Metadata-Version: 2.1
Name: patatt
Version: 0.4.9
Summary: A simple library to add cryptographic attestation to patches sent via email
Home-page: https://git.kernel.org/pub/scm/utils/patatt/patatt.git/about/
Author: Konstantin Ryabitsev
Author-email: mricon@kernel.org
License: MIT-0
Description: patatt: cryptographic patch attestation for the masses
        ======================================================
        
        This utility allows an easy way to add end-to-end cryptographic
        attestation to patches sent via mail. It does so by adapting the DKIM
        email signature standard to include cryptographic signatures via the
        X-Developer-Signature email header.
        
        If your project workflow doesn't use patches sent via email, then you
        don't need this and should simply start signing your tags and commits.
        
        Basic concepts
        --------------
        DKIM is a widely adopted standard for domain-level attestation of email
        messages. It works by hashing the message body and certain individual
        headers, and then creating a cryptographic signature of the resulting
        hash. The receiving side obtains the public key of the sending domain
        from its DNS record and checks the signature and header/body hashes. If
        the signature verifies and the resulting hashes are identical, then
        there is a high degree of assurance that neither the body of the message
        nor any of the signed headers were modified in transit.
        
        This utility uses the exact same DKIM standard to hash the headers and
        the body of the patch message, but uses a different set of fields and
        canonicalization routines:
        
        - the d= field is not used (no domain signatures involved)
        - the q= field is not used (key lookup is left to the client)
        - the c= field is not used (see below for canonicalization)
        - the i= field is optional, but MUST be the canonical email address of
          the sender, if not the same as the From: field
        
        Canonicalization
        ~~~~~~~~~~~~~~~~
        Patatt uses the "relaxed/simple" canonicalization as defined by the DKIM
        standard, but the message is first parsed by the "git-mailinfo" command
        in order to achieve the following:
        
        - normalize any content-transfer-encoding modifications (convert back
          from base64/quoted-printable/etc into 8-bit)
        - use any encountered in-body From: and Subject: headers to
          rewrite the outer message headers
        - perform the subject-line normalization in order to strip content not
          considered by git-am when applying the patch (i.e. drop [PATCH .*] and
          other bracketed prefix content)
        
        To achieve this, the message is passed through git-mailinfo with the
        following flags::
        
            cat orig.msg | git mailinfo --encoding=utf-8 --no-scissors m p > i
        
        Patatt then uses the data found in "i" to replace the From: and Subject:
        headers of the original message, and concatenates "m" and "p" back
        together to form the body of the message, which is then normalized using
        CRLF line endings and the DKIM "simple" body canonicalization (any
        trailing blank lines are removed).
        
        Any other headers included in signing are modified using the "relaxed"
        header canonicalization routines as defined in the DKIM RFC.
        
        In other words, the body and some of the headers are normalized and
        reconstituted using the "git-mailinfo" command, and then canonicalized
        using DKIM's relaxed/simple standard.
        
        Supported Signature Algorithms
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        DKIM standard mostly relies on RSA signatures, though RFC 8463 extends
        it to support ED25519 keys as well. While it is possible to use any of
        the DKIM-defined algorithms, patatt only supports the following
        two signing/hashing schemes:
        
        - ed25519-sha256: exactly as defined in RFC8463
        - openpgp-sha256: uses OpenPGP to create the signature
        
        Note: Since GnuPG supports multiple signing key algorithms,
        openpgp-sha256 signatures can be done using EDDSA keys as well. However,
        since OpenPGP output includes additional headers, the "ed25519-sha256"
        and "openpgp-sha256" schemes are not interchangeable even when ed25519
        keys are used in both cases.
        
        In the future, patatt may add support for more algorithms, especially if
        that allows incorporating TPM and U2F devices (e.g. for offloading
        credential storage and crypto operations into a sandboxed environment).
        
