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I'm attempting to establish a process for setting up a new GPG identity for myself and my threat model.

Much of it is following guides which I believe are still considered best practices:

These schemes seem to be advocating for:

  1. Setup an offline primary key that only has the "Certify" capability
  2. Create an online subkey with capabilities: "Sign"
  3. Create an online subkey with capabilities: "Authenticate"
  4. Create an online subkey with capabilities: "Encrypt"

The above guides are using RSA-4096, but based on my other readings it seems like using ECC with curve 25519 is as secure, but requires less space to store and less energy to use, so I'd like to go with that.

I was playing with the tools in a temporary GNUPGHOME, and I was able to setup an ECC primary key, but when I sent to generate the "encrypt" subkey, I noticed there didn't seem to be an encryption capability:

Possible actions for a ECDSA/EdDSA key: Sign Certify Authenticate 
Current allowed actions: Sign Certify 

   (S) Toggle the sign capability
   (A) Toggle the authenticate capability
   (Q) Finished

But if I use RSA, it seems like it is an option:

Possible actions for a RSA key: Sign Certify Encrypt Authenticate 
Current allowed actions: Sign Certify Encrypt 

   (S) Toggle the sign capability
   (E) Toggle the encrypt capability
   (A) Toggle the authenticate capability
   (Q) Finished

I wasn't able to find much online about why this is.

For reference my gpg version information is as follows:

gpg (GnuPG) 2.2.20
libgcrypt 1.8.7

Supported algorithms:
Pubkey: RSA, ELG, DSA, ECDH, ECDSA, EDDSA
Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,
        CAMELLIA128, CAMELLIA192, CAMELLIA256
Hash: SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2

In summary, my question is:

  • Why does RSA have an "encrypt" capability but ECC doesn't.
  • Is having one primary key and 3 subkeys for each functionality still best practice?
  • Should I generate an RSA subkey with an "encrypt" capability as a workaround?
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  • Make sure you're using --expert throughout (gpg --expert --full-gen-key; gpg --expert --edit-key foo@example.com) to see all the options. It seems like ECC capabilities are hidden behind --expert to avoid unintentional compatibility snafus.
    – gowenfawr
    Sep 18, 2021 at 18:45
  • The command I'm using is gpg --full-generate-key --expert --pinentry-mode loopback but the only options are: (1) RSA and RSA (default) (2) DSA and Elgamal (3) DSA (sign only) (4) RSA (sign only) (7) DSA (set your own capabilities) (8) RSA (set your own capabilities) (9) ECC and ECC (10) ECC (sign only) (11) ECC (set your own capabilities) (13) Existing key (14) Existing key from card
    – Erotemic
    Sep 18, 2021 at 18:47
  • I think I see now. Is it the case that you get the #12 ECC (encrypt only) key when you are adding subketys. And It looks like the ECC and ECC option automatically makes an encrypt-only subkey for you?
    – Erotemic
    Sep 18, 2021 at 18:55
  • Correct, option #9 (ECC and ECC) seems to create a working key with a Curve25519 subkey for encryption on my system. Is that not what you're aiming for?
    – gowenfawr
    Sep 18, 2021 at 18:56
  • I was using #8 to explicitly set capabilities. Encrypt is not an option in that case for either a primary or a subkey. #12 is not visible in primary key creation, but it is in subkey creation. For context, I'm scripting creation with an expect script in an attempt to 100% avoid all of the prompts and issues with --batch mode. It's actually working pretty well.
    – Erotemic
    Sep 18, 2021 at 19:00

1 Answer 1

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When you generate key, first you need to generate sign-only primary key. While RSA allows encryption and signing for the same key (i.e. it can have encrypting primary key), for ECC there are different key algorithms for ECC:

  • sign-only ECDSA
  • sign-only EdDSA
  • encrypt-only ECDH

Once primary key is generated, you would be able to add encrypting and signing subkey(s) via gpg --edit-key command.

This message may give more details on how to add subkeys in batch mode: https://lists.gnupg.org/pipermail/gnupg-users/2014-September/050839.html

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