是否可以（本地）使用签名子密钥对 UID 进行签名

Question

我正在尝试为系统帐户添加信任（以在使用它来使用该密钥加密数据时停止烦人的消息）。我已经设置了子密钥和一个离线主密钥：

$ gpg --edit-key AAAAAAAA
[...]
Secret key is available.

pub  4096R/AAAAAAAA  created: 2015-09-09  expires: never       usage: SC  
                     trust: ultimate      validity: ultimate
sub  4096R/BBBBBBBB  created: 2015-09-09  expires: never       usage: E   
sub  4096R/CCCCCCCC  created: 2015-09-09  expires: never       usage: S   
sub  4096R/DDDDDDDD  created: 2015-09-09  expires: never       usage: A

$ gpg --list-secret-keys
sec#  4096R/AAAAAAAA 2015-09-09
uid                  $NAME <$EMAIL>
ssb   4096R/BBBBBBBB 2015-09-09
ssb   4096R/CCCCCCCC 2015-09-09
ssb   4096R/DDDDDDDD 2015-09-09

如果我想签署一份文件，我可以使用：

$ gpg --encrypt --sign --recipient AAAAAAAA --local-user CCCCCCCC! --output out.gpg in.gpg

它完全使用指定的子密钥进行签名（尽管在这种情况下 AAAAAAAA 实际上不可用，因此在任何情况下都无法使用）。但是，如果我尝试做类似于 lsign another UID 的事情：

$ gpg --lsign-key --local-user CCCCCCCC! 'Mentor Root'                                                                                                                          

pub  4096R/DDDDDDDD  created: 2015-09-14  expires: never       usage: SC  
                     trust: undefined     validity: unknown
sub  4096R/EEEEEEEE  created: 2015-09-14  expires: never       usage: E   
[ unknown] (1). $OTHER_NAME <$OTHER_EMAIL>


pub  4096R/DDDDDDDD  created: 2015-09-14  expires: never       usage: SC  
                     trust: undefined     validity: unknown
 Primary key fingerprint: DDDD DDDD DDDD DDDD DDDD  DDDD DDDD DDDD DDDD DDDD

     $OTHER_NAME <$OTHER_EMAIL>

Are you sure that you want to sign this key with your
key "$NAME <$EMAIL>" (AAAA)

The signature will be marked as non-exportable.

Really sign? (y/N) y
gpg: secret key parts are not available
gpg: signing failed: general error

Key not changed so no update needed.

这只是 gpg 的硬性限制，还是我遗漏了某些步骤？

（或者同样有可能，我是不是完全误解了这一切的意图？）

编辑：CCCCCCCC 子项没有 C 能力只是简单吗？有没有可能有一个带C能力的子键（粗略搜索好像不是）？

Answer 1

只有主键才有认证能力C。您不能使用子密钥签署 (ceritfy) keys/user ID。

来自RFC 4880, OpenPGP, 12.1 Key Structures：

In a V4 key, the primary key MUST be a key capable of certification. The subkeys may be keys of any other type.

数学可能允许认证子密钥，但标准阻止它。

是否可以（本地）使用签名子密钥对 UID 进行签名

Is it possible to (locally) sign a UID using a signing subkey

gnupg