如何在不使用密钥服务器的情况下从 OpenPGP 智能卡获取公钥？

Question

我正在研究使用 OpenPGP 在智能卡 (Yubikey) 上生成公钥对的用例。

然后将智能卡运送给用户。 正在尝试在本地模拟这一点：

1. 在智能卡上生成密钥
2. 删除 GnuPG 主目录
3. 访问智能卡以重新生成 GnuPG 主目录

问题是在执行上述步骤后我无法测试加密文件，因为公钥似乎丢失了。 fetch 似乎不起作用。

在这个阶段，我不想在任何在线服务器上共享公钥。 删除密钥环后，有什么方法可以从智能卡中检索公钥？

以下是正在遵循的步骤：

$ gpg --card-edit                                                                                                                                                       

Reader ...........: 1050:0404:X:0
Application ID ...: D2760001240102010006046314290000
Version ..........: 2.1
Manufacturer .....: Yubico
Serial number ....: 04631429
Name of cardholder: sm sm
Language prefs ...: en
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: sm
Signature PIN ....: not forced
Key attributes ...: rsa4096 rsa4096 rsa4096
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 0
Signature key ....: 54D4 E469 7056 B390 AE72  CAA1 A507 3320 7876 0302
      created ....: 2017-10-11 13:16:52
Encryption key....: ADA3 2D7F 8D66 4F34 C04A  457C DFEB E3E4 A8F1 8611
      created ....: 2017-10-11 11:14:18
Authentication key: 18B9 7AB4 0723 46F4 C23A  3DD7 E5C0 6A93 049E F6A8
      created ....: 2017-10-11 11:14:18
General key info..: [none]

gpg/card> admin
Admin commands are allowed

gpg/card> generate
Make off-card backup of encryption key? (Y/n) n

gpg: Note: keys are already stored on the card!

Replace existing keys? (y/N) y
What keysize do you want for the Signature key? (4096) 
What keysize do you want for the Encryption key? (4096) 
What keysize do you want for the Authentication key? (4096) 
Key is valid for? (0) 0
Is this correct? (y/N) y
Real name: john doe
Email address: john.doe@foobar.com
Comment: 
You selected this USER-ID:
    "john doe <<john.doe@foobar.com>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? o

gpg: /home/xxx/.gnupg/trustdb.gpg: trustdb created
gpg: key 6825CB0EBDA94110 marked as ultimately trusted
gpg: directory '/home/xxx/.gnupg/openpgp-revocs.d' created
gpg: revocation certificate stored as '/home/xxx/.gnupg/openpgp-revocs.d/6858F119E93FB74BB561DE556825CB0EBDA94110.rev'
public and secret key created and signed.


gpg/card> list

Reader ...........: 1050:0404:X:0
Application ID ...: D2760001240102010006046314290000
Version ..........: 2.1
Manufacturer .....: Yubico
Serial number ....: 04631429
Name of cardholder: sm sm
Language prefs ...: en
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: sm
Signature PIN ....: not forced
Key attributes ...: rsa4096 rsa4096 rsa4096
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 4
Signature key ....: 6858 F119 E93F B74B B561  DE55 6825 CB0E BDA9 4110
      created ....: 2017-10-11 13:18:11
Encryption key....: BE05 7FDF 9ACD 05F0 B75A  570F 4711 4B69 A622 C1DC
      created ....: 2017-10-11 13:18:11
Authentication key: 7275 2C47 B1EF BFB5 1E6D  0E65 31C7 7DBE 2D22 7E32
      created ....: 2017-10-11 13:18:11
General key info..: pub  rsa4096/6825CB0EBDA94110 2017-10-11     john doe <<john.doe@foobar.com>
sec>  rsa4096/6825CB0EBDA94110  created: 2017-10-11  expires: never     
                                card-no: 0006 04631429
ssb>  rsa4096/31C77DBE2D227E32  created: 2017-10-11  expires: never     
                                card-no: 0006 04631429
ssb>  rsa4096/47114B69A622C1DC  created: 2017-10-11  expires: never     
                                card-no: 0006 04631429

gpg/card> quit

$ rm -rf .gnupg/

$ gpg --card-status                                                                                                                                                     
gpg: directory '/home/smalatho/.gnupg' created
gpg: new configuration file '/home/smalatho/.gnupg/dirmngr.conf' created
gpg: new configuration file '/home/smalatho/.gnupg/gpg.conf' created
gpg: keybox '/home/smalatho/.gnupg/pubring.kbx' created
Reader ...........: 1050:0404:X:0
Application ID ...: D2760001240102010006046314290000
Version ..........: 2.1
Manufacturer .....: Yubico
Serial number ....: 04631429
Name of cardholder: sm sm
Language prefs ...: en
Sex ..............: unspecified
URL of public key : [not set]
Login data .......: sm
Signature PIN ....: not forced
Key attributes ...: rsa4096 rsa4096 rsa4096
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 4
Signature key ....: 6858 F119 E93F B74B B561  DE55 6825 CB0E BDA9 4110
      created ....: 2017-10-11 13:18:11
Encryption key....: BE05 7FDF 9ACD 05F0 B75A  570F 4711 4B69 A622 C1DC
      created ....: 2017-10-11 13:18:11
Authentication key: 7275 2C47 B1EF BFB5 1E6D  0E65 31C7 7DBE 2D22 7E32
      created ....: 2017-10-11 13:18:11
General key info..: [none]

Answer 1

OpenPGP 智能卡没有存储足够的信息来重建完整的 OpenPGP 公钥。您必须单独导入公钥 - 在密钥服务器上共享它是一种解决方案，但您也可以gpg --export 密钥然后再次gpg --import 进行测试。

Answer 2

需要用户在删除GNUPGHOME目录之前手动导出公钥，然后重新导入智能卡中的公钥。

$ gpg --armor --export j.doe@example.com > public.asc
$ rm -rf ~/.gnupg
$ gpg --import public.asc

Answer 3

据 GnuPG 的维护者it is technically possible to reconstruct the public key using only information from the card 说，但这并不容易：

但是，如果您确实丢失了公钥并且需要找回它，则可以使用相同的指纹重新创建公钥。没有代码，你需要破解源代码。

你需要的是创建时间戳和公钥参数 从卡。您可以使用

收集此信息

  $ gpg-connect-agent
  > scd learn --force
  S SERIALNO D276000124010101000100xxxxxxxxxx  0
  S APPTYPE OPENPGP
[...]
  S KEY-TIME 1 1136130759
  S KEY-TIME 2 1136132140
  S KEY-TIME 3 1136131786
[...]
  OK
  > /decode
  > /hex
  > scd readkey OPENPGP.1
  D[0000]  28 31 30 3A 70 75 62 6C  69 63 2D 6B 65 79 28 33   (10:public-key(3
  D[0010]  3A 72 73 61 28 31 3A 6E  31 32 39 3A 00 D0 99 19   :rsa(1:n129:....
[...]
  OK

从 KEY-TIME 行中获取创建时间。我使用上面的/decode 和/hex 只是为了便于阅读。你应该使用

  > /datafile out
  > scd readkey OPENPGP.1
  OK
  > /bye

而是将带有公钥的 s 表达式写入文件out。 Libgcrypt 函数将这些 s 表达式作为参数。现在您需要将其提供给 gpg 以创建公钥部分和自签名。