GnuPG (한국어)

관련 항목

• pacman/Package signing
• Data-at-rest encryption
• List of applications/Security#Encryption, signing, steganography

According to the official website:

GnuPG is a complete and free implementation of the OpenPGP standard as defined by RFC4880 (also known as PGP). GnuPG allows you to encrypt and sign your data and communications; it features a versatile key management system, along with access modules for all kinds of public key directories. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications. A wealth of frontend applications and libraries are available. GnuPG also provides support for S/MIME and Secure Shell (ssh).

GnuPG의 공식 사이트로부터:

GnuPG는 (PGP로도 알려진) RFC4880에서 정의된 OpenPGP의 완전하고 자유 소프트웨어인 구현이다. GnuPG에서는 데이터와 통신에서 암호화와 서명 기능을 제공한다.

이외에도 모든 공개키 디렉토리에서 이용가능한 액세스 모듈을 비롯한 다양한 기능을 가지고 있는 키 관리 시스템을 제공한다. GnuPG 혹은 GPG는 기본적으로 다른 어플리케이션과 쉽게 연동할 수 있는 기능을 갖추고 있는 커맨드 라인 도구이며, 여러가지 프론트엔드 어플리케이션과 라이브러리가 존재한다. GnuPG는 또한 S/MIME과 ssh를 지원한다.

설치하기

gnupg 패키지를 설치하면 된다.

이는 PIN이나 암호를 입력하기 위해 GnuPG가 이용하는 대화 창을 제공하는 pinentry를 같이 설치할 것이다. 쉘 스크립트 /usr/bin/pinentry에서 어떤 pinentry 대화 창을 사용할 것인지를 지정할 수 있다. 이에 관해서는 #pinentry에 서술되어 있다.

GnuPG와 연동하여 사용가능한 프론트엔드 어플리케이션에 관해서는 List of applications/Security#Encryption, signing, steganography을 참고.

Configuration

Directory location

$GNUPGHOME is used by GnuPG to point to the directory where its configuration files are stored. By default $GNUPGHOME is not set and your $HOME is used instead; thus, you will find a ~/.gnupg directory right after installation.

To change the default location, either run gpg this way $ gpg --homedir path/to/file or set the GNUPGHOME environment variable.

Configuration files

The default configuration files are ~/.gnupg/gpg.conf and ~/.gnupg/dirmngr.conf.

By default, the gnupg directory has its permissions set to 700 and the files it contains have their permissions set to 600. Only the owner of the directory has permission to read, write, and access the files. This is for security purposes and should not be changed. In case this directory or any file inside it does not follow this security measure, you will get warnings about unsafe file and home directory permissions.

Append to these files any long options you want. Do not write the two dashes, but simply the name of the option and required arguments. You will find skeleton files in /usr/share/doc/gnupg/. These files are copied to ~/.gnupg the first time gpg is run if they do not exist there. Other examples are found in #See also.

Additionally, pacman uses a different set of configuration files for package signature verification. See Pacman/Package signing for details.

Default options for new users

If you want to setup some default options for new users, put configuration files in /etc/skel/.gnupg/. When the new user is added in system, files from here will be copied to its GnuPG home directory. There is also a simple script called addgnupghome which you can use to create new GnuPG home directories for existing users:

# addgnupghome user1 user2

This will add the respective /home/user1/.gnupg/ and /home/user2/.gnupg/ and copy the files from the skeleton directory to it. Users with existing GnuPG home directory are simply skipped.

Usage

참고:

• Whenever a user-id is required in a command, it can be specified with your key ID, fingerprint, a part of your name or email address, etc. GnuPG is flexible on this.
• Whenever a key-id is needed, it can be found adding the --keyid-format=long flag to the command. To show the master secret key for example, run gpg --list-secret-keys --keyid-format=long user-id, the key-id is the hexadecimal hash provided on the same line as sec.

Create a key pair

Generate a key pair by typing in a terminal:

$ gpg --full-gen-key

도움말: Use the --expert option for getting alternative ciphers like Wikipedia:Elliptic-curve cryptography. GnuPG supports elliptic curve keys as mentioned in GnuPG - what's new in 2.1.

The command will prompt for answers to several questions. For general use most people will want:

• The default RSA and RSA for sign and encrypt keys.
• A keysize of the default 3072 value. A larger keysize of 4096 "gives us almost nothing, while costing us quite a lot" (see why doesn’t GnuPG default to using RSA-4096).
• An expiration date: a period of one year is good enough for the average user. This way even if access is lost to the keyring, it will allow others to know that it is no longer valid. At a later stage, if necessary, the expiration date can be extended without having to re-issue a new key.
• Your name and email address. You can add multiple identities to the same key later (e.g., if you have multiple email addresses you want to associate with this key).
• no optional comment. Since the semantics of the comment field are not well-defined, it has limited value for identification.
• A secure passphrase, find some guidelines in Security#Choosing secure passwords.

참고: The name and email address you enter here will be seen by anybody who imports your key.

