neulab/tldr huggingface dataset:
Depending on the given algo the subkey may either be an encryption subkey or a signing subkey.  If an algorithm is capable of signing and encryption and such a subkey is desired, a usage string must be given.  This string is either ``default'' or ``-'' to keep the default or a comma delimited list (or space delimited list) of keywords: ``sign'' for a signing subkey, ``auth'' for an authentication subkey, and ``encr'' for an encryption subkey (``encrypt'' can be used as alias for ``encr'').  The valid combinations depend on the algorithm.
--generate-key --gen-key Generate a new key pair using the current default parameters. This is the standard command to create a new key.  In addition to the key a revocation certificate is created and stored in the ‘openpgp-revocs.d’ directory below the GnuPG home directory.
--full-generate-key --full-gen-key Generate a new key pair with dialogs for all options.  This is an extended version of --generate-key.
There is also a feature which allows you to create keys in batch mode. See the manual section ``Unattended key generation'' on how to use this.
--generate-revocation name --gen-revoke name Generate a revocation certificate for the complete key.  To only revoke a subkey or a key signature, use the --edit command.
This command merely creates the revocation certificate so that it can be used to revoke the key if that is ever needed.  To actually revoke a key the created revocation certificate needs to be merged with the key to revoke.  This is done by importing the revocation certificate using the --import command.  Then the revoked key needs to be published, which is best done by sending the key to a keyserver (command --send-key) and by exporting (--export) it to a file which is then send to frequent communication partners.
--generate-designated-revocation name --desig-revoke name Generate a designated revocation certificate for a key. This allows a user (with the permission of the keyholder) to revoke someone else's key.
--edit-key Present a menu which enables you to do most of the key management related tasks.  It expects the specification of a key on the command line.
uid n  Toggle selection of user ID or photographic user ID with index n.  Use * to select all and 0 to deselect all.
key n  Toggle selection of subkey with index n or key ID n.  Use * to select all and 0 to deselect all.
sign   Make a signature on key of user name. If the key is not yet signed by the default user (or the users given with -u), the program displays the information of the key again, together with its fingerprint and asks whether it should be signed. This question is repeated for all users specified with -u.
lsign  Same as "sign" but the signature is marked as non- exportable and will therefore never be used by others. This may be used to make keys valid only in the local environment.
nrsign Same as "sign" but the signature is marked as non-revocable and can therefore never be revoked.
tsign  Make a trust signature. This is a signature that combines the notions of certification (like a regular signature), and trust (like the "trust" command). It is generally only useful in distinct communities or groups.  For more information please read the sections ``Trust Signature'' and ``Regular Expression'' in RFC-4880.
Note that "l" (for local / non-exportable), "nr" (for non- revocable, and "t" (for trust) may be freely mixed and prefixed to "sign" to create a signature of any type desired.
If the option --only-sign-text-ids is specified, then any non-text based user ids (e.g., photo IDs) will not be selected for signing.
delsig Delete a signature. Note that it is not possible to retract a signature, once it has been send to the public (i.e. to a keyserver).  In that case you better use revsig.
revsig Revoke a signature. For every signature which has been generated by one of the secret keys, GnuPG asks whether a revocation certificate should be generated.
check  Check the signatures on all selected user IDs.  With the extra option selfsig only self-signatures are shown.
adduid Create an additional user ID.
addphoto Create a photographic user ID. This will prompt for a JPEG file that will be embedded into the user ID. Note that a very large JPEG will make for a very large key. Also note that some programs will display your JPEG unchanged (GnuPG), and some programs will scale it to fit in a dialog box (PGP).
showphoto Display the selected photographic user ID.
deluid Delete a user ID or photographic user ID.  Note that it is not possible to retract a user id, once it has been send to the public (i.e. to a keyserver).  In that case you better use revuid.
revuid Revoke a user ID or photographic user ID.
primary Flag the current user id as the primary one, removes the primary user id flag from all other user ids and sets the timestamp of all affected self-signatures one second ahead. Note that setting a photo user ID as primary makes it primary over other photo user IDs, and setting a regular user ID as primary makes it primary over other regular user IDs.
keyserver Set a preferred keyserver for the specified user ID(s). This allows other users to know where you prefer they get your key from. See --keyserver-options honor-keyserver-url for more on how this works.  Setting a value of "none" removes an existing preferred keyserver.
notation Set a name=value notation for the specified user ID(s). See --cert-notation for more on how this works. Setting a value of "none" removes all notations, setting a notation prefixed with a minus sign (-) removes that notation, and setting a notation name (without the =value) prefixed with a minus sign removes all notations with that name.
pref   List preferences from the selected user ID. This shows the actual preferences, without including any implied preferences.
showpref More verbose preferences listing for the selected user ID. This shows the preferences in effect by including the implied preferences of 3DES (cipher), SHA-1 (digest), and Uncompressed (compression) if they are not already included in the preference list. In addition, the preferred keyserver and signature notations (if any) are shown.
setpref string Set the list of user ID preferences to string for all (or just the selected) user IDs. Calling setpref with no arguments sets the preference list to the default (either built-in or set via --default-preference-list), and calling setpref with "none" as the argument sets an empty preference list. Use gpg --version to get a list of available algorithms. Note that while you can change the preferences on an attribute user ID (aka "photo ID"), GnuPG does not select keys via attribute user IDs so these preferences will not be used by GnuPG.
