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Do not trust a source-code mirror because it uses HTTPS, has the expected repository name, or displays a matching checksum. To verify a mirror, identify the project’s canonical source, pin the exact commit or release, compare the mirror’s bytes with a cryptographically strong digest, and authenticate that digest through a project-controlled channel. Then validate signatures, build provenance, transparency-log evidence, and—when practical—an independent reproducible build.
The defensible result is precise: you can show that a mirror matches a specified authoritative revision and satisfies documented authenticity and provenance checks. You cannot prove that the upstream project, signing account, or build environment was never compromised.
What a trustworthy mirror check must establish
“The files are identical” and “the legitimate project published these files” are different claims. A complete verification separates them into four questions:
- Integrity: Are the bytes in the mirror the same as the bytes identified by the trusted project revision or digest?
- Authenticity: Did an identity authorized by the project sign or publish that revision?
- Traceability: Can a built package or binary be linked to the exact source revision, builder, build type, and parameters?
- Auditability: Is there evidence that another reviewer can inspect later, such as a signed object, public transparency-log entry, or retained verification record?
Each control answers only part of the problem. A SHA-256 comparison detects changed bytes but does not authenticate the page that displayed the SHA-256 value. A valid signature proves that a key signed an object, but not that the key belongs to the project unless you have a trusted identity mapping. Provenance connects an artifact to a build process, but it is useful only when your policy checks the fields that matter.
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Step 1: Identify the canonical source and exact revision
Start by writing down the project’s authoritative repository URI. Treat a search result, package index, mirror landing page, or README link as a lead rather than proof. Confirm the canonical location through a project-controlled channel such as its official documentation, release announcement, or established developer communication.
Record an immutable reference
Branches and moving tags can change. Record the full commit ID for the source you intend to verify. If the project distributes a release archive, record the release name, archive filename, and the project-published digest. For a Git tag, record whether it is annotated and signed, and preserve the tag object’s identity rather than relying only on the tag text.
- Canonical repository URI
- Expected branch or release name
- Full expected commit ID
- Expected tag object or archive name
- Digest and algorithm, if the project publishes one
- Date and time at which the reference was obtained
CISA guidance for software and dependency inputs recommends immutable references and verification of cryptographic hashes when they are supplied. If no digest or signature is available, the fallback is a transport-integrity channel such as TLS, but that provides weaker assurance than an authenticated project signature.
Check both repositories before comparing files
Clone the canonical repository and the mirror into separate, clean directories. Fetch the same reference from each and resolve it to a commit ID. A mirror that points its main branch at the same commit as upstream has strong evidence of repository-level equality for that commit, but you should still check for missing refs, altered submodules, generated files, and release metadata.
git clone --mirror <canonical-repository-uri> upstream.git
git clone --mirror <mirror-uri> mirror.git
git -C upstream.git rev-parse <expected-ref>
git -C mirror.git rev-parse <expected-ref>Replace <expected-ref> with the immutable commit ID or a fully qualified reference you have documented. Do not treat a matching short hash as sufficient; use the complete object ID and verify that both repositories resolve the intended object type and revision.
Step 2: Compare the mirrored source bytes
A cryptographic digest is a comparison mechanism, not an authority. Generate the digest yourself from the exact source representation you intend to use and compare it with a digest obtained through an authenticated project channel.
Choose what to hash
For a Git project, decide whether the security decision concerns the commit object, the checked-out tree, or a release archive. These are related but not interchangeable:
- Commit and tree objects capture Git’s content-addressed history and metadata.
- A checked-out tree represents the files a build or review will consume, including decisions about submodules, line endings, executable bits, and ignored files.
- A release archive is the package actually distributed to users, and may contain generated files or a project-specific directory prefix.
Document the representation before hashing. Otherwise, two reviewers can calculate different digests from the same revision because one included submodules or generated files and the other did not.
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Compare repository history and trees
First compare the expected commit IDs. Then inspect the tree for differences, including submodule pointers and files outside the main branch. A mirror can preserve the visible tip while dropping tags, rewriting history, or changing a release asset.
git -C upstream.git show-ref > upstream-refs.txt
git -C mirror.git show-ref > mirror-refs.txt
git diff --no-index upstream-refs.txt mirror-refs.txtThe reference comparison is evidence about advertised refs, not proof that every object is present. For the pinned commit, export the tree in a deterministic form and hash that output. Keep the exact export procedure, Git version, archive format, and digest algorithm in your record so another reviewer can repeat it.
git -C upstream.git archive --format=tar <expected-commit> > upstream.tar
git -C mirror.git archive --format=tar <expected-commit> > mirror.tar
sha256sum upstream.tar mirror.tar
cmp upstream.tar mirror.tarUse the same archive format and commit on both sides. If the project publishes a release archive, hash that archive directly rather than assuming a newly generated archive will be byte-for-byte identical. Archive metadata, directory prefixes, compression settings, and generated files can change the digest without changing the source tree.
