TechQA.

gpgme: selecting a private key when signing data

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Using gpgme (the development library for gpg/gnupg), I'm trying to sign some data. In the key ring I have more then 1 private key so I want to select the correct one. This fails with: "Unusable secret key (117440566)". The key was generated with gnupg2 itself. When using gnupg the problem also occurs.

sec   1024R/14B7E8E6 2015-05-27
      Key fingerprint = 95C7 6C5E F839 43DA 2F32  2CF4 D2C2 5144 14B7 E8E6
uid                  testkey2 (testkey2) <[email protected]>
ssb   1024R/ED8059EA 2015-05-27

pub  rsa1024/14B7E8E6
     created: 2015-05-27  expires: never       usage: SC
     trust: ultimate      validity: ultimate
sub  rsa1024/ED8059EA
     created: 2015-05-27  expires: never       usage: E
sub  rsa1024/74D6F5C6
     created: 2015-05-31  expires: never       usage: S

First I check if there's a private key for the key selected:

gpgme_op_keylist_start(..., ..., 1);
if (gpgme_op_keylist_nex() == GPG_ERR_NO_ERROR) { proceed }

do the signing:

gpgme_new()
gpgme_set_pinentry_mode(GPGME_PINENTRY_MODE_LOOPBACK) // yes i installed v2.1
gpgme_set_passphrase_cb()
/* ...binary to gpgme_data_t... */
gpgme_data_set_encoding(GPGME_DATA_ENCODING_BINARY)
gpgme_signers_clear()
gpgme_signers_add()   // <- that key that I checked for existance earlier
if (gpgme_signers_count() != 1) { fail(); } // sanity check
gpgme_op_encrypt_sign(ctx, recipient, GPGME_ENCRYPT_ALWAYS_TRUST /* FIXME */, data_in, sig);

Now that gpgme_op_encrypt_sign always fails with that "Unusable secret key (117440566)" error. Any tips/hints?

Software versions:

gnupg    1.4.18-7
gnupg-agent      2.1.4-1
gnupg2   2.1.4-1
libgpgme++2      4:4.14.2-2+b1
libgpgme11:amd64         1.5.1-6
libgpgme11-dev   1.5.1-6
python-gnupginterface    0.3.2-9.1

I enabled debug-tracing but it doesn't help me much:

