3

My key setup is as follows:

sec  dsa3072/CA72E53A
     created: 2013-05-12  expires: never       usage: SCA 
     trust: ultimate      validity: ultimate
The following key was revoked on 2016-01-13 by DSA key CA72E53A 
ssb  elg4096/ADBBBE42
     created: 2013-05-12  revoked: 2016-01-13  usage: E   
ssb  rsa4096/6A743003
     created: 2016-01-13  expires: 2018-01-12  usage: E   
ssb  rsa4096/A57F2B30
     created: 2016-01-13  expires: 2018-01-12  usage: S   

I am trying to transition to RSA subkeys as I want to use a Yubikey for my keys, but started out with a DSA/Elgamal key.

But when adding new subkeys I don't get the option to add an RSA (or DSA) key with custom parameters. Is this a result of having a DSA master key?

gpg> addkey 
Please select what kind of key you want:
   (3) DSA (sign only)
   (4) RSA (sign only)
   (5) Elgamal (encrypt only)
   (6) RSA (encrypt only)
Your selection? 

A workaround I am considering is genereting a new keypair just for authentication purposes (RSA/RSA), not publishing the public key and uploading that authentication key onto the yubikey, but I don't know if that is supported by the Yubikey and if it has other downsides.

Is it possible to have an RSA authentication subkey with a DSA master key or am I trying to do something silly and should I just bite the bullet and transition to a new RSA master key?

I am using GnuPG version 2.1.9.

2

You need to activate the --expert mode to select arbitrary combinations of algorithms and capabilities (inside given technical limitations, DSA keys will never be able to have signing capability).

$ gpg --expert --edit-key [key-id]
[snip, key-listing]
gpg> addkey
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?

Enter 8 to create an RSA key.

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? 

Now, one after the other, enter e, s and a to disable encryption and signing and enable authentication capabilities. Finally select the key size:

RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 

Continue as usual.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.