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Is it possible to infect a file encrypted using GnuPG?

For example, if I had an encrypted file on an unsecured public ftp server, would someone be able to copy that file and attach a virus/trojan onto the file and then reupload to the server? Then when I next accessed/decrypted, I would then be infected with said virus?

I am currently storing encrypted files on public servers to store data that a few people access. I have just had a sinking feeling that I could be open to attack and possibly have this encrypted data leaked if a trojan was installed.

Jens Erat
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Freeman
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  • You mean, is it possible to modify an encrypted file and the file still be able to be decrypted? The answer to that is "no". – schroeder May 01 '16 at 20:41
  • Not even if they remove the original file then encrypt an infected file using my public key? If they encrypted an infected pdf to my public key wouldnt it encrypt and decrypt without any issues? – Freeman May 02 '16 at 09:00
  • It depends on how you are encrypting it, hence my question. If you have an encrypted file that someone can decrypt, modify then re-encrypt, then yes, the file can be infected. Is that what you are asking? – schroeder May 02 '16 at 14:55
  • No what i mean was for example, on my server i have uidInfo.gpg, which contains an file called password.pdf. Could someone remove the uidInfo.gpg and replace with their own file name uidInfo.gpg which is encrypted to my public key and inside have a password.pdf file with virus attached? – Freeman May 02 '16 at 19:10
  • it depends on how you encrypted the gpg file – schroeder May 02 '16 at 19:14
  • on which end? my end or an attackers? – Freeman May 02 '16 at 19:24

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Malware can be distributed in various fashions: as directly executable programs, or hidden in other files and formats used as containers.

If you apply a process that always decrypts a given file, replacing it with directly executable malware is not very promising, decryption will simply fail. In case of public/private key cryptography (and a known public key), of course the encrypted file could be exchanged by another, malicious file. Signing mitigates this, symmetric encryption also does (as there is no "public" key).

On the other hand, there might always exist a bug in the software reading a file: the question "How to inject executable, malicious code into PDF, JPEG, MP3, etc.?" contains some examples. A similar bug could also exist in GnuPG, triggered on decryption or verification of a signature. In this case, a malicious OpenPGP message might result in malware being executed on your computer. If a trojan is run on your computer, obviously also the encrypted content might be revealed.

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Jens Erat
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  • Right yes that makes things more clear. So if the file isnt digitally signed, someone who has my public key could reupload an infected version of my encrypted file? – Freeman May 01 '16 at 21:33
  • He could do so anyway; but a digital signature enables you to realize the file wasn't issued by yourself (missing or wrong signature, or signature issued by another key). – Jens Erat May 01 '16 at 21:59
  • Thanks. I didnt think I would be able to open an encrypted file that wasnt encrypted using my public key. I thought they would have to add my public key so I could decrypt. – Freeman May 01 '16 at 22:08
  • You won't be to decrypt it, but GnuPG has to start processing the file to realize so. Anyway, usually public keys are _public_, and available to anyone. – Jens Erat May 02 '16 at 07:43