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Security Area Internet-Draft This document defines protocols, procedures and conventions for a Generic Security Service Application Program Interface (GSS-API) security mechanism that provides key agreement without authentication of either party.

The Generic Security Service Application Program Interface (GSS-API) provides a framework for authentication and message protection services through a common programming interface. The Simple Anonymous mechanism described in this document (hereafter SAnon) is a simple protocol based on the X25519 elliptic curve Diffie–Hellman (ECDH) key agreement scheme defined in . No authentication of initiator or acceptor is provided. A potential use of SAnon is to provide a degree of privacy when bootstrapping unkeyed entities.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

The SAnon mechanism is identified by the following OID:

sanon-x25519 OBJECT IDENTIFIER ::= {iso(1)identified-organization(3)dod(6)internet(1) private(4)enterprise(1)padl(5322)gss-sanon(26) mechanisms(1)sanon-x25519(110)}

The means of discovering GSS-API peers and their supported mechanisms is out of this specification's scope. To avoid multiple layers of negotiation, SAnon is not crypto-agile. A future variant using a different key exchange algorithm would be assigned a different OID. If anonymity is not desired then SAnon MUST NOT be used. Either party can test for the presence of GSS_C_ANON_FLAG to check if anonymous authentication was performed.

The SAnon mechanism can import a variety of name types. A SAnon mechanism name is logically a boolean indicating whether it represents an anonymous identity. However SAnon SHOULD preserve the name type and string so that names round-trip through GSS_Export_name() and GSS_Import_name(). The following table indicates which names represent anonymous identities: Name type Name string Anon GSS_C_NT_USER_NAME WELLKNOWN/ANONYMOUS@WELLKNOWN:ANONYMOUS Y GSS_C_NT_HOSTBASED_SERVICE WELLKNOWN@ANONYMOUS Y GSS_C_NT_DOMAINBASED_SERVICE (see ) WELLKNOWN@ANONYMOUS@ prefix Y GSS_C_NT_ANONYMOUS Any name string Y Any other name type Any name string N

The canonical form of the anonymous name has the display string WELLKNOWN/ANONYMOUS@WELLKNOWN:ANONYMOUS and the GSS_C_NT_ANONYMOUS name type. This is the name observed by a SAnon peer: GSS_Inquire_context() on an established context MUST return this name for both parties. GSS_Canonicalize_name() SHOULD transform names representing anonymous identities to this name.

SAnon uses the mechanism-independent exported name object format defined in Section 3.2. All lengths are encoded as big-endian integers. Length Name Description 2 TOK_ID 04 01 2 MECH_OID_LEN Length of the mechanism OID MECH_OID_LEN MECH_OID The SAnon mechanism OID, in DER 4 NAME_LEN Length of the remaining fields 2 NAME_TYPE_LEN Length of the exported name type NAME_TYPE_LEN NAME_TYPE Name type OID, in DER 4 NAME_STRING_LEN Length of the exported name string NAME_STRING_LEN NAME_STRING Exported name string

The initial context token is framed per Section 1 of :

GSS-API DEFINITIONS ::= BEGIN MechType ::= OBJECT IDENTIFIER -- 1.3.6.1.4.1.5322.26.1.110 GSSAPI-Token ::= [APPLICATION 0] IMPLICIT SEQUENCE { thisMech MechType, innerToken ANY DEFINED BY thisMech -- 32 byte initiator public key } END

On the first call to GSS_Init_sec_context(), the mechanism checks for one of the following: The caller set anon_req_flag (GSS_C_ANON_FLAG) The claimant_cred_handle identity is anonymous (see ) The claimant_cred_handle is the default credential and targ_name is anonymous If none of the above are the case, the call MUST fail with GSS_S_UNAVAILABLE. (Unlike some other mechanisms, SAnon does not require GSS_C_ANON_FLAG be set to request anonymous authentication.) If proceeding, the initiator generates a fresh secret and public key pair per Section 6.1 and returns GSS_S_CONTINUE_NEEDED, indicating that a subsequent context token from the acceptor is expected. The innerToken field of the output_token contains the initiator's 32 byte public key.

Upon receiving a context token from the initiator, the acceptor validates that the token is well formed and contains a public key of the requisite length. The acceptor generates a fresh secret and public key pair. The context session key is computed as specified in . The acceptor constructs an output_token by concatenating its public key with the token emitted by calling GSS_GetMIC() with the default QOP and zero-length octet string. The output token is sent to the initiator without additional framing. The acceptor then returns GSS_S_COMPLETE, setting src_name to the canonical anonymous name. The reply_det_state (GSS_C_REPLAY_FLAG), sequence_state (GSS_C_SEQUENCE_FLAG), conf_avail (GSS_C_CONF_FLAG), integ_avail (GSS_C_INTEG_FLAG) and anon_state (GSS_C_ANON_FLAG) security context flags are set to TRUE. The context is ready to use.

