gpgsm "Encrypt failed" "Unusable public key: 53A51054BB68F7C3" root certificate missing?

[Uwe Brauer]

Hi 

I am on Ubuntu 16.04 running 


gpgsm (GnuPG) 2.1.11
libgcrypt 1.6.5
libksba 1.3.3-unknown

I am also a die hard user of emacs and use it for encrypting and
decrypting my mails.

I received a smime message from a colleague (with his public key
embedded of course).

When I tried to send him a smime encrypted and signed message I obtained
the lisp error:

Debugger entered--Lisp error: (epg-error "Encrypt failed" "Unusable public key: 53A51054BB68F7C3")

I suspect that the key was signed from an authority whose root
certificate I don't posses.

For example using thunderbird and opening his signed message, I see

I also tried to run 
gpgsm -e -r arober at ucm.es epg-bug.txt

But I receive 

gpgsm: enabled debug flags: ipc
gpgsm: can't encrypt to 'arober at ucm.es': Ambiguous name
secmem usage: 0/16384 bytes in 0 blocks


I am not sure whether this connected, but I do have old certificates of
this user.

Any help would be strongly appreciated.

Regards

Uwe Brauer 


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