Transparent encryption with ansible vault revisited

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Transparent encryption with ansible vault revisited

Michel Blanc bio photo By Michel Blanc Comment

Doing it the wrong way

Last attempt to make ansible vault encryption/decryption transparent wasn’t quite right. Decrypting files after commit wasn’t a good idea as Raphael Campardou noticed.

In search for a better idea, I eventually realized that hooks where not the right place to do it: yes, you can guard from commiting files that should be encrypted, but hacking around hooks to build a crypt/decrypt pipeline is doomed to failure.

Doing it better

While looking for alternate ways, I remembered I hacked around with git filters back in the days to see clear-text diffs for OpenOffice files.

Git let’s you apply smudge, clean and textconv filters to files which are applied this way:

  • filter/smudge: after checkout, reads blob from STDIN and outputs the workfile from STDOUT
  • filter/clean: converts the worktree file to blob upon check in
  • diff/textconv: applied before diffing files

So, for our needs, smudge and textconv are good places to decrypt, while clean is the place to encrypt.

Implementation

The implementation requires to write the 3 filters (smudge, clean, textconv) and configure your git repos to use the filters.

Those filters should be executable.

As we did in last post, we will use a .vault_password file in the project root directory containing the vault key (don’t forget to add it to your .gitignore file !). The filters fail if the file is not present.

Smudge

The problem that came up to write the smudge & clean filters is that the blob content is fed on STDIN, and ansible-vault can only encrypt/decrypt files in-place.

So we have to write the blob in a temporary file. While this is not really a problem for the smudge filter, it is for the clean filter since the temporary file contains the clear-text version of the file. The temp file is created with restricted permissions, but you’ve been warned.

Smudge’s filter job is simple: - write STDIN content to temp file - decrypt the temp file and swallow the output in a variable (using ansible-vault view after setting the PAGER to cat) - if the file was a vault encrypted file, display the variable, else, bail out.

#!/bin/sh

if [ ! -r '.vault_password' ]; then
  exit 1
fi

tmp=`mktemp`
cat > $tmp

export PAGER='cat'
CONTENT=`ansible-vault view "$tmp" --vault-password-file=.vault_password 2> /dev/null`

if echo $CONTENT | grep 'ERROR: data is not encrypted' > /dev/null; then
  echo "Looks like one file was commited clear text"
  echo "Please fix this before continuing !"
  exit 1
else
  echo $CONTENT
fi

rm $tmp

As you guessed, ansible-vault does not output errors on STDERR but on STDOUT.

Clean

The clean filter works almost the same way: - write STDIN to a temp file - encrypt the temp file in place - write the temp file to STDOUT

#!/bin/sh

if [ ! -r '.vault_password' ]; then
  exit 1
fi

tmp=`mktemp`
cat > $tmp

ansible-vault encrypt $tmp --vault-password-file=.vault_password > /dev/null 2>&1

cat "$tmp"
rm $tmp

This one was quite easy. We could also use modelines, by encrypting only if “vault: true” is present in the 4 first lines. This way, we could apply the filters to all the files. However I ditched the idea for performance reasons (see below).

Diff filter

The filter works like the smudge filter except that it uses the file name passed as a parameter.

#!/bin/sh

if [ ! -r '.vault_password' ]; then
  exit 1
fi

export PAGER='cat'
CONTENT=`ansible-vault view "$1" --vault-password-file=.vault_password 2> /dev/null`

if echo "$CONTENT" | grep 'ERROR: data is not encrypted' > /dev/null; then
  cat "$1"
else
  echo "$CONTENT"
fi

Git configuration

Attributes

Now that the various filters are out and chmoded +x, we need to set-up out git repos to use them.

For this, we need to tell git on which files we want to apply the filters, using a .gitattributes file in our project top directory.

The following .gitattributes file

*_vault* filter=vault diff=vault

will run filters on repository blobs/files that match *_vault*.

I initially intended to run the filters on all files, using modelines. However, performance was really bad, so I finally ended up removing a full wildcard (*) and restrict filter selection to specific files. You can repeat the lines ad nauseam if you want to catch multiple fileglobs.

Gitconfig

I put my filters in ~/.bin/, but the location doesn’t matter. You can event add them to the project and commit them, so everyone has them.

The following section needs to be added to the project’s .git/config file:

[filter "vault"]
  smudge = ~/.bin/smudge_vault
  clean  = ~/.bin/clean_vault

[diff "vault"]
  textconv = ~/.bin/diff_vault
 

Test

Adding a file that matches a glob in .gitattributes should now trigger transparent encryption.

Here is a sample transcript.

Big fat warning

The git cat-file part is not here for decoration. At least the first time, ensure that encryption works.

The filters

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