (D)TLS1.3.
 *
 * @param hint  The server provided Identity Hint
 * @param coap_session  The CoAP session associated with the Identity Hint
 * @param arg  The same as was passed into coap_new_client_session_psk2()
 *             in setup_data->ih_call_back_arg
 *
 * @return New coap_dtls_cpsk_info_t object or @c NULL on error.
 */
typedef const coap_dtls_cpsk_info_t *(*coap_dtls_ih_callback_t)(
                                coap_str_const_t *hint,
                                coap_session_t *coap_session,
                                void *arg);

validate_ih_call_back points to an application provided Identity Hint callback function or NULL. The application can make use of this Identity Hint information to decide what Identity and Pre-Shared Key should be used for this session. The Callback returns the new coap_dtls_cpsk_info_t on success, or NULL if the Identity Hint is unacceptable.

NOTE: The Server may not provide a hint, or a zero length hint to indicate there is no hint. In this case the initially provided Identity and Pre-Shared Key should be used.

ih_call_back_arg points to a user defined set of data that will get passed in to the validate_ih_call_back() function’s arg parameter and can be used by that function. An example would be a set of Identity Hints that map into new Identity / Pre-Shared Key to use.

SECTION: PSK Client: coap_dtls_cpsk_t: Subject Name Indicator (SNI) Definition

client_sni points to the SNI name that will be added in as a TLS extension, if not NULL. This typically is the DNS name of the server that the client is trying to contact. The server is then able to decide, based on the name in the SNI extension, whether, for example, a different Hint and/or Pre-Shared Key is to be used.

Note: Not supported by TinyDTLS.

SECTION: PSK Client: coap_dtls_cpsk_t: PSK Client Definitions

typedef struct coap_dtls_cpsk_info_t {
  coap_bin_const_t identity; /* The Identity */
  coap_bin_const_t key;      /* The Pre-Shared Key */
} coap_dtls_cpsk_info_t;

identity defines the Identity to use.

key defines the Pre-Shared Key to use

PSK SERVER INFORMATION

For PSK setup, the required information needs to be provided in the setup calls with optional application Callbacks defined to update the Identity Hint and Pre-SHared Key. Initially, the Server has to provided with an (optional) Identity Hint and a (required) Pre-Shared Key.

Libcoap will put the Hint and Pre-Shared Key as appropriate into the TLS environment.

SECTION: PSK Server: coap_dtls_spsk_t

typedef struct coap_dtls_spsk_t {
  uint8_t version; /** Set to COAP_DTLS_SPSK_SETUP_VERSION
                       to support the version of the struct */

  /* Options to enable different TLS functionality in libcoap */
  uint8_t reserved[7];             /* Reserved - must be set to 0 for
                                      future compatibility */

  /** Identity check callback function.
   * If not @p NULL, is called when the Identity is provided by the client.
   *  The appropriate Pre-Shared Key to use can then be returned.
   */
  coap_dtls_id_callback_t validate_id_call_back;
  void *id_call_back_arg;  /* Passed in to the Identity callback function */

  /** SNI check callback function.
   * If not @p NULL, called if the SNI is not previously seen and exexuted
   * prior to sending an Identity Hint back to the client so that the
   * appropriate PSK information can be used based on the requesting SNI.
   */
  coap_dtls_psk_sni_callback_t validate_sni_call_back;
  void *sni_call_back_arg;  /* Passed in to the SNI callback function */

  coap_dtls_spsk_info_t psk_info;  /* Server PSK definition */
} coap_dtls_spsk_t;

More detailed explanation of the coap_dtls_spsk_t structure follows.

WARNING: For 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 PSK definition.

SECTION: PSK Server: coap_dtls_spsk_t: Version

#define COAP_DTLS_SPSK_SETUP_VERSION 1 /**< Latest SPSK setup version */

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

SECTION: PSK Server: coap_dtls_spsk_t: Reserved

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

SECTION: PSK Server: coap_dtls_spsk_t: Identity Validation Callback

/**
 * Identity Validation callback that can be set up by
 * coap_context_set_psk2().
 * Invoked when libcoap has done the validation checks at the TLS level,
 * but the application needs to check that the Identity is allowed, and
 * needs to use the appropriate Pre-Shared Key for the (D)TLS session.
 *
 * @param identity  The client provided Identity (should be NULL terminated)
 * @param coap_session  The CoAP session associated with the Identity Hint
 * @param arg  The same as was passed into coap_context_set_psk2()
 *             in setup_data->id_call_back_arg
 *
 * @return New coap_bin_const_t Pre-Shared Key object or @c NULL on error.
 */
typedef const coap_bin_const_t *(*coap_dtls_id_callback_t)(
                                 coap_bin_const_t *identity,
                                 coap_session_t *coap_session,
                                 void *arg);

