Hello, all: In continuation of my patch attestation work, I would like to submit for your consideration an alternative implementation that is both simpler and more likely to find wider adoption due to introduction of domain-based attestation that is similar to DKIM in nature. While I consider domain-based attestation less robust than individual developer attestation, it has the upside of providing an easy way to make all patches coming from entire domains tamper-evident, once they receive the in-header signature. In other words, a patch coming from developer@xxxxxxxxxx can be verified to have been mailed via kernel.org and not altered since leaving the kernel.org SMTP server. Domain-level attestation works alongside developer-based attestation, and can also be done via email headers. It can be done either with PGP, or with individual ED25519 keys, distributed via web key directories. This proposal comes with a proof-of-concept repository implementing all the signing and verification functionality. It can be cloned from the following address: https://git.kernel.org/pub/scm/linux/kernel/git/mricon/patch-attestation-poc.git Below is the proposal itself, taken from README.rst. All examples mentioned in it can be replicated using the POC repository. ------ Header-Based Patch Attestation ============================== Author: Konstantin Ryabitsev <konstantin@xxxxxxxxxxxxxxxxxxx> Status: Alpha, soliciting comments Preamble -------- Projects participating in decentralized development continue to use RFC-2822 (email) formatted messages for code submissions and review. This remains the only widely accepted mechanism for code collaboration that does not rely on centralized infrastructure maintained by a single entity, which necessarily introduces a single point of dependency and a single point of failure. RFC-2822 formatted messages can be delivered via a variety of means. To name a few of the more common ones: - email - usenet - aggregated archives (e.g. public-inbox) Among these, email remains the most widely used transport mechanism for RFC-2822 messages, most commonly delivered via subscription-based services (mailing lists). Email and end-to-end attestation ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There are two commonly used standards for cryptographic email attestation: PGP and S/MIME. When it comes to patches sent via email, there are significant drawbacks to both: - Mailing list software may modify email body contents to add subscription information footers, causing message attestation to fail. - Attestation via detached MIME signatures may not be preserved by mailing list software that aggressively quarantines attachments. - Inline PGP attestation generally frustrates developers working with patches due to extra surrounding content and the escaping it performs for strings containing dashes at the start of the line for canonicalization purposes. - Only the body of the message is attested, leaving metadata such as "From", "Subject", and "Date" open to tampering. Git uses this metadata to formulate git commits, so leaving them unattested is suboptimal (they can be duplicated into the body of the message, but git format-patch will not do this by default). - PGP key distribution and trust delegation remains a difficult problem to solve. Even if PGP attestation is available, the developer on the receiving end of the patches may not make any use of it due to not having the sender's key in their keyring. - S/MIME certificates are increasingly difficult to obtain for developers not working in corporate environments. At the time of writing, only two commercial CAs continue to provide this service -- and only one does it for free. For these reasons, end-to-end attestation is rarely used in communities that continue to use email as their main conduit for code submissions and review. Email and domain-level attestation ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Since unsolicited emails (SPAM) frequently forge headers in order to appear to be coming from trusted sources, most major service providers have adopted DKIM (RFC-6376) to provide cryptographic attestation for header and body contents. A message that originates from gmail.com will contain a "DKIM-Signature" header that attests the contents of the following headers (among others): - from - date - message-id - subject The "DKIM-Signature" header also includes a hash of the message body (bh=) that is included in the final verification hash. When a DKIM signature is successfully verified using a public key that is published via gmail.com DNS records, this provides a degree of assurance that the email message has not been modified since leaving gmail.com infrastructure. Just as PGP and S/MIME attestation, this has