diff --git a/.github/dependabot.yml b/.github/dependabot.yml
new file mode 100644
index 0000000..9cdbb90
--- /dev/null
+++ b/.github/dependabot.yml
@@ -0,0 +1,24 @@
+version: 2
+
+updates:
+  - package-ecosystem: pip
+    directory: "/install"
+    schedule:
+      interval: daily
+    open-pull-requests-limit: 99
+    allow:
+    - dependency-type: direct
+    - dependency-type: indirect
+    rebase-strategy: "disabled"
+
+  - package-ecosystem: pip
+    directory: /
+    schedule:
+      interval: daily
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: daily
+    open-pull-requests-limit: 99
+    rebase-strategy: "disabled"
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..f2ea9c0
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,57 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  schedule:
+    - cron: '0 12 * * *'
+
+jobs:
+  test:
+    permissions:
+      # Needed to access the workflow's OIDC identity.
+      id-token: write
+    strategy:
+      matrix:
+        conf:
+          - { py: "3.7", os: "ubuntu-latest" }
+          - { py: "3.8", os: "ubuntu-latest" }
+          - { py: "3.9", os: "ubuntu-latest" }
+          - { py: "3.10", os: "ubuntu-latest" }
+          - { py: "3.11", os: "ubuntu-latest" }
+          # NOTE: We only test Windows and macOS on the latest Python;
+          # these primarily exist to ensure that we don't accidentally
+          # introduce Linux-isms into the development tooling.
+          - { py: "3.11", os: "windows-latest" }
+          - { py: "3.11", os: "macos-latest" }
+    runs-on: ${{ matrix.conf.os }}
+    steps:
+      - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3.3.0
+
+      - uses: actions/setup-python@d27e3f3d7c64b4bbf8e4abfb9b63b83e846e0435
+        with:
+          python-version: ${{ matrix.conf.py }}
+          cache: "pip"
+          cache-dependency-path: pyproject.toml
+
+      - name: deps
+        run: make dev ID_EXTRA=test
+
+      - name: test
+        run: make test TEST_ARGS="-vv --showlocals"
+
+  all-tests-pass:
+    if: always()
+
+    needs:
+    - test
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: check test jobs
+        uses: re-actors/alls-green@05ac9388f0aebcb5727afa17fcccfecd6f8ec5fe # v1.2.2
+        with:
+          jobs: ${{ toJSON(needs) }}
diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
new file mode 100644
index 0000000..ffc3b3f
--- /dev/null
+++ b/.github/workflows/lint.yml
@@ -0,0 +1,74 @@
+name: Lint
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3.3.0
+
+      # NOTE: We intentionally lint against our minimum supported Python.
+      - uses: actions/setup-python@d27e3f3d7c64b4bbf8e4abfb9b63b83e846e0435
+        with:
+          python-version: "3.7"
+          cache: "pip"
+          cache-dependency-path: pyproject.toml
+
+      - name: deps
+        run: make dev ID_EXTRA=lint
+
+      - name: lint
+        run: make lint
+
+  check-readme:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3.3.0
+
+      # NOTE: We intentionally check `--help` rendering against our minimum Python,
+      # since it changes slightly between Python versions.
+      - uses: actions/setup-python@d27e3f3d7c64b4bbf8e4abfb9b63b83e846e0435
+        with:
+          python-version: "3.7"
+          cache: "pip"
+          cache-dependency-path: pyproject.toml
+
+      - name: deps
+        run: make dev
+
+      - name: check-readme
+        run: make check-readme
+
+  licenses:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3.3.0
+      # adapted from Warehouse's bin/licenses
+      - run: |
+          for fn in $(find . -type f -name "*.py"); do
+            if [[ ! "$(head -5 $fn | grep "^ *\(#\|\*\|\/\/\) .* License\(d*\)")" ]]; then
+              echo "${fn} is missing a license"
+              exit 1
+            fi
+          done
+
+  all-lints-pass:
+    if: always()
+
+    needs:
+    - lint
+    - check-readme
+    - licenses
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: check lint jobs
+        uses: re-actors/alls-green@05ac9388f0aebcb5727afa17fcccfecd6f8ec5fe # v1.2.2
+        with:
+          jobs: ${{ toJSON(needs) }}
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
new file mode 100644
index 0000000..a1254c9
--- /dev/null
+++ b/.github/workflows/release.yml
@@ -0,0 +1,125 @@
+name: Release
+
+on:
+  release:
+    types:
+      - published
+
+permissions:  # added using https://github.com/step-security/secure-workflows
+  contents: read
+
+jobs:
+  build:
+    name: Build and sign artifacts
+    runs-on: ubuntu-latest
+    permissions:
+      id-token: write
+    outputs:
+      hashes: ${{ steps.hash.outputs.hashes }}
+    steps:
+      - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3.3.0
+
+      - uses: actions/setup-python@d27e3f3d7c64b4bbf8e4abfb9b63b83e846e0435
+        with:
+          python-version: "3.x"
+          cache: "pip"
+          cache-dependency-path: pyproject.toml
+
+      - name: deps
+        run: python -m pip install -U build sigstore
+
+      - name: build
+        run: python -m build
+
+      - name: sign
+        run: |
+          mkdir -p signing-artifacts
+
+          # Sign using the ambient OIDC identity
+          for dist in dist/*; do
+            dist_base="$(basename "${dist}")"
+
+            # NOTE: signing artifacts currently go in a separate directory,
+            # to avoid confusing the package uploader (which otherwise tries
+            # to upload them to PyPI and fails). Future versions of twine
+            # and the gh-action-pypi-publish action should support these artifacts.