WARNING: If both validate_id_call_back and validate_sni_call_back are defined, validate_id_call_back() is invoked after validate_sni_call_back(), and so if the Pre-Shared Key is changed in validate_sni_call_back(), validate_id_call_back() needs to be sure that the appropriate Pre-Shared Key is provided.

validate_id_call_back points to an application provided Identity callback function or NULL. The application can make use of this Identity information to decide what PSK should be used for this session. The Callback returns the new coap_bin_const_t Pre-Shared Key on success, or NULL if the Identity is unacceptable.

NOTE: The Client may be using a binary Identity that contains an embedded zero. However OpenSSL and GnuTLS do not currently support this.

id_call_back_arg points to a user defined set of data that will get passed in to the validate_id_call_back() function and can be used by that function. An example would be a set of Identities that map into new Pre-Shared Keys to use.

SECTION: PSK Server: coap_dtls_spsk_t: Subject Name Identifier (SNI) Callback

/**
 * PSK SNI callback that can be set up by coap_context_set_psk2().
 * Invoked when libcoap has done the validation checks at the TLS level,
 * but the application needs to check that the SNI is allowed, and needs
 * to use the appropriate PSK information for the (D)TLS session.
 *
 * @param sni  The client provided SNI
 * @param coap_session  The CoAP session associated with the SNI
 * @param arg  The same as was passed into coap_new_client_session_psk2()
 *             in setup_data->sni_call_back_arg
 *
 * @return New coap_dtls_spsk_info_t object or @c NULL on error.
 */
typedef const coap_dtls_spsk_info_t *(*coap_dtls_psk_sni_callback_t)(
                                 const char *sni,
                                 coap_session_t *coap_session,
                                 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 hence what new Hint and Pre-Shared Key to use. Thus it is possible for the coap server to host multiple domains with different Hints and PSKs allocated to each SNI domain. The Callback returns a coap_dtls_spsk_info_t pointer to the Hint and Pre-Shared Key to use for this SNI, or NULL if the connection is to get rejected. Libcoap remembers the association between a specific SNI and Hint Pre-Shared Key set and will only invoke this callback if the SNI is unknown.

Note: Not supported by TinyDTLS.

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 Hint + Pre-Shared Key sets.

SECTION: PSK Server: coap_dtls_spsk_t: PSK Information Definitions

typedef struct coap_dtls_spsk_info_t {
  coap_bin_const_t hint; /* The identity hint to use */
  coap_bin_const_t key;  /* The Pre-Shared Key to use */
} coap_dtls_spsk_info_t;

identity defines the Identity Hint to use.

key defines the Pre-Shared Key to use

PKI/RPK CLIENT AND SERVER INFORMATION

For PKI or RPK setup, if the libcoap PKI/RPK 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 (or Public Key), 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.

SECTION: PKI/RPK: coap_dtls_pki_t

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 check_common_ca;          /* 1 if peer cert is to be signed by
                                     * the same CA as the local cert */
  uint8_t allow_self_signed;        /* 1 if self-signed certs are allowed */
  uint8_t allow_self_signed;        /* 1 if self-signed certs are allowed.
                                     * Ignored if check_common_ca set */
  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 allow_bad_md_hash;        /* 1 if unsupported MD hashes are allowed */
  uint8_t allow_short_rsa_length;   /* 1 if small RSA keysizes are allowed */
  uint8_t is_rpk_not_cert;          /* 1 is RPK instead of Public Certificate.
                                     *   If set, PKI key format type cannot be
                                     *   COAP_PKI_KEY_PEM */
  uint8_t reserved[3];              /* Reserved - must be set to 0 for
                                       future compatibility */

  /** CN check callback 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 callback function */

  /** SNI check callback 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 callback function */

  /** Additional Security callback handler that is invoked when libcoap has
   * done the standard, 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: For 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/RPK definition.

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

SECTION: PKI/RPK: 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: PKI/RPK: coap_dtls_pki_t: Peer Certificate Checking

verify_peer_cert Set to 1 to check that the peer’s certificate is valid if provided, else 0. If not set, check_common_ca, allow_self_signed, allow_expired_certs, cert_chain_validation, cert_chain_verify_depth, check_cert_revocation, allow_no_crl, allow_expired_crl, allow_bad_md_hash and allow_short_rsa_length settings are all ignored.

check_common_ca Set to 1 to check that the CA that signed the peer’s certificate is the same CA that signed the local certificate else 0. If set to 1 and verify_peer_cert is set to 1, then for the server, a list of valid CAs are sent to client. For the client, the logic will check that both the client and server certificates are signed by the same CA.