important problems when it comes to patches sent via mailing lists: - If the "sender" header is included in the attestation, the DKIM signature will no longer verify due to mailing lists necessarily rewriting it for bounce handling. - ML software commonly modifies the subject header in order to insert list identification (e.g. ``[some-topic]``). Since the "subject" header is almost always included into the list of headers attested by DKIM, this causes DKIM signatures to fail verification. - ML software also routinely modifies the message body for the purposes of stripping attachments or inserting list subscription metadata. Since the bh= hash is included in the final signature hash, this results in a failed DKIM signature check. Even if all of the above does not apply and the DKIM signature is successfully verified, body canonicalization routines mandated by the DKIM RFC may result in a false-positive successful attestation for patches. The "relaxed" canonicalization instructs that all consecutive whitespace is collapsed, so patches for languages like Python or GNU Make where whitespace is syntactically significant may have different code result in the same hash. DKIM works well enough for end-to-end email attestation, but has important drawbacks for domain-level attestation of patches, especially when they are delivered via mailing lists. Proposal -------- The goal of this document is to propose a scheme that would provide cryptographic attestation for all message contents necessary for trusted distributed code collaboration. It draws on the success of the DKIM standard in order to adapt (and adopt) it for this purpose. Anatomy of an email patch ~~~~~~~~~~~~~~~~~~~~~~~~~ A patch submitted via an RFC-2822 formatted message consists of the following three significant parts: - *metadata*, which includes the Author, Email, Subject, and Date of the submission - *commit message*, which describes what the change is supposed to accomplish - *diff content*, which is structured data that should be applied to the codebase in order to implement the changes proposed Patch submissions also routinely provide additional content that may have significance to the author or to the reviewer, but is not preserved in the codebase after patches are applied, such as: - information describing changes between revisions - statistics about what files are changed (diffstat) - structured data indicating tree dependencies (base-commit) - author's signature and software version info - mailing list subscription metadata Our goal is to provide attestation for the significant parts and ignore the parts that are not preserved after code is committed to a git repository. Three hashes per patch ~~~~~~~~~~~~~~~~~~~~~~ Instead of creating a single attestation hash, we create a separate hash for each meaningful part of the patch submission: - i: patch metadata - m: commit message - p: diff content This allows the person performing verification to identify which part of the submission has been altered since being signed. A change to a commit message may be explained by the addition of a ``Signed-off-by`` (or similar) trailer, so the developer performing the review may ignore a failure in the "m" hash if the other two hashes are passing. Similarly, a patch that goes through a chain of maintainers will necessarily have its commit message modified by the inclusion of various provenance trailers. Having a separate hash for the patch content and patch metadata provides a way to track whether or not any of the submaintainers made changes to the patch code, or just to the commit message, as is generally expected. To generate the three parts, we rely on the ``git mailinfo`` command, that does most of what we need:: git mailinfo m p > i < email.msg The above command will produce three files that closely match what we are looking for, but require a bit of extra processing to remove content that is likely to be altered in SMTP transmission. To get the "m" hash, we take the "m" file as-is:: sha256sum m To get the "i" hash, we remove the "Date" header from the output, because it can be modified by git during format-patch or send-email stages (or, infrequently, by SMTP relays). We only take the "Author", "Email", and "Subject" headers:: egrep '^(Author|Email|Subject)' i | sha256sum The "p" file requires most work, as it contains data from the "below the cut" portion of the commit message (usually, diffstat and revision information), plus trailing content such as signatures or mailing list subscription info. All of this is stripped away to leave just the diff content. Unfortunately, there is no way to do it with git itself, so we use manual parsing of the diff structure to perform this operation. Why not use git