NAME

     fido2-assert — get/verify a FIDO2 assertion

SYNOPSIS

     fido2-assert -G [-bdhpruv] [-t option] [-i input_file] [-o output_file] device
     fido2-assert -V [-dhpv] [-i input_file] key_file [type]

DESCRIPTION

     fido2-assert gets or verifies a FIDO2 assertion.

     The input of fido2-assert is defined by the parameters of the assertion to be
     obtained/verified.  See the INPUT FORMAT section for details.

     The output of fido2-assert is defined by the result of the selected operation.  See the
     OUTPUT FORMAT section for details.

     If an assertion is successfully obtained or verified, fido2-assert exits 0.  Otherwise,
     fido2-assert exits 1.

     The options are as follows:

     -G      Tells fido2-assert to obtain a new assertion from device.

     -V      Tells fido2-assert to verify an assertion using the PEM-encoded public key in
             key_file of type type, where type may be es256 (denoting ECDSA over NIST P-256 with
             SHA-256), rs256 (denoting 2048-bit RSA with PKCS#1.5 padding and SHA-256), or eddsa
             (denoting EDDSA over Curve25519 with SHA-512).  If type is not specified, es256 is
             assumed.

     -b      Request the credential's “largeBlobKey”, a 32-byte symmetric key associated with the
             asserted credential.

     -h      If obtaining an assertion, enable the FIDO2 hmac-secret extension.  If verifying an
             assertion, check whether the extension data bit was signed by the authenticator.

     -d      Causes fido2-assert to emit debugging output on stderr.

     -i input_file
             Tells fido2-assert to read the parameters of the assertion from input_file instead
             of stdin.

     -o output_file
             Tells fido2-assert to write output on output_file instead of stdout.

     -p      If obtaining an assertion, request user presence.  If verifying an assertion, check
             whether the user presence bit was signed by the authenticator.

     -r      Obtain an assertion using a resident credential.  If -r is specified, fido2-assert
             will not expect a credential id in its input, and may output multiple assertions.
             Resident credentials are called “discoverable credentials” in CTAP 2.1.

     -t option
             Toggles a key/value option, where option is a string of the form “key=value”.  The
             options supported at present are:

             up=true|false
                     Asks the authenticator for user presence to be enabled or disabled.

             uv=true|false
                     Asks the authenticator for user verification to be enabled or disabled.

             pin=true|false
                     Tells fido2-assert whether to prompt for a PIN and request user
                     verification.

             The -t option may be specified multiple times.

     -u      Obtain an assertion using U2F.  By default, fido2-assert will use FIDO2 if supported
             by the authenticator, and fallback to U2F otherwise.

     -v      If obtaining an assertion, prompt the user for a PIN and request user verification
             from the authenticator.  If verifying an assertion, check whether the user
             verification bit was signed by the authenticator.

     If a tty is available, fido2-assert will use it to obtain the PIN.  Otherwise, stdin is
     used.

INPUT FORMAT

     The input of fido2-assert consists of base64 blobs and UTF-8 strings separated by newline
     characters ('\n').

     When obtaining an assertion, fido2-assert expects its input to consist of:

           1.   client data hash (base64 blob);
           2.   relying party id (UTF-8 string);
           3.   credential id, if credential not resident (base64 blob);
           4.   hmac salt, if the FIDO2 hmac-secret extension is enabled (base64 blob);

     When verifying an assertion, fido2-assert expects its input to consist of:

           1.   client data hash (base64 blob);
           2.   relying party id (UTF-8 string);
           3.   authenticator data (base64 blob);
           4.   assertion signature (base64 blob);

     UTF-8 strings passed to fido2-assert must not contain embedded newline or NUL characters.

OUTPUT FORMAT

     The output of fido2-assert consists of base64 blobs and UTF-8 strings separated by newline
     characters ('\n').

     For each generated assertion, fido2-assert outputs:

           1.   client data hash (base64 blob);
           2.   relying party id (UTF-8 string);
           3.   authenticator data (base64 blob);
           4.   assertion signature (base64 blob);
           5.   user id, if credential resident (base64 blob);
           6.   hmac secret, if the FIDO2 hmac-secret extension is enabled (base64 blob);
           7.   the credential's associated 32-byte symmetric key (“largeBlobKey”), if requested
                (base64 blob).

     When verifying an assertion, fido2-assert produces no output.

EXAMPLES

     Assuming cred contains a es256 credential created according to the steps outlined in
     fido2-cred(1), obtain an assertion from an authenticator at /dev/hidraw5 and verify it:

           $ echo assertion challenge | openssl sha256 -binary | base64 > assert_param
           $ echo relying party >> assert_param
           $ head -1 cred >> assert_param
           $ tail -n +2 cred > pubkey
           $ fido2-assert -G -i assert_param /dev/hidraw5 | fido2-assert -V pubkey es256

SEE ALSO

     fido2-cred(1), fido2-token(1)