- Yubikey first-time setup
- Install GnuPG from the official site.
- Note, do not install GPG4Win. These instructions may or may not work with the opinionated default configuration of GPG4Win.
- Install the
Simple installer for the current GnuPGinstaller, as this is known to work, and is the distribution that these instructions were tested with. - If you have a GnuPG version installed already, note that at least 2.3 is required to be able to control the User Interaction Function (uif) features of the Yubikey through
gpg
- Install, or extract manually
- Extract the installer with 7-zip
- Copy the folder contents of the package to
%LOCALAPPDATA%\Programs\gnupg- To confirm that the directories were laid out correctly, ensure that
%LOCALAPPDATA%\Programs\gnupg\bin\gpg.exeexists
- To confirm that the directories were laid out correctly, ensure that
- Edit the PATH environment variable for your account to include the new gpg location
- Start ->
variables for your account-> "Edit environment variables for your account" - Add
%LOCALAPPDATA%\Programs\gnupg\binon a new line to thePATHenvironment variable in the top list (user variables), and not the bottom list (system variables)
- Start ->
Plug in your USB key to your computer, and confirm that the above worked by opening a Powershell prompt:
PS C:\Users\MyUser> gpg --card-status
Reader ...........: Yubico YubiKey OTP FIDO CCID 0
Application ID ...: D2760001240100000006179161830000
Application type .: OpenPGP
Version ..........: 0.0
Manufacturer .....: Yubico
Serial number ....: 17916183
Name of cardholder: [not set]
Language prefs ...: [not set]
Salutation .......:
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: not forced
Key attributes ...: rsa4096 rsa4096 rsa4096
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 0
KDF setting ......: off
UIF setting ......: Sign=off Decrypt=off Auth=off
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]You can get extended card details from the gpg-card utility, which lists additional information such as the card firmware version and OpenPGP module version:
PS C:\Users\MyUser> gpg-card
Reader ...........: Yubico YubiKey OTP FIDO CCID 0
Card type ........: yubikey
Card firmware ....: 5.4.3
Serial number ....: D2760001240100000006179161830000
Application type .: OpenPGP
Version ..........: 3.4
Displayed s/n ....: 17 916 183
Manufacturer .....: Yubico (6)
Name of cardholder: [not set]
Language prefs ...: [not set]
Salutation .......: [not set]
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: forced
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 3 3
Signature counter : 4
Capabilities .....: key-import algo-change button priv-data
KDF setting ......: on
UIF setting ......: Sign=on Decrypt=on Auth=on
Signature key ....: 49E599DB45969009FDD76E1D8E0806429B5BD1C2
keyref .....: OPENPGP.1 (sign,cert)
algorithm ..: brainpoolP512r1
stored fpr .: 576FA375139B2871414656DAD0658E1F80938677
created ....: 2021-11-15 21:11:08
Encryption key....: 7ED669F8514F18D9B2BED5611C893ACECD8F6023
keyref .....: OPENPGP.2 (encr)
algorithm ..: brainpoolP512r1
stored fpr .: 9F8A78352B3CE1BC867BCFB62F6F001478A29A2F
created ....: 2021-11-15 21:11:08
Authentication key: 5C771F3DB090AF6AAB277AB02BED944F1BB587A5
keyref .....: OPENPGP.3 (sign,auth)
algorithm ..: brainpoolP512r1
stored fpr .: F69909CA4F86E30189F4CA15F4289EAF9A3F140A
created ....: 2021-11-15 21:11:08
All of the below sections will start with gpg --edit-card to enter the interactive GPG card editor prompt.
Note that the some of the commands we run will prompt you for a PIN. The default PINs are:
- Unlock PIN:
123456 - Admin PIN:
12345678
The Unlock PIN is used to perform operations on the card using the private key, and is required to be entered the first time an operation is attempted after the gpg process opens the card for use.
The Admin PIN is used to change the state of the card, and to unblock the Unlock PIN if too many incorrect pin attempts are made.
There is also the Reset Code which can be used to reset the key in the event that the admin pin has been locked out.
PS C:\Users\MyUser\Desktop> gpg --edit-card
...
gpg/card> admin
Admin commands are allowed
gpg/card> passwd
gpg: OpenPGP card no. D2760001240100000006179161830000 detected
1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit
Your selection? 1
PIN changed.
1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit
Your selection? 3
PIN changed.
1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit
Your selection? 4
Reset Code set.
1 - change PIN
2 - unblock PIN
3 - change Admin PIN
4 - set the Reset Code
Q - quit
Your selection? qThe short answer: Choose RSA 4096
Why: It is ubiquitous, well understood, secure for 5-10 years, and fast enough (~1200ms per encryption operation)
If you want to be able to sign hundreds of commits when rebasing a code tree, use ECC and Curve 25519.
