Generating 2048-bit RSA keys¶

In this section, we describe how to generate 2048-bit RSA keys.

Key length of RSA¶

In 2005, NIST (National Institute of Standards and Technology, USA) has issued the first revision of NIST Special Publication 800-57, “Recommendation for Key Management”.

In 800-57, NIST advises that 1024-bit RSA keys will no longer be viable after 2010 and advises moving to 2048-bit RSA keys. NIST advises that 2048-bit keys should be viable until 2030.

As of 2010, GnuPG’s default for generating RSA key is 2048-bit.

Some people have preference on RSA 4096-bit keys, considering “longer is better”.

However, “longer is better” is not always true. When it’s long, it requires more computational resource, memory and storage, and it consumes more power for nomal usages. These days, many people has enough computational resource, that would be true, but less is better for power consumption.

For security, the key length is just a single factor. We had and will have algorithm issues, too. It is true that it’s difficult to update our public keys, but this problem wouldn’t be solved by just have longer keys.

We deliberately support only RSA 2048-bit keys for Gnuk, considering device computation power and host software constraints.

Thus, the key size is 2048-bit in the examples below.

Generating keys on host PC¶

Here is the example session to generate main key and a subkey for encryption.

I invoke GnuPG with --gen-key option.

$gpg --gen-key gpg (GnuPG) 1.4.11; Copyright (C) 2010 Free Software Foundation, Inc. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.  and GnuPG asks kind of key. Select RSA and RSA. 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 RSA keys may be between 1024 and 4096 bits long.  and select 2048-bit (as Gnuk Token only supports this). What keysize do you want? (2048) Requested keysize is 2048 bits  and select expiration of the key. 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  Confirm key types, bitsize and expiration. Is this correct? (y/N) y  Then enter user ID. You need a user ID to identify your key; the software constructs the user ID from the Real Name, Comment and Email Address in this form: "Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>" Real name: Niibe Yutaka Email address: gniibe@fsij.org Comment: You selected this USER-ID: "Niibe Yutaka <gniibe@fsij.org>" Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o  and enter passphrase for this key on host PC. Note that this is a passphrase for the key on host PC. It is different thing to the passphrase of Gnuk Token. We enter two same inputs two times (once for passphrase input, and another for confirmation). You need a Passphrase to protect your secret key. <PASSWORD-KEY-ON-PC>  Then, GnuPG generate keys. It takes some time. 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. ...+++++ +++++ 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 15 more bytes) ...+++++ gpg: key 4CA7BABE 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 pub 2048R/4CA7BABE 2010-10-15 Key fingerprint = 1241 24BD 3B48 62AF 7A0A 42F1 00B4 5EBD 4CA7 BABE uid Niibe Yutaka <gniibe@fsij.org> sub 2048R/084239CF 2010-10-15$


Done.

Then, we create authentication subkey. Authentication subkey is not that common, but very useful (for SSH authentication). As it is not that common, we need --expert option for GnuPG.

$gpg --expert --edit-key 4CA7BABE gpg (GnuPG) 1.4.11; Copyright (C) 2010 Free Software Foundation, Inc. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Secret key is available. pub 2048R/4CA7BABE created: 2010-10-15 expires: never usage: SC trust: ultimate validity: ultimate sub 2048R/084239CF created: 2010-10-15 expires: never usage: E [ultimate] (1). Niibe Yutaka <gniibe@fsij.org> gpg>  Here, it displays that there are main key and a subkey. It prompts sub-command with gpg> . Here, we enter addkey sub-command. Then, we enter the passphrase of key on host PC. It’s the one we entered above as <PASSWORD-KEY-ON-PC>. gpg> addkey Key is protected. You need a passphrase to unlock the secret key for user: "Niibe Yutaka <gniibe@fsij.org>" 2048-bit RSA key, ID 4CA7BABE, created 2010-10-15 <PASSWORD-KEY-ON-PC> gpg: gpg-agent is not available in this session  GnuPG asks kind of key. We select RSA (set your own capabilities). Please select what kind of key you want: (3) DSA (sign only) (4) RSA (sign only) (5) Elgamal (encrypt only) (6) RSA (encrypt only) (7) DSA (set your own capabilities) (8) RSA (set your own capabilities) Your selection? 8  And select Authenticate for the capabilities for this key. Initially, it’s Sign and Encrypt. I need to deselect Sign and Encrypt, and select Authenticate. To do that, I enter s, e, and a. Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: Sign Encrypt (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? s Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: Encrypt (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? e Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? a Possible actions for a RSA key: Sign Encrypt Authenticate Current allowed actions: Authenticate (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished  OK, we set the capability of Authenticate. We enter q to finish setting capabilities. Your selection? q  GnuPG asks bitsize and expiration, we enter 2048 for bitsize and no expiration. Then, we confirm that we really create the key. RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (2048) Requested keysize is 2048 bits 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 Really create? (y/N) y  Then, GnuPG generate the key. 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. .......+++++ +++++ pub 2048R/4CA7BABE created: 2010-10-15 expires: never usage: SC trust: ultimate validity: ultimate sub 2048R/084239CF created: 2010-10-15 expires: never usage: E sub 2048R/5BB065DC created: 2010-10-22 expires: never usage: A [ultimate] (1). Niibe Yutaka <gniibe@fsij.org> gpg>  We save the key (to the storage of the host PC. gpg> save$


Now, we have three keys (one primary key for signature and certification, subkey for encryption, and another subkey for authentication).

Publishing public key¶

We make a file for the public key by --export option of GnuPG.

$gpg --armor --output <YOUR-KEY>.asc --export <YOUR-KEY-ID>  We can publish the file by web server. Or we can publish the key to a keyserver, by invoking GnuPG with --send-keys option. $ gpg --keyserver pool.sks-keyservers.net --send-keys <YOUR-KEY-ID>


Here, pool.sks-keyservers.net is a keyserver, which is widely used.

Backup the private key¶

There are some ways to back up private key, such that backup .gnupg directory entirely, or use of paperkey, etc. Here, we describe backup by ASCII file. ASCII file is good, because it has less risk on transfer. Binary file has a risk to be modified on transfer.

Note that the key on host PC is protected by passphrase (which is <PASSWORD-KEY-ON-PC> in the example above). Using the key from the backup needs this passphrase. It is common that people will forget passphrase for backup. Never forget it. You have been warned.

To make ASCII backup for private key, invokde GnuPG with --armor option and --export-secret-keys specifying the key identifier.

$gpg --armor --output <YOUR-SECRET>.asc --export-secret-keys <YOUR-KEY-ID>  From the backup, we can recover privet key by invoking GnuPG with --import option. $ gpg --import <YOUR-SECRET>.asc