List keys

To list keys in your public key ring:

$ gpg --list-keys

To list keys in your secret key ring:

$ gpg --list-secret-keys

Export your public key

GnuPG's main usage is to ensure confidentiality of exchanged messages via public-key cryptography. With it each user distributes the public key of their keyring, which can be used by others to encrypt messages to the user. The private key must always be kept private, otherwise confidentiality is broken. See Wikipedia:Public-key cryptography for examples about the message exchange.

So, in order for others to send encrypted messages to you, they need your public key.

To generate an ASCII version of a user's public key to file public.key (e.g. to distribute it by e-mail):

$ gpg --export --armor --output public.key user-id

Alternatively, or in addition, you can #Use a keyserver to share your key.

도움말: Add --no-emit-version to avoid printing the version number, or add the corresponding setting to your configuration file.

Import a public key

In order to encrypt messages to others, as well as verify their signatures, you need their public key. To import a public key with file name public.key to your public key ring:

$ gpg --import public.key

Alternatively, #Use a keyserver to find a public key.

If you wish to import a key ID to install a specific Arch Linux package, see pacman/Package signing#Managing the keyring and Makepkg#Signature checking.

Use a keyserver

Sending keys

You can register your key with a public PGP key server, so that others can retrieve it without having to contact you directly:

$ gpg --send-keys key-id

경고: Once a key has been submitted to a keyserver, it cannot be deleted from the server. The reason is explained in the MIT PGP Public Key Server FAQ.

참고: The associated email address, once published publicly, could be the target of spammers and in this case anti-spam filtering may be necessary.

Searching and receiving keys

To find out details of a key on the keyserver, without importing it, do:

$ gpg --search-keys user-id

To import a key from a key server:

$ gpg --recv-keys key-id

경고:

• You should verify the authenticity of the retrieved public key by comparing its fingerprint with one that the owner published on an independent source(s) (e.g., contacting the person directly). See Wikipedia:Public key fingerprint for more information.
• It is recommended to use the long key ID or the full fingerprint when receiving a key. Using a short ID may encounter collisions. All keys will be imported that have the short ID, see fake keys found in the wild for such example.

도움말: Adding auto-key-retrieve to gpg.conf will automatically fetch keys from the key server as needed, but this can be considered a privacy violation; see "web bug" in gpg(1).

Key servers

The most common keyservers are:

• SKS Keyserver Pool^{[dead link 2021-05-13 ⓘ]}: federated, no verification, keys cannot be deleted.
• Mailvelope Keyserver: central, verification of email IDs, keys can be deleted.
• keys.openpgp.org: central, verification of email IDs, keys can be deleted, no third-party signatures (i.e. no Web of Trust support).

More are listed at Wikipedia:Key server (cryptographic)#Keyserver examples.

An alternative key server can be specified with the keyserver option in one of the #Configuration files, for instance:

~/.gnupg/dirmngr.conf

keyserver hkp://pool.sks-keyservers.net

A temporary use of another server is handy when the regular one does not work as it should. It can be achieved by, for example,

$ gpg --keyserver https://keys.openpgp.org/ --search-keys 931FF8E79F0876134EDDBDCCA87FF9DF48BF1C90

도움말:

• If receiving fails with the message gpg: keyserver receive failed: General error, and you use the default hkps keyserver pool, make sure set the HKPS pool verification certificate with hkp-cacert /usr/share/gnupg/sks-keyservers.netCA.pem in your dirmngr.conf and kill the old dirmngr process.
• If your network blocks connection to port 11371 used for hkp, you may need to specify port 80, i.e. pool.sks-keyservers.net:80.
• If receiving fails with the message gpg: keyserver receive failed: Connection refused, try using a different DNS server.
• You can connect to the keyserver over Tor with Tor#Torsocks. Or using the --use-tor command line option. See [1] for more information.
• You can connect to a keyserver using a proxy by setting the http_proxy environment variable and setting honor-http-proxy in dirmngr.conf. Alternatively, set http-proxy host[:port] in the configuration file to override the environment variable of the same name. Restart the dirmngr.service user service for the changes to take effect.

Web Key Directory

The Web Key Service (WKS) protocol is a new standard for key distribution, where the email domain provides its own key server called Web Key Directory (WKD). When encrypting to an email address (e.g. user@example.com), GnuPG (>=2.1.16) will query the domain (example.com) via HTTPS for the public OpenPGP key if it is not already in the local keyring. The option auto-key-locate will locate a key using the WKD protocol if there is no key on the local keyring for this email address.

# gpg --recipient user@example.org --auto-key-locate --encrypt doc

See the GnuPG Wiki for a list of email providers that support WKD. If you control the domain of your email address yourself, you can follow this guide to enable WKD for your domain. To check if your key can be found in the WKD you can use this webinterface.

Encrypt and decrypt

Asymmetric

You need to #Import a public key of a user before encrypting (option -e/--encrypt) a file or message to that recipient (option -r/--recipient). Additionally you need to #Create a key pair if you have not already done so.