        X-Developer-Key header
        ~~~~~~~~~~~~~~~~~~~~~~
        Patatt adds a separate ``X-Developer-Key:`` header with public key
        information. It is merely informational and ISN'T and SHOULDN'T be used
        for performing any kind of message validation (for obvious reasons). It
        is included to make it easier for maintainers to obtain the
        contributor's public key before performing whatever necessary
        verification steps prior to its inclusion into their individual or
        project-wide keyrings.
        
        This also allows keeping a historical record of contributor keys via
        list archive services such as lore.kernel.org and others.
        
        Getting started as contributor
        ------------------------------
        It is very easy to start signing your patches with patatt.
        
        Installing
        ~~~~~~~~~~
        You can install from pip::
        
            pip install --user patatt
        
        Make sure your PATH includes $HOME/.local/bin.
        
        Alternatively, you can clone this repository and symlink patatt.sh into
        your path::
        
            cd bin
            ln -s ~/path/to/patatt/patatt.sh patatt
        
        After this, you should be able to run ``patatt --help`` without
        specifying the full path to the repository.
        
        Using PGP
        ~~~~~~~~~
        If you already have a PGP key, you can simply start using it to sign
        patches. Add the following to your ~/.gitconfig::
        
            [patatt]
                signingkey = openpgp:KEYID
        
        The KEYID should be the 16-character identifier of your key, for
        example::
        
            [patatt]
                signingkey = openpgp:E63EDCA9329DD07E
        
        Using ed25519
        ~~~~~~~~~~~~~
        If you don't already have a PGP key, you can opt to generate and use a
        new ed25519 key instead (see below for some considerations on pros and
        cons of PGP vs ed25519 keys).
        
        To generate a new keypair, run::
        
            patatt genkey
        
        You will see an output similar to the following::
        
            Generating a new ed25519 keypair
            Wrote: /home/user/.local/share/patatt/private/20210505.key
            Wrote: /home/user/.local/share/patatt/public/20210505.pub
            Wrote: /home/user/.local/share/patatt/public/ed25519/example.org/user/default
            Add the following to your .git/config (or global ~/.gitconfig):
            ---
            [patatt]
                signingkey = ed25519:20210505
            ---
            Next, communicate the contents of the following file to the
            repository keyring maintainers for inclusion into the project:
            /home/user/.local/share/patatt/public/20210505.pub
        
        Please make sure to back up your new private key, located in
        ``~/.local/share/patatt/private``. It is short enough to simply
        print/write out for storing offline.
        
        Next, just do as instructions say. If the project for which you are
        contributing patches already uses patatt attestation, please work with
        the project maintainers to add your public key to the repository. If
        they aren't yet using patatt, just start signing your patches and
        hopefully the project will start keeping its own keyring in the future.
        
        Testing if it's working
        ~~~~~~~~~~~~~~~~~~~~~~~
        To test if it's working::
        
            $ git format-patch -1 --stdout | patatt sign > /tmp/test
        
        If you didn't get an error message, then the process was successful. You
        can review /tmp/test to see that ``X-Developer-Signature`` and
        ``X-Developer-Key`` headers were successfully added.
        
        You can now validate your own message::
        
            $ patatt validate /tmp/test
        
        Automatic signing via the sendemail-validate hook
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        If everything is working well, you can start automatically signing all
        outgoing patches sent via git-send-email. Inside the repo you want enabled
        for signing, run::
        
            $ patatt install-hook
        
        Or you can do it manually::
        
            $ echo 'patatt sign --hook "${1}"' > "$(git rev-parse --git-dir)/hooks/sendemail-validate"
            $ chmod a+x "$(git rev-parse --git-dir)/hooks/sendemail-validate"
        
        PGP vs ed25519 keys considerations
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        If you don't already have a PGP key, you may wonder whether it makes
        sense to create a new PGP key or start using standalone ed25519 keys.
        
