PROTOCOL.u2f - platform/external/openssh - Gitiles

 This document describes OpenSSH's support for U2F/FIDO security keys.

 Background
 ----------

 U2F is an open standard for two-factor authentication hardware, widely
 used for user authentication to websites. U2F tokens are ubiquitous,
 available from a number of manufacturers and are currently by far the
 cheapest way for users to achieve hardware-backed credential storage.

 The U2F protocol however cannot be trivially used as an SSH protocol key
 type as both the inputs to the signature operation and the resultant
 signature differ from those specified for SSH. For similar reasons,
 integration of U2F devices cannot be achieved via the PKCS#11 API.

 U2F also offers a number of features that are attractive in the context
 of SSH authentication. They can be configured to require indication
 of "user presence" for each signature operation (typically achieved
 by requiring the user touch the key). They also offer an attestation
 mechanism at key enrollment time that can be used to prove that a
 given key is backed by hardware. Finally the signature format includes
 a monotonic signature counter that can be used (at scale) to detect
 concurrent use of a private key, should it be extracted from hardware.

 U2F private keys are generated through an enrollment operation,
 which takes an application ID - a URL-like string, typically "ssh:"
 in this case, but a HTTP origin for the case of web authentication,
 and a challenge string (typically randomly generated). The enrollment
 operation returns a public key, a key handle that must be used to invoke
 the hardware-backed private key, some flags and signed attestation
 information that may be used to verify that a private key is hosted on a
 particular hardware instance.

 It is common for U2F hardware to derive private keys from the key handle
 in conjunction with a small per-device secret that is unique to the
 hardware, thus requiring little on-device storage for an effectively
 unlimited number of supported keys. This drives the requirement that
 the key handle be supplied for each signature operation. U2F tokens
 primarily use ECDSA signatures in the NIST-P256 field, though the FIDO2
 standard specified additional key types include one based on Ed25519.

 SSH U2F Key formats
 -------------------

 OpenSSH integrates U2F as new key and corresponding certificate types:

 	sk-ecdsa-sha2-nistp256@openssh.com
 	sk-ecdsa-sha2-nistp256-cert-v01@openssh.com
 	sk-ssh-ed25519@openssh.com
 	sk-ssh-ed25519-cert-v01@openssh.com

 These key types are supported only for user authentication with the
 "publickey" method. They are not used for host-based user authentication
 or server host key authentication.

 While each uses ecdsa-sha256-nistp256 as the underlying signature primitive,
 keys require extra information in the public and private keys, and in
 the signature object itself. As such they cannot be made compatible with
 the existing ecdsa-sha2-nistp* key types.

 The format of a sk-ecdsa-sha2-nistp256@openssh.com public key is:

 	string		"sk-ecdsa-sha2-nistp256@openssh.com"
 	string		curve name
 	ec_point	Q
 	string		application (user-specified, but typically "ssh:")

 The corresponding private key contains:

 	string		"sk-ecdsa-sha2-nistp256@openssh.com"
 	string		curve name
 	ec_point	Q
 	string		application (user-specified, but typically "ssh:")
 	uint8		flags
 	string		key_handle
 	string		reserved

 The format of a sk-ssh-ed25519@openssh.com public key is:

 	string		"sk-ssh-ed25519@openssh.com"
 	string		public key
 	string		application (user-specified, but typically "ssh:")

 With a private half consisting of:

 	string		"sk-ssh-ed25519@openssh.com"
 	string		public key
 	string		application (user-specified, but typically "ssh:")
 	uint8		flags
 	string		key_handle
 	string		reserved

 The certificate form for SSH U2F keys appends the usual certificate
 information to the public key:

 	string		"sk-ecdsa-sha2-nistp256-cert-v01@openssh.com"
 	string		nonce
 	string		curve name
 	ec_point	Q
 	string		application
 	uint64		serial
 	uint32		type
 	string		key id
 	string		valid principals
 	uint64		valid after
 	uint64		valid before
 	string		critical options
 	string		extensions
 	string		reserved
 	string		signature key
 	string		signature

 and for security key ed25519 certificates:

 	string		"sk-ssh-ed25519-cert-v01@openssh.com"
 	string		nonce
 	string		public key
 	string		application
 	uint64		serial
 	uint32		type
 	string		key id
 	string		valid principals
 	uint64		valid after
 	uint64		valid before
 	string		critical options
 	string		extensions
 	string		reserved
 	string		signature key
 	string		signature

 Both security key certificates use the following encoding for private keys:

 	string		type (e.g. "sk-ssh-ed25519-cert-v01@openssh.com")
 	string		pubkey (the above key/cert structure)
 	string		application
 	uint8		flags
 	string		key_handle
 	string		reserved

 During key generation, the hardware also returns attestation information
 that may be used to cryptographically prove that a given key is
 hardware-backed. Unfortunately, the protocol required for this proof is
 not privacy-preserving and may be used to identify U2F tokens with at
 least manufacturer and batch number granularity. For this reason, we
 choose not to include this information in the public key or save it by
 default.