To encrypt a file with the name doc, use:

$ gpg --recipient user-id --encrypt doc

To decrypt (option -d/--decrypt) a file with the name doc.gpg encrypted with your public key, use:

$ gpg --output doc --decrypt doc.gpg

gpg will prompt you for your passphrase and then decrypt and write the data from doc.gpg to doc. If you omit the -o/--output option, gpg will write the decrypted data to stdout.

도움말:

• Add --armor to encrypt a file using ASCII armor, suitable for copying and pasting a message in text format.
• Use -R user-id or --hidden-recipient user-id instead of -r to not put the recipient key IDs in the encrypted message. This helps to hide the receivers of the message and is a limited countermeasure against traffic analysis.
• Add --no-emit-version to avoid printing the version number, or add the corresponding setting to your configuration file.
• You can use GnuPG to encrypt your sensitive documents by using your own user-id as recipient or by using the --default-recipient-self flag; however, you can only do this one file at a time, although you can always tarball various files and then encrypt the tarball. See also Data-at-rest encryption#Available methods if you want to encrypt directories or a whole file-system.

Symmetric

Symmetric encryption does not require the generation of a key pair and can be used to simply encrypt data with a passphrase. Simply use -c/--symmetric to perform symmetric encryption:

$ gpg -c doc

The following example:

• Encrypts doc with a symmetric cipher using a passphrase
• Uses the AES-256 cipher algorithm to encrypt the passphrase
• Uses the SHA-512 digest algorithm to mangle the passphrase
• Mangles the passphrase for 65536 iterations

$ gpg -c --s2k-cipher-algo AES256 --s2k-digest-algo SHA512 --s2k-count 65536 doc

To decrypt a symmetrically encrypted doc.gpg using a passphrase and output decrypted contents into the same directory as doc do:

$ gpg --output doc --decrypt doc.gpg

Key maintenance

Backup your private key

To backup your private key do the following:

$ gpg --export-secret-keys --armor --output privkey.asc user-id

Note the above command will require that you enter the passphrase for the key. This is because otherwise anyone who gains access to the above exported file would be able to encrypt and sign documents as if they were you without needing to know your passphrase.

경고: The passphrase is usually the weakest link in protecting your secret key. Place the private key in a safe place on a different system/device, such as a locked container or encrypted drive. It is the only safety you have to regain control to your keyring in case of, for example, a drive failure, theft or worse.

To import the backup of your private key:

$ gpg --import privkey.asc

도움말: Paperkey can be used to export private keys as human readable text or machine readable barcodes that can be printed on paper and archived.

Backup your revocation certificate

Revocation certificates are automatically generated for newly generated keys. These are by default located in ~/.gnupg/openpgp-revocs.d/. The filename of the certificate is the fingerprint of the key it will revoke. The revocation certificates can also be generated manually by the user later using:

$ gpg --gen-revoke --armor --output revcert.asc user-id

This certificate can be used to #Revoke a key if it is ever lost or compromised. The backup will be useful if you have no longer access to the secret key and are therefore not able to generate a new revocation certificate with the above command. It is short enough to be printed out and typed in by hand if necessary.

경고: Anyone with access to the revocation certificate can revoke the key publicly, this action cannot be undone. Protect your revocation certificate like you protect your secret key.

Edit your key

Running the gpg --edit-key user-id command will present a menu which enables you to do most of your key management related tasks.

Type help in the edit key sub menu to show the complete list of commands. Some useful ones:

> passwd       # change the passphrase
> clean        # compact any user ID that is no longer usable (e.g revoked or expired)
> revkey       # revoke a key
> addkey       # add a subkey to this key
> expire       # change the key expiration time
> adduid       # add additional names, comments, and email addresses
> addphoto     # add photo to key (must be JPG, 240x288 recommended, enter full path to image when prompted)

도움말: If you have multiple email accounts you can add each one of them as an identity, using adduid command. You can then set your favourite one as primary.

Exporting subkey

If you plan to use the same key across multiple devices, you may want to strip out your master key and only keep the bare minimum encryption subkey on less secure systems.

First, find out which subkey you want to export.

$ gpg --list-secret-keys --with-subkey-fingerprint

Select only that subkey to export.

$ gpg -a --export-secret-subkeys [subkey id]! > /tmp/subkey.gpg

경고: If you forget to add the !, all of your subkeys will be exported.

At this point you could stop, but it is most likely a good idea to change the passphrase as well. Import the key into a temporary folder.

$ gpg --homedir /tmp/gpg --import /tmp/subkey.gpg
$ gpg --homedir /tmp/gpg --edit-key user-id
> passwd
> save
$ gpg --homedir /tmp/gpg -a --export-secret-subkeys [subkey id]! > /tmp/subkey.altpass.gpg

참고: You will get a warning that the master key was not available and the password was not changed, but that can safely be ignored as the subkey password was.

At this point, you can now use /tmp/subkey.altpass.gpg on your other devices.

Extending expiration date

경고: Never delete your expired or revoked subkeys unless you have a good reason. Doing so will cause you to lose the ability to decrypt files encrypted with the old subkey. Please only delete expired or revoked keys from other users to clean your keyring.