+            python -m sigstore sign "${dist}" \
+              --output-signature signing-artifacts/"${dist_base}.sig" \
+              --output-certificate signing-artifacts/"${dist_base}.crt"
+
+          done
+
+      - name: Generate hashes for provenance
+        shell: bash
+        id: hash
+        run: |
+          # sha256sum generates sha256 hash for all artifacts.
+          # base64 -w0 encodes to base64 and outputs on a single line.
+          # sha256sum artifact1 artifact2 ... | base64 -w0
+          echo "hashes=$(sha256sum ./dist/* | base64 -w0)" >> $GITHUB_OUTPUT
+
+      - name: Upload built packages
+        uses: actions/upload-artifact@0b7f8abb1508181956e8e162db84b466c27e18ce # v3.1.2
+        with:
+          name: built-packages
+          path: ./dist/
+          if-no-files-found: warn
+
+      - name: Upload signing-artifacts
+        uses: actions/upload-artifact@0b7f8abb1508181956e8e162db84b466c27e18ce # v3.1.2
+        with:
+          name: signing-artifacts
+          path: signing-artifacts/
+          if-no-files-found: warn
+
+  generate-provenance:
+    needs: [build]
+    name: Generate build provenance
+    permissions:
+      actions: read   # To read the workflow path.
+      id-token: write # To sign the provenance.
+      contents: write # To add assets to a release.
+    # Currently this action needs to be referred by tag. More details at:
+    # https://github.com/slsa-framework/slsa-github-generator#verification-of-provenance
+    uses: slsa-framework/slsa-github-generator/.github/workflows/generator_generic_slsa3.yml@v1.2.1
+    with:
+      attestation-name: provenance-sigstore-${{ github.event.release.tag_name }}.intoto.jsonl
+      base64-subjects: "${{ needs.build.outputs.hashes }}"
+      compile-generator: true # Workaround for https://github.com/slsa-framework/slsa-github-generator/issues/1163
+      upload-assets: true
+
+  release-pypi:
+    needs: [build, generate-provenance]
+    runs-on: ubuntu-latest
+    permissions: {}
+    steps:
+      - name: Download artifacts directories # goes to current working directory
+        uses: actions/download-artifact@9bc31d5ccc31df68ecc42ccf4149144866c47d8a # v3.0.2
+
+      - name: publish
+        uses: pypa/gh-action-pypi-publish@c7f29f7adef1a245bd91520e94867e5c6eedddcc
+        with:
+          user: __token__
+          password: ${{ secrets.PYPI_TOKEN }}
+          packages_dir: built-packages/
+
+  release-github:
+    needs: [build, generate-provenance]
+    runs-on: ubuntu-latest
+    permissions:
+      # Needed to upload release assets.
+      contents: write
+    steps:
+      - name: Download artifacts directories # goes to current working directory
+        uses: actions/download-artifact@9bc31d5ccc31df68ecc42ccf4149144866c47d8a # v3.0.2
+
+      - name: Upload artifacts to GitHub
+        # Confusingly, this action also supports updating releases, not
+        # just creating them. This is what we want here, since we've manually
+        # created the release that triggered the action.
+        uses: softprops/action-gh-release@de2c0eb89ae2a093876385947365aca7b0e5f844 # v1
+        with:
+          # signing-artifacts/ contains the signatures and certificates.
+          files: |
+            built-packages/*
+            signing-artifacts/*
diff --git a/.github/workflows/scorecards-analysis.yml b/.github/workflows/scorecards-analysis.yml
new file mode 100644
index 0000000..235ce03
--- /dev/null
+++ b/.github/workflows/scorecards-analysis.yml
@@ -0,0 +1,57 @@
+name: Scorecards supply-chain security
+on:
+  # Only the default branch is supported.
+  workflow_dispatch: # Manual
+  branch_protection_rule:
+  schedule:
+    - cron: '30 4 * * 0'
+  push:
+    branches: [ main ]
+
+# Declare default permissions as read only.
+permissions: read-all
+
+jobs:
+  analysis:
+    name: Scorecards analysis
+    runs-on: ubuntu-latest
+    permissions:
+      # Needed to upload the results to code-scanning dashboard.
+      security-events: write
+      actions: read
+      contents: read
+      # Needed to access GitHub's OIDC token which ensures the uploaded results integrity.
+      id-token: write
+    steps:
+      - name: "Checkout code"
+        uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3.3.0
+        with:
+          persist-credentials: false
+
+      - name: "Run analysis"
+        uses: ossf/scorecard-action@e38b1902ae4f44df626f11ba0734b14fb91f8f86 # v2.1.2
+        with:
+          results_file: results.sarif
+          results_format: sarif
+          # Read-only PAT token. To create it,
+          # follow the steps in https://github.com/ossf/scorecard-action#pat-token-creation.
+          repo_token: ${{ secrets.SCORECARD_TOKEN }}
+          # Publish the results to enable scorecard badges. For more details, see
+          # https://github.com/ossf/scorecard-action#publishing-results.
+          # For private repositories, `publish_results` will automatically be set to `false`,
+          # regardless of the value entered here.
+          publish_results: true
+
+      # Upload the results as artifacts (optional).
+      - name: "Upload artifact"
+        uses: actions/upload-artifact@6673cd052c4cd6fcf4b4e6e60ea986c889389535 # v2.3.1
+        with:
+          name: SARIF file
+          path: results.sarif
+          retention-days: 5
+
+      # Upload the results to GitHub's code scanning dashboard.
