GNU/Linux Crypto: Passwords

This entry is part 6 of 10 in the series GNU/Linux Crypto.

It’s now becoming more widely known that using guessable passwords or using the same password for more than one account is a serious security risk, because an attacker able to control one account (such as an email account) can do a lot of damage. If an attacker gets the hash of your password from some web service, you want to be assured that the hash will be very difficult to reverse, and even if it can be reversed, that it’s unique and won’t give them access to any of your other accounts.

This growing awareness has contributed to the popularity of password managers, tools designed to securely generate, store, and retrieve passwords, encrypted with a master password or passphrase. In some cases these are locally stored, such as KeePass, and in others they are stored on a web service, such as LastPass. Both are good tools, and work well with GNU/Linux. I personally have some reservations about LastPass as I don’t want my passwords stored on a third party service, and I don’t trust JavaScript encryption.

Interestingly, because we now have a tidy GnuPG setup to handle the encryption ourselves, another option is the pass(1) tool, billing itself as “the standard UNIX password manager”. It’s little more than a shell script and some bash(1) completions wrapped around existing tools like git(1), gpg2(1), pwgen(1), tree(1), and xclip(1), and your choice of $EDITOR. If you’re not already invested in an existing password management method, you might find this a good first application of your new cryptography setup, and a great minimal approach to secure password storage accessible from the command line (and therefore SSH).

On Debian-derived systems, it’s available as part of the pass package:

# apt-get install pass

This includes a manual:

$ man pass

Instructions for installing on other operating systems are also available on the site. Releases are also available for download, and a link to the development repository. If you use this, make sure you have the required tools outlined above installed as well, although xclip(1) is only needed if you run the X Windows system.

Setup

We can get an overview of what pass(1) can do by invoking it with no arguments:

$ pass

To start, we’ll initialize our password store. For your own passwords, you will want to do this as your own user rather than root. Because pass(1) uses GnuPG for its encryption, we also need to tell it the ID of the appropriate key to use. Remember, you can find this eight-digit hex code by typing gpg --list-secret-keys. A unique string identifying your private key such as your name or email address may also work.

$ pass init 0x77BB8872
mkdir: created directory ‘/home/tom/.password-store’
Password store initialized for 0x77BB8872.

Indeed, we note the directory ~/.password-store has been created, although it’s presently empty except for the .gpg-id file recording our key ID:

$ find .password-store
.password-store
.password-store/.gpg-id

Inserting

We’ll insert an existing password of ours with pass insert, giving it a descriptive hierarchical name:

$ pass insert google.com/gmail/example@gmail.com
mkdir: created directory ‘/home/tom/.password-store/google.com’
mkdir: created directory ‘/home/tom/.password-store/google.com/gmail’
Enter password for google.com/gmail/example@gmail.com:
Retype password for google.com/gmail/example@gmail.com:

The password is read from the command line, encrypted, and placed in ~/.password-store:

$ find .password-store
.password-store
.password-store/google.com
.password-store/google.com/gmail
.password-store/google.com/gmail/example@gmail.com.gpg
.password-store/.gpg-id

Notice that pass(1) creates a directory structure for us automatically. We can get a nice view of the password store with pass with no arguments:

$ pass
Password Store
└── google.com
    └── gmail
            └── example@gmail.com

Generating

If you’d like it to generate a new secure random password for you, you can use generate instead, including a password length as the last argument:

$ pass generate google.com/gmail/example@gmail.com 16
The generated password to google.com/gmail/example@gmail.com is:
!Q%i$$&q1+JJi-|X

If you have some service that doesn’t cooperate with symbols in passwords, you can add the -n option to this call:

$ pass generate -n google.com/gmail/example@gmail.com 16
The generated password to google.com/gmail/example@gmail.com is:
pJeF18CrZEZzI59D

pass(1) uses pwgen(1) for this password generation. In each case, the password is automatically inserted into the password store for you.

If we need to change an existing password, we can either overwrite it with insert again, or use the edit operation to invoke our choice of $EDITOR:

$ pass edit google.com/gmail/example@gmail.com

If you do this, you may like to be careful that your editor is not configured to keep backups or swap files in plain text of documents it edits in temporary directories or memory filesystems. If you’re using Vim, I wrote a plugin in an attempt to solve this problem.

Note that adding or overwriting passwords does not require your passphrase; only retrieval and editing does, consistent with how GnuPG normally works.