<0x1927>  gpgme_debug: level=4
<0x1927>  gpgme_check_version: call: 0=(nil), req_version=(null), VERSION=1.5.1
<0x1927>  gpgme_check_version_internal: call: 0=(nil), req_version=(null), offset_sig_validity=60
<0x1927>  gpgme_set_locale: enter: ctx=(nil), category=0, value=C
<0x1927>  gpgme_set_locale: leave
<0x1927>  gpgme_set_locale: enter: ctx=(nil), category=5, value=C
<0x1927>  gpgme_set_locale: leave
<0x1927>  gpgme-dinfo: gpgconf='/usr/bin/gpgconf'
<0x1927>  gpgme-dinfo:     gpg='/usr/bin/gpg2'
<0x1927>  gpgme-dinfo:   gpgsm='/usr/bin/gpgsm'
<0x1927>  gpgme-dinfo: homedir='/home/folkert/.gnupg'
<0x1927>  gpgme-dinfo:   agent='/home/folkert/.gnupg/S.gpg-agent'
<0x1927>  gpgme-dinfo:   uisrv='/home/folkert/.gnupg/S.uiserver'
<0x1927>  gpgme_new: enter: r_ctx=0x7fff5afd07a8
<0x1927>  gpgme_new: leave: ctx=0x20c0810
<0x1927>  gpgme_op_keylist_start: enter: ctx=0x20c0810, pattern=0BF38589, secret_only=1
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c0810, fd 4, dir=1 -> tag=0x20c26b0
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c0810, fd 6, dir=1 -> tag=0x20c2800
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c0c10, event 0x7fd8b1a20ad0, type 0, type_data (nil)
<0x1927>  gpgme_op_keylist_start: leave
<0x1927>  gpgme_op_keylist_next: enter: ctx=0x20c0810
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2820, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2820, handler (0x20c0c10, 6)
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = (nil), line = sec:u:2048:1:CC73A8A60BF38589:1433443717:::u:::scESC::::::
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c2850, line = fpr:::::::::20CD3FF80DA6C1E46CD9F135CC73A8A60BF38589:
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c2850, line = uid:u::::1433443717::9963CFDE0C8920AD077B06A281992C4008E67E4F::testkey3 (testkey3) <[email protected]>:
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c2850, line = ssb:u:2048:1:22317805D48C1491:1433443717::::::e::::::
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c2850, line = fpr:::::::::FB6FFB7D8BEC710A745DE86C22317805D48C1491:
<0x1927>    _gpgme_run_io_cb: call: item=0x20c26d0, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c26d0, handler (0x20c0c10, 4)
<0x1927>      _gpgme_remove_io_cb: call: data=0x20c26b0, setting fd 0x4 (item=0x20c26d0) done
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2820, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2820, handler (0x20c0c10, 6)
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c2850, line = (null)
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c0c10, event 0x7fd8b1a20ad0, type 2, type_data 0x20c2850
<0x1927>      _gpgme_remove_io_cb: call: data=0x20c2800, setting fd 0x6 (item=0x20c2820) done
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c0c10, event 0x7fd8b1a20ad0, type 1, type_data 0x7fff5afd0700
<0x1927>  gpgme_op_keylist_next: leave: key=0x20c2850 (20CD3FF80DA6C1E46CD9F135CC73A8A60BF38589)
<0x1927>  gpgme_release: call: ctx=0x20c0810
<0x1927>  gpgme_new: enter: r_ctx=0x7fff5afd0818
<0x1927>  gpgme_new: leave: ctx=0x20c2210
<0x1927>  gpgme_op_keylist_start: enter: ctx=0x20c2210, pattern=4BE78BDCF3F5352CF624A6DF3AD6F8118300CC02, secret_only=0
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c2210, fd 4, dir=1 -> tag=0x20c1f50
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c2210, fd 6, dir=1 -> tag=0x20c1fa0
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c0c10, event 0x7fd8b1a20ad0, type 0, type_data (nil)
<0x1927>  gpgme_op_keylist_start: leave
<0x1927>  gpgme_op_keylist_next: enter: ctx=0x20c2210