When setting preferences, you should list the algorithms in the order which you'd like to see them used by someone else when encrypting a message to your key.  If you don't include 3DES, it will be automatically added at the end. Note that there are many factors that go into choosing an algorithm (for example, your key may not be the only recipient), and so the remote OpenPGP application being used to send to you may or may not follow your exact chosen order for a given message.  It will, however, only choose an algorithm that is present on the preference list of every recipient key.  See also the INTEROPERABILITY WITH OTHER OPENPGP PROGRAMS section below.
addkey Add a subkey to this key.
addcardkey Generate a subkey on a card and add it to this key.
keytocard Transfer the selected secret subkey (or the primary key if no subkey has been selected) to a smartcard. The secret key in the keyring will be replaced by a stub if the key could be stored successfully on the card and you use the save command later. Only certain key types may be transferred to the card. A sub menu allows you to select on what card to store the key. Note that it is not possible to get that key back from the card - if the card gets broken your secret key will be lost unless you have a backup somewhere.
bkuptocard file Restore the given file to a card. This command may be used to restore a backup key (as generated during card initialization) to a new card. In almost all cases this will be the encryption key. You should use this command only with the corresponding public key and make sure that the file given as argument is indeed the backup to restore. You should then select 2 to restore as encryption key.  You will first be asked to enter the passphrase of the backup key and then for the Admin PIN of the card.
delkey Remove a subkey (secondary key). Note that it is not possible to retract a subkey, once it has been send to the public (i.e. to a keyserver).  In that case you better use revkey.  Also note that this only deletes the public part of a key.
revkey Revoke a subkey.
expire Change the key or subkey expiration time. If a subkey is selected, the expiration time of this subkey will be changed. With no selection, the key expiration of the primary key is changed.
trust  Change the owner trust value for the key. This updates the trust-db immediately and no save is required.
disable enable Disable or enable an entire key. A disabled key can not normally be used for encryption.
addrevoker Add a designated revoker to the key. This takes one optional argument: "sensitive". If a designated revoker is marked as sensitive, it will not be exported by default (see export-options).
passwd Change the passphrase of the secret key.
toggle This is dummy command which exists only for backward compatibility.
clean  Compact (by removing all signatures except the selfsig) any user ID that is no longer usable (e.g. revoked, or expired). Then, remove any signatures that are not usable by the trust calculations.  Specifically, this removes any signature that does not validate, any signature that is superseded by a later signature, revoked signatures, and signatures issued by keys that are not present on the keyring.
minimize Make the key as small as possible. This removes all signatures from each user ID except for the most recent self-signature.
change-usage Change the usage flags (capabilities) of the primary key or of subkeys.  These usage flags (e.g. Certify, Sign, Authenticate, Encrypt) are set during key creation. Sometimes it is useful to have the opportunity to change them (for example to add Authenticate) after they have been created.  Please take care when doing this; the allowed usage flags depend on the key algorithm.
cross-certify Add cross-certification signatures to signing subkeys that may not currently have them. Cross-certification signatures protect against a subtle attack against signing subkeys. See --require-cross-certification.  All new keys generated have this signature by default, so this command is only useful to bring older keys up to date.
save   Save all changes to the keyring and quit.
quit   Quit the program without updating the keyring.
The listing shows you the key with its secondary keys and all user IDs.  The primary user ID is indicated by a dot, and selected keys or user IDs are indicated by an asterisk.  The trust value is displayed with the primary key: "trust" is the assigned owner trust and "validity" is the calculated validity of the key. Validity values are also displayed for all user IDs.  For possible values of trust, see: [trust-values].
--sign-key name Signs a public key with your secret key. This is a shortcut version of the subcommand "sign" from --edit.
--lsign-key name Signs a public key with your secret key but marks it as non- exportable. This is a shortcut version of the subcommand "lsign" from --edit-key.
--quick-sign-key fpr [names] --quick-lsign-key fpr [names] Directly sign a key from the passphrase without any further user interaction.  The fpr must be the verified primary fingerprint of a key in the local keyring. If no names are given, all useful user ids are signed; with given [names] only useful user ids matching one of theses names are signed.  By default, or if a name is prefixed with a '*', a case insensitive substring match is used. If a name is prefixed with a '=' a case sensitive exact match is done.
The command --quick-lsign-key marks the signatures as non- exportable.  If such a non-exportable signature already exists the --quick-sign-key turns it into a exportable signature.  If you need to update an existing signature, for example to add or change notation data, you need to use the option --force-sign-key.