It is good practice to set an expiration date on your subkeys, so that if you lose access to the key (e.g. you forget the passphrase) the key will not continue to be used indefinitely by others. When the key expires, it is relatively straight-forward to extend the expiration date:

$ gpg --edit-key user-id
> expire

You will be prompted for a new expiration date, as well as the passphrase for your secret key, which is used to sign the new expiration date.

Repeat this for any further subkeys that have expired:

> key 1
> expire

Finally, save the changes and quit:

> save

Update it to a keyserver.

$ gpg --keyserver keyserver.ubuntu.com --send-keys key-id

Alternatively, if you use this key on multiple computers, you can export the public key (with new signed expiration dates) and import it on those machines:

$ gpg --export --output pubkey.gpg user-id
$ gpg --import pubkey.gpg

There is no need to re-export your secret key or update your backups: the master secret key itself never expires, and the signature of the expiration date left on the public key and subkeys is all that is needed.

Rotating subkeys

경고: Never delete your expired or revoked subkeys unless you have a good reason. Doing so will cause you to lose the ability to decrypt files encrypted with the old subkey. Please only delete expired or revoked keys from other users to clean your keyring.

Alternatively, if you prefer to stop using subkeys entirely once they have expired, you can create new ones. Do this a few weeks in advance to allow others to update their keyring.

도움말: You do not need to create a new key simply because it is expired. You can extend the expiration date, see the section #Extending expiration date.

Create new subkey (repeat for both signing and encrypting key)

$ gpg --edit-key user-id
> addkey

And answer the following questions it asks (see #Create a key pair for suggested settings).

Save changes

> save

Update it to a keyserver.

$ gpg --keyserver pgp.mit.edu --send-keys user-id

You will also need to export a fresh copy of your secret keys for backup purposes. See the section #Backup your private key for details on how to do this.

도움말: Revoking expired subkeys is unnecessary and arguably bad form. If you are constantly revoking keys, it may cause others to lack confidence in you.

Revoke a key

Key revocation should be performed if the key is compromised, superseded, no longer used, or you forget your passphrase. This is done by merging the key with the revocation certificate of the key.

If you have no longer access to your keypair, first #Import a public key to import your own key. Then, to revoke the key, import the file saved in #Backup your revocation certificate:

 $ gpg --import revcert.asc

Now the revocation needs to be made public. #Use a keyserver to send the revoked key to a public PGP server if you used one in the past, otherwise, export the revoked key to a file and distribute it to your communication partners.

Signatures

Signatures certify and timestamp documents. If the document is modified, verification of the signature will fail. Unlike encryption which uses public keys to encrypt a document, signatures are created with the user's private key. The recipient of a signed document then verifies the signature using the sender's public key.

Create a signature

Sign a file

To sign a file use the -s/--sign flag:

$ gpg --output doc.sig --sign doc

doc.sig contains both the compressed content of the original file doc and the signature in a binary format, but the file is not encrypted. However, you can combine signing with encrypting.

Clearsign a file or message

To sign a file without compressing it into binary format use:

$ gpg --output doc.sig --clearsign doc

Here both the content of the original file doc and the signature are stored in human-readable form in doc.sig.

Make a detached signature

To create a separate signature file to be distributed separately from the document or file itself, use the --detach-sig flag:

$ gpg --output doc.sig --detach-sig doc

Here the signature is stored in doc.sig, but the contents of doc are not stored in it. This method is often used in distributing software projects to allow users to verify that the program has not been modified by a third party.

Verify a signature

To verify a signature use the --verify flag:

$ gpg --verify doc.sig

where doc.sig is the signed file containing the signature you wish to verify.

If you are verifying a detached signature, both the signed data file and the signature file must be present when verifying. For example, to verify Arch Linux's latest iso you would do:

$ gpg --verify archlinux-version.iso.sig

where archlinux-version.iso must be located in the same directory.

You can also specify the signed data file with a second argument:

$ gpg --verify archlinux-version.iso.sig /path/to/archlinux-version.iso

If a file has been encrypted in addition to being signed, simply decrypt the file and its signature will also be verified.

gpg-agent

gpg-agent is mostly used as daemon to request and cache the password for the keychain. This is useful if GnuPG is used from an external program like a mail client. gnupg comes with systemd user sockets which are enabled by default. These sockets are gpg-agent.socket, gpg-agent-extra.socket, gpg-agent-browser.socket, gpg-agent-ssh.socket, and dirmngr.socket.

• The main gpg-agent.socket is used by gpg to connect to the gpg-agent daemon.
• The intended use for the gpg-agent-extra.socket on a local system is to set up a Unix domain socket forwarding from a remote system. This enables to use gpg on the remote system without exposing the private keys to the remote system. See gpg-agent(1) for details.
• The gpg-agent-browser.socket allows web browsers to access the gpg-agent daemon.
• The gpg-agent-ssh.socket can be used by SSH to cache SSH keys added by the ssh-add program. See #SSH agent for the necessary configuration.
• The dirmngr.socket starts a GnuPG daemon handling connections to keyservers.