 Attestation information is very useful however in an organisational
 context, where it may be used by a CA as part of certificate
 issuance. In this case, exposure to the CA of hardware identity is
 desirable. To support this case, OpenSSH optionally allows retaining the
 attestation information at the time of key generation. It will take the
 following format:

 	string		"sk-attest-v00"
 	uint32		version		(1 for U2F, 2 for FIDO2 in future)
 	string		attestation certificate
 	string		enrollment signature

 SSH U2F signatures
 ------------------

 In addition to the message to be signed, the U2F signature operation
 requires the key handle and a few additional parameters. The signature
 is signed over a blob that consists of:

 	byte[32]	SHA256(application)
 	byte		flags (including "user present", extensions present)
 	uint32		counter
 	byte[]		extensions
 	byte[32]	SHA256(message)

 No extensons are yet defined for SSH use. If any are defined in the future,
 it will be possible to infer their presence from the contents of the "flags"
 value.

 The signature returned from U2F hardware takes the following format:

 	byte		flags (including "user present")
 	uint32		counter
 	byte[]		ecdsa_signature (in X9.62 format).

 For use in the SSH protocol, we wish to avoid server-side parsing of ASN.1
 format data in the pre-authentication attack surface. Therefore, the
 signature format used on the wire in SSH2_USERAUTH_REQUEST packets will
 be reformatted to better match the existing signature encoding:

 	string		"sk-ecdsa-sha2-nistp256@openssh.com"
 	string		ecdsa_signature
 	byte		flags
 	uint32		counter

 Where the "ecdsa_signature" field follows the RFC5656 ECDSA signature
 encoding:

 	mpint		r
 	mpint		s

 For Ed25519 keys the signature is encoded as:

 	string		"sk-ssh-ed25519@openssh.com"
 	string		signature
 	byte		flags
 	uint32		counter

 ssh-agent protocol extensions
 -----------------------------

 ssh-agent requires a protocol extension to support U2F keys. At
 present the closest analogue to Security Keys in ssh-agent are PKCS#11
 tokens, insofar as they require a middleware library to communicate with
 the device that holds the keys. Unfortunately, the protocol message used
 to add PKCS#11 keys to ssh-agent does not include any way to send the
 key handle to the agent as U2F keys require.

 To avoid this, without having to add wholly new messages to the agent
 protocol, we will use the existing SSH2_AGENTC_ADD_ID_CONSTRAINED message
 with a new key constraint extension to encode a path to the middleware
 library for the key. The format of this constraint extension would be:

 	byte		SSH_AGENT_CONSTRAIN_EXTENSION
 	string		sk-provider@openssh.com
 	string		middleware path

 This constraint-based approach does not present any compatibility
 problems.

 OpenSSH integration
 -------------------

 U2F tokens may be attached via a number of means, including USB and NFC.
 The USB interface is standardised around a HID protocol, but we want to
 be able to support other transports as well as dummy implementations for
 regress testing. For this reason, OpenSSH shall support a dynamically-
 loaded middleware libraries to communicate with security keys, but offer
 support for the common case of USB HID security keys internally.

 The middleware library need only expose a handful of functions:

 	/* Flags */
 	#define SSH_SK_USER_PRESENCE_REQD	0x01

 	/* Algs */
 	#define SSH_SK_ECDSA                   0x00
 	#define SSH_SK_ED25519                 0x01

 	struct sk_enroll_response {
 		uint8_t *public_key;
 		size_t public_key_len;
 		uint8_t *key_handle;
 		size_t key_handle_len;
 		uint8_t *signature;
 		size_t signature_len;
 		uint8_t *attestation_cert;
 		size_t attestation_cert_len;
 	};

 	struct sk_sign_response {
 		uint8_t flags;
 		uint32_t counter;
 		uint8_t *sig_r;
 		size_t sig_r_len;
 		uint8_t *sig_s;
 		size_t sig_s_len;
 	};

 	/* Return the version of the middleware API */
 	uint32_t sk_api_version(void);

 	/* Enroll a U2F key (private key generation) */
 	int sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
 	    const char *application, uint8_t flags,
 	    struct sk_enroll_response **enroll_response);

 	/* Sign a challenge */
 	int sk_sign(int alg, const uint8_t *message, size_t message_len,
 	    const char *application,
 	    const uint8_t *key_handle, size_t key_handle_len,
 	    uint8_t flags, struct sk_sign_response **sign_response);

 In OpenSSH, these will be invoked by using a similar mechanism to
 ssh-pkcs11-helper to provide address-space containment of the
 middleware from ssh-agent.
	This document describes OpenSSH's support for U2F/FIDO security keys.