patch-id? ~~~~~~~~~~~~~~~~~~~~~~~~~ Git provides a command to generate a "patch-id" that can be used to quickly identify similar patches. To generate the patch-id hash, git performs several canonicalization routines that make this hash unsuitable for attestation purposes: - it collapses all repeating whitespace - it removes all line numbers from diff contents It is possible for a malicious actor to create two patches that generate identical patch-id hashes but have drastically different results when applied to the codebase. For more info, see discussion here: - https://lore.kernel.org/git/20200210164115.x4gciujyjisivfgi@chatter.i7.local/ X-Patch-Hashes header ~~~~~~~~~~~~~~~~~~~~~ After the i, m, p hashes are generated, we insert them into the email message as a separate header. You can use the proof-of-concept code included to generate one yourself:: $ ./main.py hashes-hdr Using emails/unsigned.eml as message source --- HEADER STARTS --- X-Patch-Hashes: v=1; h=sha256; i=pkD5Pg8+cndZAzQQzo3RBSOOUzZM3GYWxiFIKFGIKe0=; m=yW4TvC/DGWCUJTa11Aw1b/2ZAXobsLD45aLA/440yQI=; p=iJdYN6+isP/3HmQaf1IiG7OfA1vzRxXlPGZtvecS484= Running POC code ~~~~~~~~~~~~~~~~ The POC code is written in Python and requires an extra set of libraries in order to work. To get going, please do the following:: $ python3 -mvenv .venv $ source .venv/bin/activate $ pip install --upgrade pip $ pip install -r requirements.txt Domain-level attestation ------------------------ Once the X-Patch-Hashes header is generated and inserted into the email, it will need to be signed in order to be useful for attestation purposes. Adding domain-level signatures during SMTP processing is the simplest way to accomplish this, as it would allow entire companies to automatically attest all patches sent out via their infrastructure. This can be easily done by introducing a patch-attestation milter that would automatically analyze body contents and generate the X-Patch-Hashes header if it finds that the message contains a patch (unless this header is already present). This milter can then either create its own cryptographic signature or let the usual DKIM-signing infrastructure create the necessary attestation. Using vanilla DKIM ~~~~~~~~~~~~~~~~~~ Vanilla DKIM is well-suited for this purpose, as it was specifically created to sign email headers. The following changes will need to be made to the configuration for it to be useful: - add "x-patch-hashes" to the list of signed headers - ensure that "sender" is not included - potentially, exclude "subject" from the list of signed headers, in order to hedge against mailing lists that add ``[topic]`` to all email subjects Here's how it looks with the POC command, using the bundled rsa.key:: $ ./main.py sign-dkim Signing: plain DKIM Using emails/unsigned.eml as message source Using rsa.key to sign --- MESSAGE STARTS --- [...] X-Patch-Hashes: v=1; h=sha256; i=pkD5Pg8+cndZAzQQzo3RBSOOUzZM3GYWxiFIKFGIKe0=; m=yW4TvC/DGWCUJTa11Aw1b/2ZAXobsLD45aLA/440yQI=; p=iJdYN6+isP/3HmQaf1IiG7OfA1vzRxXlPGZtvecS484= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=example.org; i=@example.org; q=dns/txt; s=patches; t=1600264001; h=from : date : x-patch-hashes; bh=g2Sv1ZR+jIrWukzdXbqb+aeiqyFQOBLDQY6z0BBnGg4=; b=pphhMzvqehfxDDLx/OqjbrP6HnMjhlklrQacWqwf5bpZ3cVZ00z5D+BcwpzsKnpQF7c7A 2FmO6Mtjtn/lVRwppIF+tlph46sLE9XfdS+60X6Bzzxu1u/l0uieQ+cIT3DjUuejfxVpvIE Zd4oAeVHD/OWRTJrWGYzrK3e+9UpIZJnxRkJLNj9OKOCwZDiGobM6+NusTWduqjYLRlMXXt EvRbs8QXsTkoTttngM5DwSFRXC7zYSprKxbL6i/DdE+GM+iN2UQk10lpVfhYXtDBoKX1/vX CXb77/X1ug1/ktfYU1xEDUU/NrovqnAfcJHCAL2lHomznoi/IYBC1qfR5t2w== [...] Note, that the b= value will be different for you since the timestamp is included into the hashed content and will be different each time the code runs. This header was created by a generic DKIM implementation (dkimpy), commonly used in production via the popular dkimpy-milter daemon. This POC also includes a few example emails signed by the kernel.org DKIM key. You can run the POC verification yourself:: $ ./main.py -m emails/korg-signed-dkim.eml verify Using emails/korg-signed-dkim.eml as message source Verifying: Plain DKIM DNS-lookup: default._domainkey.kernel.org. PASS : identity and domain match From header PASS : time drift between Date and t (2 days, 23:24:18) PASS : DKIM signature for d=kernel.org, s=default ----- --------------- PASS : metadata PASS : commit message PASS : diff content ----- --------------- PASS : All hashes verified As you can see, the verification steps will check several things: - that the DKIM signature passes verification (this is done as dictated by the RFC -- by normalizing and concatenating all signed headers, plus the DKIM-signature header itself, minus the signature content following b=) - that the x-patch-hashes header is included