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I often use PGP (aka GnuPG, GPG) to encrypt/decrypt/sign/verify emails exchanged with a friend.

Is there a similar solution (ideally re-using the same standards) for conversation with 3 persons or more?

Requirements:

  • Encrypt/decrypt
  • Sign/verify
  • Open source
  • Clients for Linux/Mac/Windows/Android/iOS
  • Without any special client support, you can always simply encrypt and email replies to each member of the discussion separately. In other words, write your reply, then send it once for each member of the discussion. A SERIOUS pain in the neck if there's more than 3 or 4 people in the discussion. – Michael Kohne Sep 8 '14 at 13:24
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    Why are you looking for an alternative to PGP? I've never seen a PGP email interface that was restricted to a single recipient. – Gilles 'SO- stop being evil' Sep 9 '14 at 22:09
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You can use GnuPG.

The manual writes:

The --recipient option is used once for each recipient and takes an extra argument specifying the public key to which the document should be encrypted.

Of course, it encrypts and decrypts, it signs and verifies, is Free Software, and is available for many platforms.

  • So, if I add N --recipient arguments, it will create N PGP blocks, and each recipient will be able to decrypt only one of these blocks? – Nicolas Raoul Sep 10 '14 at 3:14
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    Yes. It will encrypt your data symmetrically and that symmetric key asymmetrically for each recipient. See security.stackexchange.com/a/8251/55222 for more details. – Frederick Nord Sep 10 '14 at 9:26

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