	Background
	----------

	U2F is an open standard for two-factor authentication hardware, widely
	used for user authentication to websites. U2F tokens are ubiquitous,
	available from a number of manufacturers and are currently by far the
	cheapest way for users to achieve hardware-backed credential storage.

	The U2F protocol however cannot be trivially used as an SSH protocol key
	type as both the inputs to the signature operation and the resultant
	signature differ from those specified for SSH. For similar reasons,
	integration of U2F devices cannot be achieved via the PKCS#11 API.

	U2F also offers a number of features that are attractive in the context
	of SSH authentication. They can be configured to require indication
	of "user presence" for each signature operation (typically achieved
	by requiring the user touch the key). They also offer an attestation
	mechanism at key enrollment time that can be used to prove that a
	given key is backed by hardware. Finally the signature format includes
	a monotonic signature counter that can be used (at scale) to detect
	concurrent use of a private key, should it be extracted from hardware.

	U2F private keys are generated through an enrollment operation,
	which takes an application ID - a URL-like string, typically "ssh:"
	in this case, but a HTTP origin for the case of web authentication,
	and a challenge string (typically randomly generated). The enrollment
	operation returns a public key, a key handle that must be used to invoke
	the hardware-backed private key, some flags and signed attestation
	information that may be used to verify that a private key is hosted on a
	particular hardware instance.

	It is common for U2F hardware to derive private keys from the key handle
	in conjunction with a small per-device secret that is unique to the
	hardware, thus requiring little on-device storage for an effectively
	unlimited number of supported keys. This drives the requirement that
	the key handle be supplied for each signature operation. U2F tokens
	primarily use ECDSA signatures in the NIST-P256 field, though the FIDO2
	standard specified additional key types include one based on Ed25519.

	SSH U2F Key formats
	-------------------

	OpenSSH integrates U2F as new key and corresponding certificate types:

	sk-ecdsa-sha2-nistp256@openssh.com
	sk-ecdsa-sha2-nistp256-cert-v01@openssh.com
	sk-ssh-ed25519@openssh.com
	sk-ssh-ed25519-cert-v01@openssh.com

	These key types are supported only for user authentication with the
	"publickey" method. They are not used for host-based user authentication
	or server host key authentication.

	While each uses ecdsa-sha256-nistp256 as the underlying signature primitive,
	keys require extra information in the public and private keys, and in
	the signature object itself. As such they cannot be made compatible with
	the existing ecdsa-sha2-nistp* key types.

	The format of a sk-ecdsa-sha2-nistp256@openssh.com public key is:

	string "sk-ecdsa-sha2-nistp256@openssh.com"
	string curve name
	ec_point Q
	string application (user-specified, but typically "ssh:")

	The corresponding private key contains:

	string "sk-ecdsa-sha2-nistp256@openssh.com"
	string curve name
	ec_point Q
	string application (user-specified, but typically "ssh:")
	uint8 flags
	string key_handle
	string reserved

	The format of a sk-ssh-ed25519@openssh.com public key is:

	string "sk-ssh-ed25519@openssh.com"
	string public key
	string application (user-specified, but typically "ssh:")

	With a private half consisting of:

	string "sk-ssh-ed25519@openssh.com"
	string public key
	string application (user-specified, but typically "ssh:")
	uint8 flags
	string key_handle
	string reserved

	The certificate form for SSH U2F keys appends the usual certificate
	information to the public key:

	string "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com"
	string nonce
	string curve name
	ec_point Q
	string application
	uint64 serial
	uint32 type
	string key id
	string valid principals
	uint64 valid after
	uint64 valid before
	string critical options
	string extensions
	string reserved
	string signature key
	string signature

	and for security key ed25519 certificates:

	string "sk-ssh-ed25519-cert-v01@openssh.com"
	string nonce
	string public key
	string application
	uint64 serial
	uint32 type
	string key id
	string valid principals
	uint64 valid after
	uint64 valid before
	string critical options
	string extensions
	string reserved
	string signature key
	string signature

	Both security key certificates use the following encoding for private keys:

	string type (e.g. "sk-ssh-ed25519-cert-v01@openssh.com")
	string pubkey (the above key/cert structure)
	string application
	uint8 flags
	string key_handle
	string reserved

	During key generation, the hardware also returns attestation information
	that may be used to cryptographically prove that a given key is
	hardware-backed. Unfortunately, the protocol required for this proof is
	not privacy-preserving and may be used to identify U2F tokens with at
	least manufacturer and batch number granularity. For this reason, we
	choose not to include this information in the public key or save it by
	default.