+      - name: "Upload to code-scanning"
+        uses: github/codeql-action/upload-sarif@515828d97454b8354517688ddc5b48402b723750 # v1.0.26
+        with:
+          sarif_file: results.sarif
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..dbed0b0
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,9 @@
+env/
+pip-wheel-metadata/
+*.egg-info/
+__pycache__/
+.coverage*
+html/
+dist/
+.python-version
+build
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..efad111
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,12 @@
+# Changelog
+
+All notable changes to `id` will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+
+## [Unreleased]
+
+* Initial split from https://github.com/sigstore/sigstore-python
+
+<!--Release URLs -->
+[Unreleased]: https://github.com/di/sigstore-python/compare/v1.0.0...HEAD
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..e1775ee
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,136 @@
+Contributing to id
+==================
+
+Thank you for your interest in contributing to `id`!
+
+The information below will help you set up a local development environment,
+as well as performing common development tasks.
+
+## Requirements
+
+`id`'s only development environment requirement *should* be Python 3.7
+or newer. Development and testing is actively performed on macOS and Linux,
+but Windows and other supported platforms that are supported by Python
+should also work.
+
+If you're on a system that has GNU Make, you can use the convenience targets
+included in the `Makefile` that comes in the `id` repository detailed
+below. But this isn't required; all steps can be done without Make.
+
+## Development steps
+
+First, clone this repository:
+
+```bash
+git clone https://github.com/di/id
+cd id
+```
+
+Then, use one of the `Makefile` targets to run a task. The first time this is
+run, this will also set up the local development virtual environment, and will
+install `id` as an editable package into this environment.
+
+Any changes you make to the `id` source tree will take effect
+immediately in the virtual environment.
+
+### Linting
+
+You can lint locally with:
+
+```bash
+make lint
+```
+
+`id` is automatically linted and formatted with a collection of tools:
+
+* [`black`](https://github.com/psf/black): Code formatting
+* [`isort`](https://github.com/PyCQA/isort): Import sorting, ordering
+* [`ruff`](https://github.com/charliermarsh/ruff): PEP-8 linting, style enforcement
+* [`mypy`](https://mypy.readthedocs.io/en/stable/): Static type checking
+* [`bandit`](https://github.com/PyCQA/bandit): Security issue scanning
+* [`interrogate`](https://interrogate.readthedocs.io/en/latest/): Documentation coverage
+
+
+To automatically apply any lint-suggested changes, you can run:
+
+```bash
+make reformat
+```
+
+### Testing
+
+You can run the tests locally with:
+
+```bash
+make test
+```
+
+You can also filter by a pattern (uses `pytest -k`):
+
+```bash
+make test TESTS=test_version
+```
+
+To test a specific file:
+
+```bash
+make test T=path/to/file.py
+```
+
+`id` has a [`pytest`](https://docs.pytest.org/)-based unit test suite,
+including code coverage with [`coverage.py`](https://coverage.readthedocs.io/).
+
+### Releasing
+
+**NOTE**: If you're a non-maintaining contributor, you don't need the steps
+here! They're documented for completeness and for onboarding future maintainers.
+
+Releases of `id` are managed with [`bump`](https://github.com/di/bump)
+and GitHub Actions.
+
+```bash
+# default release (patch bump)
+make release
+
+# override the default
+# vX.Y.Z -> vX.Y.Z-rc.0
+make release BUMP_ARGS="--pre rc.0"
+
+# vX.Y.Z -> vN.0.0
+make release BUMP_ARGS="--major"
+```
+
+`make release` will fail if there are any untracked changes in the source tree.
+
+If `make release` succeeds, you'll see an output like this:
+
+```
+RUN ME MANUALLY: git push origin main && git push origin vX.Y.Z
+```
+
+Run that last command sequence to complete the release.
+
+## Development practices
+
+Here are some guidelines to follow if you're working on a new feature or changes to
+`id`'s internal APIs:
+
+* *Keep the `id` APIs as private as possible*. Nearly all of `id`'s
+APIs should be private and treated as unstable and unsuitable for public use.
+If you're adding a new module to the source tree, prefix the filename with an
+underscore to emphasize that it's an internal (e.g., `id/_foo.py` instead of
+`id/foo.py`).
+
+* *Perform judicious debug logging.* `id` uses the standard Python
+[`logging`](https://docs.python.org/3/library/logging.html) module. Use
+`logger.debug` early and often -- users who experience errors can submit better
+bug reports when their debug logs include helpful context!
+
+* *Update the [CHANGELOG](./CHANGELOG.md)*. If your changes are public or result
+in changes to `id`'s CLI, please record them under the "Unreleased" section,
+with an entry in an appropriate subsection ("Added", "Changed", "Removed", or "Fixed").
+
+* Ensure your commits are signed off, as `id` uses the
+[DCO](https://en.wikipedia.org/wiki/Developer_Certificate_of_Origin).
+You can do it using `git commit -s`, or `git commit -s --amend` if you want to
+amend already existing commits.
diff --git a/COPYRIGHT.txt b/COPYRIGHT.txt
new file mode 100644
index 0000000..5f2c003
--- /dev/null
+++ b/COPYRIGHT.txt
@@ -0,0 +1,13 @@
+Copyright 2022 The Sigstore Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..d645695
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,202 @@
+
+                                 Apache License
+                           Version 2.0, January 2004
+                        http://www.apache.org/licenses/
+
+   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+   1. Definitions.
+
+      "License" shall mean the terms and conditions for use, reproduction,
+      and distribution as defined by Sections 1 through 9 of this document.