        Reasons to choose PGP:
        
        - you can protect the PGP private key with a passphrase (gpg-agent will
          manage it for you so you only need to enter it once per session)
        - you can move your PGP key to an OpenPGP-compliant smartcard to further
          protect your key from being leaked/stolen
        - you can use PGP keys to sign git tags/commits, not just mailed patches
        
        If you choose to create a new PGP key, you can use the following guide:
        https://github.com/lfit/itpol/blob/master/protecting-code-integrity.md
        
        Reasons to choose a standalone ed25519 key:
        
        - much smaller signatures, especially compared to PGP RSA keys
        - implements the DKIM ed25519 signing standard
        - faster operation
        
        If you choose ed25519 keys, you will need to make sure that PyNaCl is
        installed (pip install should have already taken care of it for you).
        
        Getting started as a project maintainer
        ---------------------------------------
        Patatt implements basic signature validation, but it's a tool aimed
        primarily at contributors. If you are processing mailed-in patches, then
        you should look into using b4, which aims at making the entire process
        easier. B4 properly recognizes X-Developer-Signature headers starting
        with version 0.7.0 and uses the patatt library as well.
        
        - https://pypi.org/project/b4/
        
        That said, keyring management as discussed below applies both to patatt
        and b4, so you can read on for an overview.
        
        In-git pubkey management
        ~~~~~~~~~~~~~~~~~~~~~~~~
        The trickiest part of all decentralized PKI schemes is not the crypto
        itself, but public key distribution and management. PGP famously tried
        to solve this problem by relying on cross-key certification and
        keyservers, but the results were not encouraging.
        
        On the other hand, within the context of git repositories, we already
        have a suitable mechanism for distributing developer public keys, which
        is the repository itself. Consider this:
        
        - git is already decentralized and can be mirrored to multiple
          locations, avoiding any single points of failure
        - all contents are already versioned and key additions/removals can be
          audited and "git blame'd"
        - git commits themselves can be cryptographically signed, which allows a
          small subset of developers to act as "trusted introducers" to many
          other contributors (mimicking the "keysigning" process)
        
        The idea of using git itself for keyring management was originally
        suggested by the did:git project, though we do not currently implement
        the proposed standard itself.
        
        - https://github.com/dhuseby/did-git-spec/blob/master/did-git-spec.md
        
        Keyring structure
        ~~~~~~~~~~~~~~~~~
        The keyring is structured as follows::
        
            - dir: topdir (e.g. ".keys")
              |
              - dir: keytype (e.g. "ed25519" or "openpgp")
                |
                - dir: address-domainname (e.g. "example.org")
                  |
                  - dir: address-localpart (e.g. "developer")
                    |
                    - file: selector (e.g. "default")
        
        The main reasoning behind this structure was to make it easy for
        multiple project maintainers to manage keys without causing any
        unnecessary git merge complications. Keeping all public keys in
        individual files helps achieve this goal.
        
        For example, let's take the following signature::
        
            From: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
            X-Developer-Signature: v=1; a=ed25519-sha256; t=1620240207; l=2577;
             h=from:subject; bh=yqviDBgyf3/dQgHcBe3B7fTP39SuKnYInPBxnOiuGcA=;
             b=Xzd0287MvPE9NLX7xbQ6xnyrvqQOMK01mxHnrPmm1f6O7KKyogc8YH6IAlwIPdo+jk1CkdYYQsyZ
             sS0cJdX2B4uTmV9mxOe7hssjtjLcj5/NU9zAw6WJARybaNAKH8rv
        
        The key would be found in the following subpath::
        
            .keys/ed25519/linuxfoundation.org/konstantin/default
        
        If i= and s= fields are specified in the signature, as below::
        
            X-Developer-Signature: v=1; a=ed25519-sha256; t=1620244687; l=12645;
             i=mricon@kernel.org; s=20210505; h=from:subject;
             bh=KRCBcYiMdeoSX0l1XJ2YzP/uJhmym3Pi6CmbN9fs4aM=;
             b=sSY2vXzju7zU3KK4VQ5vFa5iPpDr3nrf221lnpq2+uuXmCODlAsgoqDmjKUBmbPtlY1Bcb2N0XZQ
             0KX+OShCAAwB5U1dtFtRnB/mgVibMxwl68A7OivGIVYe491yll5q
        