This command uses reasonable defaults and thus does not provide the full flexibility of the "sign" subcommand from --edit-key. Its intended use is to help unattended key signing by utilizing a list of verified fingerprints.
--quick-add-uid user-id new-user-id This command adds a new user id to an existing key.  In contrast to the interactive sub-command adduid of --edit-key the new-user- id is added verbatim with only leading and trailing white space removed, it is expected to be UTF-8 encoded, and no checks on its form are applied.
--quick-revoke-uid user-id user-id-to-revoke This command revokes a user ID on an existing key.  It cannot be used to revoke the last user ID on key (some non-revoked user ID must remain), with revocation reason ``User ID is no longer valid''.  If you want to specify a different revocation reason, or to supply supplementary revocation text, you should use the interactive sub-command revuid of --edit-key.
--quick-revoke-sig fpr signing-fpr [names] This command revokes the key signatures made by signing-fpr from the key specified by the fingerprint fpr.  With names given only the signatures on user ids of the key matching any of the given names are affected (see --quick-sign-key).  If a revocation already exists a notice is printed instead of creating a new revocation; no error is returned in this case.  Note that key signature revocations may be superseded by a newer key signature and in turn again revoked.
--quick-set-primary-uid user-id primary-user-id This command sets or updates the primary user ID flag on an existing key.  user-id specifies the key and primary-user-id the user ID which shall be flagged as the primary user ID.  The primary user ID flag is removed from all other user ids and the timestamp of all affected self-signatures is set one second ahead.
--change-passphrase user-id --passwd user-id Change the passphrase of the secret key belonging to the certificate specified as user-id.  This is a shortcut for the sub- command passwd of the edit key menu.  When using together with the option --dry-run this will not actually change the passphrase but check that the current passphrase is correct.
gpg features a bunch of options to control the exact behaviour and to change the default configuration.
Long options can be put in an options file (default "~/.gnupg/gpg.conf"). Short option names will not work - for example, "armor" is a valid option for the options file, while "a" is not. Do not write the 2 dashes, but simply the name of the option and any required arguments. Lines with a hash ('#') as the first non-white-space character are ignored. Commands may be put in this file too, but that is not generally useful as the command will execute automatically with every execution of gpg.
Please remember that option parsing stops as soon as a non-option is encountered, you can explicitly stop parsing by using the special option --.
How to change the configuration
These options are used to change the configuration and most of them are usually found in the option file.
--default-key name Use name as the default key to sign with. If this option is not used, the default key is the first key found in the secret keyring.  Note that -u or --local-user overrides this option. This option may be given multiple times.  In this case, the last key for which a secret key is available is used.  If there is no secret key available for any of the specified values, GnuPG will not emit an error message but continue as if this option wasn't given.
--default-recipient name Use name as default recipient if option --recipient is not used and don't ask if this is a valid one. name must be non-empty.
--default-recipient-self Use the default key as default recipient if option --recipient is not used and don't ask if this is a valid one. The default key is the first one from the secret keyring or the one set with --default-key.
--no-default-recipient Reset --default-recipient and --default-recipient-self.  Should not be used in an option file.
-v, --verbose Give more information during processing. If used twice, the input data is listed in detail.
--no-verbose Reset verbose level to 0.  Should not be used in an option file.
-q, --quiet Try to be as quiet as possible.  Should not be used in an option file.
--batch --no-batch Use batch mode.  Never ask, do not allow interactive commands. --no-batch disables this option.  Note that even with a filename given on the command line, gpg might still need to read from STDIN (in particular if gpg figures that the input is a detached signature and no data file has been specified).  Thus if you do not want to feed data via STDIN, you should connect STDIN to ‘/dev/null’.
It is highly recommended to use this option along with the options --status-fd and --with-colons for any unattended use of gpg. Should not be used in an option file.
--no-tty Make sure that the TTY (terminal) is never used for any output. This option is needed in some cases because GnuPG sometimes prints warnings to the TTY even if --batch is used.
--yes  Assume "yes" on most questions.  Should not be used in an option file.
--no   Assume "no" on most questions.  Should not be used in an option file.
--list-options parameters This is a space or comma delimited string that gives options used when listing keys and signatures (that is, --list-keys, --check- signatures, --list-public-keys, --list-secret-keys, and the --edit-key functions).  Options can be prepended with a no- (after the two dashes) to give the opposite meaning.  The options are:
show-photos Causes --list-keys, --check-signatures, --list-public-keys, and --list-secret-keys to display any photo IDs attached to the key.  Defaults to no. See also --photo-viewer.  Does not work with --with-colons: see --attribute-fd for the appropriate way to get photo data for scripts and other frontends.
show-usage Show usage information for keys and subkeys in the standard key listing.  This is a list of letters indicating the allowed usage for a key (E=encryption, S=signing, C=certification, A=authentication).  Defaults to yes.
show-policy-urls Show policy URLs in the  --check-signatures listings. Defaults to no.
