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--- pgpfan:agevspgp [2021/02/13 13:22] – [Data Recovery] Typo caused by Dokuwiki formatting b.walzer
+++ pgpfan:agevspgp [2021/12/30 23:01] (current) – [File Substitution] Typo. b.walzer
@@ Line 34: / Line 34: @@
 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.
+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 allows an attacker to terminate your process at any point they choose. The assumption here is that no data or partial 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:
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 ====Data Recovery====
-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:
+To be clear, "recovery" here is the sifting of good data from bad. Neither age or GnuPG can regenerate good data from redundant bad data.
+Here is what this would look like in practice using the example of a single flipped bit((Please note that the single flipped bit here is not a realistic example and that in practice damage tends to encompass one or more media blocks. Such blocks tend to be multiples of 512 bytes. Often the blocks are entirely missing.)) near the start of the encrypted file. First age:
   $ age -d -i key.txt -o totc_out.txt totc.txt.age
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   -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.
+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. There is no recovery mode or utility available for age which makes the recovery capability provided by your compression utility((The commonly used bzip2 compression utility for example is specifically designed to be recoverable and comes with the ''%%bzip2recover%%'' command.)) worthless.
+The published standard for age (([[https://docs.google.com/document/d/11yHom20CrsuX8KQJXBBw04s80Unjv8zCg_A7sPAX_9Y/preview|A simple file encryption tool & format]])) is incomplete. I was not able to determine if a recovery utility was even possible as the standard does not provide a description of how the 64KB chunks relate to one another. Creating a recovery utility would be harder due to this lack of documentation.
+It is quite common for media problems to result in the loss of one or more media blocks. Since the 64k chunk used by age is likely going to be larger, an age recovery process would require a search for the start of the next chunk.
 Now GnuPG:
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   ...
-So we have lost 5 words out of 800KB of text((GnuPG noticed that I was encrypting text and helpfully compressed it thus eliminating my example. The addition of a "%%--compress-algo none%%" option at encrypt time was required. Almost all backups are separately compressed. GnuPG will not recompress input. So GnuPG still gets credit for good recoverablity for the backup case using default options.)). 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.
+So we have lost 5 words out of 800KB of text((GnuPG noticed that I was encrypting text and helpfully compressed it thus eliminating my example. The addition of a ''%%--compress-algo none%%'' option at encrypt time was required. Almost all backups are separately compressed. GnuPG will not recompress input. So GnuPG still gets credit for good recoverablity for the backup case using default options.)). 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. The "chunk" in this case is only going to be something like 16 bytes which is going to be shorter than the media block so there is no search required to find the next good "chunk". As a result, no recovery mode or utility is required for GnuPG in almost all cases. Recovery is automatic with a provided warning and returned error.
 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.
-====Signed Backups====
+The ultimate point here is that "blowing up with an error" might not be the best way to handle this sort of situation in a command line utility. A lesson from application programming is that it is often better to complete the operation and return the error at the end.
+====File Substitution====
+Public key encryption has a generic consideration that is quite relevant here. Anyone with access to the public key can easily create a file that will pass any file modification tests ... because after all //that// file has not been modified after creation. You can try to keep the public key secret but a public key design does not guarantee that the public key can not be derived from the encrypted material. That guarantee only applies to the secret key. So an attacker can skip the bother of attempting to overcome the authentication and just replace the file with whatever they want.
+The traditional fix for this weakness is to cryptographically sign the file ... and, once again, age does not do signatures. An age user would need to go through the process of finding a separate signing utility and would have to apply it correctly while using a separate signature file. A GnuPG user would only have to add a ''%%-s%%'' option while encrypting and would have an integrated signature protected by the encryption. No further effort would be required on the part of the GnuPG user. A good signature message would appear automatically after decryption.
-Doing backups in this way means that you have to keep the public key secret((There is no hard guarantee that the public key can not be derived from the encrypted material somehow. The guarantee is that the //private// key can not be derived in this sort of system)). 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.
 =====Conclusion=====