<0x1927>    _gpgme_run_io_cb: call: item=0x20c1fc0, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c1fc0, handler (0x20c0c10, 6)
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = (nil), line = tru::0:1433443869:2410285847:3:1:5
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = (nil), line = pub:-:1024:17:3AD6F8118300CC02:1039074767:::-:::scESC::::::
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = 0x20c2b70, line = fpr:::::::::4BE78BDCF3F5352CF624A6DF3AD6F8118300CC02:
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = 0x20c2b70, line = uid:-::::1203999932::275AAD3E991F1962AD510CC96760907BE70FE668::Bla <bla@com>:
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = 0x20c2b70, line = uid:-::::1203999938::59689891229F1817EF66BFC63D9D0BB2F45F5209::Bla <bla@com>:
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = 0x20c2b70, line = uid:r::::::8A709552E7AB85B53DDAE18A48C0978E5EBF5547::Bla <bla@com>:
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = 0x20c2b70, line = sub:-:2048:16:942E547C12A6B1C2:1039075030::::::e::::::
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = 0x20c2b70, line = fpr:::::::::E69EF5226BBF7EC14F1D7D96942E547C12A6B1C2:
<0x1927>    _gpgme_run_io_cb: call: item=0x20c1f70, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c1f70, handler (0x20c0c10, 4)
<0x1927>      _gpgme_remove_io_cb: call: data=0x20c1f50, setting fd 0x4 (item=0x20c1f70) done
<0x1927>    _gpgme_run_io_cb: call: item=0x20c1fc0, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c1fc0, handler (0x20c0c10, 6)
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c2210, key = 0x20c2b70, line = (null)
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c0c10, event 0x7fd8b1a20ad0, type 2, type_data 0x20c2b70
<0x1927>      _gpgme_remove_io_cb: call: data=0x20c1fa0, setting fd 0x6 (item=0x20c1fc0) done
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c0c10, event 0x7fd8b1a20ad0, type 1, type_data 0x7fff5afd0760
<0x1927>  gpgme_op_keylist_next: leave: key=0x20c2b70 (4BE78BDCF3F5352CF624A6DF3AD6F8118300CC02)
<0x1927>  gpgme_release: call: ctx=0x20c2210
<0x1927>  gpgme_new: enter: r_ctx=0x7fff5afd07c8
<0x1927>  gpgme_new: leave: ctx=0x20c2550
<0x1927>  gpgme_set_passphrase_cb: call: ctx=0x20c2550, passphrase_cb=(nil)/(nil)
<0x1927>  gpgme_set_pinentry_mode: call: ctx=0x20c2550, pinentry_mode=4
<0x1927>  gpgme_set_passphrase_cb: call: ctx=0x20c2550, passphrase_cb=0x403420/0x20c0058
<0x1927>  gpgme_set_passphrase_cb: call: ctx=0x20c2550, passphrase_cb=0x403420/0x20c0058
<0x1927>  gpgme_new: enter: r_ctx=0x7fff5afd0768
<0x1927>  gpgme_new: leave: ctx=0x20c0810
<0x1927>  gpgme_op_keylist_start: enter: ctx=0x20c0810, pattern=0BF38589, secret_only=0
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c0810, fd 4, dir=1 -> tag=0x20c4fd0
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c0810, fd 6, dir=1 -> tag=0x20c5120
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c1d50, event 0x7fd8b1a20ad0, type 0, type_data (nil)
<0x1927>  gpgme_op_keylist_start: leave
<0x1927>  gpgme_op_keylist_next: enter: ctx=0x20c0810
<0x1927>    _gpgme_run_io_cb: call: item=0x20c5140, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c5140, handler (0x20c1d50, 6)
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = (nil), line = tru::0:1433443869:2410285847:3:1:5
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = (nil), line = pub:u:2048:1:CC73A8A60BF38589:1433443717:::u:::scESC:::::: 
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c5170, line = fpr:::::::::20CD3FF80DA6C1E46CD9F135CC73A8A60BF38589:  
