Context Token Encapsulate/Decapsulate and OID Comparison Functions for the Generic Security Service Application Program Interface (GSS-API)

The Generic Security Service Application Program Interface (GSS-API) is a framework that provides security services to applications using a variety of authentication mechanisms. There are widely implemented C bindings for the abstract interface. For initial context tokens a mechanism-independent token format may be used, see section 3.1 of . Some protocols, e.g., SASL GS2 , need the ability to add and remove this token header which contains some ASN.1 tags, a length and the mechanism OID to and from context tokens. This document adds two GSS-API interfaces (GSS_Encapsulate_token and GSS_Decapsulate_token) so that GSS-API libraries can provide this functionality. Being able to compare OIDs is useful, for example when validating that a negotiated mechanism matched the requested one. This document adds one GSS-API interface (GSS_OID_equal) for this purpose. The intention is that text from this specification should be possible to use for implementation documentation, and for this reason this entire document should be considered a code component.

The document uses terms from, and is structured in a similar way as, and . The normative reference to is for the C types "gss_const_buffer_t" and "gss_const_OID", nothing else from that document is required to implement this document.

Inputs: o input_token OCTET STRING -- buffer with token data to encapsulate o token_oid OBJECT IDENTIFIER -- object identifier of mechanism for the token Outputs: o major_status INTEGER o output_token OCTET STRING -- Encapsulated token data; caller must release with GSS_Release_buffer() Return major_status codes: o GSS_S_COMPLETE indicates successful completion, and that output parameters holds correct information. o GSS_S_FAILURE indicates that encapsulation failed for reasons unspecified at the GSS-API level. GSS_Encapsulate_token() is used to add the mechanism-independent token header to GSS-API context token data.

OM_uint32 gss_encapsulate_token ( gss_const_buffer_t input_token, gss_const_OID token_oid, gss_buffer_t output_token) Purpose: Add the mechanism-independent token header to GSS-API context token data. Parameters: input_token buffer, opaque, read Buffer with GSS-API context token data. token_oid Object ID, read Object identifier of token. output_token buffer, opaque, modify Encapsulated token data; caller must release with gss_release_buffer(). Function value: GSS status code GSS_S_COMPLETE Indicates successful completion, and that output parameters holds correct information. GSS_S_FAILURE Indicates that encapsulation failed for reasons unspecified at the GSS-API level.

Inputs: o input_token OCTET STRING -- buffer with token to decapsulate o token_oid OBJECT IDENTIFIER -- expected object identifier of token Outputs: o major_status INTEGER o output_token OCTET STRING -- Decapsulated token data; caller must release with GSS_Release_buffer() Return major_status codes: o GSS_S_COMPLETE indicates successful completion, and that output parameters holds correct information. o GSS_S_DEFECTIVE_TOKEN means that the token failed consistency checks (e.g., OID mismatch or ASN.1 DER length errors). o GSS_S_FAILURE indicates that decapsulation failed for reasons unspecified at the GSS-API level. GSS_Decapsulate_token() is used to remove the mechanism- independent token header from an initial GSS-API context token.

OM_uint32 gss_decapsulate_token ( gss_const_buffer_t input_token, gss_const_OID token_oid, gss_buffer_t output_token) Purpose: Remove the mechanism-independent token header from an initial GSS-API context token. Parameters: input_token buffer, opaque, read Buffer with GSS-API context token. token_oid Object ID, read Expected object identifier of token. output_token buffer, opaque, modify Decapsulated token data; caller must release with gss_release_buffer(). Function value: GSS status code GSS_S_COMPLETE Indicates successful completion, and that output parameters holds correct information. GSS_S_DEFECTIVE_TOKEN Means that the token failed consistency checks (e.g., OID mismatch or ASN.1 DER length errors). GSS_S_FAILURE Indicates that decapsulation failed for reasons unspecified at the GSS-API level.

Inputs: o first_oid OBJECT IDENTIFIER -- first object identifier to compare o second_oid OBJECT IDENTIFIER -- second object identifier to compare Return codes: o non-0 when neither OID is GSS_C_NO_OID and the two OIDs are equal. o 0 when the two OIDs are not identical or either OID is equal to GSS_C_NO_OID. GSS_OID_equal() is used to add compare two OIDs for equality. The value GSS_C_NO_OID will not match any OID, including GSS_C_NO_OID itself.

extern int gss_oid_equal ( gss_const_OID first_oid, gss_const_OID second_oid ) Purpose: Compare two OIDs for equality. The value GSS_C_NO_OID will not match any OID, including GSS_C_NO_OID itself. Parameters: first_oid Object ID, read First object identifier to compare. second_oid Object ID, read Second object identifier to compare. Function value: GSS status code non-0 Neither OID is GSS_C_NO_OID and the two OIDs are equal. 0 When the two OIDs are not identical or either OID is equal to GSS_C_NO_OID.

For the GSS_Encapsulate_token function, if the "input_token" buffer is the 3 bytes octet sequence "foo" and the "token_oid" OID is 1.2.840.113554.1.2.2, which encoded corresponds to the 9 bytes long octet sequence (using C notation) "\x2a\x86\x48\x86\xf7\x12\x01\x02\x02", the output should be the 16 byte long octet sequence (again in C notation) "\x60\x0e\x06\x09\x2a\x86\x48\x86\xf7\x12\x01\x02\x02\x66\x6f\x6f". These values may be used to also test the GSS_Decapsulate_token interface.

Greg Hudson pointed out the 'const' problem with the C bindings in earlier versions of this document, and Luke Howard suggested to resolve it by using the RFC 5587 types. Stephen Farrell suggested several editorial improvements and the security consideration regarding absent security features of the encapsulation function. Chris Lonvick suggested some improvements.

None.

The security considerations of the base GSS-API specification () and the base C bindings () are inherited. Encapsulation of data does not provide any kind of integrity or confidentiality. Implementations needs to treat input as potentially untrustworthy for purposes of dereferencing memory objects to avoid security vulnerabilities. In particular, ASN.1 DER length fields is a common source of mistakes.