Description
Yesterday I tried to add a new signing subkey to my PGP key using gpg --expert --edit-key <fpr>:
$ gpg --expert --edit-key C7E8A6AD4181CFD2D382B1238C920482C5CDA5FB Secret key is available. sec ed25519/0x8C920482C5CDA5FB created: 2022-04-01 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xD98102F24C067CA5 created: 2022-04-01 expired: 2023-04-01 usage: S card-no: 0006 04267031 ssb rsa4096/0x6566AAFFA15425EE created: 2022-04-01 expires: never usage: E card-no: 0006 04267031 ssb rsa4096/0x753468F4BE25063B created: 2023-04-03 expired: 2024-04-02 usage: S card-no: 0006 04267031 [ultimate] (1). ... 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) (10) ECC (sign only) (11) ECC (set your own capabilities) (12) ECC (encrypt only) (13) Existing key (14) Existing key from card Your selection? 8 Possible actions for this 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? e Possible actions for this RSA key: Sign Encrypt Authenticate Current allowed actions: Sign (S) Toggle the sign capability (E) Toggle the encrypt capability (A) Toggle the authenticate capability (Q) Finished Your selection? q RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (3072) 4096 Requested keysize is 4096 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) 1y Key expires at Do 03 Apr 2025 14:07:03 CEST Is this correct? (y/N) y Really create? (y/N) y 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. sec ed25519/0x8C920482C5CDA5FB created: 2022-04-01 expires: never usage: C trust: ultimate validity: ultimate ssb rsa4096/0xD98102F24C067CA5 created: 2022-04-01 expired: 2023-04-01 usage: S card-no: 0006 04267031 ssb rsa4096/0x6566AAFFA15425EE created: 2022-04-01 expires: never usage: E card-no: 0006 04267031 ssb rsa4096/0x753468F4BE25063B created: 2023-04-03 expired: 2024-04-02 usage: S card-no: 0006 04267031 ssb rsa4096/0xF29354429642DC16 created: 2024-04-03 expires: 2025-04-03 usage: SR [ultimate] (1). ...
As you can see, I chose "RSA (set your own capabilities)" and then removed the encryption capability, so that according to gpg, the only remaining key usage flag was S. However, after completing the key generation, the output showed the key usage flags SR.
Compare that to the result when "RSA (sign only) is chosen; this time, only the S flag is set:
gpg> newkey
Invalid command (try "help")
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)
(10) ECC (sign only)
(11) ECC (set your own capabilities)
(12) ECC (encrypt only)
(13) Existing key
(14) Existing key from card
Your selection? 4
RSA keys may be between 1024 and 4096 bits long.
What keysize do you want? (3072) 4096
Requested keysize is 4096 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) 1y
Key expires at Do 03 Apr 2025 14:14:06 CEST
Is this correct? (y/N) y
Really create? (y/N) y
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.
sec ed25519/0x8C920482C5CDA5FB
created: 2022-04-01 expires: never usage: C
trust: ultimate validity: ultimate
ssb rsa4096/0xD98102F24C067CA5
created: 2022-04-01 expired: 2023-04-01 usage: S
card-no: 0006 04267031
ssb rsa4096/0x6566AAFFA15425EE
created: 2022-04-01 expires: never usage: E
card-no: 0006 04267031
ssb rsa4096/0x753468F4BE25063B
created: 2023-04-03 expired: 2024-04-02 usage: S
card-no: 0006 04267031
ssb rsa4096/0xC9E8EAB911C67BB8
created: 2024-04-03 expires: 2025-04-03 usage: S
[ultimate] (1). ...Expected Behavior
The "set your own capabilities" submenu should know about and display the R key usage flag, and provide a way to toggle it.
Actual Behavior
The "set your own capabilities" submenu does not appear to know about the R key usage flag at all, and silently sets it on newly created keys.
I have not checked what happens if other key types are chosen.
Environment
OS: Arch Linux x86_64
Kernel: Linux 6.6.23-1-lts
GnuPG Version: 2.4.5