Retrieval

This password can now be retrieved and echoed onto the command line given the appropriate passphrase:

$ pass google.com/gmail/example@gmail.com
(...gpg-agent pinentry prompt...)
Tr0ub4dor&3

If you’re using X windows and have xclip(1) installed, you can put the password on the clipboard temporarily to paste into web forms:

$ pass -c google.com/gmail/example@gmail.com
Copied google.com/gmail/example@gmail.com to clipboard. Will clear in 45 seconds.

In each case, note that if you have the bash completion installed and working, you should be able to complete the full path to the passwords with Tab, just as if you were directly browsing a directory hierarchy.

Deletion

If we no longer need the password, we can remove it with pass rm:

$ pass rm google.com/gmail/example@gmail.com
Are you sure you would like to delete google.com/gmail/example@gmail.com? [y/N] y
removed ‘/home/tom/.password-store/google.com/gmail/example@gmail.com.gpg’

We can delete whole directories of passwords with pass rm -r:

$ pass rm -r google.com
Are you sure you would like to delete google.com? [y/N] y
removed ‘/home/tom/.password-store/google.com/gmail/example@gmail.com.gpg’
removed directory: ‘/home/tom/.password-store/google.com/gmail’
removed directory: ‘/home/tom/.password-store/google.com’

Version control

To keep historical passwords, including deleted ones if we find we do need them again one day, we can set up some automatic version control on the directory with pass git init:

$ pass git init
Initialized empty Git repository in /home/tom/.password-store/.git/
[master (root-commit) 0ebb933] Added current contents of password store.
 1 file changed, 1 insertion(+)
 create mode 100644 .gpg-id

This will update the repository every time the password store is changed, meaning we can be confident we’ll be able to retrieve old passwords we’ve replaced or deleted:

$ pass insert google.com/gmail/newexample@gmail.com
mkdir: created directory ‘/home/tom/.password-store/google.com’
mkdir: created directory ‘/home/tom/.password-store/google.com/gmail’
Enter password for google.com/gmail/newexample@gmail.com:
Retype password for google.com/gmail/newexample@gmail.com:
[master 00971b6] Added given password for google.com/gmail/newexample@gmail.com to store.
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 google.com/gmail/newexample@gmail.com.gpg

Backups

Because the password files are all encrypted only to your GnuPG key, you can relatively safely back up the store on remote and third-party sites simply by copying the ~/.password-store directory. If the filenames themselves contain sensitive information, such as private usernames or sites, you might like to back up an encrypted tarball of the store instead:

$ tar -cz .password-store \
    | gpg --sign --encrypt -r 0x77BB8872 \
    > password-store-backup.tar.gz.gpg

This directory can be restored in a similar way:

$ gpg --decrypt \
    < password-store-backup.tar.gz.gpg \
    | tar -xz 

GNU/Linux Crypto: GnuPG Usage

This entry is part 3 of 10 in the series GNU/Linux Crypto.

With our private and public key generated and stored, we can start using a few of GnuPG’s features to sign, verify, encrypt, and decrypt files and messages for distribution over untrusted channels like the internet.

Signing a message or text file

We’ll start by signing a simple text file, using the --clearsign option. This includes the signature in the message, which we can then distribute to people to read. Here’s the contents of message.txt:

This is a public message from Tom Ryder.

We’ll sign that with our new private key like so:

$ gpg --clearsign message.txt

We’re prompted for our passphrase for the private key:

You need a passphrase to unlock the secret key for
user: "Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>"
4096-bit RSA key, ID 040FE79B, created 2013-03-23

Having provided that, the file message.txt.asc is created, with PGP sections and a plaintext ASCII signature:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

This is a public message from Tom Ryder.
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.10 (GNU/Linux)
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=WMzO
-----END PGP SIGNATURE-----

Note that the message itself is plainly readable; this message isn’t encrypted, it’s just verified as having been written by a particular person, and not altered since it was written.