+
+      "Licensor" shall mean the copyright owner or entity authorized by
+      the copyright owner that is granting the License.
+
+      "Legal Entity" shall mean the union of the acting entity and all
+      other entities that control, are controlled by, or are under common
+      control with that entity. For the purposes of this definition,
+      "control" means (i) the power, direct or indirect, to cause the
+      direction or management of such entity, whether by contract or
+      otherwise, or (ii) ownership of fifty percent (50%) or more of the
+      outstanding shares, or (iii) beneficial ownership of such entity.
+
+      "You" (or "Your") shall mean an individual or Legal Entity
+      exercising permissions granted by this License.
+
+      "Source" form shall mean the preferred form for making modifications,
+      including but not limited to software source code, documentation
+      source, and configuration files.
+
+      "Object" form shall mean any form resulting from mechanical
+      transformation or translation of a Source form, including but
+      not limited to compiled object code, generated documentation,
+      and conversions to other media types.
+
+      "Work" shall mean the work of authorship, whether in Source or
+      Object form, made available under the License, as indicated by a
+      copyright notice that is included in or attached to the work
+      (an example is provided in the Appendix below).
+
+      "Derivative Works" shall mean any work, whether in Source or Object
+      form, that is based on (or derived from) the Work and for which the
+      editorial revisions, annotations, elaborations, or other modifications
+      represent, as a whole, an original work of authorship. For the purposes
+      of this License, Derivative Works shall not include works that remain
+      separable from, or merely link (or bind by name) to the interfaces of,
+      the Work and Derivative Works thereof.
+
+      "Contribution" shall mean any work of authorship, including
+      the original version of the Work and any modifications or additions
+      to that Work or Derivative Works thereof, that is intentionally
+      submitted to Licensor for inclusion in the Work by the copyright owner
+      or by an individual or Legal Entity authorized to submit on behalf of
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+      to the Licensor or its representatives, including but not limited to
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+      "Contributor" shall mean Licensor and any individual or Legal Entity
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+      subsequently incorporated within the Work.
+
+   2. Grant of Copyright License. Subject to the terms and conditions of
+      this License, each Contributor hereby grants to You a perpetual,
+      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+      copyright license to reproduce, prepare Derivative Works of,
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+      Work and such Derivative Works in Source or Object form.
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+   3. Grant of Patent License. Subject to the terms and conditions of
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+      or a Contribution incorporated within the Work constitutes direct
+      or contributory patent infringement, then any patent licenses
+      granted to You under this License for that Work shall terminate
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+
+   4. Redistribution. You may reproduce and distribute copies of the
+      Work or Derivative Works thereof in any medium, with or without
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+      meet the following conditions:
+
+      (a) You must give any other recipients of the Work or
+          Derivative Works a copy of this License; and
+
+      (b) You must cause any modified files to carry prominent notices
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+
+      (c) You must retain, in the Source form of any Derivative Works
+          that You distribute, all copyright, patent, trademark, and
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+
+      (d) If the Work includes a "NOTICE" text file as part of its
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+   5. Submission of Contributions. Unless You explicitly state otherwise,
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+      by You to the Licensor shall be under the terms and conditions of
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+   8. Limitation of Liability. In no event and under no legal theory,
+      whether in tort (including negligence), contract, or otherwise,
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+      negligent acts) or agreed to in writing, shall any Contributor be
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+      incurred by, or claims asserted against, such Contributor by reason
+      of your accepting any such warranty or additional liability.
+
+   END OF TERMS AND CONDITIONS
+
+   APPENDIX: How to apply the Apache License to your work.
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+      To apply the Apache License to your work, attach the following
+      boilerplate notice, with the fields enclosed by brackets "[]"
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+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
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+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
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diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..aa45ecb
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,114 @@
+SHELL := /bin/bash
+
+PY_MODULE := id
+
+ALL_PY_SRCS := $(shell find $(PY_MODULE) -name '*.py') \
+	$(shell find test -name '*.py')
+
+# Optionally overriden by the user, if they're using a virtual environment manager.
+VENV ?= env
+
+# On Windows, venv scripts/shims are under `Scripts` instead of `bin`.
+VENV_BIN := $(VENV)/bin
+ifeq ($(OS),Windows_NT)
+	VENV_BIN := $(VENV)/Scripts
+endif
+
+# Optionally overridden by the user in the `release` target.
+BUMP_ARGS :=
+
+# Optionally overridden by the user in the `test` target.
+TESTS ?=
+
+# Optionally overridden by the user/CI, to limit the installation to a specific
+# subset of development dependencies.
+ID_EXTRA := dev
+
+# If the user selects a specific test pattern to run, set `pytest` to fail fast
+# and only run tests that match the pattern.
+# Otherwise, run all tests and enable coverage assertions, since we expect
+# complete test coverage.
+ifneq ($(TESTS),)
+	TEST_ARGS := -x -k $(TESTS) $(TEST_ARGS)
+	COV_ARGS :=
+else
+	TEST_ARGS := $(TEST_ARGS)
+# TODO: Reenable coverage testing
+#	COV_ARGS := --fail-under 100
+endif
+
+ifneq ($(T),)
+	T := $(T)
+else
+	T := test/unit
+endif
+
+.PHONY: all
+all:
+	@echo "Run my targets individually!"
