I would like to ask this question as I'm not the first person which has this behavior.

I'm trying to encrypt a simple message using GnuPG using a public key I've received from a friend, he's using DSA 3072 but for some reason, whenever I try to encrypt using his public key, I get

gpg: <mailaddr>: skipped: unusable public key
gpg: s: encryption failed: unusable public key

Here's the output of gpg --list-key when I lookup my friend's mail

pub   3072D/fingerprint some-date-here
uid                  Name1 <mail1>
uid                  Name2 <mail2>

Here's my GPG --version output

gpg --version
gpg (GnuPG) 1.4.19
Copyright (C) 2015 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: ~/.gnupg
Supported algorithms:
Hash: MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2

I'm asking here as I recently got mail from people claiming the same (I'm also using DSA 3072 on my key). Can anyone shed some light on this topic ?


DSA is a signature algorithm; its keys are not usable for asymmetric encryption. In the OpenPGP format, algorithms for encryption are RSA and El-Gamal.

Normally, when you create your keypair, you are actually producing several key pairs: a "master" key, and some "sub-keys" for signing and for encrypting. The master key signs the sub-keys. When you say that you use "DSA 3072", you really mean that your master key uses DSA 3072. Similarly for your friend.

  • Ok, thanks for the semantic correction. but why can't I use "gpg --reciepent "person" --encrypt " ? why does it give an error ?
    – gpgfr
    May 26 '15 at 14:45
  • The idea is that, for some reason, you have a copy of the recipient's master key (usable for signatures only) but not of his encryption subkey. gpg only sees the signature key, and cannot use it for encryption.
    – Tom Leek
    May 26 '15 at 14:49
  • which software should i use instead of gpg then in order to encrypt using it ?
    – gpgfr
    May 26 '15 at 14:52
  • also, doesn't "Pubkey: ... DSA" states that my version of GPG supports DSA?
    – gpgfr
    May 26 '15 at 15:01
  • GPG supports DSA and DSA is a signature algorithm, so GPG will be perfectly happy to verify signatures using that key. That's the point of DSA being a signature algorithm and not an encryption algorithm: you can sign with it, you cannot encrypt with it.
    – Tom Leek
    May 26 '15 at 15:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.