Now anyone who has our public key on their keyring (as we ourselves do) can verify that it was actually us who wrote this message:

$ gpg --verify message.txt.asc
gpg: Signature made Sat 23 Mar 2013 14:32:17 NZDT using RSA key ID 040FE79B
gpg: Good signature from "Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>"

If anybody tampers with the message, even something like removing a period from the end of a sentence, the verification will fail, suggesting the message was tampered with:

$ gpg --verify message.txt.asc
gpg: Signature made Sat 23 Mar 2013 14:32:17 NZDT using RSA key ID 040FE79B
gpg: BAD signature from "Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>"

Signing and verifying binary files

For all other files, we likely need to make the signature file separate with a detached signature:

$ gpg --armor --detach-sign archive.tar.gz

This produces a file archive.tar.gz.asc in the same directory, containing the signature. We use --armor to make the signature in ASCII, which makes for a longer file but easier distribution online.

In this case, both the file and the signature are required for verification; put the signature file first when you check this:

$ gpg --verify archive.tar.gz.asc archive.tar.gz

You can use this method to verify software downloads from trusted sources, such as the Apache HTTPD development team. First, we would download and import all their public keys at the URL they nominate:

$ wget http://www.apache.org/dist/httpd/KEYS
$ gpg --import KEYS

We could then download an Apache HTTPD release, along with its key, from an arbitrary mirror:

$ wget http://www.example.com/apache/httpd/httpd-2.4.4.tar.gz
$ wget https://www.apache.org/dist/httpd/httpd-2.4.4.tar.gz.asc

We can then use the key and signature to verify that it’s an uncompromised copy of the original file signed by the developers:

$ gpg --verify httpd-2.4.4.tar.gz.asc httpd-2.4.4.tar.gz
gpg: Signature made Tue 19 Feb 2013 09:28:39 NZDT using RSA key ID 791485A8
gpg: Good signature from "Jim Jagielski (Release Signing Key) <jim@apache.org>"
gpg:                 aka "Jim Jagielski <jim@jaguNET.com>"
gpg:                 aka "Jim Jagielski <jim@jimjag.com>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: A93D 62EC C3C8 EA12 DB22  0EC9 34EA 76E6 7914 85A8

Note that the gpg output cautions that this is still not perfect assurance that the release actually came from Jim Jagielski, because we’ve never met him and can’t absolutely, definitely say that this is his public key. Looking him up on public keyservers, we can however see a lot of other Apache developers have signed his key, which looks promising, but do we know who they are?

Despite the lack of absolute certainty, when downloading from mirrors this is a lot better (and harder to exploit) than simply downloading without validating or checksumming at all, given that the signature and the KEYS file were downloaded from Apache’s own site.

You will need to decide for yourself how sure you need to be whether a person’s public key really corresponds to them. This might extend to the point of arranging to meet them with government-issued identification!

Encrypting a file

We can encrypt a file so that only nominated people can decrypt and read it. In this case, we encrypt it not with our own private key, but with the recipient’s public key. This means that they will be able to decrypt it using their own private key.

Here’s the contents of secret-message.txt:

This is a secret message from Tom Ryder.

Now we need at least one recipient. Let’s say this message was intended for my friend John Public. He’s given me his public key in a file called john-public.asc on a USB drive in person; he even brought along his birth certificate and driver’s license (which is weird, because I’ve known him since I was four).

To start with, I’ll import his key into my keychain:

$ gpg --import john-public.asc
gpg: key 695195A5: public key "John Public (Main key) <johnpublic@example.com>" imported
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)

Now I can encrypt the message for only John to read. I like to use the 8-digit hex code for the key for --recipient, to make sure I’ve got the right person. You can see it in the output above, or in the output of gpg --list-keys.

$ gpg --armor --recipient 695195A5 --encrypt secret-message.txt

The encrypted message is written to secret-message.txt.asc:

-----BEGIN PGP MESSAGE-----
Version: GnuPG v1.4.10 (GNU/Linux)

hQEMAxiBb8eWSupuAQgAgOUQvqbTh60N6RQhDtP/bY9l+gjm4Grx5XcuhgQqK6pn
YtyPTKcpHdPK679lhbv0vE0RYe7pL+nBOngU1hCQYuGbRDZDxIXTIZW/rBvXbtHA
jgeSxrquad2totfh2nc7upePVCqXncPrLraJyDJBLLMrBHVvmOZymDabJbemOFuq
A/NbcmT3+osptvaEPFdlbgAW+J3vGxXMuQQYkT8GSnuutfEhZRb7SEL1ktaXwaMc
AA6NAan5ak7nCyDDHhDSDFMS9SQQHd8TDvQPF6OzRXlq26EOFD8HvlbDcgc51lbS
+N5nWaHM/CiuPh9dIOEV0H4Y8WDBdgkxp6kXKQfqb9JzAdwQ047r82SJAA7MSqCS
HRVtCRf5SNM12HqTRzF9XXum4uG+HXT6Bpy+K/lYpLgmHcHoUVKh8c2OcGaCHWQh
UC9B+aaThKdkxUfD/9tVIRmugjutgj7KdtDTGm+qLeCoJqp6HK5z5SX8Ha+P6/P5
hxinyw==
=kqUG
-----END PGP MESSAGE-----