참고: If you use non-default GnuPG #Directory location, you will need to edit all socket files to use the values of gpgconf --list-dirs.

Configuration

gpg-agent can be configured via ~/.gnupg/gpg-agent.conf file. The configuration options are listed in gpg-agent(1). For example you can change cache ttl for unused keys:

~/.gnupg/gpg-agent.conf

default-cache-ttl 3600

도움말: To cache your passphrase for the whole session, please run the following command:

$ /usr/lib/gnupg/gpg-preset-passphrase --preset XXXXX

where XXXXX is the keygrip. You can get its value when running gpg --with-keygrip -K. The passphrase will be stored until gpg-agent is restarted. If you set up default-cache-ttl value, it will take precedence.

Reload the agent

After changing the configuration, reload the agent using gpg-connect-agent:

$ gpg-connect-agent reloadagent /bye

The command should print OK.

However in some cases only the restart may not be sufficient, like when keep-screen has been added to the agent configuration. In this case you firstly need to kill the ongoing gpg-agent process and then you can restart it as was explained above.

pinentry

gpg-agent can be configured via the pinentry-program stanza to use a particular pinentry user interface when prompting the user for a passphrase. For example:

~/.gnupg/gpg-agent.conf

pinentry-program /usr/bin/pinentry-curses

There are other pinentry programs that you can choose from - see pacman -Ql pinentry | grep /usr/bin/.

도움말: In order to use /usr/bin/pinentry-kwallet you have to install the kwalletcli^AUR package.

Remember to reload the agent after making changes to the configuration.

Cache passwords

max-cache-ttl and default-cache-ttl defines how many seconds gpg-agent should cache the passwords. To enter a password once a session, set them to something very high, for instance:

gpg-agent.conf

max-cache-ttl 60480000
default-cache-ttl 60480000

For password caching in SSH emulation mode, set default-cache-ttl-ssh and max-cache-ttl-ssh instead, for example:

gpg-agent.conf

default-cache-ttl-ssh 60480000
max-cache-ttl-ssh 60480000

Unattended passphrase

Starting with GnuPG 2.1.0 the use of gpg-agent and pinentry is required, which may break backwards compatibility for passphrases piped in from STDIN using the --passphrase-fd 0 commandline option. In order to have the same type of functionality as the older releases two things must be done:

First, edit the gpg-agent configuration to allow loopback pinentry mode:

~/.gnupg/gpg-agent.conf

allow-loopback-pinentry

Reload the agent if it is running to let the change take effect.

Second, either the application needs to be updated to include a commandline parameter to use loopback mode like so:

$ gpg --pinentry-mode loopback ...

...or if this is not possible, add the option to the configuration:

~/.gnupg/gpg.conf

pinentry-mode loopback

참고: The upstream author indicates setting pinentry-mode loopback in gpg.conf may break other usage, using the commandline option should be preferred if at all possible. [2]

SSH agent

gpg-agent has OpenSSH agent emulation. If you already use the GnuPG suite, you might consider using its agent to also cache your SSH keys. Additionally, some users may prefer the PIN entry dialog GnuPG agent provides as part of its passphrase management.

Set SSH_AUTH_SOCK

You have to set SSH_AUTH_SOCK so that SSH will use gpg-agent instead of ssh-agent. To make sure each process can find your gpg-agent instance regardless of e.g. the type of shell it is child of use pam_env.

~/.pam_environment

SSH_AGENT_PID	DEFAULT=
SSH_AUTH_SOCK	DEFAULT="${XDG_RUNTIME_DIR}/gnupg/S.gpg-agent.ssh"

참고:

• If you set your SSH_AUTH_SOCK manually (such as in this pam_env example), keep in mind that your socket location may be different if you are using a custom GNUPGHOME. You can use the following bash example, or change SSH_AUTH_SOCK to the value of gpgconf --list-dirs agent-ssh-socket.
• If GNOME Keyring is installed, it is necessary to deactivate its ssh component. Otherwise, it will overwrite SSH_AUTH_SOCK.

Alternatively, depend on Bash. This works for non-standard socket locations as well:

~/.bashrc

unset SSH_AGENT_PID
if [ "${gnupg_SSH_AUTH_SOCK_by:-0}" -ne $$ ]; then
  export SSH_AUTH_SOCK="$(gpgconf --list-dirs agent-ssh-socket)"
fi

참고: The test involving the gnupg_SSH_AUTH_SOCK_by variable is for the case where the agent is started as gpg-agent --daemon /bin/sh, in which case the shell inherits the SSH_AUTH_SOCK variable from the parent, gpg-agent [3].

Configure pinentry to use the correct TTY

Also set the GPG_TTY and refresh the TTY in case user has switched into an X session as stated in gpg-agent(1). For example:

~/.bashrc

export GPG_TTY=$(tty)
gpg-connect-agent updatestartuptty /bye >/dev/null

Add SSH keys

Once gpg-agent is running you can use ssh-add to approve keys, following the same steps as for ssh-agent. The list of approved keys is stored in the ~/.gnupg/sshcontrol file.