show-notations show-std-notations show-user-notations Show all, IETF standard, or user-defined signature notations in the --check-signatures listings. Defaults to no.
show-keyserver-urls Show any preferred keyserver URL in the --check-signatures listings. Defaults to no.
show-uid-validity Display the calculated validity of user IDs during key listings.  Defaults to yes.
show-unusable-uids Show revoked and expired user IDs in key listings. Defaults to no.
show-unusable-subkeys Show revoked and expired subkeys in key listings. Defaults to no.
show-keyring Display the keyring name at the head of key listings to show which keyring a given key resides on. Defaults to no.
show-sig-expire Show signature expiration dates (if any) during --check- signatures listings. Defaults to no.
show-sig-subpackets Include signature subpackets in the key listing. This option can take an optional argument list of the subpackets to list. If no argument is passed, list all subpackets. Defaults to no. This option is only meaningful when using --with-colons along with --check-signatures.
show-only-fpr-mbox For each user-id which has a valid mail address print only the fingerprint followed by the mail address.
--verify-options parameters This is a space or comma delimited string that gives options used when verifying signatures. Options can be prepended with a `no-' to give the opposite meaning. The options are:
show-photos Display any photo IDs present on the key that issued the signature.  Defaults to no. See also --photo-viewer.
show-policy-urls Show policy URLs in the signature being verified. Defaults to yes.
show-notations show-std-notations show-user-notations Show all, IETF standard, or user-defined signature notations in the signature being verified. Defaults to IETF standard.
show-keyserver-urls Show any preferred keyserver URL in the signature being verified.  Defaults to yes.
show-uid-validity Display the calculated validity of the user IDs on the key that issued the signature. Defaults to yes.
show-unusable-uids Show revoked and expired user IDs during signature verification.  Defaults to no.
show-primary-uid-only Show only the primary user ID during signature verification.  That is all the AKA lines as well as photo Ids are not shown with the signature verification status.
pka-lookups Enable PKA lookups to verify sender addresses. Note that PKA is based on DNS, and so enabling this option may disclose information on when and what signatures are verified or to whom data is encrypted. This is similar to the "web bug" described for the --auto-key-retrieve option.
pka-trust-increase Raise the trust in a signature to full if the signature passes PKA validation. This option is only meaningful if pka-lookups is set.
--enable-large-rsa --disable-large-rsa With --generate-key and --batch, enable the creation of RSA secret keys as large as 8192 bit.  Note: 8192 bit is more than is generally recommended.  These large keys don't significantly improve security, but they are more expensive to use, and their signatures and certifications are larger.  This option is only available if the binary was build with large-secmem support.
--enable-dsa2 --disable-dsa2 Enable hash truncation for all DSA keys even for old DSA Keys up to 1024 bit.  This is also the default with --openpgp.  Note that older versions of GnuPG also required this flag to allow the generation of DSA larger than 1024 bit.
--photo-viewer string This is the command line that should be run to view a photo ID. "%i" will be expanded to a filename containing the photo. "%I" does the same, except the file will not be deleted once the viewer exits.  Other flags are "%k" for the key ID, "%K" for the long key ID, "%f" for the key fingerprint, "%t" for the extension of the image type (e.g. "jpg"), "%T" for the MIME type of the image (e.g. "image/jpeg"), "%v" for the single-character calculated validity of the image being viewed (e.g. "f"), "%V" for the calculated validity as a string (e.g.  "full"), "%U" for a base32 encoded hash of the user ID, and "%%" for an actual percent sign. If neither %i or %I are present, then the photo will be supplied to the viewer on standard input.
On Unix the default viewer is xloadimage -fork -quiet -title 'KeyID 0x%k' STDIN with a fallback to display -title 'KeyID 0x%k' %i and finally to xdg-open %i.  On Windows !ShellExecute 400 %i is used; here the command is a meta command to use that API call followed by a wait time in milliseconds which is used to give the viewer time to read the temporary image file before gpg deletes it again.  Note that if your image viewer program is not secure, then executing it from gpg does not make it secure.
--exec-path string Sets a list of directories to search for photo viewers If not provided photo viewers use the PATH environment variable.
--keyring file Add file to the current list of keyrings. If file begins with a tilde and a slash, these are replaced by the $HOME directory. If the filename does not contain a slash, it is assumed to be in the GnuPG home directory ("~/.gnupg" unless --homedir or $GNUPGHOME is used).
Note that this adds a keyring to the current list. If the intent is to use the specified keyring alone, use --keyring along with --no-default-keyring.
If the option --no-keyring has been used no keyrings will be used at all.
--primary-keyring file This is a varian of --keyring and designates file as the primary public keyring. This means that newly imported keys (via --import or keyserver --recv-from) will go to this keyring.
--secret-keyring file This is an obsolete option and ignored.  All secret keys are stored in the ‘private-keys-v1.d’ directory below the GnuPG home directory.