Note that even I can’t read it, because I didn’t list myself as a recipient, and I don’t have access to John’s private key:

tom@tombox:~$ gpg --decrypt secret-message.txt.asc
gpg: encrypted with 2048-bit RSA key, ID 964AEA6E, created 2013-03-10
    "John Public (Main key) <johnpublic@example.com>"
gpg: decryption failed: secret key not available

However, on John’s computer, using his private key, he can decrypt and read it:

john@johnbox:~$ gpg --decrypt secret-message.txt.asc
gpg: encrypted with 2048-bit RSA key, ID 964AEA6E, created 2013-03-10
    "John Public (Main key) <johnpublic@example.com>"
This is a private, secret message from Tom Ryder.

If I wanted to make sure I could read the message too, I’d add my own public key to identify myself as a recipient when I encrypt it. Then either of us will be able to read it with our private keys (independently of the other):

$ gpg --recipient 695195A5 --recipient 040FE79B \
    --armor --encrypt secret-message.txt

Just to be thorough, we can sign the message as well to prove it came from us:

$ gpg --recipient 695195A5 --recipient 040FE79B \
    --armor --sign --encrypt secret-message.txt

Then when John runs the --decrypt, gpg will automatically verify the signature for us too, provided he has my public key in his keyring:

$ gpg --decrypt secret-message.txt.asc
gpg: encrypted with 2048-bit RSA key, ID 964AEA6E, created 2013-03-10
    "John Public (Main key) <johnpublic@example.com>"
gpg: encrypted with 4096-bit RSA key, ID AA159E5B, created 2013-03-23
    "Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>"
This is a private, secret message from Tom Ryder.
gpg: Signature made Sat 23 Mar 2013 17:23:20 NZDT using RSA key ID 040FE79B
gpg: Good signature from "Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>"

These are all the basic functions of GnuPG that will be useful to most people. We haven’t considered here submitting keys to public servers, or participating in the web of trust; you should only look into this once you’re happy with how your key setup is working, and are ready to publish your key for public use.

GNU/Linux Crypto: GnuPG Keys

This entry is part 2 of 10 in the series GNU/Linux Crypto.

Many tools that use cryptography on GNU/Linux and the internet revolve around the Pretty Good Privacy software standard (OpenPGP). The GNU Privacy Guard (GnuPG or GPG) is a popular free software implementation of this standard.

You can install GnuPG with its gpg(1) frontend on Debian like so:

# apt-get install gnupg

You can do a lot of very cool things with GPG, but it boils down to four central ideas:

  • Generation of keypairs, randomly-generated and mathematically linked pairs of files, one of which is kept permanently secret (the private key) and one of which is published (the public key). This is the basis of asymmetric key cryptography.
  • Managing keys, both your own public and private key, along with other people’s public keys, so that you can verify others’ messages and files, or encrypt them so that only those people can read them. This might include publishing your public key to online keyservers, and getting people to sign it to confirm that the key is really yours.
  • Signing files and messages with your private key to enable others to verify that a file or message was authored or sighted by you, and not edited in transmission over untrusted channels like the internet. The message itself remains readable to everybody.
  • Encrypting files and messages with other people’s public keys, so that only those people can decrypt and read them with their private keys. You can also sign such messages with your own private key so that people can verify that it was sent by you.

We’ll run through the fundamentals of each of these. We won’t concern ourselves too much with the mathematics or algorithms behind these operations; the Wikipedia article for asymmetric key cryptography explains this very well for those curious for further details.

Generating a keypair

Let’s start by generating a 4096-bit RSA keypair, which should be more than sufficient for almost everyone at the time of writing. We’ll observe a few of the best practices recommended for the Debian developers.

Doing this on a private, up-to-date desktop machine is best, as it’s easier to generate entropy this way. It’s still possible on an SSH-only headless server, but you may have to resort to less cryptographically sound methods to generate proper randomness.