Once your key is approved, you will get a pinentry dialog every time your passphrase is needed. For password caching see #Cache passwords.

Using a PGP key for SSH authentication

You can also use your PGP key as an SSH key. This requires a key with the Authentication capability (see #Custom capabilities). There are various benefits gained by using a PGP key for SSH authentication, including:

• Reduced key maintenance, as you will no longer need to maintain an SSH key.
• The ability to store the authentication key on a smartcard. GnuPG will automatically detect the key when the card is available, and add it to the agent (check with ssh-add -l or ssh-add -L). The comment for the key should be something like: openpgp:key-id or cardno:card-id.

To retrieve the public key part of your GPG/SSH key, run gpg --export-ssh-key gpg-key.

Unless you have your GPG key on a keycard, you need to add your key to $GNUPGHOME/sshcontrol to be recognized as a SSH key. If your key is on a keycard, its keygrip is added to sshcontrol implicitly. If not, get the keygrip of your key this way:

$ gpg --list-keys --with-keygrip

sub   rsa4096 2018-07-25 [A]
      Keygrip = 1531C8084D16DC4C36911F1585AF0ACE7AAFD7E7

Then edit sshcontrol like this. Adding the keygrip is a one-time action; you will not need to edit the file again, unless you are adding additional keys.

$GNUPGHOME/sshcontrol

1531C8084D16DC4C36911F1585AF0ACE7AAFD7E7

Smartcards

GnuPG uses scdaemon as an interface to your smartcard reader, please refer to the man page scdaemon(1) for details.

GnuPG only setups

참고: To allow scdaemon direct access to USB smartcard readers the optional dependency libusb-compat must be installed

If you do not plan to use other cards but those based on GnuPG, you should check the reader-port parameter in ~/.gnupg/scdaemon.conf. The value '0' refers to the first available serial port reader and a value of '32768' (default) refers to the first USB reader.

GnuPG with pcscd (PCSC Lite)

pcscd(8) is a daemon which handles access to smartcard (SCard API). If GnuPG's scdaemon fails to connect the smartcard directly (e.g. by using its integrated CCID support), it will fallback and try to find a smartcard using the PCSC Lite driver.

To use pscsd install pcsclite and ccid. Then start and/or enable pcscd.service. Alternatively start and/or enable pcscd.socket to activate the daemon when needed.

Always use pcscd

If you are using any smartcard with an opensc driver (e.g.: ID cards from some countries) you should pay some attention to GnuPG configuration. Out of the box you might receive a message like this when using gpg --card-status

gpg: selecting openpgp failed: ec=6.108

By default, scdaemon will try to connect directly to the device. This connection will fail if the reader is being used by another process. For example: the pcscd daemon used by OpenSC. To cope with this situation we should use the same underlying driver as opensc so they can work well together. In order to point scdaemon to use pcscd you should remove reader-port from ~/.gnupg/scdaemon.conf, specify the location to libpcsclite.so library and disable ccid so we make sure that we use pcscd:

~/.gnupg/scdaemon.conf

pcsc-driver /usr/lib/libpcsclite.so
card-timeout 5
disable-ccid

Please check scdaemon(1) if you do not use OpenSC.

Shared access with pcscd

GnuPG scdaemon is the only popular pcscd client that uses PCSC_SHARE_EXCLUSIVE flag when connecting to pcscd. Other clients like OpenSC PKCS#11 that are used by browsers and programs listed in Electronic identification are using PCSC_SHARE_SHARED that allows simultaneous access to single smartcard. pcscd will not give exclusive access to smartcard while there are other clients connected. This means that to use GnuPG smartcard features you must before have to close all your open browser windows or do some other inconvenient operations. There is a out of tree patch in GPGTools/MacGPG2 git repo that enables scdaemon to use shared access but GnuPG developers are against allowing this because when one pcscd client authenticates the smartcard then some other malicious pcscd clients could do authenticated operations with the card without you knowing. You can read full mailing list thread here.

If you accept the security risk then you can use the patch from GPGTools/MacGPG2 git repo or use gnupg-scdaemon-shared-access^{AUR[broken link: package not found]} package. After patching your scdaemon you can enable shared access by modifying your scdaemon.conf file and adding shared-access line end of it.

Multi applet smart cards

When using YubiKeys or other multi applet USB dongles with OpenSC PKCS#11 may run into problems where OpenSC switches your Yubikey from OpenPGP to PIV applet, breaking the scdaemon.

You can hack around the problem by forcing OpenSC to also use the OpenPGP applet. Open /etc/opensc.conf file, search for Yubikey and change the driver = "PIV-II"; line to driver = "openpgp";. If there is no such entry, use pcsc_scan. Search for the Answer to Reset ATR: 12 34 56 78 90 AB CD .... Then create a new entry.

/etc/opensc.conf

...
card_atr 12:23:34:45:67:89:ab:cd:... {
    name = "YubiKey Neo";
    driver = "openpgp"
}
...

