Annotation of src/usr.bin/ssh/PROTOCOL.u2f, Revision 1.8
1.1 djm 1: This document describes OpenSSH's support for U2F/FIDO security keys.
2:
3: Background
4: ----------
5:
6: U2F is an open standard for two-factor authentication hardware, widely
7: used for user authentication to websites. U2F tokens are ubiquitous,
8: available from a number of manufacturers and are currently by far the
9: cheapest way for users to achieve hardware-backed credential storage.
10:
11: The U2F protocol however cannot be trivially used as an SSH protocol key
12: type as both the inputs to the signature operation and the resultant
13: signature differ from those specified for SSH. For similar reasons,
14: integration of U2F devices cannot be achieved via the PKCS#11 API.
15:
16: U2F also offers a number of features that are attractive in the context
17: of SSH authentication. They can be configured to require indication
18: of "user presence" for each signature operation (typically achieved
19: by requiring the user touch the key). They also offer an attestation
20: mechanism at key enrollment time that can be used to prove that a
21: given key is backed by hardware. Finally the signature format includes
22: a monotonic signature counter that can be used (at scale) to detect
23: concurrent use of a private key, should it be extracted from hardware.
24:
1.2 naddy 25: U2F private keys are generated through an enrollment operation,
1.1 djm 26: which takes an application ID - a URL-like string, typically "ssh:"
27: in this case, but a HTTP origin for the case of web authentication,
28: and a challenge string (typically randomly generated). The enrollment
29: operation returns a public key, a key handle that must be used to invoke
30: the hardware-backed private key, some flags and signed attestation
1.2 naddy 31: information that may be used to verify that a private key is hosted on a
1.1 djm 32: particular hardware instance.
33:
34: It is common for U2F hardware to derive private keys from the key handle
35: in conjunction with a small per-device secret that is unique to the
36: hardware, thus requiring little on-device storage for an effectively
37: unlimited number of supported keys. This drives the requirement that
38: the key handle be supplied for each signature operation. U2F tokens
1.7 djm 39: primarily use ECDSA signatures in the NIST-P256 field, though the FIDO2
40: standard specified additional key types include one based on Ed25519.
1.1 djm 41:
42: SSH U2F Key formats
43: -------------------
44:
1.7 djm 45: OpenSSH integrates U2F as new key and corresponding certificate types:
1.1 djm 46:
47: sk-ecdsa-sha2-nistp256@openssh.com
48: sk-ecdsa-sha2-nistp256-cert-v01@openssh.com
1.7 djm 49: sk-ssh-ed25519@openssh.com
50: sk-ssh-ed25519-cert-v01@openssh.com
1.1 djm 51:
52: These key types are supported only for user authentication with the
53: "publickey" method. They are not used for host-based user authentication
54: or server host key authentication.
55:
56: While each uses ecdsa-sha256-nistp256 as the underlying signature primitive,
57: keys require extra information in the public and private keys, and in
58: the signature object itself. As such they cannot be made compatible with
59: the existing ecdsa-sha2-nistp* key types.
60:
61: The format of a sk-ecdsa-sha2-nistp256@openssh.com public key is:
62:
63: string "sk-ecdsa-sha2-nistp256@openssh.com"
1.5 djm 64: string curve name
1.1 djm 65: ec_point Q
66: string application (user-specified, but typically "ssh:")
67:
68: The corresponding private key contains:
69:
70: string "sk-ecdsa-sha2-nistp256@openssh.com"
1.5 djm 71: string curve name
1.1 djm 72: ec_point Q
73: string application (user-specified, but typically "ssh:")
1.6 djm 74: uint8 flags
1.1 djm 75: string key_handle
76: string reserved
77:
1.7 djm 78: The format of a sk-ssh-ed25519@openssh.com public key is:
79:
80: string "sk-ssh-ed25519@openssh.com"
81: string public key
82: string application (user-specified, but typically "ssh:")
83:
84: With a private half consisting of:
85:
86: string "sk-ssh-ed25519@openssh.com"
87: string public key
88: string application (user-specified, but typically "ssh:")
89: uint32 flags
90: string key_handle
91: string reserved
92:
93: The certificate form for SSH U2F keys appends the usual certificate
1.1 djm 94: information to the public key:
95:
1.2 naddy 96: string "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com"
1.1 djm 97: string nonce
1.5 djm 98: string curve name
1.1 djm 99: ec_point Q
100: string application
101: uint64 serial
102: uint32 type
103: string key id
104: string valid principals
105: uint64 valid after
106: uint64 valid before
107: string critical options
108: string extensions
109: string reserved
110: string signature key
111: string signature
112:
1.7 djm 113: string "sk-ssh-ed25519-cert-v01@openssh.com"
114: string nonce
115: string public key
116: string application
117: uint64 serial
118: uint32 type
119: string key id
120: string valid principals
121: uint64 valid after
122: uint64 valid before
123: string critical options
124: string extensions
125: string reserved
126: string signature key
127: string signature
128:
1.1 djm 129: During key generation, the hardware also returns attestation information
130: that may be used to cryptographically prove that a given key is
131: hardware-backed. Unfortunately, the protocol required for this proof is
132: not privacy-preserving and may be used to identify U2F tokens with at
133: least manufacturer and batch number granularity. For this reason, we
134: choose not to include this information in the public key or save it by
135: default.