Create or edit the file ~/.gnupg/gpg.conf on your system, and add the following lines:

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

These lines tell GnuPG to use the cryptographically stronger SHA256 hashing algorithm for signatures in preference to the long-broken SHA1 algorithm.

With that done, we can get down to generating some keys:

$ gpg --gen-key

You will be prompted to choose the type of keypair you want. The default ought to be RSA and RSA, which means we’ll generate one master key for signing, and one subkey for encryption:

Please select what kind of key you want:
   (1) RSA and RSA (default)
   (2) DSA and Elgamal
   (3) DSA (sign only)
   (4) RSA (sign only)
Your selection? 1

For the key length, choose the maximum 4096 bit RSA:

What keysize do you want? (2048) 4096
Requested keysize is 4096 bits

The expiry date is up to you. Good practice is to set an expiry date about a year out, because as long as you have access to the private key material, you can update the expiry date indefinitely, even if it’s already expired. For this particular example, we’ll set an expiry date one year out:

Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years
Key is valid for? (0) 1y
Key expires at Wed 21 Jan 2015 12:24:57 NZDT
Is this correct? (y/N) y

Next, we’re prompted for some basic information to name the key. In almost all circumstances you should use your real name, as without a real-world means to actually verify your identity, public keys are much less useful long-term. For the comment, you can include the key’s purpose, or your public aliases, or any other information relevant to the key:

Real name: Tom Ryder
Email address: tom@sanctum.geek.nz
Comment: Test Key Only
You selected this USER-ID:
    "Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O

Key passphrase

Next, we’re prompted for a passphrase to encrypt the key, so that if it ever falls into the wrong hands, nobody will be able to use it without knowing the passphrase.

You need a Passphrase to protect your secret key.

Choose a sequence of random words, or possibly a unique sentence you can easily memorise in any language, the longer the better. Don’t choose anything that might be feasibly guessable, like proverbs or movie quotes. You will also need to remember how you typed the passphrase exactly; I recommend using all-lowercase and no punctuation. Wikipedia has a few guidelines here.

You’ll need to type the passphrase twice to confirm it, and it won’t echo on your terminal, much as if you were typing a password.

Entropy generation

Finally, the system will prompt us to generate some entropy:

We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.

Not enough random bytes available.  Please do some other work to give
the OS a chance to collect more entropy! (Need 283 more bytes)

This step is necessary for the computer to generate sufficient random information to ensure that the private key being generated could not feasibly be reproduced. Moving the mouse around and using the keyboard on a desktop system is ideal, but generating any kind of hardware activity (including spinning disks up) should do the trick. Running expensive find(1) operations over a filesystem (with contents that couldn’t be reasonably predicted or guessed) helps too.

This step benefits from patience. You might find discussion online about forcing the use of the non-blocking PRNG random device /dev/urandom instead, using a tool like rngd(1). This definitely speeds up the process, but if you’re going to be using your key for anything serious, I recommend actually interacting with the computer and using hardware noise to seed the randomness adequately, if you can.

When adequate entropy is read and the key generation is done, you’ll be presented with some details for your master signing key pair and its encrypting subkey pair, and the private and public keys for each are automatically added to your keyring for use:

gpg: /home/tom/.gnupg/trustdb.gpg: trustdb created
gpg: key 040FE79B marked as ultimately trusted
public and secret key created and signed.
gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   1  signed:   0  trust: 0-, 0q, 0n, 0m, 0f, 1u
pub   4096R/040FE79B 2013-03-23
    Key fingerprint = 7A28 5ADA 7680 6813 48DF  401B 6207 438A 040F E79B
uid                  Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>
sub   4096R/AA159E5B 2013-03-23

Managing keys

With this done, we have our own keys added to the private and public keychain:

$ gpg --list-secret-keys
/home/tom/.gnupg/secring.gpg
----------------------------
sec   4096R/040FE79B 2013-03-23
uid                  Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>
ssb   4096R/AA159E5B 2013-03-23

$ gpg --list-public-keys
/home/tom/.gnupg/pubring.gpg
----------------------------
pub   4096R/040FE79B 2013-03-23
uid                  Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>
sub   4096R/AA159E5B 2013-03-23

The directory ~/.gnupg contains the managed keys. It’s very, very important to keep this directory private and to back it up securely, preferably to removable media that you keep in some physically secure place. Don’t lose it!