At present, both RSA and ECC are similar in cryptographic strength and safety, for common key lengths. Specifically, Table 2 in the NIST Special Publication 800-57 Part 1 lists security strengths (in effective bits of key) for various key lengths and algorithms. Reproduced in part here, noting the key lengths in bits that are equivalent security, with an additional row interpolated for a 4096-bit RSA key:
| Strength | Symmetric | RSA/DSA | ECC |
|---|---|---|---|
| 112 | 3TDEA | 2048 | 224-255 |
| 128 | AES-128 | 3072 | 256-383 (e.g. Curve 25519) |
| ~170 | N/A | 4096 | ~360 |
| 192 | AES-192 | 7680 | 384-511 |
| 256 | AES-256 | 15360 | >512 (e.g. NIST P-521, Brainpool P-512) |
This suggests that, at present, a 4096-bit RSA key is stronger than a 256 bit ECC key, but not a NIST P-521 key.
ECC keys are subject to an additional safety constraint as a result of the ECC curve that is chosen for various algorithms. The choice of ECC curve is made using the results of SafeCurves, which evaluates known cryptographic primitives for security across known attack and compromise/implementation areas.
Specifically, of the ECC curves supported by the Yubikey 5-series, only Curve 25519 is both safe and strong. While Brainpool P-512 (r1) is likely safe and definitely stronger, it cannot be used for SSH authentication. The NIST P-521 curve is possibly safe, and definitely strongly, there is significant lack of rigidity in how its parameters were chosen with open concerns (note: the validity of these concerns is also in questionable. c.f. Tin Foil Hat) about potential cryptographic weaknesses inherent by design. The NIST curves can be used for SSH authentication.
NOTE: The list of all supported ECC curves on the Yubikey 5-series cards with sufficient firmware revisions can be found here, as well as extensive details on additional SmartCard features of the keys, including acting a hardware random number generator.
It is notable that the GnuPG team has defaulted to using ECC as the default key type when generating new keys.
ECC curves provide tradeoffs of value depending on your use cases:
- RSA4096 is strong, ubiquitous, and the "simple" choice, with a long operation time (~1200ms per decryption operation)
- ECC on Curve 25519 is very well supported, and has a small strength penalty for a significant performance improvement over all other options. (~100ms per decryption operation)
- Choose this option if you expect to be using your key for signing large numbers of items, for example when rebasing commit trees with hundreds of commits non-interactively.
- ECC on curve NIST-P 521 is the strongest option available, offering security until at least 2030 according to NIST, with a mid-range performance (~500ms per decryption operation), with strong support across a broad range of applications (including SSH). The tradeoff is that there is unverified claims that cryptographic backdoors are possible.
- Choose this option if you want the strongest possible keypair, and are not concerned with unsubstantiated rumours of cryptographic backdoors.
All three keypairs can be used for SSH authentication, whereas Brainpool curves cannot be used for SSH authentication.
# Nore the use of the --expert flag, which is required to see all available curves when choosing an ECC key type
PS C:\Users\MyUser> gpg --edit-card --expert
...
gpg/card> admin
Admin commands are allowed
gpg/card> key-attr
Changing card key attribute for: Signature key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 2
Please select which elliptic curve you want:
(1) Curve 25519 *default*
(2) Curve 448
(3) NIST P-256
(4) NIST P-384
(5) NIST P-521
(6) Brainpool P-256
(7) Brainpool P-384
(8) Brainpool P-512
(9) secp256k1
Your selection? 1
The card will now be re-configured to generate a key of type: ed25519
Note: There is no guarantee that the card supports the requested
key type or size. If the key generation does not succeed,
please check the documentation of your card to see which
key types and sizes are supported.
Changing card key attribute for: Encryption key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 2
Please select which elliptic curve you want:
(1) Curve 25519 *default*
(2) Curve 448
(3) NIST P-256
(4) NIST P-384
(5) NIST P-521
(6) Brainpool P-256
(7) Brainpool P-384
(8) Brainpool P-512
(9) secp256k1
Your selection? 1
The card will now be re-configured to generate a key of type: cv25519
Changing card key attribute for: Authentication key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 2
Please select which elliptic curve you want:
(1) Curve 25519 *default*
(2) Curve 448
(3) NIST P-256
(4) NIST P-384
(5) NIST P-521
(6) Brainpool P-256
(7) Brainpool P-384
(8) Brainpool P-512
(9) secp256k1
Your selection? 1
The card will now be re-configured to generate a key of type: ed25519gpg/card> admin
Admin commands are allowed
gpg/card> key-attr
Changing card key attribute for: Signature key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 1
What keysize do you want? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
Changing card key attribute for: Encryption key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 1
What keysize do you want? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
Changing card key attribute for: Authentication key
Please select what kind of key you want:
(1) RSA
(2) ECC
Your selection? 1
What keysize do you want? (2048) 4096
The card will now be re-configured to generate a key of 4096 bits
gpg/card>Now the key will generate 4096-bit keys, as opposed to the default 2048-bit keys.
When generating keys, we make specific note to not create off-key backup, and instead generate the keys entirely on the card, and never export them (the private key cannot be exported from the card after generation). This ensures that the card performs the generation efforts, and that the private key never exists anywhere except on the Yubikey itself.
