BlackBox (Security) Archiv

Curve25519:
Only the Nitrokey Start controls the elliptical curve Curve25519, which is one of the SaveCurves - the only curve where the choice of curve is completely transparent and therefore rear doors can practically be excluded. The other Nitrokeys only support the algorithms NIST P, Brainpool, and/or SECG/Koblitz. So if you really want to use ECC keys, you should currently use a Nitrokey Start - and update its firmware, as it may be affected by a vulnerability.

Compatibility:
In order for ECC keys to work smoothly in practice and, for example, to be used for e-mail encryption, all communication partners must use at least GnuPG 2.1 or newer. If this is not the case, problems may occur.

Apart from that, the use of ECC has a decisive advantage over RSA:
With smaller key lengths, the procedure is just as secure as longer RSA keys and is much faster in practice - especially on the security tokens, where all crypto operations (encryption, decryption, authentication, etc.) take place directly on the hardware, this is noticeable.

6.2 RSA-4096-Bit

The greater the RSA key length used, the longer a crypto operation on the smart card will take. For example, if you want to open an encrypted email, using an RSA-2048-bit key is much faster than using an RSA-4096-bit key. However, it is not only the Federal Office for Information Security (BSI) that recommends not using RSA keys of 2048-bit length from 2022 at the latest.

So you can still choose between RSA-3072-bit and RSA-4096-bit. The GnuPG project has an interesting answer to the question Why do people advise against using RSA-4096? in the FAQ:

"Almost always when people use 4096-bit RSA they’re doing so because they believe RSA-4096 to be much stronger than it is. The United States’ National Institute of Standards and Technology (NIST) states that RSA-2048 gives roughly 112 bits of security and RSA-3072 gives roughly 128. There is no formal recommendation on where RSA-4096 lies, but the general consensus is that it would come in somewhere around 140 bits – 28 bits of improvement over RSA-2048. This is an improvement so marginal that it’s really not worth mentioning.

If you need more security than RSA-2048 offers, the way to go would be to switch to elliptical curve cryptography – not to continue using RSA."

Do what, then? Since RSA does not support Perfect Forward Secrecy, I recommend using at least RSA-3072-bit or RSA-4096-bit. The decision may be at the expense of speed, but in view of the security gain it is a reasonable compromise.

7. conclusion

Both the GPG master key (sign / certify) and the two subkeys for encryption and authentication are now stored on the Nitrokey. Thus all three key slots of the OpenPGP smart card are occupied. The advantage of the variant shown is that if the nitrokey is lost or defective, a backup of the keys can be imported.

In the next part of the article series we will use the Nitrokey for the exchange of encrypted e-mails based on GnuPG. We will use the free email client Thunderbird in combination with the add-on Enigmail to decrypt / encrypt and sign emails.

Source (German) and more info on Nitrokey (part 2):
https://www.kuketz-blog.de/gnupg-schluesselerstellung-und-smartcard-transfer-nitrokey-teil2/

Nitrokey part 1:
https://t.iss.one/BlackBox_Archiv/404

#Nitrokey #SecurityKeys #usb #guide #kuketz #part2
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