+
+$(VENV)/pyvenv.cfg: pyproject.toml
+	# Create our Python 3 virtual environment
+	python3 -m venv $(VENV)
+	$(VENV_BIN)/python -m pip install --upgrade pip
+	$(VENV_BIN)/python -m pip install -e .[$(ID_EXTRA)]
+
+.PHONY: dev
+dev: $(VENV)/pyvenv.cfg
+
+.PHONY: run
+run: $(VENV)/pyvenv.cfg
+	@. $(VENV_BIN)/activate && python -m id $(ARGS)
+
+.PHONY: lint
+lint: $(VENV)/pyvenv.cfg
+	. $(VENV_BIN)/activate && \
+		black --check $(ALL_PY_SRCS) && \
+		isort --check $(ALL_PY_SRCS) && \
+		ruff $(ALL_PY_SRCS) && \
+		mypy $(PY_MODULE) && \
+		bandit -c pyproject.toml -r $(PY_MODULE) && \
+		interrogate --fail-under 80 -c pyproject.toml $(PY_MODULE)
+
+.PHONY: reformat
+reformat: $(VENV)/pyvenv.cfg
+	. $(VENV_BIN)/activate && \
+		ruff --fix $(ALL_PY_SRCS) && \
+		black $(ALL_PY_SRCS) && \
+		isort $(ALL_PY_SRCS)
+
+.PHONY: test
+test: $(VENV)/pyvenv.cfg
+	. $(VENV_BIN)/activate && \
+		pytest --cov=$(PY_MODULE) $(T) $(TEST_ARGS) && \
+		python -m coverage report -m $(COV_ARGS)
+
+.PHONY: package
+package: $(VENV)/pyvenv.cfg
+	. $(VENV_BIN)/activate && \
+		python3 -m build
+
+.PHONY: release
+release: $(VENV)/pyvenv.cfg
+	@. $(VENV_BIN)/activate && \
+		NEXT_VERSION=$$(bump $(BUMP_ARGS)) && \
+		git add $(PY_MODULE)/_version.py && git diff --quiet --exit-code && \
+		git commit -m "version: v$${NEXT_VERSION}" && \
+		git tag v$${NEXT_VERSION} && \
+		echo "RUN ME MANUALLY: git push origin main && git push origin v$${NEXT_VERSION}"
+
+.PHONY: check-readme
+check-readme:
+	# id --help
+	@diff \
+	  <( \
+	    awk '/@begin-id-help@/{f=1;next} /@end-id-help@/{f=0} f' \
+	      < README.md | sed '1d;$$d' \
+	  ) \
+	  <( \
+	    $(MAKE) -s run ARGS="--help" \
+	  )
+
+.PHONY: edit
+edit:
+	$(EDITOR) $(ALL_PY_SRCS)
diff --git a/README.md b/README.md
index e69de29..0e9fc79 100644
--- a/README.md
+++ b/README.md
@@ -0,0 +1,82 @@
+id
+==
+
+<!--- @begin-badges@ --->
+![CI](https://github.com/di/id/workflows/CI/badge.svg)
+[![PyPI version](https://badge.fury.io/py/id.svg)](https://pypi.org/project/id)
+[![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/di/id/badge)](https://api.securityscorecards.dev/projects/github.com/di/id)
+[![SLSA](https://slsa.dev/images/gh-badge-level3.svg)](https://slsa.dev/)
+<!--- @end-badges@ --->
+
+`id` is a Python tool for generating OIDC identities. It can automatically
+detect and produce OIDC on an number of environments, including GitHub Actions
+and Google Cloud.
+
+## Installation
+
+`id` requires Python 3.7 or newer, and can be installed directly via `pip`:
+
+```console
+python -m pip install id
+```
+
+## Usage
+
+You can run `id` as a Python module via `python -m`:
+
+```console
+python -m id --help
+```
+
+Top-level:
+
+<!-- @begin-id-help@ -->
+```
+usage: id [-h] [-V] [-v] audience
+
+a tool for generating OIDC identities
+
+positional arguments:
+  audience       the OIDC audience to use
+
+optional arguments:
+  -h, --help     show this help message and exit
+  -V, --version  show program's version number and exit
+  -v, --verbose  run with additional debug logging; supply multiple times to
+                 increase verbosity (default: 0)
+```
+<!-- @end-id-help@ -->
+
+For Python API usage, there is a single importable function, `detect_credential`:
+
+>>> from id import detect_credential
+>>> detect_credential(audience='something')
+'<OIDC token>'
+
+This function requires an `audience` parameter, which is used when generating
+the OIDC token. This should be set to the intended audience for the token.
+
+## Supported environments
+
+`id` currently supports ambient credential detection in the following environments:
+
+* [GitHub Actions](https://docs.github.com/en/actions/deployment/security-hardening-your-deployments/about-security-hardening-with-openid-connect)
+* [Google Cloud]
+  * [Cloud Run](https://cloud.google.com/run/docs/securing/service-identity)
+  * [Google Kubernetes Engine](https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity)
+  * [Compute Engine](https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances)
+  * and more
+
+## Licensing
+
+`id` is licensed under the Apache 2.0 License.
+
+## Contributing
+
+See [the contributing docs](https://github.com/di/id/blob/main/CONTRIBUTING.md) for details.
+
+### SLSA Provenance
+This project emits a SLSA provenance on its release! This enables you to verify the integrity
+of the downloaded artifacts and ensured that the binary's code really comes from this source code.