        Then the path would reflect those parameters::
        
            .keys/ed25519/kernel.org/mricon/20210505
        
        In the case of ed25519 keys, the contents of the file are just the
        base64-encoded public key itself. For openpgp keys, the format should be
        the ascii-armored public key export, for example obtained by using the
        following command::
        
            gpg -a --export --export-options export-minimal keyid
        
        Whose keys to add to the keyring
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        It does not really make sense to require cryptographic attestation for
        patches submitted by occasional contributors. The only keys added to the
        keyring should be those of the core maintainers who have push access to
        the "canonical" repository location, plus the keys belonging to regular
        contributors with a long-term ongoing relationship with the project.
        
        Managing the keyring: small teams
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        For smaller repositories with a handful of core maintainers, it makes
        sense to keep the keyring in the main branch, together with all other
        project files.
        
        Managing the keyring: large teams
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        For large teams with thousands of regular contributors and teams of
        subsystem maintainers (e.g. the Linux kernel), it does not make sense to
        have a centrally managed keyring tracked in the main repository.
        Instead, each subsystem maintainer team should manage their own keyring
        in a separate ref of their own repository.
        
        For example, to create a blank new ref called ``refs/meta/keyring``::
        
            git symbolic-ref HEAD refs/meta/keyring
            git reset --hard
            mkdir ed25519 openpgp
        
        Individual public key files can then be added and committed following
        the same structure as described above. Keeping the keyring outside the
        regular development branch ensures that it doesn't interfere with
        submitted pull requests or git-format-patch operations. Keeping the ref
        under ``refs/meta/`` will hide it from most GUI interfaces, but if that
        is not the goal, then it can be stored in ``refs/heads`` just like any
        other branch.
        
        To commit and push the files after adding them, regular git operations
        should be used::
        
            git commit -asS
            git push origin HEAD:refs/meta/keyring
            # Switch back to the development environment
            git checkout regular-branch
        
        To make changes to an existing keyring ref, a similar workflow can be
        used::
        
            git fetch origin refs/meta/keyring
            # Verify that the commit is signed
            git verify-commit FETCH_HEAD
            git checkout FETCH_HEAD
            # make any changes to the keys
            git commit -asS
            git push origin HEAD:refs/meta/keyring
            git checkout regular-branch
        
        Alternatively, if key additions/updates are frequent enough, the remote
        ref can be checked out into its own workdir and set up for proper
        remote tracking.
        
        Telling patatt where to find the keyring(s)
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        To use the keyring with patatt or b4, just tell them which paths to
        check, via the ``keyringsrc`` setting (can be specified multiple
        times and will be checked in the listed order)::
        
            [patatt]
                # Empty ref means "use currently checked out ref in this repo"
                keyringsrc = ref:::.keys
                # Use a dedicated ref in this repo called refs/meta/keyring
                keyringsrc = ref::refs/meta/keyring:
                # Use a ref in a different repo
                keyringsrc = ref:~/path/to/another/repo:refs/heads/main:.keys
                # Use a regular dir on disk
                keyringsrc = ~/git/korg-pgpkeys/.keyring
        
        For b4, use the same configuration under the ``[b4]`` section.
        
        External and local-only keyrings
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        Any path on disk can be used for a keyring location, and some will
        always be checked just in case. The following locations are added by
        default::
        
            ref:::.keys
            ref:::.local-keys
            ref::refs/meta/keyring:
            $XDG_DATA_HOME/patatt/public
        
        The ":::" means "whatever ref is checked out in the current repo",
        and $XDG_DATA_HOME usually points at $HOME/.local/share.
        
        Getting support and contributing patches
        ----------------------------------------
        Please send patches and support requests to tools@linux.kernel.org.
        
        Submissions must be made under the terms of the Linux Foundation
        certificate of contribution and should include a Signed-off-by: line.
        Please read the DCO file for full legal definition of what that implies.
        
        Frequently seen commentary
        --------------------------
        Why is this library even needed? Why not...
        
