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diff --git a/PROTOCOL.u2f b/PROTOCOL.u2f new file mode 100644 index 0000000..f8ca56b --- /dev/null +++ b/PROTOCOL.u2f @@ -0,0 +1,309 @@ +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 specifies additional key types, including one based on Ed25519. + +Use of U2F security keys does not automatically imply multi-factor +authentication. From sshd's perspective, a security key constitutes a +single factor of authentication, even if protected by a PIN or biometric +authentication. To enable multi-factor authentication in ssh, please +refer to the AuthenticationMethods option in sshd_config(5). + + +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 + +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 useful for out-of-band key and certificate +registration workflows, e.g. proving to a CA that a key is backed +by trusted hardware before it will issue a certificate. To support this +case, OpenSSH optionally allows retaining the attestation information +at the time of key generation. It will take the following format: + + string "ssh-sk-attest-v01" + string attestation certificate + string enrollment signature + string authenticator data (CBOR encoded) + uint32 reserved flags + string reserved string + +A previous version of this format, emitted prior to OpenSSH 8.4 omitted +the authenticator data. + + string "ssh-sk-attest-v00" + string attestation certificate + string enrollment signature + uint32 reserved flags + string reserved string + +OpenSSH treats the attestation certificate and enrollment signatures as +opaque objects and does no interpretation of them itself. + +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 extensions 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 + +webauthn signatures +------------------- + +The W3C/FIDO webauthn[1] standard defines a mechanism for a web browser to +interact with FIDO authentication tokens. This standard builds upon the +FIDO standards, but requires different signature contents to raw FIDO +messages. OpenSSH supports ECDSA/p256 webauthn signatures through the +"webauthn-sk-ecdsa-sha2-nistp256@openssh.com" signature algorithm. + +The wire encoding for a webauthn-sk-ecdsa-sha2-nistp256@openssh.com +signature is similar to the sk-ecdsa-sha2-nistp256@openssh.com format: + + string "webauthn-sk-ecdsa-sha2-nistp256@openssh.com" + string ecdsa_signature + byte flags + uint32 counter + string origin + string clientData + string extensions + +Where "origin" is the HTTP origin making the signature, "clientData" is +the JSON-like structure signed by the browser and "extensions" are any +extensions used in making the signature. + +[1] https://www.w3.org/TR/webauthn-2/ + +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 and +numbers listed in sk-api.h. Included in the defined numbers is a +SSH_SK_VERSION_MAJOR that should be incremented for each incompatible +API change. + +miscellaneous options may be passed to the middleware as a NULL- +terminated array of pointers to struct sk_option. The middleware may +ignore unsupported or unknown options unless the "required" flag is set, +in which case it should return failure if an unsupported option is +requested. + +At present the following options names are supported: + + "device" + + Specifies a specific FIDO device on which to perform the + operation. The value in this field is interpreted by the + middleware but it would be typical to specify a path to + a /dev node for the device in question. + + "user" + + Specifies the FIDO2 username used when enrolling a key, + overriding OpenSSH's default of using an all-zero username. + +In OpenSSH, the middleware will be invoked by using a similar mechanism to +ssh-pkcs11-helper to provide address-space containment of the +middleware from ssh-agent. + +$OpenBSD: PROTOCOL.u2f,v 1.26 2020/09/09 03:08:01 djm Exp $ |