136:
137: Attestation information is very useful however in an organisational
1.2 naddy 138: context, where it may be used by a CA as part of certificate
1.1 djm 139: issuance. In this case, exposure to the CA of hardware identity is
140: desirable. To support this case, OpenSSH optionally allows retaining the
141: attestation information at the time of key generation. It will take the
142: following format:
143:
144: string "sk-attest-v00"
145: uint32 version (1 for U2F, 2 for FIDO2 in future)
146: string attestation certificate
147: string enrollment signature
148:
149: SSH U2F signatures
150: ------------------
151:
152: In addition to the message to be signed, the U2F signature operation
153: requires a few additional parameters:
154:
155: byte control bits (e.g. "user presence required" flag)
156: byte[32] SHA256(message)
157: byte[32] SHA256(application)
158: byte key_handle length
159: byte[] key_handle
160:
161: This signature is signed over a blob that consists of:
162:
163: byte[32] SHA256(application)
164: byte flags (including "user present", extensions present)
165: uint32 counter
166: byte[] extensions
167: byte[32] SHA256(message)
168:
169: The signature returned from U2F hardware takes the following format:
170:
171: byte flags (including "user present")
172: uint32 counter
173: byte[32] ecdsa_signature (in X9.62 format).
174:
175: For use in the SSH protocol, we wish to avoid server-side parsing of ASN.1
176: format data in the pre-authentication attack surface. Therefore, the
177: signature format used on the wire in SSH2_USERAUTH_REQUEST packets will
1.8 ! djm 178: be reformatted to better match the existing signature encoding:
1.1 djm 179:
1.8 ! djm 180: string "sk-ecdsa-sha2-nistp256@openssh.com"
! 181: string ecdsa_signature
1.1 djm 182: byte flags
183: uint32 counter
184:
1.8 ! djm 185: Where the "ecdsa_signature" field follows the RFC5656 ECDSA signature
! 186: encoding:
! 187:
! 188: mpint r
! 189: mpint s
1.1 djm 190:
1.4 markus 191: For Ed25519 keys the signature is encoded as:
192:
193: string "sk-ssh-ed25519@openssh.com"
194: string signature
195: byte flags
196: uint32 counter
197:
198:
1.1 djm 199: ssh-agent protocol extensions
200: -----------------------------
201:
1.2 naddy 202: ssh-agent requires a protocol extension to support U2F keys. At
1.1 djm 203: present the closest analogue to Security Keys in ssh-agent are PKCS#11
204: tokens, insofar as they require a middleware library to communicate with
205: the device that holds the keys. Unfortunately, the protocol message used
206: to add PKCS#11 keys to ssh-agent does not include any way to send the
207: key handle to the agent as U2F keys require.
208:
1.2 naddy 209: To avoid this, without having to add wholly new messages to the agent
210: protocol, we will use the existing SSH2_AGENTC_ADD_ID_CONSTRAINED message
211: with a new key constraint extension to encode a path to the middleware
1.1 djm 212: library for the key. The format of this constraint extension would be:
213:
214: byte SSH_AGENT_CONSTRAIN_EXTENSION
215: string sk@openssh.com
216: string middleware path
217:
218: This constraint-based approach does not present any compatibility
219: problems.
220:
221: OpenSSH integration
222: -------------------
223:
224: U2F tokens may be attached via a number of means, including USB and NFC.
225: The USB interface is standardised around a HID protocol, but we want to
226: be able to support other transports as well as dummy implementations for
1.7 djm 227: regress testing. For this reason, OpenSSH shall support a dynamically-
228: loaded middleware libraries to communicate with security keys, but offer
229: support for the common case of USB HID security keys internally.
1.1 djm 230:
231: The middleware library need only expose a handful of functions:
232:
233: /* Flags */
234: #define SSH_SK_USER_PRESENCE_REQD 0x01
235:
1.3 markus 236: /* Algs */
237: #define SSH_SK_ECDSA 0x00
238: #define SSH_SK_ED25519 0x01
239:
1.1 djm 240: struct sk_enroll_response {
241: uint8_t *public_key;
242: size_t public_key_len;
243: uint8_t *key_handle;
244: size_t key_handle_len;
245: uint8_t *signature;
246: size_t signature_len;
247: uint8_t *attestation_cert;
248: size_t attestation_cert_len;
249: };
250:
251: struct sk_sign_response {
252: uint8_t flags;
253: uint32_t counter;
254: uint8_t *sig_r;
255: size_t sig_r_len;
256: uint8_t *sig_s;
257: size_t sig_s_len;
258: };
259:
260: /* Return the version of the middleware API */
261: uint32_t sk_api_version(void);
262:
263: /* Enroll a U2F key (private key generation) */
1.3 markus 264: int sk_enroll(int alg, const uint8_t *challenge, size_t challenge_len,
1.1 djm 265: const char *application, uint8_t flags,
266: struct sk_enroll_response **enroll_response);
267:
268: /* Sign a challenge */
1.3 markus 269: int sk_sign(int alg, const uint8_t *message, size_t message_len,
1.1 djm 270: const char *application,
271: const uint8_t *key_handle, size_t key_handle_len,
272: uint8_t flags, struct sk_sign_response **sign_response);
273:
274: In OpenSSH, these will be invoked by generalising the existing
275: ssh-pkcs11-helper mechanism to provide containment of the middleware from
276: ssh-agent.
277:
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