+
+To do so, please follow the instructions [here](https://github.com/slsa-framework/slsa-github-generator#verification-of-provenance).
diff --git a/cloudbuild.yaml b/cloudbuild.yaml
new file mode 100644
index 0000000..46b76a0
--- /dev/null
+++ b/cloudbuild.yaml
@@ -0,0 +1,12 @@
+steps:
+  # Install dependencies
+  - name: python
+    entrypoint: python
+    args: ["-m", "pip", "install", ".", "--user"]
+
+  # Generate ambient GCP credentials
+  - name: python
+    entrypoint: python
+    args: ["-m", "id", "--audience", "throwaway"]
+    env:
+      - "GOOGLE_SERVICE_ACCOUNT_NAME=sigstore-python-test@projectsigstore.iam.gserviceaccount.com"
diff --git a/id/__init__.py b/id/__init__.py
new file mode 100644
index 0000000..afe63ac
--- /dev/null
+++ b/id/__init__.py
@@ -0,0 +1,67 @@
+# Copyright 2022 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+API for retrieving OIDC tokens.
+"""
+
+from __future__ import annotations
+
+from typing import Callable, List, Optional
+
+__version__ = "1.0.0"
+
+
+class IdentityError(Exception):
+    """
+    Raised on any OIDC token format or claim error.
+    """
+
+    pass
+
+
+class AmbientCredentialError(IdentityError):
+    """
+    Raised when an ambient credential should be present, but
+    can't be retrieved (e.g. network failure).
+    """
+
+    pass
+
+
+class GitHubOidcPermissionCredentialError(AmbientCredentialError):
+    """
+    Raised when the current GitHub Actions environment doesn't have permission
+    to retrieve an OIDC token.
+    """
+
+    pass
+
+
+def detect_credential(audience: str) -> Optional[str]:
+    """
+    Try each ambient credential detector, returning the first one to succeed
+    or `None` if all fail.
+
+    Raises `AmbientCredentialError` if any detector fails internally (i.e.
+    detects a credential, but cannot retrieve it).
+    """
+    from ._internal.oidc.ambient import detect_gcp, detect_github
+
+    detectors: List[Callable[..., Optional[str]]] = [detect_github, detect_gcp]
+    for detector in detectors:
+        credential = detector(audience)
+        if credential is not None:
+            return credential
+    return None
diff --git a/id/__main__.py b/id/__main__.py
new file mode 100644
index 0000000..c9fe5ab
--- /dev/null
+++ b/id/__main__.py
@@ -0,0 +1,77 @@
+# Copyright 2022 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+The `python -m id` entrypoint.
+"""
+import argparse
+import logging
+import os
+
+from . import __version__
+
+logging.basicConfig()
+logger = logging.getLogger(__name__)
+
+# NOTE: We configure the top package logger, rather than the root logger,
+# to avoid overly verbose logging in third-party code by default.
+package_logger = logging.getLogger("id")
+package_logger.setLevel(os.environ.get("ID_LOGLEVEL", "INFO").upper())
+
+
+def _parser() -> argparse.ArgumentParser:
+    parser = argparse.ArgumentParser(
+        prog="id",
+        description="a tool for generating OIDC identities",
+        formatter_class=argparse.ArgumentDefaultsHelpFormatter,
+    )
+    parser.add_argument(
+        "-V", "--version", action="version", version=f"%(prog)s {__version__}"
+    )
+    parser.add_argument(
+        "-v",
+        "--verbose",
+        action="count",
+        default=0,
+        help="run with additional debug logging; supply multiple times to increase verbosity",
+    )
+    parser.add_argument(
+        "audience",
+        type=str,
+        default=os.getenv("ID_OIDC_AUDIENCE"),
+        help="the OIDC audience to use",
+    )
+
+    return parser
+
+
+def main() -> None:
+    parser = _parser()
+    args = parser.parse_args()
+
+    # Configure logging upfront, so that we don't miss anything.
+    if args.verbose >= 1:
+        package_logger.setLevel("DEBUG")
+    if args.verbose >= 2:
+        logging.getLogger().setLevel("DEBUG")
+
+    logger.debug(f"parsed arguments {args}")
+
+    from . import detect_credential
+
+    print(detect_credential(args.audience))
+
+
+if __name__ == "__main__":  # pragma: no cover
+    main()
diff --git a/id/_internal/oidc/__init__.py b/id/_internal/oidc/__init__.py
new file mode 100644
index 0000000..88cb71f
--- /dev/null
+++ b/id/_internal/oidc/__init__.py
@@ -0,0 +1,13 @@
+# Copyright 2022 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
diff --git a/id/_internal/oidc/ambient.py b/id/_internal/oidc/ambient.py
new file mode 100644
index 0000000..c52c204
--- /dev/null
+++ b/id/_internal/oidc/ambient.py
@@ -0,0 +1,190 @@
+# Copyright 2022 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+Ambient OIDC credential detection.
+"""
+
+import logging
+import os
+from typing import Optional
+
+import requests
+from pydantic import BaseModel, StrictStr
+
+from ... import AmbientCredentialError, GitHubOidcPermissionCredentialError
+
+logger = logging.getLogger(__name__)
+
+_GCP_PRODUCT_NAME_FILE = "/sys/class/dmi/id/product_name"
+_GCP_TOKEN_REQUEST_URL = "http://metadata/computeMetadata/v1/instance/service-accounts/default/token"  # noqa # nosec B105
+_GCP_IDENTITY_REQUEST_URL = "http://metadata/computeMetadata/v1/instance/service-accounts/default/identity"  # noqa
+_GCP_GENERATEIDTOKEN_REQUEST_URL = "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/{}:generateIdToken"  # noqa
+
+
+class _GitHubTokenPayload(BaseModel):
+    """
+    A trivial model for GitHub's OIDC token endpoint payload.