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Authenticate the digest
The strongest comparison is between your locally computed digest and a value delivered through a channel controlled by the project. A checksum copied from an unauthenticated mirror page can be altered along with the files. HTTPS protects the connection to a particular server; it does not establish that the server is the legitimate publisher or that its content was not replaced before delivery.
Look for a signed release statement, a signed tag, a signed archive, or a project-controlled announcement that identifies both the revision and digest. Record the URL or channel, retrieval date, digest algorithm, and the identity that authenticated the statement. If the project offers only an unsigned checksum, treat it as useful for detecting accidental corruption, not as proof of publisher authenticity.
Step 3: Verify signed tags, commits, and archives
Signatures add authorship or release evidence to the byte comparison. They do not remove the need to identify the expected signer.
Signed Git tags
For a signed tag, verify the tag object and inspect the signer identity. A mathematically valid signature is insufficient if it was made by an unexpected key, an expired project role, or an account that has no documented authority to publish the release.
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git tag -v <release-tag>Check that the signature covers the expected commit, the tag name matches the release you selected, and the key fingerprint matches a trusted project identity obtained independently of the mirror. Preserve the verification output and the key fingerprint. Do not silently accept a new signing key; require a documented key transition or an additional trusted confirmation.
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Signed commits and archives
Projects may sign individual commits or detached release archives instead of tags. Verify the signature against the exact object or file you will use, then apply the same identity test. A signature on a parent commit does not automatically authenticate every later descendant, and a signed source archive does not necessarily authenticate a separately generated binary.
Signing protects the object named by the signature. It does not prove that the project’s infrastructure was uncompromised before signing, that the reviewed commit was intended, or that a binary was built from that source. Those are separate provenance and policy questions.
Step 4: Verify Sigstore identity and transparency evidence
When a project uses Sigstore, verification combines an artifact signature with an identity-bearing certificate and a public transparency record. Sigstore documentation describes signing events as being recorded in a tamper-resistant public log.
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- The artifact signature is valid for the exact file or digest.
- The signing certificate chains to the expected certificate authority.
- The certificate identity—such as the project workflow, repository, or authorized email identity—matches your policy.
- The certificate was valid for the signing event and has not been rejected by your trust policy.
- The transparency-log inclusion proof is present and corresponds to the signature.
Identity is the key policy decision. A valid certificate for an unrelated repository or personal account does not authenticate the mirror you are reviewing. Write down the expected repository, workflow or identity pattern, and any allowed signing time or environment before accepting the result.
Step 5: Verify build provenance for binaries and packages
Matching source code is not the same as proving that a binary came from that source. If the mirror is used to build a package, inspect the signed provenance attached to the artifact.
Fields to compare in SLSA provenance
SLSA verification guidance requires more than checking that provenance exists. Compare the statement with expected values for:
- Canonical source repository: It must identify the authoritative repository, not merely the mirror that supplied your checkout.
- Exact source revision: The digest or commit must be the immutable revision you approved.
- Builder identity: Confirm that the builder is an approved service or environment.
- Build type: Ensure the process is the expected workflow or build definition.
- External parameters: Check options, targets, dependencies, and other inputs that can change the output.
- Provenance signature: Verify the signer and trust root before relying on any field.
SLSA’s own guidance emphasizes that “provenance doesn’t do anything unless somebody inspects it.” A signed statement that names the wrong source repository or an unapproved builder is not a successful verification.
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Verify that the project’s layout defines meaningful steps, authorized functionaries, expected materials, and required order. An attestation is only as useful as that policy: a workflow that omits the packaging step cannot prove that packaging preserved the reviewed source.
Step 6: Add an independent reproducible build
A reproducible build gives you an independent computation: two builders use the same approved source and inputs and produce bit-for-bit identical output. This strengthens confidence that the artifact corresponds to the documented process, especially when builders are operated by different teams.
What reproducibility can and cannot prove
- It can show that independent builders produced identical bytes from the declared inputs.
- It can expose hidden timestamps, nondeterministic ordering, undeclared network access, or environment-dependent output.
- It does not authenticate the source revision by itself.
- It does not prove that the reproduced revision was the project’s intended revision.
- It does not replace signed provenance, signer identity, or a policy for trusted builders.
Use reproducibility as an additional layer after authenticating the source and provenance. Retain the build instructions, toolchain versions, dependency lock data, environment details, and resulting digest. If independent outputs differ, treat that as a failed assurance check until you explain the difference; do not select the output that merely looks more plausible.