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. If check_common_ca is set, then a self-signed certificate will not be allowed.

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

SECTION: PKI/RPK: coap_dtls_pki_t: 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: PKI/RPK: coap_dtls_pki_t: 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, else 0.

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

SECTION: PKI/RPK: coap_dtls_pki_t: Other

allow_bad_md_hash Set to 1 if unsupported MD hashes are allowed, else 0.

allow_short_rsa_length Set to 1 if small RSA keysizes are allowed, else 0.

is_rpk_not_cert Set to 1 if the Certificate is actually a Raw Public Key. If set, PKI key format type cannot be COAP_PKI_KEY_PEM. If set, check_common_ca, allow_self_signed, allow_expired_certs, cert_chain_validation, cert_chain_verify_depth, check_cert_revocation, allow_no_crl, allow_expired_crl, allow_bad_md_hash and allow_short_rsa_length settings are all ignored.

SECTION: PKI/RPK: coap_dtls_pki_t: Reserved

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

SECTION: PKI/RPK: coap_dtls_pki_t: Common Name (CN) Callback

#define COAP_DTLS_RPK_CERT_CN "RPK"

/**
 * CN Validation callback 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.
 * NOTE: If using RPK, then the Public Key does not contain a CN, but the
 * content of COAP_DTLS_RPK_CERT_CN is presented for the @p cn parameter.
 *
 * @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 int 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: PKI/RPK: coap_dtls_pki_t: 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 file keys */
    coap_pki_key_pem_buf_t pem_buf; /* for PEM memory keys */
    coap_pki_key_asn1_t asn1;       /* for ASN.1 (DER) memory keys */
    coap_pki_key_pkcs11_t pkcs11;   /* for PKCS11 keys */
  } key;
} coap_dtls_key_t;

/**
 * SNI Validation callback 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: PKI/RPK: coap_dtls_pki_t: 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,
                                          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: PKI/RPK: coap_dtls_pki_t: 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: PKI/RPK: coap_dtls_pki_t: Key Type Definition

typedef enum coap_pki_key_t {
  COAP_PKI_KEY_PEM,     /* The PKI key type is PEM file */
  COAP_PKI_KEY_ASN1,    /* The PKI key type is ASN.1 (DER) buffer */
  COAP_PKI_KEY_PEM_BUF, /* The PKI key type is PEM buffer */
  COAP_PKI_KEY_PKCS11,  /* The PKI key type is PKCS11 (DER) */
} coap_pki_key_t;

key_type defines the format that the certificates / keys are provided in. This can be COAP_PKI_KEY_PEM, COAP_PKI_KEY_PEM_BUF, COAP_PKI_KEY_ASN1 or COAP_PKI_KEY_PKCS11.

SECTION: PKI: coap_dtls_pki_t: 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 */
  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 one CA (that has 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: PKI/RPK: coap_dtls_pki_t: PEM Memory Key Definitions

typedef struct coap_pki_key_pem_buf_t {
  const uint8_t *ca_cert;     /* PEM buffer Common CA Cert */
  const uint8_t *public_cert; /* PEM buffer Public Cert, or Public Key if RPK */
  const uint8_t *private_key; /* PEM buffer Private Key */
                                 If RPK and 'EC PRIVATE KEY' this can be used
                                 for both the public_cert and private_key */
  size_t ca_cert_len;         /* PEM buffer CA Cert length */
  size_t public_cert_len;     /* PEM buffer Public Cert length */
  size_t private_key_len;     /* PEM buffer Private Key length */
} coap_pki_key_pem_buf_t;

key.pem_buf.ca_cert points to the CA location in memory which will be in PEM format, or NULL. This file should only contain one CA (that has 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_buf.ca_cert_len is the length of the CA.

key.pem_buf.public_cert points to the public certificate (or public key if RPK) location in memory which will be in PEM format.

key.pem_buf.public_cert_len is the length of the public certificate.

key.pem_buf.private_key points to the private key location in memory which will be in PEM format. This data cannot be password protected. If RPK and EC PRIVATE KEY this can be used for both the public_cert and private_key.

key.pem_buf.private_key is the length of the private key.

  • Note:* The PEM buffer Certs and Key should be be NULL terminated strings for performance reasons (to save a potential buffer copy) and the length include this NULL terminator. It is not a requirement to have the NULL terminator though and the length must then reflect the actual data size.