+
+    This exists solely to provide nice error handling.
+    """
+
+    value: StrictStr
+
+
+def detect_github(audience: str) -> Optional[str]:
+    """
+    Detect and return a GitHub Actions ambient OIDC credential.
+
+    Returns `None` if the context is not a GitHub Actions environment.
+
+    Raises if the environment is GitHub Actions, but is incorrect or
+    insufficiently permissioned for an OIDC credential.
+    """
+
+    logger.debug("GitHub: looking for OIDC credentials")
+    if not os.getenv("GITHUB_ACTIONS"):
+        logger.debug("GitHub: environment doesn't look like a GH action; giving up")
+        return None
+
+    # If we're running on a GitHub Action, we need to issue a GET request
+    # to a special URL with a special bearer token. Both are stored in
+    # the environment and are only present if the workflow has sufficient permissions.
+    req_token = os.getenv("ACTIONS_ID_TOKEN_REQUEST_TOKEN")
+    if not req_token:
+        raise GitHubOidcPermissionCredentialError(
+            "GitHub: missing or insufficient OIDC token permissions, the "
+            "ACTIONS_ID_TOKEN_REQUEST_TOKEN environment variable was unset"
+        )
+    req_url = os.getenv("ACTIONS_ID_TOKEN_REQUEST_URL")
+    if not req_url:
+        raise GitHubOidcPermissionCredentialError(
+            "GitHub: missing or insufficient OIDC token permissions, the "
+            "ACTIONS_ID_TOKEN_REQUEST_URL environment variable was unset"
+        )
+
+    logger.debug("GitHub: requesting OIDC token")
+    resp = requests.get(
+        req_url,
+        params={"audience": audience},
+        headers={"Authorization": f"bearer {req_token}"},
+    )
+    try:
+        resp.raise_for_status()
+    except requests.HTTPError as http_error:
+        raise AmbientCredentialError(
+            f"GitHub: OIDC token request failed (code={resp.status_code})"
+        ) from http_error
+
+    try:
+        body = resp.json()
+        payload = _GitHubTokenPayload(**body)
+    except Exception as e:
+        raise AmbientCredentialError("GitHub: malformed or incomplete JSON") from e
+
+    logger.debug("GCP: successfully requested OIDC token")
+    return payload.value
+
+
+def detect_gcp(audience: str) -> Optional[str]:
+    """
+    Detect an return a Google Cloud Platform ambient OIDC credential.
+
+    Returns `None` if the context is not a GCP environment.
+
+    Raises if the environment is GCP, but is incorrect or
+    insufficiently permissioned for an OIDC credential.
+    """
+    logger.debug("GCP: looking for OIDC credentials")
+
+    service_account_name = os.getenv("GOOGLE_SERVICE_ACCOUNT_NAME")
+    if service_account_name:
+        logger.debug("GCP: GOOGLE_SERVICE_ACCOUNT_NAME set; attempting impersonation")
+
+        logger.debug("GCP: requesting access token")
+        resp = requests.get(
+            _GCP_TOKEN_REQUEST_URL,
+            params={"scopes": "https://www.googleapis.com/auth/cloud-platform"},
+            headers={"Metadata-Flavor": "Google"},
+        )
+        try:
+            resp.raise_for_status()
+        except requests.HTTPError as http_error:
+            raise AmbientCredentialError(
+                f"GCP: access token request failed (code={resp.status_code})"
+            ) from http_error
+
+        access_token = resp.json().get("access_token")
+
+        if not access_token:
+            raise AmbientCredentialError("GCP: access token missing from response")
+
+        resp = requests.post(
+            _GCP_GENERATEIDTOKEN_REQUEST_URL.format(service_account_name),
+            json={"audience": audience, "includeEmail": True},
+            headers={
+                "Authorization": f"Bearer {access_token}",
+            },
+        )
+
+        logger.debug("GCP: requesting OIDC token")
+        try:
+            resp.raise_for_status()
+        except requests.HTTPError as http_error:
+            raise AmbientCredentialError(
+                f"GCP: OIDC token request failed (code={resp.status_code})"
+            ) from http_error
+
+        oidc_token: str = resp.json().get("token")
+
+        if not oidc_token:
+            raise AmbientCredentialError("GCP: OIDC token missing from response")
+
+        logger.debug("GCP: successfully requested OIDC token")
+        return oidc_token
+
+    else:
+        logger.debug("GCP: GOOGLE_SERVICE_ACCOUNT_NAME not set; skipping impersonation")
+
+        try:
+            with open(_GCP_PRODUCT_NAME_FILE) as f:
+                name = f.read().strip()
+        except OSError:
+            logger.debug(
+                "GCP: environment doesn't have GCP product name file; giving up"
+            )
+            return None
+
+        if name not in {"Google", "Google Compute Engine"}:
+            logger.debug(
+                f"GCP: product name file exists, but product name is {name!r}; giving up"
+            )
+            return None
+
+        logger.debug("GCP: requesting OIDC token")
+        resp = requests.get(
+            _GCP_IDENTITY_REQUEST_URL,
+            params={"audience": audience, "format": "full"},
+            headers={"Metadata-Flavor": "Google"},
+        )
+
+        try:
+            resp.raise_for_status()
+        except requests.HTTPError as http_error:
+            raise AmbientCredentialError(
+                f"GCP: OIDC token request failed (code={resp.status_code})"
+            ) from http_error
+
+        logger.debug("GCP: successfully requested OIDC token")
+        return resp.text
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..4f8a83e
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,106 @@
+[build-system]
+requires = ["flit_core >=3.2,<4"]
+build-backend = "flit_core.buildapi"
+
+[project]
+name = "id"
+dynamic = ["version"]
+description = "A tool for generating OIDC identities"
+readme = "README.md"
+license = { file = "LICENSE" }
+authors = [
+  { name = "Sigstore Authors", email = "sigstore-dev@googlegroups.com" }
+]
+classifiers = [
+  "License :: OSI Approved :: Apache Software License",
+  "Programming Language :: Python :: 3 :: Only",
+  "Programming Language :: Python :: 3",
+  "Programming Language :: Python :: 3.7",
+  "Programming Language :: Python :: 3.8",
+  "Programming Language :: Python :: 3.9",
+  "Programming Language :: Python :: 3.10",
+  "Programming Language :: Python :: 3.11",
+  "Development Status :: 4 - Beta",
+  "Intended Audience :: Developers",
+  "Topic :: Security",
+  "Topic :: Security :: Cryptography",
+]
+dependencies = [
+  "pydantic",
+  "requests",
+]
+requires-python = ">=3.7"
+
+[project.urls]
+Homepage = "https://pypi.org/project/id/"
+Issues = "https://github.com/di/id/issues"
+Source = "https://github.com/di/id"
+
+[project.optional-dependencies]
+test = [
+  "pytest",
+  "pytest-cov",
+  "pretend",
+  "coverage[toml]",
+]
+lint = [
+  "bandit",
+  "black",
+  "isort",
+  "interrogate",
+  "mypy",
+  # NOTE(ww): ruff is under active development, so we pin conservatively here
+  # and let Dependabot periodically perform this update.