Upon receiving the acceptor context token and verifying it is well formed, the initiator extracts the acceptor's public key (being the first 32 bytes of the input token) and computes the context session key per . The initiator calls GSS_VerifyMIC() with the MIC extracted from the context token and the zero-length octet string. If successful, the initiator returns GSS_S_COMPLETE to the caller, to indicate the initiator is authenticated and the context is ready for use. No output token is emitted. Supported security context flags are as for the acceptor context. The flags returned to the caller are the intersection of supported and requested flags, combined with anon_state (GSS_C_ANON_FLAG) which is set unconditionally.

The per-message tokens definitions are imported from Section 4.2. The base key used to derive specific keys for signing and sealing messages is defined in . The encryption and checksum algorithms use the aes128-cts-hmac-sha256-128 encryption type defined in . The AcceptorSubkey flag as defined in Section 4.2.2 MUST be set.

Context deletion tokens are empty in this mechanism. The behavior of GSS_Delete_sec_context() is as specified in Section 4.3.

The context session key is known as the base key, and is computed using a key derivation function from Section 5.1 (using HMAC as the PRF):

base key = HMAC-SHA-256(K1, i | label | 0x00 | context | L)

where: the output of X25519(local secret key, peer public key) as specified in Section 6.1 the constant 0x00000001, representing the iteration count expressed in big-endian binary representation of 4 bytes the string "sanon-x25519" (without quotation marks) initiator public key | acceptor public key | channel binding application data (if present) the constant 0x00000080, being length in bits of the key to be outputted expressed in big-endian binary representation of 4 bytes The inclusion of channel bindings in the key derivation function means that the acceptor cannot ignore initiator channel bindings; this differs from some other mechanisms. The base key provides the acceptor-asserted subkey defined in Section 2 and is used to generate keys for per-message tokens and the GSS-API PRF. Its encryption type is aes128-cts-hmac-sha256-128 per . The algorithm protocol parameters are as given in Section 5.

The GSS-API pseudo-random function for this mechanism imports the definitions from , using the base key for both GSS_C_PRF_KEY_FULL and GSS_C_PRF_KEY_PARTIAL usages.

This document defines a GSS-API security mechanism, and therefore deals in security and has security considerations text embedded throughout. This section only addresses security considerations associated with the SAnon mechanism described in this document. It does not address security considerations associated with the GSS-API itself. This mechanism provides only for key agreement. It does not authenticate the identity of either party. It MUST NOT be selected if either party requires identification of its peer. The anonymous identity is not a unary one: implementations MUST ensure that GSS_Compare_name() does not compare two anonymous names as being identical (see Section 2.4.3). This caution applies also to non-anonymous names if the implementation does not support round-tripping of imported export names.

AuriStor, Inc funded the design of this protocol, along with an implementation for the Heimdal GSS-API library. Jeffrey Altman, Greg Hudson, Simon Josefsson, and Nicolas Williams provided valuable feedback on this document.

&RFC2119; &RFC2743; &RFC3961; &RFC4121; &RFC4401; &RFC7748; &RFC8009; National Institute of Standards and Technology &NegoEx; &RFC4178; &RFC4757; &RFC5179; &RFC5587; &RFC8062;

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The mechanism attributes for this mechanism are: GSS_C_MA_MECH_CONCRETE GSS_C_MA_ITOK_FRAMED GSS_C_MA_AUTH_INIT_ANON GSS_C_MA_AUTH_TARG_ANON GSS_C_MA_INTEG_PROT GSS_C_MA_CONF_PROT GSS_C_MA_MIC GSS_C_MA_WRAP GSS_C_MA_REPLAY_DET GSS_C_MA_OOS_DET GSS_C_MA_CBINDINGS GSS_C_MA_PFS GSS_C_MA_CTX_TRANS

When SAnon is negotiated by , the authentication scheme identifier is DEE384FF-1086-4E86-BE78-B94170BFD376. The initiator and acceptor keys for NegoEx checksum generation and verification are derived using the GSS-API PRF (see ), with the input data "sanon-x25519-initiator-negoex-key" and "sanon-x25519-acceptor-negoex-key" respectively (without quotation marks). The initiator metadata, if present, contains a set of GSS-API flags encoded as a 4 byte little endian integer. This is used to convey to the acceptor any Windows-specific GSS-API flags (see Section 7.1). Other GSS-API flags MUST NOT be present in the metadata. If the GSS-API implementation supports both SPNEGO and NegoEx, SAnon SHOULD be advertised by both to maximise interoperability.