On Mon, 7 Sep 2020, pedro martelletto wrote:
> > I haven't tried to parse the attestation blob for FIDO2 devices,
> > so it's entirely possible that it doesn't work.
> >
> > We rely on libfido2 to obtain the attestation cert from the token,
> > so if it has functions that let us get at the rest of what we need
> > then it wouldn't be too much work to append/prepend that extra
> > information to the blob.
> >
> > Pedro, do you have any insight here?
> >
> > -d
>
> I agree with Ian -- we need to persist the contents of
> fido_cred_authdata_ptr(), or an external verifier won't be able to form a
> complete picture of what was signed.
>
> Alternatively, we could persist only the AAGUID and the signature counter, and
> let the verifier reconstruct authdata. Since authdata isn't strictly bounded
> (its last constituent part is a CBOR map of variable content and length),
> reconstructing it can get tricky.
Thanks for the pointers - I think the attached patch should do it.
-d
diff --git a/PROTOCOL.u2f b/PROTOCOL.u2f
index 5b8a062..f11494c 100644
--- a/PROTOCOL.u2f
+++ b/PROTOCOL.u2f
@@ -154,6 +154,16 @@ 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 authenicator data
+ 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
diff --git a/sk-api.h b/sk-api.h
index 6bf7c09..7d40679 100644
--- a/sk-api.h
+++ b/sk-api.h
@@ -45,6 +45,8 @@ struct sk_enroll_response {
size_t signature_len;
uint8_t *attestation_cert;
size_t attestation_cert_len;
+ uint8_t *authdata;
+ size_t authdata_len;
};
struct sk_sign_response {
@@ -70,7 +72,7 @@ struct sk_option {
uint8_t required;
};
-#define SSH_SK_VERSION_MAJOR 0x00060000 /* current API version */
+#define SSH_SK_VERSION_MAJOR 0x00070000 /* current API version */
#define SSH_SK_VERSION_MAJOR_MASK 0xffff0000
/* Return the version of the middleware API */
diff --git a/sk-usbhid.c b/sk-usbhid.c
index 9dde5c1..e36930e 100644
--- a/sk-usbhid.c
+++ b/sk-usbhid.c
@@ -781,6 +781,16 @@ sk_enroll(uint32_t alg, const uint8_t *challenge, size_t challenge_len,
memcpy(response->attestation_cert, ptr, len);
response->attestation_cert_len = len;
}
+ if ((ptr = fido_cred_authdata_ptr(cred)) != NULL) {
+ len = fido_cred_authdata_len(cred);
+ debug3("%s: authdata len=%zu", __func__, len);
+ if ((response->authdata = calloc(1, len)) == NULL) {
+ skdebug(__func__, "calloc authdata failed");
+ goto out;
+ }
+ memcpy(response->authdata, ptr, len);
+ response->authdata_len = len;
+ }
*enroll_response = response;
response = NULL;
ret = 0;
@@ -791,6 +801,7 @@ sk_enroll(uint32_t alg, const uint8_t *challenge, size_t challenge_len,
free(response->key_handle);
free(response->signature);
free(response->attestation_cert);
+ free(response->authdata);
free(response);
}
sk_close(sk);
diff --git a/ssh-sk.c b/ssh-sk.c
index 6a2422f..cd1645a 100644
--- a/ssh-sk.c
+++ b/ssh-sk.c
@@ -409,6 +409,30 @@ make_options(const char *device, const char *user_id,
return ret;
}
+
+static int
+fill_attestation_cert(struct sk_enroll_response *resp, struct sshbuf *attest)
+{
+ int r;
+
+ if (attest == NULL)
+ return 0; /* nothing to do */
+ if ((r = sshbuf_put_cstring(attest, "ssh-sk-attest-v01")) != 0 ||
+ (r = sshbuf_put_string(attest,
+ resp->attestation_cert, resp->attestation_cert_len)) != 0 ||
+ (r = sshbuf_put_string(attest,
+ resp->signature, resp->signature_len)) != 0 ||
+ (r = sshbuf_put_string(attest,
+ resp->authdata, resp->authdata_len)) != 0 ||
+ (r = sshbuf_put_u32(attest, 0)) != 0 || /* resvd flags */
+ (r = sshbuf_put_string(attest, NULL, 0)) != 0 /* resvd */) {
+ error("%s: buffer error: %s", __func__, ssh_err(r));
+ return r;
+ }
+ /* success */
+ return 0;
+}
+
int
sshsk_enroll(int type, const char *provider_path, const char *device,
const char *application, const char *userid, uint8_t flags,
@@ -496,19 +520,9 @@ sshsk_enroll(int type, const char *provider_path, const char *device,
goto out;
/* Optionally fill in the attestation information */
- if (attest != NULL) {
- if ((r = sshbuf_put_cstring(attest,
- "ssh-sk-attest-v00")) != 0 ||
- (r = sshbuf_put_string(attest,
- resp->attestation_cert, resp->attestation_cert_len)) != 0 ||
- (r = sshbuf_put_string(attest,
- resp->signature, resp->signature_len)) != 0 ||
- (r = sshbuf_put_u32(attest, 0)) != 0 || /* resvd flags */
- (r = sshbuf_put_string(attest, NULL, 0)) != 0 /* resvd */) {
- error("%s: buffer error: %s", __func__, ssh_err(r));
- goto out;
- }
- }
+ if ((r = fill_attestation_cert(resp, attest)) != 0)
+ goto out;
+
/* success */
*keyp = key;
key = NULL; /* transferred */
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