--trustdb-name file Use file instead of the default trustdb. If file begins with a tilde and a slash, these are replaced by the $HOME directory. If the filename does not contain a slash, it is assumed to be in the GnuPG home directory (‘~/.gnupg’ if --homedir or $GNUPGHOME is not used).
--homedir dir Set the name of the home directory to dir. If this option is not used, the home directory defaults to ‘~/.gnupg’.  It is only recognized when given on the command line.  It also overrides any home directory stated through the environment variable ‘GNUPGHOME’ or (on Windows systems) by means of the Registry entry HKCU\Software\GNU\GnuPG:HomeDir.
On Windows systems it is possible to install GnuPG as a portable application.  In this case only this command line option is considered, all other ways to set a home directory are ignored.
To install GnuPG as a portable application under Windows, create an empty file named ‘gpgconf.ctl’ in the same directory as the tool ‘gpgconf.exe’.  The root of the installation is then that directory; or, if ‘gpgconf.exe’ has been installed directly below a directory named ‘bin’, its parent directory.  You also need to make sure that the following directories exist and are writable: ‘ROOT/home’ for the GnuPG home and ‘ROOT/usr/var/cache/gnupg’ for internal cache files.
--display-charset name Set the name of the native character set. This is used to convert some informational strings like user IDs to the proper UTF-8 encoding.  Note that this has nothing to do with the character set of data to be encrypted or signed; GnuPG does not recode user- supplied data. If this option is not used, the default character set is determined from the current locale. A verbosity level of 3 shows the chosen set.  This option should not be used on Windows. Valid values for name are:
iso-8859-1 This is the Latin 1 set.
iso-8859-2 The Latin 2 set.
iso-8859-15 This is currently an alias for the Latin 1 set.
koi8-r The usual Russian set (RFC-1489).
utf-8  Bypass all translations and assume that the OS uses native UTF-8 encoding.
--utf8-strings --no-utf8-strings Assume that command line arguments are given as UTF-8 strings. The default (--no-utf8-strings) is to assume that arguments are encoded in the character set as specified by --display-charset. These options affect all following arguments. Both options may be used multiple times.  This option should not be used in an option file.
This option has no effect on Windows.  There the internal used UTF-8 encoding is translated for console input and output.  The command line arguments are expected as Unicode and translated to UTF-8.  Thus when calling this program from another, make sure to use the Unicode version of CreateProcess.
--options file Read options from file and do not try to read them from the default options file in the homedir (see --homedir). This option is ignored if used in an options file.
--no-options Shortcut for --options /dev/null. This option is detected before an attempt to open an option file.  Using this option will also prevent the creation of a ‘~/.gnupg’ homedir.
-z n --compress-level n --bzip2-compress-level n Set compression level to n for the ZIP and ZLIB compression algorithms. The default is to use the default compression level of zlib (normally 6). --bzip2-compress-level sets the compression level for the BZIP2 compression algorithm (defaulting to 6 as well). This is a different option from --compress-level since BZIP2 uses a significant amount of memory for each additional compression level.  -z sets both. A value of 0 for n disables compression.
--bzip2-decompress-lowmem Use a different decompression method for BZIP2 compressed files. This alternate method uses a bit more than half the memory, but also runs at half the speed. This is useful under extreme low memory circumstances when the file was originally compressed at a high --bzip2-compress-level.
--mangle-dos-filenames --no-mangle-dos-filenames Older version of Windows cannot handle filenames with more than one dot. --mangle-dos-filenames causes GnuPG to replace (rather than add to) the extension of an output filename to avoid this problem. This option is off by default and has no effect on non- Windows platforms.
--ask-cert-level --no-ask-cert-level When making a key signature, prompt for a certification level. If this option is not specified, the certification level used is set via --default-cert-level. See --default-cert-level for information on the specific levels and how they are used. --no-ask-cert-level disables this option. This option defaults to no.
--default-cert-level n The default to use for the check level when signing a key.
0 means you make no particular claim as to how carefully you verified the key.
1 means you believe the key is owned by the person who claims to own it but you could not, or did not verify the key at all. This is useful for a "persona" verification, where you sign the key of a pseudonymous user.
2 means you did casual verification of the key. For example, this could mean that you verified the key fingerprint and checked the user ID on the key against a photo ID.
3 means you did extensive verification of the key. For example, this could mean that you verified the key fingerprint with the owner of the key in person, and that you checked, by means of a hard to forge document with a photo ID (such as a passport) that the name of the key owner matches the name in the user ID on the key, and finally that you verified (by exchange of email) that the email address on the key belongs to the key owner.
Note that the examples given above for levels 2 and 3 are just that: examples. In the end, it is up to you to decide just what "casual" and "extensive" mean to you.
This option defaults to 0 (no particular claim).
--min-cert-level When building the trust database, treat any signatures with a certification level below this as invalid. Defaults to 2, which disregards level 1 signatures. Note that level 0 "no particular claim" signatures are always accepted.