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c5170, line = uid:u::::1433443717::9963CFDE0C8920AD077B06A281992C4008E67E4F::testkey3 (testkey3) <[email protected]>:
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c5170, line = sub:u:2048:1:22317805D48C1491:1433443717::::::e::::::
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c5170, line = fpr:::::::::FB6FFB7D8BEC710A745DE86C22317805D48C1491:
<0x1927>    _gpgme_run_io_cb: call: item=0x20c4ff0, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c4ff0, handler (0x20c1d50, 4)
<0x1927>      _gpgme_remove_io_cb: call: data=0x20c4fd0, setting fd 0x4 (item=0x20c4ff0) done
<0x1927>    _gpgme_run_io_cb: call: item=0x20c5140, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c5140, handler (0x20c1d50, 6)
<0x1927>    gpgme:keylist_colon_handler: call: ctx=0x20c0810, key = 0x20c5170, line = (null)
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c1d50, event 0x7fd8b1a20ad0, type 2, type_data 0x20c5170
<0x1927>      _gpgme_remove_io_cb: call: data=0x20c5120, setting fd 0x6 (item=0x20c5140) done
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c1d50, event 0x7fd8b1a20ad0, type 1, type_data 0x7fff5afd06c0
<0x1927>  gpgme_op_keylist_next: leave: key=0x20c5170 (20CD3FF80DA6C1E46CD9F135CC73A8A60BF38589)
<0x1927>  gpgme_release: call: ctx=0x20c0810
<0x1927>  gpgme_signers_clear: call: ctx=0x20c2550
<0x1927>  gpgme_signers_add: enter: ctx=0x20c2550, key=0x20c2850 (20CD3FF80DA6C1E46CD9F135CC73A8A60BF38589)
<0x1927>  gpgme_signers_add: leave
<0x1927>  gpgme_op_encrypt_sign: enter: ctx=0x20c2550, flags=0x1, plain=0x20c2ed0, cipher=0x20c3f20
<0x1927>  gpgme_op_encrypt_sign: check: ctx=0x20c2550, recipient[0] = 0x20c5170 (20CD3FF80DA6C1E46CD9F135CC73A8A60BF38589)
<0x1927>    gpgme_sig_notation_get: call: ctx=0x20c2550, ctx->sig_notations=(nil)
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c2550, fd 4, dir=1 -> tag=0x20c2070
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c2550, fd 8, dir=1 -> tag=0x20c21c0
<0x1927>    _gpgme_add_io_cb: call: ctx=0x20c2550, fd 11, dir=0 -> tag=0x20c2210
<0x1927>    gpgme:gpg_io_event: call: gpg=0x20c1d50, event 0x7fd8b1a20ad0, type 0, type_data (nil)
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2230, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2230, handler (0x20c2ed0, 11)
<0x1927>    _gpgme_data_outbound_handler: enter: dh=0x20c2ed0, fd=0xb
<0x1927>    _gpgme_data_outbound_handler: leave
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2230, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2230, handler (0x20c2ed0, 11)
<0x1927>    _gpgme_data_outbound_handler: enter: dh=0x20c2ed0, fd=0xb
<0x1927>        _gpgme_remove_io_cb: call: data=0x20c2210, setting fd 0xb (item=0x20c2230) done
<0x1927>    _gpgme_data_outbound_handler: leave
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2090, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2090, handler (0x20c1d50, 4)
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2090, need to check
<0x1927>    _gpgme_run_io_cb: call: item=0x20c2090, handler (0x20c1d50, 4)
<0x1927>    _gpgme_cancel_with_err: enter: ctx=0x20c2550, ctx_err=117440566, op_err=0
<0x1927>        _gpgme_remove_io_cb: call: data=0x20c2070, setting fd 0x4 (item=0x20c2090) done
<0x1927>        _gpgme_remove_io_cb: call: data=0x20c21c0, setting fd 0x8 (item=0x20c21e0) done
<0x1927>      gpgme:gpg_io_event: call: gpg=0x20c1d50, event 0x7fd8b1a20ad0, type 1, type_data 0x7fff5afd06c0
<0x1927>    _gpgme_cancel_with_err: leave
<0x1927>  gpgme_op_encrypt_sign: error: Unusable secret key <GPGME>
<0x1927>  gpgme_release: call: ctx=0x20c2550