in the content attested by DKIM - that the domain (d=) and identity (i=) values match what is in the From: field of the email message - that time drift between the Date header and the timestamp of the signature is reasonable - that all patch hashes that we generate match the hashes in the signed header Note, that this check specifically excludes verifying the body hash (bh=) value, for the reasons described in the previous section concerning DKIM drawbacks. Also, since we excluded "subject" from the list of signed headers, the verification will succeed even with usual mailman-induced changes to the email content:: $ ./main.py -m emails/korg-signed-dkim-with-ml-junk.eml verify Using emails/korg-signed-dkim-with-ml-junk.eml as message source Verifying: Plain DKIM DNS-lookup: default._domainkey.kernel.org. PASS : identity and domain match From header PASS : time drift between Date and t (2 days, 23:24:18) PASS : DKIM signature for d=kernel.org, s=default ----- --------------- PASS : metadata PASS : commit message PASS : diff content ----- --------------- PASS : All hashes verified However, since we include the subject of the commit (as git sees it) into the "i" hash, any changes to the subject header that aren't extra prefixes like ``[topic]`` will result in verification failure:: $ ./main.py -m emails/korg-signed-dkim-changed-subject.eml verify Using emails/korg-signed-dkim-changed-subject.eml as message source Verifying: Plain DKIM DNS-lookup: default._domainkey.kernel.org. PASS : identity and domain match From header PASS : time drift between Date and t (2 days, 23:24:18) PASS : DKIM signature for d=kernel.org, s=default ----- --------------- FAIL : metadata PASS : commit message PASS : diff content ----- --------------- FAIL : Some or all hashes failed verification Using the X-Patch-Sig header ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There may be several reasons why you may not want to use DKIM for the purpose of attesting the X-Patch-Hashes header: - you may not have sufficient control over the infrastructure performing DKIM signing, for example if your company uses a commercial upstream relayhost that performs DKIM signing for your domain - you may not want to exclude the "subject" header from your DKIM configuration, as it reduces the overall scope of your email attestation - you may not want to rely on DNS for the purposes of public key lookups, since DNS records are easily spoofed (and DNSSec adoption is still very low) For these reasons, we also introduce a separate "X-Patch-Sig" header that acts as a compatible subset of the DKIM RFC: - we only use the "x-patch-hashes" header, omitting the need for the h= record, and always normalize it as "relaxed" - we omit the bh= field entirely - we omit the v= field, since we will rely on the v= value in the X-Patch-Hashes header for versioning info - we add the m= field to indicate the signature mode (dk, wk, pgp, wkd, discussed below) - for the purposes of the POC, we hardcode the algorithm to ed25519-sha256, though other algorithms like rsa-sha256 or rsa-sha512 can be easily implemented The signature is generated in the exact same way as the DKIM signature, by concatenating the x-patch-hashes header and the x-patch-sig header (after normalizing them using the "relaxed" mode), obviously excluding the content that follows b=. Here's the result of running the POC code, using the bundled dk.key:: $ ./main.py sign-dk Signing: X-Patch-Sig header using dk mode Using emails/unsigned.eml as message source --- MESSAGE STARTS --- [...] X-Patch-Hashes: v=1; h=sha256; i=pkD5Pg8+cndZAzQQzo3RBSOOUzZM3GYWxiFIKFGIKe0=; m=yW4TvC/DGWCUJTa11Aw1b/2ZAXobsLD45aLA/440yQI=; p=iJdYN6+isP/3HmQaf1IiG7OfA1vzRxXlPGZtvecS484= X-Patch-Sig: m=dk; d=example.org; i=@example.org; s=patches; t=1600268242; a=ed25519-sha256; b=Ot3276T9ebQJ5Rzof7TNjz70IVpq9y/4ggevAO9iHVDg3P2tgBesuu2w/6mRIZ6m7mYuy22fNUW 3hmxYCG9VCegq3sEw9y0B7Poj6fvA6ZBcza41HhCNxb5J44UFgnDM [...] DK Mode ~~~~~~~ The DK mode is fully compatible with the DKIM standard and will perform the exact same DNS query to look up the public key for the selector specified:: $ ./main.py -m emails/korg-signed-dk.eml verify Using emails/korg-signed-dk.eml as message source Verifying: X-Patch-Sig (mode=dk) DNS-lookup: patches._domainkey.kernel.org. PASS : identity and domain match From header PASS : time drift between Date and t (4 days, 5:56:18) PASS : mode=dk signature verified for: d=kernel.org, i=@kernel.org, s=patches ----- --------------- PASS : metadata PASS : commit message PASS : diff content ----- --------------- PASS : All hashes verified WK Mode ~~~~~~~ Instead of looking up the public key using DNS, we perform a HTTPS lookup instead. This has the advantages of being more secure, but