SECTION: PKI/RPK: coap_dtls_pki_t: 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 (DER) Public Cert, or Public Key if RPK */
  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 (or public key if RPK).

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.

SECTION: PKI: coap_dtls_pki_t: PKCS11 Key Definitions

typedef struct coap_pki_key_pkcs11_t {
  const char *ca;            /* pkcs11: URI for Common CA Certificate */
  const char *public_cert;   /* pkcs11: URI for Public Cert */
  const char *private_key;   /* pkcs11: URI for Private Key */
  const char *pin;           /* pin to access PKCS11.  If NULL, then
                                pin-value= parameter must be set in
                                pkcs11: URI as a query. */
} coap_pki_key_pkcs11_t;

key.pkcs11.ca is a pkcs11: URI for the CA certificate or NULL. This is for the CA (that has 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. An example URI is pkcs11:pkcs11:token=My%20Token;id=%aa%bb%cc%dd which is for token My Token and (hex) id of aabbccdd.

key.pkcs11.public_cert is a pkcs11: URI for the Public Certificate which was signed by key.pkcs11.ca or NULL.

key.pkcs11.private_key is a pkcs11: URI for the Private Key for the public certificate defined by key.pkcs11.public_cert or NULL.

key.pkcs11.user_pin is the user pin used to unlock the token or NULL. If NULL, the pin can be defined on the other pkcs11: URI entries by using pin-value=XXX as a query - e.g. pkcs11:pkcs11:token=My%20Token;id=%aa%bb%cc%dd?pin-value=XXX where XXX is the user pin.

EXAMPLES

CoAP Server DTLS PKI Setup

#include <coap3/coap.h>

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

/**
 * CN Validation callback 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.
 * NOTE: If using RPK, then the Public Key does not contain a CN, but "RPK"
 * is presented for the cn parameter.
 *
 * @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 *c_session,
                   unsigned depth,
                   int validated,
                   void *arg
) {
  valid_cns_t *valid_cn_list = (valid_cns_t*)arg;
  int i;
  /* Remove (void) definition if variable is used */
  (void)asn1_public_cert;
  (void)asn1_length;
  (void)c_session;
  (void)depth;
  (void)validated;

  /* 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 callback 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_pki_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;
  /* See coap_block(3) */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


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

  dtls_pki.version                 = COAP_DTLS_PKI_SETUP_VERSION;
  dtls_pki.verify_peer_cert        = 1;
  dtls_pki.check_common_ca         = 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.allow_bad_md_hash       = 0;
  dtls_pki.allow_short_rsa_length  = 0;
  dtls_pki.is_rpk_not_cert         = 0; /* Set to 1 if RPK */
  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_pki_sni_callback;
  dtls_pki.sni_call_back_arg       = valid_sni_list;
  dtls_pki.additional_tls_setup_call_back = NULL;
  dtls_pki.client_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;
  }

  /* Initialize resources - See coap_resource(3) init_resources() example */

  return context;
}

CoAP Server DTLS PSK Setup

#include <coap3/coap.h>

typedef struct id_def_t {
  char *hint_match;
  coap_bin_const_t id;
  coap_bin_const_t key;
} id_def_t;

typedef struct valid_ids_t {
  size_t count;
  id_def_t *id_list;
} valid_ids_t;

/*
 * PSK Identity Pre-Shared Key selection Callback function
 */
static const coap_bin_const_t *
verify_id_callback(coap_bin_const_t *identity,
                   coap_session_t *c_session,
                   void *arg
) {
  valid_ids_t *valid_id_list = (valid_ids_t*)arg;
  const coap_bin_const_t *s_psk_hint = coap_session_get_psk_hint(c_session);
  size_t i;

  /* Check that the Identity is valid */
  for (i = 0; i < valid_id_list->count; i++) {
    if (s_psk_hint &&
        strcmp((const char *)s_psk_hint->s,
               valid_id_list->id_list[i].hint_match)) {
      continue;
    }
    if (coap_binary_equal(identity, &valid_id_list->id_list[i].id)) {
      return &valid_id_list->id_list[i].key;
    }
  }
  return NULL;
}

typedef struct sni_psk_def_t {
  char* sni;
  coap_dtls_spsk_info_t psk_info;
} sni_psk_def_t;

typedef struct valid_psk_snis_t {
  int count;
  sni_psk_def_t *sni_list;
} valid_psk_snis_t;

