Channel Bindings

For many applications, basic context establishment is sufficient to assure proper authentication of a context initiator. In cases where additional security is desired, the GSS-API offers the use of channel bindings. Channel bindings are tags that identify the particular data channel being used — that is, the origin and endpoint (initiator and acceptor) of the context. Because these tags are specific to the originator and recipient applications, they offer more proof of a valid identity.

Channel bindings are pointed to by the gss_channel_bindings_t data type, which is a pointer to a gss_channel_bindings_struct structure as shown in Example 1–7:

Example 1–7 gss_channel_bindings_t

typedef struct gss_channel_bindings_struct {
OM_uint32       initiator_addrtype;
gss_buffer_desc initiator_address;
OM_uint32       acceptor_addrtype;
gss_buffer_desc acceptor_address;
gss_buffer_desc application_data;
} *gss_channel_bindings_t;

The first two fields are the address of the initiator along with an address type that identifies the format in which the initiator's address is being sent. For example, the inititiator_addrtype might be sent to GSS_C_AF_INET to indicate that the initiator_address is in the form of an Internet address — that is, an IP address. Similarly, the third and fourth fields indicate the address and address type of the acceptor. The final field, application_data, can be used by the application as it wants (it's good programming practice to set it to GSS_C_NO_BUFFER if you're not going to use it). If an application does not want to specify an address, it should set its address type field to GSS_C_AF_NULLADDR. Address Types for Channel Bindings has a list of valid address type values.

These address types indicate address families, rather than specific addressing formats. For address families that contain several alternative address forms, the initiator_address and acceptor_address fields must contain sufficient information to determine which address form is used. When not otherwise specified, addresses should be specified in network byte-order (that is, native byte-ordering for the address family).

To establish a context using channel bindings, the input_chan_bindings argument for gss_init_sec_context() should point to an allocated channel bindings structure. The function concatenates the structure's fields into an octet string, calculates a MIC over this string, and binds the MIC to the output token produced by gss_init_sec_context(). The application then sends the token to the context acceptor, which receives it and calls gss_accept_sec_context(). (See Context Acceptance (Server).) gss_accept_sec_context() calculates a MIC on the received channel bindings and returns GSS_C_BAD_BINDINGS if the MIC does not match.

Because gss_accept_sec_context() returns the transmitted channel bindings, an acceptor can do its own security checking based on the received channel binding values. For example, it might check the value of application_data against code words kept in a secure database. However, in many cases this is “overkill.”

An underlying mechanism might or might not provide confidentiality for channel binding information. Therefore, an application should not include sensitive information as part of channel bindings unless it knows that confidentiality is ensured. The application might check the ret_flags argument of gss_init_sec_context() or gss_accept_sec_context(), especially for the values GSS_C_CONF_FLAG and GSS_C_PROT_READY_FLAG in order to determine if confidentiality is available. See Context Initiation (Client) or Context Acceptance (Server) for information on ret_flags.

Individual mechanisms can impose additional constraints on addresses and address types that can appear in channel bindings. For example, a mechanism can verify that the initiator_address field of the channel bindings presented to gss_init_sec_context() contains the correct network address of the host system. Portable applications should therefore ensure that they either provide correct information for the address fields or omit addressing information, specifying GSS_C_AF_NULLADDR as the address types.