--trusted-key long key ID or fingerprint Assume that the specified key (which should be given as fingerprint) is as trustworthy as one of your own secret keys. This option is useful if you don't want to keep your secret keys (or one of them) online but still want to be able to check the validity of a given recipient's or signator's key.  If the given key is not locally available but an LDAP keyserver is configured the missing key is imported from that server.
--trust-model {pgp|classic|tofu|tofu+pgp|direct|always|auto} Set what trust model GnuPG should follow. The models are:
pgp    This is the Web of Trust combined with trust signatures as used in PGP 5.x and later. This is the default trust model when creating a new trust database.
classic This is the standard Web of Trust as introduced by PGP 2.
TOFU stands for Trust On First Use.  In this trust model, the first time a key is seen, it is memorized.  If later another key with a user id with the same email address is seen, both keys are marked as suspect.  In that case, the next time either is used, a warning is displayed describing the conflict, why it might have occurred (either the user generated a new key and failed to cross sign the old and new keys, the key is forgery, or a man-in-the-middle attack is being attempted), and the user is prompted to manually confirm the validity of the key in question.
Because a potential attacker is able to control the email address and thereby circumvent the conflict detection algorithm by using an email address that is similar in appearance to a trusted email address, whenever a message is verified, statistics about the number of messages signed with the key are shown.  In this way, a user can easily identify attacks using fake keys for regular correspondents.
When compared with the Web of Trust, TOFU offers significantly weaker security guarantees.  In particular, TOFU only helps ensure consistency (that is, that the binding between a key and email address doesn't change).  A major advantage of TOFU is that it requires little maintenance to use correctly.  To use the web of trust properly, you need to actively sign keys and mark users as trusted introducers.  This is a time-consuming process and anecdotal evidence suggests that even security-conscious users rarely take the time to do this thoroughly and instead rely on an ad-hoc TOFU process.
In the TOFU model, policies are associated with bindings between keys and email addresses (which are extracted from user ids and normalized).  There are five policies, which can be set manually using the --tofu-policy option.  The default policy can be set using the --tofu-default-policy option.
The TOFU policies are: auto, good, unknown, bad and ask. The auto policy is used by default (unless overridden by --tofu-default-policy) and marks a binding as marginally trusted.  The good, unknown and bad policies mark a binding as fully trusted, as having unknown trust or as having trust never, respectively.  The unknown policy is useful for just using TOFU to detect conflicts, but to never assign positive trust to a binding.  The final policy, ask prompts the user to indicate the binding's trust.  If batch mode is enabled (or input is inappropriate in the context), then the user is not prompted and the undefined trust level is returned.
tofu+pgp This trust model combines TOFU with the Web of Trust.  This is done by computing the trust level for each model and then taking the maximum trust level where the trust levels are ordered as follows: unknown < undefined < marginal < fully < ultimate < expired < never.
By setting --tofu-default-policy=unknown, this model can be used to implement the web of trust with TOFU's conflict detection algorithm, but without its assignment of positive trust values, which some security-conscious users don't like.
direct Key validity is set directly by the user and not calculated via the Web of Trust.  This model is solely based on the key and does not distinguish user IDs.  Note that when changing to another trust model the trust values assigned to a key are transformed into ownertrust values, which also indicate how you trust the owner of the key to sign other keys.
always Skip key validation and assume that used keys are always fully valid. You generally won't use this unless you are using some external validation scheme. This option also suppresses the "[uncertain]" tag printed with signature checks when there is no evidence that the user ID is bound to the key.  Note that this trust model still does not allow the use of expired, revoked, or disabled keys.
auto   Select the trust model depending on whatever the internal trust database says. This is the default model if such a database already exists.  Note that a tofu trust model is not considered here and must be enabled explicitly.
--auto-key-locate mechanisms --no-auto-key-locate GnuPG can automatically locate and retrieve keys as needed using this option.  This happens when encrypting to an email address (in the "user@example.com" form), and there are no "user@example.com" keys on the local keyring.  This option takes any number of the mechanisms listed below, in the order they are to be tried. Instead of listing the mechanisms as comma delimited arguments, the option may also be given several times to add more mechanism. The option --no-auto-key-locate or the mechanism "clear" resets the list.  The default is "local,wkd".
cert   Locate a key using DNS CERT, as specified in RFC-4398.
pka    Locate a key using DNS PKA.
dane   Locate a key using DANE, as specified in draft-ietf-dane- openpgpkey-05.txt.
wkd    Locate a key using the Web Key Directory protocol.
ldap   Using DNS Service Discovery, check the domain in question for any LDAP keyservers to use.  If this fails, attempt to locate the key using the PGP Universal method of checking ‘ldap://keys.(thedomain)’.
ntds   Locate the key using the Active Directory (Windows only). This method also allows to search by fingerprint using the command --locate-external-key.
keyserver Locate a key using a keyserver.  This method also allows to search by fingerprint using the command --locate-external- key if any of the configured keyservers is an LDAP server.
keyserver-URL In addition, a keyserver URL as used in the dirmngr configuration may be used here to query that particular keyserver.  This method also allows to search by fingerprint using the command --locate-external-key if the URL specifies an LDAP server.
local  Locate the key using the local keyrings.  This mechanism allows the user to select the order a local key lookup is done.  Thus using ‘--auto-key-locate local’ is identical to --no-auto-key-locate.
nodefault This flag disables the standard local key lookup, done before any of the mechanisms defined by the --auto-key- locate are tried.  The position of this mechanism in the list does not matter.  It is not required if local is also used.
clear  Clear all defined mechanisms.  This is useful to override mechanisms given in a config file.  Note that a nodefault in mechanisms will also be cleared unless it is given after the clear.