        Why not simply PGP-sign all patches?
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        PGP-signing patches causes important problems for reviewers. If a patch
        is inline-signed, then this not only adds textual headers/footers, but
        adds additional escaping in the protected body, converting all '^-'
        sequences into '^- -', which corrupts patches.
        
        MIME-signing is better, but has several other downsides:
        
        - messages are now sent as multipart mime structures, which causes some
          tooling to no longer properly handle the patch content
        - the signature attachments may be stripped/quarantined by email
          gateways that don't properly recognize OpenPGP mime signatures
        - the From/Subject headers are rarely included into protected content,
          even though they are crucial parts of what ends up going into a git
          commit
        
        These considerations have resulted in many projects specifically
        requesting that patches should NOT be sent PGP-signed.
        
        Why not just rely on proper code review?
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        Code review is a crucial step of the development process and patatt does
        not aim to replace it. However, there are several areas where the
        process can be abused by malicious parties in the absence of end-to-end
        cryptographic attestation:
        
        1. A maintainer who struggles with code review volume may delegate parts
           of their duties to a submaintainer. If that person submits aggregated
           patch series to the maintainer after performing that work, there must
           be a mechanism to ensure that none of the reviewed patches have been
           modified between when they were reviewed by the trusted submaintainer
           and when the upstream developer applies them to their tree. Up to
           now, the only mechanism to ensure this was via signed pull requests
           -- with patatt this is now also possible with regular patch series.
        
        2. It is important to ensure that what developer reviews is what
           actually ends up being applied to their git tree. Linux development
           process consists of collecting follow-up trailers (Tested-by,
           Reviewed-by, etc), so various tooling exists to aggregate these
           trailers and create the collated patch series containing all
           follow-up tags (see b4, patchwork, etc). Patatt signing provides a
           mechanism to ensure that what that developer reviewed and approved
           and what they applied to their tree is the exact same code and hasn't
           been maliciously modified in-between review and "git am" (e.g. by
           archival services such as lore.kernel.org, mail hosting providers,
           someone with access to the developer's inbox, etc).
        
        3. An attacker may attempt to impersonate a well-known developer by
           submitting malicious code, perhaps with the hope that it receives
           less scrutiny and is accepted without rigorous code review. Even if
           this attempt is unsuccessful (and it most likely would be), this may
           cause unnecessary reputation damage to the person being impersonated.
           Cryptographic signatures (and lack thereof) will help the developer
           quickly establish that the attack was performed without their
           involvement.
        
        Why not just rely on DKIM?
        ~~~~~~~~~~~~~~~~~~~~~~~~~~
        DKIM standard is great, but there are several places where it falls a
        bit short when it comes to patch attestation:
        
        1. The signing is done by the mail gateways that may or may not be
           properly checking that the "From:" header matches the identity of the
           authenticated user. For example, a service that allows free account
           registration may not check that alice@example.org sends outgoing
           email with "bob@example.org" in the "From:" field, which would allow
           Alice to impersonate Bob and have the messages arrive with a valid
           DKIM signature.
        
        2. DKIM is usually seen as merely a spam reduction mechanism, so there's
           usually little incentive for infrastructure administrators to be too
           strict about how they handle the private keys used for DKIM signing.
           Most likely, they are just stored on disk without a passphrase and
           accessible by the SMTP daemon.
        
        3. DKIM's "relaxed" canonicalization standard for message bodies
           replaces all multiple whitespace characters with a single space
           before the body hash is signed. This poses significant problems for
           patches where whitespace is syntactically significant (Python,
           Makefiles, etc). A "return True" with a different indent will pass
           DKIM signature check and may introduce a serious security
           vulnerability.
        
        4. DKIM doesn't prevent typosquatting attacks. For example, an attacker
           attempting to impersonate known.developer@companyname.com may send an
           email from known.developer@company-name.com or any other
           similar-looking address or domain, with valid DKIM signatures in
           every case.
        
Keywords: git,patches,attestation
Platform: UNKNOWN
Requires-Python: >=3.6
Description-Content-Type: text/x-rst