/*
 * PSK Subject Name Identifier (SNI) callback verifier
 */
static const coap_dtls_spsk_info_t *
verify_psk_sni_callback(const char *sni,
                        coap_session_t *c_session,
                        void *arg
) {
  valid_psk_snis_t *valid_sni_list = (valid_psk_snis_t *)arg;
  int i;
  /* Remove (void) definition if variable is used */
  (void)c_session;

  /* 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].psk_info;
    }
  }
  return NULL;
}

static coap_context_t *
setup_server_context_psk (const char *hint,
                          const uint8_t *key,
                          unsigned int key_len,
                          valid_ids_t *valid_id_list,
                          valid_psk_snis_t *valid_sni_list
) {
  coap_endpoint_t *endpoint;
  coap_address_t listen_addr;
  coap_context_t *context;
  coap_dtls_spsk_t dtls_psk;

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

  context = coap_new_context(NULL);
  if (!context)
    return NULL;
  /* See coap_block(3) */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


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

  /* see coap_encryption(3) */
  dtls_psk.version                 = COAP_DTLS_SPSK_SETUP_VERSION;
  dtls_psk.validate_id_call_back   = verify_id_callback;
  dtls_psk.id_call_back_arg        = valid_id_list;
  dtls_psk.validate_sni_call_back  = verify_psk_sni_callback;
  dtls_psk.sni_call_back_arg       = valid_sni_list;
  dtls_psk.psk_info.hint.s         = (const uint8_t*)hint;
  dtls_psk.psk_info.hint.length    = hint ? strlen(hint) : 0;
  dtls_psk.psk_info.key.s          = key;
  dtls_psk.psk_info.key.length     = key_len;

  if (coap_context_set_psk2(context, &dtls_psk)) {
    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);

  endpoint = coap_new_endpoint(context, &listen_addr, COAP_PROTO_DTLS);
  if (!endpoint) {
    coap_free_context(context);
    return NULL;
  }

  /* Initialize resources - See coap_resource(3) init_resources() example */

  return context;
}

CoAP Client DTLS PSK Setup

#include <coap3/coap.h>

#include <stdio.h>

#ifndef min
#define min(a,b) ((a) < (b) ? (a) : (b))
#endif

static const coap_dtls_cpsk_info_t *
verify_ih_callback(coap_str_const_t *hint,
                   coap_session_t *c_session,
                   void *arg
) {
  coap_dtls_cpsk_info_t *psk_info = (coap_dtls_cpsk_info_t *)arg;
  /* Remove (void) definition if variable is used */
  (void)c_session;

  coap_log(LOG_INFO, "Identity Hint '%.*s' provided\n", (int)hint->length, hint->s);

  /* Just use the defined information for now as passed in by arg */
  return psk_info;
}

static coap_dtls_cpsk_t dtls_psk;
static char client_sni[256];

static coap_session_t *
setup_client_session_psk (const char *uri,
                          struct in_addr ip_address,
                          const uint8_t *identity,
                          unsigned int identity_len,
                          const uint8_t *key,
                          unsigned int key_len
) {
  coap_session_t *session;
  coap_address_t server;
  /* See coap_context(3) */
  coap_context_t *context = coap_new_context(NULL);

  if (!context)
    return NULL;
  /* See coap_block(3) */
  coap_context_set_block_mode(context,
                              COAP_BLOCK_USE_LIBCOAP | COAP_BLOCK_SINGLE_BODY);


  coap_address_init(&server);
  server.addr.sa.sa_family = AF_INET;
  server.addr.sin.sin_addr = ip_address;
  server.addr.sin.sin_port = htons (5684);

  /* See coap_encryption(3) */
  memset (&dtls_psk, 0, sizeof(dtls_psk));
  dtls_psk.version = COAP_DTLS_CPSK_SETUP_VERSION;
  dtls_psk.validate_ih_call_back = verify_ih_callback;
  dtls_psk.ih_call_back_arg = &dtls_psk.psk_info;
  if (uri)
    memcpy(client_sni, uri, min(strlen(uri), sizeof(client_sni)-1));
  else
    memcpy(client_sni, "localhost", 9);
  dtls_psk.client_sni = client_sni;
  dtls_psk.psk_info.identity.s = identity;
  dtls_psk.psk_info.identity.length = identity_len;
  dtls_psk.psk_info.key.s = key;
  dtls_psk.psk_info.key.length = key_len;
  session = coap_new_client_session_psk2(context, NULL, &server,
                                        COAP_PROTO_DTLS, &dtls_psk);
  if (!session) {
    coap_free_context(context);
    return NULL;
  }
  /* The context is in session->context */
  return session;
}

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 or raise an issue on GitHub at https://github.com/obgm/libcoap/issues

AUTHORS

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

coap_encryption(3)