EDIT

As requested by @kylehuff, here's the code for the key selection:

search_key_result_t gpgme::find_key(const std::string & key_id, const bool priv_key_only, gpgme_key_t *k, std::string *const error)
  {
          error -> clear();

          *k = NULL;

          gpgme_ctx_t ctx = NULL;
          if (!my_gpgme_new(&ctx, false, error))
                  return SK_ERROR;

          gpgme_error_t err = gpgme_op_keylist_start(ctx, key_id.c_str(), priv_key_only ? 1 : 0);
          if (err != GPG_ERR_NO_ERROR)
          {
                  error -> append(format("Problem searching for %s: %s (%d)", key_id.c_str(), gpg_strerror(err), err));
                  gpgme_release(ctx);
                  return SK_ERROR;
          }

          err = gpgme_op_keylist_next(ctx, k);
          if (err == GPG_ERR_EOF)
                  return SK_NOT_FOUND;

          if (err != GPG_ERR_NO_ERROR)
          {
                  error -> append(format("Problem finding %s: %s (%d)", key_id.c_str(), gpg_strerror(err), err));
                  gpgme_release(ctx);
                  return SK_ERROR;
          }

          gpgme_release(ctx);

          return SK_FOUND;
  }

Then in the constructor I do:

std::string error;
  if (find_key(my_key_id, true, &my_key, &error) != SK_FOUND)
          error_exit(false, "Cannot find key %s: %s", my_key_id.c_str(), error.c_str());

and when it is time to sign:

          if (find_key(target_uid, false, &recipient[0], error) != SK_FOUND)
                  break;

          gpgme_signers_clear(ctx);
          err = gpgme_signers_add(ctx, my_key);
          if (err != GPG_ERR_NO_ERROR)
          {
                  error -> append(format("gpgme_signers_add(%s) failed: %s (%d)", my_key_id.c_str(), gpg_strerror(err), err));
                  break;
          }

          int n_signers = gpgme_signers_count(ctx);
          if (n_signers != 1)
          {
                  error -> append(format("Number of signers (%d) not expected number (1)", n_signers));
                  break;
          }

          err = gpgme_op_encrypt_sign(ctx, recipient, GPGME_ENCRYPT_ALWAYS_TRUST /* FIXME */, data_in, sig);
          if (err != GPG_ERR_NO_ERROR)
          {
                  error -> append(format("gpgme_op_encrypt failed: %s (%d)", gpg_strerror(err), err));
                  break;
          }

@kylehuff, is this what you requested? Thanks

EDIT

Here's the listing for 20CD3FF80DA6C1E46CD9F135CC73A8A60BF38589:

tru::0:1433443869:2410285847:3:1:5
pub:u:2048:1:CC73A8A60BF38589:2015-06-04:::u:testkey3 (testkey3) <[email protected]>::scESC:
sub:u:2048:1:22317805D48C1491:2015-06-04::::::e:

EDIT

folkert@travelmate:~$ gpg2 --local-user 14B7E8E6 --sign bla.txt

folkert@travelmate:~$ gpg --verify bla.txt.gpg gpg: Signature made Thu 18 Jun 2015 07:18:17 PM UTC using RSA key ID 74D6F5C6 gpg: Good signature from "testkey2 (testkey2) "

and with edit key I can see that 74d6f5c6 is indeed the sign sub-key:

sub 1024R/74D6F5C6 created: 2015-05-31 expires: never usage: S

So I'm a bit surprised that this doesn't work for gpgme. Not doing the find but directly the gpgme_get_key() gives the same error.

gnupg gpgme
Original Q&A
1

There are 1 answers

6
kylehuff On

It is difficult to tell what is going on, even with the sample, but here are few possible scenarios -

Something being done in the method my_gpgme_new(&ctx, false, error) could be wayward. What is that function doing exactly? Why not use the standard gpgme_new method?

In the sample provided, you are using the function search_key_result_t gpgme::find_key(); is there a need for multiple signers? if so, at least for testing, why not simplify it and just use gpgme_get_key()? i.e.

  gpgme_error_t err;
  gpgme_key_t key;

  err = gpgme_get_key (ctx, key_string, &key, 1);
  if (err) {
    // .. error handling
  }
  gpgme_signers_clear();
  err = gpgme_signers_add (ctx, key);
  gpgme_key_unref (key);

  int n_signers = gpgme_signers_count(ctx);
  if (n_signers != 1) {
    // .. error handling
  }

  err = gpgme_op_encrypt_sign(ctx, recipient, ....);

Additionally with that method, it appears you are adding that method to the gpgme namesapce. Is it possible you trampling on in-built method in the gpgme namespace as well?

As for the recipient object, how is that constructed? It should be a null terminated gpgme_key_t structure of gpgme_key_ts, even if it is only a single recipient. i.e.

  gpgme_key_t recipients[2] = { NULL, NULL };
  err = gpgme_get_key (ctx, recipient_fpr.c_str(), &recipients[0], 0);
  if (err) { // .. error handling }

Alternately, if the recipients are provided as an array:

  gpgme_key_t recipients = new gpgme_key_t[recip_array.size()];
  err = gpgme_get_key (ctx, recipient_fpr.c_str(), &recipients[0], 0);
  if (err) { // .. error handling }
  recipients[recip_array.size()] = NULL; // null terminate the array

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