After that you can test with pkcs11-tool -O --login that the OpenPGP applet is selected by default. Other PKCS#11 clients like browsers may need to be restarted for that change to be applied.

Tips and tricks

Different algorithm

You may want to use stronger algorithms:

~/.gnupg/gpg.conf

...

personal-digest-preferences SHA512
cert-digest-algo SHA512
default-preference-list SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed
personal-cipher-preferences TWOFISH CAMELLIA256 AES 3DES

In the latest version of GnuPG, the default algorithms used are SHA256 and AES, both of which are secure enough for most people. However, if you are using a version of GnuPG older than 2.1, or if you want an even higher level of security, then you should follow the above step.

Encrypt a password

It can be useful to encrypt some password, so it will not be written in clear on a configuration file. A good example is your email password.

First create a file with your password. You need to leave one empty line after the password, otherwise gpg will return an error message when evaluating the file.

Then run:

$ gpg -e -a -r user-id your_password_file

-e is for encrypt, -a for armor (ASCII output), -r for recipient user ID.

You will be left with a new your_password_file.asc file.

도움말: pass automates this process.

Change trust model

By default GnuPG uses the Web of Trust as the trust model. You can change this to Trust on first use by adding --trust-model=tofu when adding a key or adding this option to your GnuPG configuration file. More details are in this email to the GnuPG list.

Hide all recipient id's

By default the recipient's key ID is in the encrypted message. This can be removed at encryption time for a recipient by using hidden-recipient user-id. To remove it for all recipients add throw-keyids to your configuration file. This helps to hide the receivers of the message and is a limited countermeasure against traffic analysis. (Using a little social engineering anyone who is able to decrypt the message can check whether one of the other recipients is the one he suspects.) On the receiving side, it may slow down the decryption process because all available secret keys must be tried (e.g. with --try-secret-key user-id).

Using caff for keysigning parties

To allow users to validate keys on the keyservers and in their keyrings (i.e. make sure they are from whom they claim to be), PGP/GPG uses the Web of Trust. Keysigning parties allow users to get together at a physical location to validate keys. The Zimmermann-Sassaman key-signing protocol is a way of making these very effective. Here you will find a how-to article.

For an easier process of signing keys and sending signatures to the owners after a keysigning party, you can use the tool caff. It can be installed from the AUR with the package caff-git^AUR.

To send the signatures to their owners you need a working MTA. If you do not have already one, install msmtp.

Always show long ID's and fingerprints

To always show long key ID's add keyid-format 0xlong to your configuration file. To always show full fingerprints of keys, add with-fingerprint to your configuration file.

Custom capabilities

For further customization also possible to set custom capabilities to your keys. The following capabilities are available:

• Certify (only for master keys) - allows the key to create subkeys, mandatory for master keys.
• Sign - allows the key to create cryptographic signatures that others can verify with the public key.
• Encrypt - allows anyone to encrypt data with the public key, that only the private key can decrypt.
• Authenticate - allows the key to authenticate with various non-GnuPG programs. The key can be used as e.g. an SSH key.

It is possible to specify the capabilities of the master key, by running:

$ gpg --full-generate-key --expert

And select an option that allows you to set your own capabilities.

Comparably, to specify custom capabilities for subkeys, add the --expert flag to gpg --edit-key, see #Edit your key for more information.

Troubleshooting

Not enough random bytes available

When generating a key, gpg can run into this error:

Not enough random bytes available. Please do some other work to give the OS a chance to collect more entropy!

To check the available entropy, check the kernel parameters:

$ cat /proc/sys/kernel/random/entropy_avail

A healthy Linux system with a lot of entropy available will have return close to the full 4,096 bits of entropy. If the value returned is less than 200, the system is running low on entropy.

To solve it, remember you do not often need to create keys and best just do what the message suggests (e.g. create disk activity, move the mouse, edit the wiki - all will create entropy). If that does not help, check which service is using up the entropy and consider stopping it for the time. If that is no alternative, see Random number generation#Alternatives.

su

When using pinentry, you must have the proper permissions of the terminal device (e.g. /dev/tty1) in use. However, with su (or sudo), the ownership stays with the original user, not the new one. This means that pinentry will fail with a Permission denied error, even as root. If this happens when attempting to use ssh, an error like sign_and_send_pubkey: signing failed: agent refused operation will be returned. The fix is to change the permissions of the device at some point before the use of pinentry (i.e. using gpg with an agent). If doing gpg as root, simply change the ownership to root right before using gpg:

# chown root /dev/ttyN  # where N is the current tty

and then change it back after using gpg the first time. The equivalent is true with /dev/pts/.

참고: The owner of tty must match with the user for which pinentry is running. Being part of the group tty is not enough.

도움말: If you run gpg with script it will use a new tty with the correct ownership:

# script -q -c "gpg --gen-key" /dev/null

Agent complains end of file

If the pinentry program is /usr/bin/pinentry-gnome3, it needs a DBus session bus to run properly. See General troubleshooting#Session permissions for details.

Alternatively, you can use a variety of different options described in #pinentry.