requires caching, TTL expiration, and proxy configuration by the client, plus is more fragile due to the less distributed nature of the web as opposed to the distributed and fault-tolerant implementation of DNS. The query is performed to the domain name specified in the signature, using the following rule:: https://[domain]/.well-known/_domainkey/[selector].txt The contents of the txt file are the same as the contents of the TXT record. We have it configured for kernel.org and you can perform a verification lookup using the provided example:: $ ./main.py -m emails/korg-signed-wk.eml verify Using emails/korg-signed-wk.eml as message source Verifying: X-Patch-Sig (mode=wk) Retrieving: https://kernel.org/.well-known/_domainkey/patches.txt PASS : identity and domain match From header PASS : time drift between Date and t (4 days, 6:18:45) PASS : mode=wk signature verified for: d=kernel.org, i=@kernel.org, s=patches ----- --------------- PASS : metadata PASS : commit message PASS : diff content ----- --------------- PASS : All hashes verified Developer-level attestation --------------------------- The domain-level attestation has significant advantages, but also important drawbacks: - advantage: it allows auto-enrolling entire companies, without the need for individual developers to make any changes to their usual routines - advantage: it piggybacks on the existing DKIM standard, which has a proven success record - disadvantage: it requires changes to the IT infrastructure, including adding a new milter daemon to the authenticated SMTP relay, which has security and stability implications - disadvantage: it requires explicit trust that the infrastructure performing the hashing and signing has not been compromised by malicious attackers - disadvantage: it allows someone with access to a compromised account to send out patches purporting to be coming from an official employee of the company - disadvantage: it is not useful to unaffiliated developers sending patches from generic email addresses (gmail, yahoo, hotmail, etc). These disadvantages can be mitigated by allowing individual developers to provide their own signatures, using the "pgp" and "wkd" modes of the X-Patch-Sig header. PGP mode ~~~~~~~~ Many open-source projects already provide a mechanism for developers to exchange and use PGP keys for the purposes of code attestation (e.g. via signed git tags and git commits). We can easily use GnuPG to provide the signature content of the X-Patch-Sig header. Here is an example from the bundled emails/mricon-signed-pgp.eml:: X-Patch-Hashes: v=1; h=sha256; i=pkD5Pg8+cndZAzQQzo3RBSOOUzZM3GYWxiFIKFGIKe0=; m=yW4TvC/DGWCUJTa11Aw1b/2ZAXobsLD45aLA/440yQI=; p=iJdYN6+isP/3HmQaf1IiG7OfA1vzRxXlPGZtvecS484= X-Patch-Sig: m=pgp; i=mricon@xxxxxxxxxx; s=0xE63EDCA9329DD07E; b=iHUEABYIAB0WIQR2vl2yUnHhSB5njDW2xBzjVmSZbAUCX1+/nQAKCRC2xBzjVmSZbFiQAQD42c l5It3AVJbtkwbY5XZxb9I9YuvvX3L3buU+EwjumwD9HBH8t6xcavIKQF6dwKjsmhwJnDj1tCfaxg 3WRdUllgM= Since a lot of the attesting information is already embedded into the PGP signature itself, the header structure is different from the "dk" or "wk" mode: - we don't need to know the domain, since we won't be doing any lookups on our own (GnuPG can handle this, if configured) - the selector field identifies the public key ID of the certification subkey, for ease of lookups - the identity field is informational only, but can be used by GnuPG to perform WKD lookups, if it matches the From header (not implemented in the POC) - the timestamp field is missing, since this data is embedded into the PGP signature itself On the verification side, if the key specified by the selector is already present in the verifier's default keyring, we will verify that the signature is GOOD, VALID, and that it is either TRUST_FULLY or TRUST_ULTIMATE. If the key is not present in the verifier's default keyring, the POC will check if there is a matching entry in .keys/openpgp/keys/[keyid].asc, and if so, will use .keys/openpgp/pubring.kbx for performing the verification. In this case, TRUST_* fields are not used, as they will always be "unknown". In-git key distribution is discussed further below. WKD mode (EXPERIMENTAL) ~~~~~~~~~~~~~~~~~~~~~~~ I wanted to provide a way for developers to use a WK-like mode for public key lookups as an alternative to PGP. The signature is generated just like for the domain-level WK mode, using the ed25519 key provided by each individual developer. Here's the POC running with the bundled "ingit.key":: $ ./main.py sign-wkd Signing: X-Patch-Sig header using wkd mode Using emails/unsigned.eml as message source --- MESSAGE STARTS --- [...] X-Patch-Hashes: v=1; h=sha256; i=pkD5Pg8+cndZAzQQzo3RBSOOUzZM3GYWxiFIKFGIKe0=; m=yW4TvC/DGWCUJTa11Aw1b/2ZAXobsLD45aLA/440yQI=; p=iJdYN6+isP/3HmQaf1IiG7OfA1vzRxXlPGZtvecS484= X-Patch-Sig: m=wkd; d=example.org; i=dev@xxxxxxxxxx; s=patches; t=1600270651; a=ed25519-sha256; b=/s2WOrzK2tmqCYj3x22uck6Yi6V1ODX+PZiE2TLstSoVDGvTAaYoPZwmO7IKbUC148KEeGVXB0W g+wGNtQn3AmUsvnoX0Jppqc5ei6GDzr0yMQKzEbUt0DkPrd/Y000b [...] It is very similar to content created in the "dk" or "wk" mode, except the identity field includes the entire email address of the developer. When we verify the attestation, we will do the following: - check if that key is available in .keys/devkey/[domain]/[local]/[selector].txt - if it is not present, we perform a https query to https://[domain]/.well-known/devkey/[zbase32-encoded-hash-of-local]/[selector].txt The hashing and zbase32-encoding is taken to be compatible with openpgp's WKD implementation and is done to prevent someone from easily finding out everyone's email addresses from unprotected directory listings. You can run the verification using the POC example. Here's the run without using the in-git matching key:: $ ./main.py -m emails/mricon-signed-wkd.eml verify Using emails/mricon-signed-wkd.eml as message source Verifying: X-Patch-Sig (mode=wkd) Retrieving: https://kernel.org/.well-known/devkey/sapsizz4qsj4zmmscbz9f7y8cunt496y/patches.txt PASS : identity and domain match From header PASS : time drift between Date and t (4 days, 6:58:47) PASS : mode=wkd signature verified for: d=kernel.org, i=mricon@xxxxxxxxxx, s=patches ----- --------------- PASS : metadata PASS : commit message PASS : diff content ----- --------------- PASS : All hashes verified Here is the same, but using the public key provided in the git repository itself:: $ ./main.py -m emails/dev-signed-wkd-ingit.eml verify Using emails/dev-signed-wkd-ingit.eml as message source Verifying: X-Patch-Sig (mode=wkd) Loading: WKD key from /var/home/user/work/git/patch-attestation-poc/.keys/devkey/kernel.org/dev/patches.txt PASS : identity and domain match From header PASS : time drift between Date and t (4 days, 7:28:47) PASS : mode=wkd signature verified for: d=kernel.org, i=dev@xxxxxxxxxx, s=patches ----- --------------- PASS : metadata PASS : commit message PASS : diff content ----- --------------- PASS : All hashes verified The structure and nature of the WKD mechanism is entirely up for discussion (along with everything else in this proposal). Automating developer attestation -------------------------------- The easiest way to automate developer attestation is by providing a sendmail-compatible "attest-and-send" utility that can be a drop-in command settable via git's sendemail.smtpServer config setting. It would be automatically invoked whenever git-send-email runs and would inject the X-Patch-Hashes and X-Patch-Sig headers before sending the emails to the SMTP server specified via the rest of the sendemail configuration options. In addition to creating these headers, this tool can also automatically add all emails going through it to the developer's personal public-inbox archive that can act as a separate source of patch data in addition to mail delivered via SMTP and mailing lists. Public keys bundled with git repos ---------------------------------- Delegated trust is hard and securely bootstrapping your trusted identities is even harder. There are existing proposals to include developer keys as part of the git repository itself in order to make it possible for someone to quickly bootstrap their keyring with trusted identities. Obviously, this introduces a chicken-and-egg problem of getting your source of trust from the thing you're trying to attest in the first place. However, no mechanism short of in-person meetings is able to provide perfect levels of assurance, so in-git key distribution remains as good a source of bootstrap trust as any. The implementation in this POC is naive and shouldn't be used for serious purposes. An emerging proposal like did:git (https://github.com/dhuseby/did-git-spec/blob/master/did-git-spec.md) is a more thoroughly considered approach and should probably be preferred. Where should verification be performed -------------------------------------- Signature verification should be performed by the maintainer evaluating the patches they received for inclusion into the git repository. The POC already pulls in "b4" as a dependency for the patch hashing routines, and I intend to add the header-based verification mechanisms in the future release of b4, once this proposal is thoroughly discussed. Similarly, browser and other email client plugins can be written to indicate to the developer whether the patches they are viewing pass signature verification. If this proposal is adopted, we can come up with implementations for Gmail, Mutt and Emacs, which should cover a significant number of end-user tools.