In most contexts in GnuPG, you can refer to a key by the name of its owner, or by its eight-digit hex ID. I prefer the latter method. Here, the short ID of my main key is 040FE79B. While you shouldn’t use this for any actual verification, it’s sufficiently unique that you can use it to identify a specific key on your keyring with which you want to work.

For example, if we want to provide someone with a copy of our public key, a friendly way to do so is to export it in ASCII format with --armor, providing the appropriate key’s short ID:

$ gpg --armor --export 040FE79B > tom-ryder.public.asc

While you can export private keys the same way with --export-secret-key, you should never, ever provide anyone with your private key, so this shouldn’t be necessary.

Revocation certificate

After generating your keys, you should generate a revocation certificate:

$ gpg --output revoke.asc --gen-revoke 040FE79B

sec  4096R/040FE79B 2013-03-23 Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>

Create a revocation certificate for this key? (y/N) y
Please select the reason for the revocation:
0 = No reason specified
1 = Key has been compromised
2 = Key is superseded
3 = Key is no longer used
Q = Cancel
(Probably you want to select 1 here)
Your decision? 1
Enter an optional description; end it with an empty line:
>
Reason for revocation: Key has been compromised
(No description given)
Is this okay? (y/N) y

You need a passphrase to unlock the secret key for
user: "Tom Ryder (Test Key Only) <tom@sanctum.geek.nz>"
4096-bit RSA key, ID 040FE79B, created 2013-03-23

ASCII armored output forced.
Revocation certificate created.

Please move it to a medium which you can hide away; if Mallory gets
access to this certificate he can use it to make your key unusable.
It is smart to print this certificate and store it away, just in case
your media become unreadable.  But have some caution:  The print system of
your machine might store the data and make it available to others!

You should store the resulting revoke.asc file somewhere safe. You can use this certificate to revoke your key later on if the private key is ever compromised, so that people know the key should no longer be used or trusted. You may even like to print it out and keep a hard copy, as the output of gpg suggests.

With the above setup done, we can proceed with some basic usage of GnuPG, as discussed in the next article.

Subkeys

In the output of both commands, you’ll note we actually have two private and two public keys. The sub line refers to the encryption subkey automatically generated for you. The master key is used for cryptographic signing, and the subkey for encryption; this is how GnuPG does things by default with RSA keypairs.

For extra security, it might be appropriate to physically remove the master private key from your computer, and instead use a second generated subkey for signing files as well. This is desirable because it allows you to keep the master key secure on some removable media (preferably with a backup), and not loaded on your main computer in case you get compromised.

This means you can sign and encrypt files as normal with your signing subkey and encryption subkey. If those keys ever get compromised, you can simply revoke them and generate new ones with your uncompromised master key; everyone who has signed your public master key or otherwise indicated they trust it will not have to do that all over again.

For details on how to do this, I suggest reading the Debian Wiki article on subkey management. However, it’s not necessary for performing basic GPG operations.

Thanks to commenter coldtobi for recommending setting a key expiry.

GNU/Linux Crypto: Introduction

This entry is part 1 of 10 in the series GNU/Linux Crypto.

Most of this series has been independently translated into Portuguese by Rafael Beraldo. Thanks very much, Rafael!

With the growing popularity of operating systems with Linux kernels that do not primarily use GNU components in the base system, this series was retitled to “GNU/Linux Crypto” in May 2017 for less ambiguity and to give proper credit to the GNU Project.

Cryptography for authentication and encryption is a complex and frequently changing field, and for somebody new to using it, it can be hard to know where to start. If you’re a GNU/Linux user comfortable with the terminal, but unfamiliar with the cryptographic tools available to you on open source UNIX-like operating systems, this series of posts aims at getting you set up with some basic tools that will allow you to keep your own information secure, to authenticate conveniently and safely with remote servers, and to work with signed and encrypted files online.

I’ll be working on Debian GNU/Linux, but most of these tools should adapt well to other open source UNIX-likes, including BSD. Please feel free to comment on the articles with details relevant to your own implementations, or with extra security considerations for interested readers.

As a disclaimer, I’m not myself an expert on cryptographic algorithms or key security. If you are, and you find an error or security problem with any of my explanations or suggestions, please let me know and I will correct it and credit you.

I’ll be covering the following topics:

If you already know about a specific topic, feel free to skip around through the other articles.