KGpg configuration permissions

There have been issues with kgpg being able to access the ~/.gnupg/ options. One issue might be a result of a deprecated options file, see the bug report.

GNOME on Wayland overrides SSH agent socket

For Wayland sessions, gnome-session sets SSH_AUTH_SOCK to the standard gnome-keyring socket, $XDG_RUNTIME_DIR/keyring/ssh. This overrides any value set in ~/.pam_environmment or systemd unit files.

See GNOME/Keyring#Disable keyring daemon components on how to disable this behavior.

mutt

Mutt might not use gpg-agent correctly, you need to set an environment variable GPG_AGENT_INFO (the content does not matter) when running mutt. Be also sure to enable password caching correctly, see #Cache passwords.

See this forum thread.

"Lost" keys, upgrading to gnupg version 2.1

When gpg --list-keys fails to show keys that used to be there, and applications complain about missing or invalid keys, some keys may not have been migrated to the new format.

Please read GnuPG invalid packet workaround. Basically, it says that there is a bug with keys in the old pubring.gpg and secring.gpg files, which have now been superseded by the new pubring.kbx file and the private-keys-v1.d/ subdirectory and files. Your missing keys can be recovered with the following commands:

$ cd
$ cp -r .gnupg gnupgOLD
$ gpg --export-ownertrust > otrust.txt
$ gpg --import .gnupg/pubring.gpg
$ gpg --import-ownertrust otrust.txt
$ gpg --list-keys

gpg hanged for all keyservers (when trying to receive keys)

If gpg hanged with a certain keyserver when trying to receive keys, you might need to kill dirmngr in order to get access to other keyservers which are actually working, otherwise it might keeping hanging for all of them.

Smartcard not detected

Your user might not have the permission to access the smartcard which results in a card error to be thrown, even though the card is correctly set up and inserted.

One possible solution is to add a new group scard including the users who need access to the smartcard.

Then use udev rules, similar to the following:

/etc/udev/rules.d/71-gnupg-ccid.rules

ACTION=="add", SUBSYSTEM=="usb", ENV{ID_VENDOR_ID}=="1050", ENV{ID_MODEL_ID}=="0116|0111", MODE="660", GROUP="scard"

One needs to adapt VENDOR and MODEL according to the lsusb output, the above example is for a YubikeyNEO.

server 'gpg-agent' is older than us (x < y)

This warning appears if gnupg is upgraded and the old gpg-agent is still running. Restart the user's gpg-agent.socket (i.e., use the --user flag when restarting).

IPC connect call failed

Make sure gpg-agent and dirmngr are not running with killall gpg-agent dirmngr and the $GNUPGHOME/crls.d/ folder has permission set to 700.

If your keyring is stored on a vFat filesystem (e.g. a USB drive), gpg-agent will fail to create the required sockets (vFat does not support sockets), you can create redirects to a location that handles sockets, e.g. /dev/shm:

# export GNUPGHOME=/custom/gpg/home
# printf '%%Assuan%%\nsocket=/dev/shm/S.gpg-agent\n' > $GNUPGHOME/S.gpg-agent
# printf '%%Assuan%%\nsocket=/dev/shm/S.gpg-agent.browser\n' > $GNUPGHOME/S.gpg-agent.browser
# printf '%%Assuan%%\nsocket=/dev/shm/S.gpg-agent.extra\n' > $GNUPGHOME/S.gpg-agent.extra
# printf '%%Assuan%%\nsocket=/dev/shm/S.gpg-agent.ssh\n' > $GNUPGHOME/S.gpg-agent.ssh

Test that gpg-agent starts successfully with gpg-agent --daemon.

Mitigating Poisoned PGP Certificates

In June 2019, an unknown attacker spammed several high-profile PGP certificates with tens of thousands (or hundreds of thousands) of signatures (CVE-2019-13050) and uploaded these signatures to the SKS keyservers. The existence of these poisoned certificates in a keyring causes gpg to hang with the following message:

gpg: removing stale lockfile (created by 7055)

Possible mitigation involves removing the poisoned certificate as per this blog post.

Invalid IPC response and Inappropriate ioctl for device

The default pinentry program is /usr/bin/pinentry-gtk-2. If gtk2 is unavailable, pinentry falls back to /usr/bin/pinentry-curses and causes signing to fail:

gpg: signing failed: Inappropriate ioctl for device
gpg: [stdin]: clear-sign failed: Inappropriate ioctl for device

You need to set the GPG_TTY environment variable for the pinentry programs /usr/bin/pinentry-tty and /usr/bin/pinentry-curses.

$ export GPG_TTY=$(tty)

See also

• GNU Privacy Guard Homepage
• Alan Eliasen's GPG Tutorial
• RFC4880 "OpenPGP Message Format"
• gpg.conf recommendations and best practices
• Creating GPG Keys (Fedora)
• OpenPGP subkeys in Debian
• Protecting code integrity with PGP
• A more comprehensive gpg Tutorial
• /r/GPGpractice - a subreddit to practice using GnuPG.