--auto-key-import --no-auto-key-import This is an offline mechanism to get a missing key for signature verification and for later encryption to this key.  If this option is enabled and a signature includes an embedded key, that key is used to verify the signature and on verification success that key is imported. The default is --no-auto-key-import.
On the sender (signing) site the option --include-key-block needs to be used to put the public part of the signing key as “Key Block subpacket” into the signature.
--auto-key-retrieve --no-auto-key-retrieve These options enable or disable the automatic retrieving of keys from a keyserver when verifying signatures made by keys that are not on the local keyring.  The default is --no-auto-key-retrieve.
The order of methods tried to lookup the key is:
1. If the option --auto-key-import is set and the signatures includes an embedded key, that key is used to verify the signature and on verification success that key is imported.
2. If a preferred keyserver is specified in the signature and the option honor-keyserver-url is active (which is not the default), that keyserver is tried.  Note that the creator of the signature uses the option --sig-keyserver-url to specify the preferred keyserver for data signatures.
3. If the signature has the Signer's UID set (e.g. using --sender while creating the signature) a Web Key Directory (WKD) lookup is done.  This is the default configuration but can be disabled by removing WKD from the auto-key-locate list or by using the option --disable-signer-uid.
4. If the option honor-pka-record is active, the legacy PKA method is used.
5. If any keyserver is configured and the Issuer Fingerprint is part of the signature (since GnuPG 2.1.16), the configured keyservers are tried.
Note that this option makes a "web bug" like behavior possible. Keyserver or Web Key Directory operators can see which keys you request, so by sending you a message signed by a brand new key (which you naturally will not have on your local keyring), the operator can tell both your IP address and the time when you verified the signature.
--keyid-format {none|short|0xshort|long|0xlong} Select how to display key IDs.  "none" does not show the key ID at all but shows the fingerprint in a separate line.  "short" is the traditional 8-character key ID.  "long" is the more accurate (but less convenient) 16-character key ID.  Add an "0x" to either to include an "0x" at the beginning of the key ID, as in 0x99242560. Note that this option is ignored if the option --with-colons is used.
--keyserver name This option is deprecated - please use the --keyserver in ‘dirmngr.conf’ instead.
Use name as your keyserver. This is the server that --receive- keys, --send-keys, and --search-keys will communicate with to receive keys from, send keys to, and search for keys on. The format of the name is a URI: `scheme:[//]keyservername[:port]' The scheme is the type of keyserver: "hkp"/"hkps" for the HTTP (or compatible) keyservers or "ldap"/"ldaps" for the LDAP keyservers. Note that your particular installation of GnuPG may have other keyserver types available as well. Keyserver schemes are case- insensitive.
Most keyservers synchronize with each other, so there is generally no need to send keys to more than one server. The keyserver hkp://keys.gnupg.net uses round robin DNS to give a different keyserver each time you use it.
--keyserver-options {name=value} This is a space or comma delimited string that gives options for the keyserver. Options can be prefixed with a `no-' to give the opposite meaning. Valid import-options or export-options may be used here as well to apply to importing (--recv-key) or exporting (--send-key) a key from a keyserver. While not all options are available for all keyserver types, some common options are:
include-revoked When searching for a key with --search-keys, include keys that are marked on the keyserver as revoked. Note that not all keyservers differentiate between revoked and unrevoked keys, and for such keyservers this option is meaningless. Note also that most keyservers do not have cryptographic verification of key revocations, and so turning this option off may result in skipping keys that are incorrectly marked as revoked.
include-disabled When searching for a key with --search-keys, include keys that are marked on the keyserver as disabled. Note that this option is not used with HKP keyservers.
auto-key-retrieve This is an obsolete alias for the option auto-key-retrieve. Please do not use it; it will be removed in future versions..
honor-keyserver-url When using --refresh-keys, if the key in question has a preferred keyserver URL, then use that preferred keyserver to refresh the key from. In addition, if auto-key-retrieve is set, and the signature being verified has a preferred keyserver URL, then use that preferred keyserver to fetch the key from. Note that this option introduces a "web bug": The creator of the key can see when the keys is refreshed. Thus this option is not enabled by default.
honor-pka-record If --auto-key-retrieve is used, and the signature being verified has a PKA record, then use the PKA information to fetch the key. Defaults to "yes".
include-subkeys When receiving a key, include subkeys as potential targets. Note that this option is not used with HKP keyservers, as they do not support retrieving keys by subkey id.
timeout http-proxy=value verbose debug check-cert
ca-cert-file These options have no more function since GnuPG 2.1.  Use the dirmngr configuration options instead.