	Attestation information is very useful however in an organisational
	context, where it may be used by a CA as part of certificate
	issuance. In this case, exposure to the CA of hardware identity is
	desirable. To support this case, OpenSSH optionally allows retaining the
	attestation information at the time of key generation. It will take the
	following format:

	string "sk-attest-v00"
	uint32 version (1 for U2F, 2 for FIDO2 in future)
	string attestation certificate
	string enrollment signature

	SSH U2F signatures
	------------------

	In addition to the message to be signed, the U2F signature operation
	requires the key handle and a few additional parameters. The signature
	is signed over a blob that consists of:

	byte[32] SHA256(application)
	byte flags (including "user present", extensions present)
	uint32 counter
	byte[] extensions
	byte[32] SHA256(message)

	No extensons are yet defined for SSH use. If any are defined in the future,
	it will be possible to infer their presence from the contents of the "flags"
	value.

	The signature returned from U2F hardware takes the following format:

	byte flags (including "user present")
	uint32 counter
	byte[] ecdsa_signature (in X9.62 format).

	For use in the SSH protocol, we wish to avoid server-side parsing of ASN.1
	format data in the pre-authentication attack surface. Therefore, the
	signature format used on the wire in SSH2_USERAUTH_REQUEST packets will
	be reformatted to better match the existing signature encoding:

	string "sk-ecdsa-sha2-nistp256@openssh.com"
	string ecdsa_signature
	byte flags
	uint32 counter

	Where the "ecdsa_signature" field follows the RFC5656 ECDSA signature
	encoding:

	mpint r
	mpint s

	For Ed25519 keys the signature is encoded as:

	string "sk-ssh-ed25519@openssh.com"
	string signature
	byte flags
	uint32 counter

	ssh-agent protocol extensions
	-----------------------------

	ssh-agent requires a protocol extension to support U2F keys. At
	present the closest analogue to Security Keys in ssh-agent are PKCS#11
	tokens, insofar as they require a middleware library to communicate with
	the device that holds the keys. Unfortunately, the protocol message used
	to add PKCS#11 keys to ssh-agent does not include any way to send the
	key handle to the agent as U2F keys require.

	To avoid this, without having to add wholly new messages to the agent
	protocol, we will use the existing SSH2_AGENTC_ADD_ID_CONSTRAINED message
	with a new key constraint extension to encode a path to the middleware
	library for the key. The format of this constraint extension would be:

	byte SSH_AGENT_CONSTRAIN_EXTENSION
	string sk-provider@openssh.com
	string middleware path

	This constraint-based approach does not present any compatibility
	problems.

	OpenSSH integration
	-------------------

	U2F tokens may be attached via a number of means, including USB and NFC.
	The USB interface is standardised around a HID protocol, but we want to
	be able to support other transports as well as dummy implementations for
	regress testing. For this reason, OpenSSH shall support a dynamically-
	loaded middleware libraries to communicate with security keys, but offer
	support for the common case of USB HID security keys internally.

	The middleware library need only expose a handful of functions:

	/* Flags */
	#define SSH_SK_USER_PRESENCE_REQD 0x01

	/* Algs */
	#define SSH_SK_ECDSA 0x00
	#define SSH_SK_ED25519 0x01

	struct sk_enroll_response {
	uint8_t *public_key;
	size_t public_key_len;
	uint8_t *key_handle;
	size_t key_handle_len;
	uint8_t *signature;
	size_t signature_len;
	uint8_t *attestation_cert;
	size_t attestation_cert_len;
	};

	struct sk_sign_response {
	uint8_t flags;
	uint32_t counter;
	uint8_t *sig_r;
	size_t sig_r_len;
	uint8_t *sig_s;
	size_t sig_s_len;
	};

	/* Return the version of the middleware API */
	uint32_t sk_api_version(void);

	/* Enroll a U2F key (private key generation) */
	int sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
	const char *application, uint8_t flags,
	struct sk_enroll_response **enroll_response);

	/* Sign a challenge */
	int sk_sign(int alg, const uint8_t *message, size_t message_len,
	const char *application,
	const uint8_t *key_handle, size_t key_handle_len,
	uint8_t flags, struct sk_sign_response **sign_response);

	In OpenSSH, these will be invoked by using a similar mechanism to
	ssh-pkcs11-helper to provide address-space containment of the
	middleware from ssh-agent.