Age is a utility and library to do public key and symmetrical encryption/decryption in a simple and straightforward way1). The idea that age could replace OpenPGP comes up from time to time.
OpenPGP based systems are used for multiple purposes…
In terms of actual use, this is probably the most important function of OpenPGP. The value here is mostly the well supported and standardized signature standard provided by OpenPGP. Age does not do signing at all.
Age is obviously not intended for this purpose.
Messaging is all about identities. Age provides no identity management at all. Again, it doesn't even do signatures.
Age is obviously not intended for this purpose either.
Email already has two working and widely implemented encryption standards (OpenPGP and S/MIME). Messaging is mostly about standards. Even if someone were to build out age to do messaging it would be irresponsible to promote it as a third standard for email.
This (and encrypted long term storage in general) is where age has purpose. Let's say you use GnuPG as an OpenPGP implementation to encrypt a backup, that backup becomes corrupt and then you use this command line to decrypt it and unarchive it all in one go:
$ gpg2 -d backup.tgz.pgp | tar xz
GnuPG then dumps the unencrypted file, corruption and all, to tar. When it hits the end it does an integrity check which would fail with this message:
gpg: WARNING: encrypted message has been manipulated!
Then GnuPG would return an error.
The issue here is that tar might of had to deal with garbage for input. A potentially more serious issue would be if a malicious entity had enough knowledge of the unencrypted backup to allow them to modify files and/or file locations in a purposeful way. This might be a concern if you store your backups on a server that is not under your control.
Age does an integrity check every 64KB along the encrypted stream. If the integrity check fails then it does not release the bad 64KB chunk and errors out thus truncating the output. This truncation might cause a problem in some cases, but the assumption is that no data is better than bad data.
The solution to the GnuPG bad data and the age truncated data is the same. Check the integrity of the input file before attempting to do anything with it. I would think this step would be mandatory in a case where there is a possibility that someone might attempt to modify your encrypted file(s). A manual integrity check could be as simple as running GnuPG and dumping the output to see if an error message shows up:
$ gpg2 -d backup.tgz.pgp >/dev/null
Here is what this would look like in practice using the example of a single flipped bit near the start of the encrypted file. First age:
$ age -d -i key.txt -o totc_out.txt totc.txt.age Error: chacha20poly1305: message authentication failed [ Did age not do what you expected? Could an error be more useful? Tell us: https://filippo.io/age/report ]
$ ls -l totc_out.txt -rw-r--r-- 1 operator operator 0 Dec 25 12:53 totc_out.txt
Age has returned a zero length file. Those that have spent time maintaining backups for others might be suffering some vicarious anxiety at this point. A single bit error has nerfed the entire backup. The knowledge that there is no recovery mode or utility available for age will probably not help with that anxiety. I was not able to determine if such a recovery utility was possible as there does not seem to be enough of a description available to determine how the 64KB chunks relate to one another.
$ gpg2 -d totc.txt.pgp >totc_out.txt gpg: encrypted with 2048-bit RSA key, ID B456C3BB5A48A0EA, created 2020-06-01 "BACKUP <backup@store>" gpg: WARNING: encrypted message has been manipulated!
$ ls -l totc_out.txt -rw-r--r-- 1 operator operator 807615 Dec 25 15:26 totc_out.txt
Here is the damage to the text:
... which passed within his view, at a distance of some fifty or sixty yards. It is likely enough that, rooted in the woods of France and Norway, there were growing trees, wheo <CC>tR<EF>ZQ^Z^\'s^Q<E1>.3ACs put to death, already marked by the Woodman, Fate, to come down and be sawn into boards, to make a certain movable framework with a sack and a knife in ...
So we have lost 5 words out of 800KB of text2). This is an example of a somewhat unknown and under appreciated feature of the type of encryption used in OpenPGP. It is self-synchronizing in the face of corruption.
At this point you might want to consider how often longer term backups end up with bad sections. Then consider how many attackers exist with sufficient skill, combined with sufficient knowledge of the organization and content of your backups to do reliable changes. Attackers that are willing to have their efforts revealed after the first attempt. This would seem to be where OpenPGP is superior to age.
Doing backups in this way means that you have to keep the public key secret. Otherwise an attacker could replace something like a backup with a completely new file that would pass the integrity check(s). If the public key can not be kept secret then the backup would have to be signed. That would be as simple as adding a “-s” option when encrypting using GnuPG. Since age does not do signatures you would have to use a separate utility and would end up with a separate signature file to deal with. I find it slightly ironic that in most situations GnuPG would be the most available and easiest choice for that added signature. This could be another place where OpenPGP is superior to age.
Age should be considered where a simple encryption utility is needed that will not under any circumstances release unauthenticated data. Where that behaviour is undesirable, such as for encrypted backups, something based on OpenPGP would be preferred.