The key is set not to expire intentionally, as this is a master key, and master key expiration is generally not recommended.
gpg/card> generate
Make off-card backup of encryption key? (Y/n) n
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) 0
Key does not expire at all
Is this correct? (y/N) y
GnuPG needs to construct a user ID to identify your key.
Real name: John Doe
Email address: [email protected]
Comment: ORG-YK5-1
You selected this USER-ID:
"John Doe (ORG-YK5-1) <[email protected]>"
Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
gpg: C:\\Users\\MyUser\\AppData\\Roaming\\gnupg\\trustdb.gpg: trustdb created
gpg: key 43D4EEE22B19002B marked as ultimately trusted
gpg: directory 'C:\\Users\\MyUser\\AppData\\Roaming\\gnupg\\openpgp-revocs.d' created
gpg: revocation certificate stored as 'C:\\Users\\MyUser\\AppData\\Roaming\\gnupg\\openpgp-revocs.d\\C699DA9ECB5907E10E829CEC43D4EEE22B19002B.rev'
public and secret key created and signed.
THe Yubikey supports requiring a physical press on the brass circle on the top of the card during private key operations, and enabling this is good practice. This requires that a physical press is required to permit any private key operation for signing (1), decryption (2), and authentication (3).
gpg/card> admin
Admin commands are allowed
gpg/card> uif 1 on
gpg/card> uif 2 on
gpg/card> uif 3 onBy default, the PIN is required on the first operation each time the key is inserted into a system or access by a gpg process. The forcesig property forces the PIN to be entered on every operation. This can make many workflows less convenient, however when in use in untrusted environments, this should be enabled.
gpg/card> admin
Admin commands are allowed
gpg/card> forcesiggpg/card> admin
Admin commands are allowed
gpg/card> name
Cardholder's surname: Doe
Cardholder's given name: John
gpg/card> lang
Language preferences: en
gpg/card> salutation
Salutation (M = Mr., F = Ms., or space): M
The keypair public key can be uploaded to a public location, such as GItHub, SharePoint, OneDrive, Google Drive, or any other location, and then referenced by setting it in the url property of the card. This is useful for sharing the public key of your keypair with others.
PS C:\Users\MikeHimbeault\Desktop> gpg --card-status
Reader ...........: Yubico YubiKey OTP FIDO CCID 0
Application ID ...: D2760001240100000006179161830000
Application type .: OpenPGP
Version ..........: 0.0
Manufacturer .....: Yubico
Serial number ....: 17916183
Name of cardholder: John Doe
Language prefs ...: en
Salutation .......: Mr.
URL of public key : [not set]
Login data .......: [not set]
Signature PIN ....: forced
Key attributes ...: ed25519 cv25519 ed25519
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 5
KDF setting ......: off
UIF setting ......: Sign=on Decrypt=on Auth=on
Signature key ....: C699 DA9E CB59 07E1 0E82 9CEC 43D4 EEE2 2B19 002B
created ....: 2021-11-10 16:34:51
Encryption key....: 25BB 6BC8 7505 7D3A 382A 5E84 3D32 2F7C E55C 258E
created ....: 2021-11-10 16:34:51
Authentication key: E010 C1C1 976C 7CCC 98F2 A1AB E74A 4EB2 16F7 9ADA
created ....: 2021-11-10 16:34:51
General key info..: pub ed25519/43D4EEE22B19002B 2021-11-10 John Doe (ORG-YK5-1) <john.doe@example.com>
sec> ed25519/43D4EEE22B19002B created: 2021-11-10 expires: never
card-no: 0006 17916183
ssb> ed25519/E74A4EB216F79ADA created: 2021-11-10 expires: never
card-no: 0006 17916183
ssb> cv25519/3D322F7CE55C258E created: 2021-11-10 expires: never
card-no: 0006 17916183As part of your ~/.gitconfig (or %USERPROFILE%\.gitconfig), add some lines like the following (note that we use the same key IDs from previous, so you can see which part of your key information you want to be using):
[user]
email = [email protected]
name = John Doe
signingkey = 43D4EEE22B19002B
[gpg]
program = C:\\Users\\JohnDoe\\AppData\\Local\\Programs\\gnupg\\bin\\gpg.exe
[commit]
gpgsign = true
GitHub supports strong warnings if commits are not signed, and can be enabled through Vigilant Mode.
This section is a work in progress, however these are some references that can help.
Note that using Yubikeys on Windows as SSH private keys is challenging, especially if you want to use it within WSL:
- https://ryanlue.com/posts/2017-06-29-gpg-for-ssh-auth
- https://incenp.org/notes/2014/gnupg-for-ssh-authentication.html
- Don't forget about
echo enable-ssh-support >> ~/.gnuph/gpg-agent.confand~/.gnupg/sshcontrolneeding the keygrips of your Authentication-Only keys- A good treatment on the different kinds of key fingerprints
- Yubikey has some useful documentation on exporting GPG public keys as SSH key fingerprints for inclusion into server-side
authorized_keysfiles.