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| Control | Integrity | Authenticity | Traceability | Auditability | Operational cost | What can still go wrong |
|---|---|---|---|---|---|---|
| Cryptographic hash | Detects changed bytes when compared with a trusted digest | Does not identify who published the digest | Only the representation and digest are linked | Depends on how the digest was retained and delivered | Low; requires a consistent hashing procedure | A compromised or unauthenticated publisher can provide a matching false digest |
| Signed tag, commit, or archive | Authenticates the signed object | Links the object to a signing identity | Usually identifies a revision or release object | Signature and key history can be reviewed | Key management and identity mapping are required | An authorized key or account may be compromised, or the signer may not be trusted for this release |
| Sigstore | Authenticates the signed artifact digest | Adds certificate identity and a trust chain | Can bind the signature to a repository or workflow identity | Provides public transparency-log evidence | Requires certificate, policy, and log verification | A valid identity may still be outside your approved policy |
| SLSA provenance | Describes the artifact and declared build inputs | Depends on a trusted provenance signer and builder | Links artifact, source, builder, build type, and parameters | Creates a reviewable signed record | Requires policy for expected fields and trust roots | Unchecked or incorrect expectations can make a valid statement misleading |
| in-toto attestations | Protects material handoffs between defined steps | Identifies authorized actors for those steps | Models the intended order and materials | Provides step-by-step attestations | Requires a meaningful layout and maintained keys | Missing or weak steps leave gaps in the claimed supply chain |
| Reproducible builds | Shows independent outputs are identical | Does not authenticate source or builders by itself | Relates outputs to declared inputs and process | Independent rebuilders provide corroboration | Can require specialized build environments and maintenance | An unintended source revision can be reproduced perfectly |
Common false assurances and how to correct them
“The mirror uses HTTPS, so it is safe.”
HTTPS protects data in transit between you and the mirror’s server. It does not prove that the server is the canonical project, that its administrator did not replace files, or that the upstream project approved the revision. Use HTTPS as transport protection, then add immutable references and authenticated project evidence.
“The repository name and branch look right.”
Names are easy to copy and branches move. Compare the full commit ID, inspect refs and submodules, and authenticate the revision through a signature or project-controlled digest.
“The SHA-256 values match.”
A matching digest proves that the compared bytes are identical to each other. It does not prove that either value came from the legitimate project. Obtain the expected digest through a signed or otherwise trusted channel.
“The signature is valid.”
Validity is a cryptographic result, not a trust decision. Confirm the signer fingerprint or certificate identity against an independently established project authority and check that the signature covers the exact object you approved.
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Provenance may prove how a binary was built without proving that the mirror supplied the intended source. Compare the provenance’s canonical repository and exact revision with your source verification record.
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“The build is reproducible, so the source is authentic.”
Reproducibility demonstrates consistent recomputation. It cannot distinguish an intended revision from a compromised or mistakenly selected revision. Authenticate the source first.
A practical verification record
Store an evidence bundle that another reviewer can rerun without relying on your memory:
- Canonical repository URI and the channel used to establish it.
- Expected release, tag, or full commit ID.
- Mirror URI, retrieval date, and mirror-resolved commit ID.
- Exact tree or archive representation that was hashed.
- Digest algorithm, computed digest, and the project-published digest.
- Reference comparison output, including tags, branches, and submodule pointers relevant to the build.
- Signature output, signer fingerprint or certificate identity, and the trust source for that identity.
- Sigstore certificate and transparency-log evidence, when applicable.
- SLSA or in-toto provenance record, expected values, and verification result.
- Reproducible-build inputs, builder identities, toolchain versions, and output digests, when performed.
- Verification-tool versions, operator, date, and any exceptions or unresolved differences.
Keep the record with the reviewed source or artifact. A future reviewer should be able to tell exactly what was trusted, what was measured, and which assumptions remain.
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When to stop and reject the mirror
Reject or quarantine the mirror when any of these conditions holds:
- The canonical source or expected revision cannot be established.
- The mirror resolves to a different commit, tree, archive digest, or submodule state.
- The digest matches only an unauthenticated page or mirror-provided statement.
- The signature is valid but the signer identity is not approved or cannot be mapped to project authority.
- Provenance names a different source repository, revision, builder, build type, or required parameter.
- Transparency-log evidence is missing or does not correspond to the signature.
- Independent builds differ and the cause is unexplained.
Do not “fix” a discrepancy by checking another mirror until one happens to agree. Preserve the conflicting evidence, contact the project through an established channel, and verify the corrected revision from the beginning.
The assurance claim you can safely make
After completing the checks, state the result narrowly: “This mirror matches commit R from the canonical repository, its tree or archive matches digest D, the digest or release is authenticated by identity I, and the documented provenance and reproducibility checks passed under policy P on date T.”
Avoid claiming that the mirror is absolutely safe. An upstream repository, authorized signing account, build worker, or dependency can be compromised while still producing internally consistent hashes, signatures, provenance, and reproducible outputs. Layered verification reduces that risk by making each assumption explicit and independently reviewable.
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