The default list of options is: "self-sigs-only, repair-keys, repair-pks- subkey-bug, export-attributes, honor-pka-record". However, if the actual used source is an LDAP server "no-self-sigs-only" is assumed unless "self-sigs-only" has been explictly configured.
--completes-needed n Number of completely trusted users to introduce a new key signer (defaults to 1).
--marginals-needed n Number of marginally trusted users to introduce a new key signer (defaults to 3)
--tofu-default-policy {auto|good|unknown|bad|ask} The default TOFU policy (defaults to auto).  For more information about the meaning of this option, see: [trust-model-tofu].
--max-cert-depth n Maximum depth of a certification chain (default is 5).
--no-sig-cache Do not cache the verification status of key signatures.  Caching gives a much better performance in key listings. However, if you suspect that your public keyring is not safe against write modifications, you can use this option to disable the caching. It probably does not make sense to disable it because all kind of damage can be done if someone else has write access to your public keyring.
--auto-check-trustdb --no-auto-check-trustdb If GnuPG feels that its information about the Web of Trust has to be updated, it automatically runs the --check-trustdb command internally.  This may be a time consuming process. --no-auto- check-trustdb disables this option.
--use-agent --no-use-agent This is dummy option. gpg always requires the agent.
--gpg-agent-info This is dummy option. It has no effect when used with gpg.
--agent-program file Specify an agent program to be used for secret key operations. The default value is determined by running gpgconf with the option --list-dirs.  Note that the pipe symbol (|) is used for a regression test suite hack and may thus not be used in the file name.
--dirmngr-program file Specify a dirmngr program to be used for keyserver access.  The default value is ‘/usr/bin/dirmngr’.
--disable-dirmngr Entirely disable the use of the Dirmngr.
--no-autostart Do not start the gpg-agent or the dirmngr if it has not yet been started and its service is required.  This option is mostly useful on machines where the connection to gpg-agent has been redirected to another machines.  If dirmngr is required on the remote machine, it may be started manually using gpgconf --launch dirmngr.
--lock-once Lock the databases the first time a lock is requested and do not release the lock until the process terminates.
--lock-multiple Release the locks every time a lock is no longer needed. Use this to override a previous --lock-once from a config file.
--lock-never Disable locking entirely. This option should be used only in very special environments, where it can be assured that only one process is accessing those files. A bootable floppy with a stand- alone encryption system will probably use this. Improper usage of this option may lead to data and key corruption.
--exit-on-status-write-error This option will cause write errors on the status FD to immediately terminate the process. That should in fact be the default but it never worked this way and thus we need an option to enable this, so that the change won't break applications which close their end of a status fd connected pipe too early. Using this option along with --enable-progress-filter may be used to cleanly cancel long running gpg operations.
--limit-card-insert-tries n With n greater than 0 the number of prompts asking to insert a smartcard gets limited to N-1. Thus with a value of 1 gpg won't at all ask to insert a card if none has been inserted at startup. This option is useful in the configuration file in case an application does not know about the smartcard support and waits ad infinitum for an inserted card.
--no-random-seed-file GnuPG uses a file to store its internal random pool over invocations.  This makes random generation faster; however sometimes write operations are not desired. This option can be used to achieve that with the cost of slower random generation.
--no-greeting Suppress the initial copyright message.
--no-secmem-warning Suppress the warning about "using insecure memory".
--no-permission-warning Suppress the warning about unsafe file and home directory (--homedir) permissions. Note that the permission checks that GnuPG performs are not intended to be authoritative, but rather they simply warn about certain common permission problems. Do not assume that the lack of a warning means that your system is secure.
Note that the warning for unsafe --homedir permissions cannot be suppressed in the gpg.conf file, as this would allow an attacker to place an unsafe gpg.conf file in place, and use this file to suppress warnings about itself. The --homedir permissions warning may only be suppressed on the command line.
--require-secmem --no-require-secmem Refuse to run if GnuPG cannot get secure memory. Defaults to no (i.e. run, but give a warning).
--require-cross-certification --no-require-cross-certification When verifying a signature made from a subkey, ensure that the cross certification "back signature" on the subkey is present and valid.  This protects against a subtle attack against subkeys that can sign.  Defaults to --require-cross-certification for gpg.
--expert --no-expert Allow the user to do certain nonsensical or "silly" things like signing an expired or revoked key, or certain potentially incompatible things like generating unusual key types. This also disables certain warning messages about potentially incompatible actions. As the name implies, this option is for experts only. If you don't fully understand the implications of what it allows you to do, leave this off. --no-expert disables this option.
Key related options
--recipient name -r     Encrypt for user id name. If this option or --hidden-recipient is not specified, GnuPG asks for the user-id unless --default- recipient is given.