+  "ruff < 0.0.224",
+  "types-requests",
+  # Needed for protocol typing in 3.7; remove when our minimum Python is 3.8.
+  "typing-extensions; python_version < '3.8'",
+]
+dev = [
+  "build",
+  "bump >= 1.3.2",
+  "id[test,lint]",
+]
+
+[tool.isort]
+multi_line_output = 3
+known_first_party = "id"
+include_trailing_comma = true
+
+[tool.interrogate]
+# don't enforce documentation coverage for packaging, testing, the virtual
+# environment, or the CLI (which is documented separately).
+ignore-semiprivate = true
+ignore-private = true
+# Ignore nested classes for docstring coverage because we use them primarily
+# for pydantic model configuration.
+ignore-nested-classes = true
+fail-under = 100
+
+[tool.mypy]
+allow_redefinition = true
+check_untyped_defs = true
+disallow_incomplete_defs = true
+disallow_untyped_defs = true
+ignore_missing_imports = true
+no_implicit_optional = true
+show_error_codes = true
+sqlite_cache = true
+strict_equality = true
+warn_no_return = true
+warn_redundant_casts = true
+warn_return_any = true
+warn_unreachable = true
+warn_unused_configs = true
+warn_unused_ignores = true
+plugins = ["pydantic.mypy"]
+
+[tool.bandit]
+exclude_dirs = ["./test"]
+
+[tool.ruff]
+line-length = 100
+# TODO: Enable "UP" here once Pydantic allows us to:
+# See: https://github.com/pydantic/pydantic/issues/4146
+select = ["E", "F", "W"]
+target-version = "py37"
diff --git a/test/unit/__init__.py b/test/unit/__init__.py
new file mode 100644
index 0000000..88cb71f
--- /dev/null
+++ b/test/unit/__init__.py
@@ -0,0 +1,13 @@
+# Copyright 2022 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
diff --git a/test/unit/internal/__init__.py b/test/unit/internal/__init__.py
new file mode 100644
index 0000000..88cb71f
--- /dev/null
+++ b/test/unit/internal/__init__.py
@@ -0,0 +1,13 @@
+# Copyright 2022 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
diff --git a/test/unit/internal/oidc/__init__.py b/test/unit/internal/oidc/__init__.py
new file mode 100644
index 0000000..88cb71f
--- /dev/null
+++ b/test/unit/internal/oidc/__init__.py
@@ -0,0 +1,13 @@
+# Copyright 2022 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
diff --git a/test/unit/internal/oidc/test_ambient.py b/test/unit/internal/oidc/test_ambient.py
new file mode 100644
index 0000000..dafe989
--- /dev/null
+++ b/test/unit/internal/oidc/test_ambient.py
@@ -0,0 +1,417 @@
+# Copyright 2022 The Sigstore Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import pretend
+import pytest
+from requests import HTTPError
+
+from id import detect_credential
+from id._internal.oidc import ambient
+
+
+def test_detect_credential_none(monkeypatch):
+    detect_none = pretend.call_recorder(lambda audience: None)
+    monkeypatch.setattr(ambient, "detect_github", detect_none)
+    monkeypatch.setattr(ambient, "detect_gcp", detect_none)
+    assert detect_credential("some-audience") is None
+
+
+def test_detect_credential(monkeypatch):
+    detect_github = pretend.call_recorder(lambda audience: "fakejwt")
+    monkeypatch.setattr(ambient, "detect_github", detect_github)
+
+    assert detect_credential("some-audience") == "fakejwt"
+
+
+def test_detect_github_bad_env(monkeypatch):
+    # We might actually be running in a CI, so explicitly remove this.
+    monkeypatch.delenv("GITHUB_ACTIONS", raising=False)
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    assert ambient.detect_github("some-audience") is None
+    assert logger.debug.calls == [
+        pretend.call("GitHub: looking for OIDC credentials"),
+        pretend.call("GitHub: environment doesn't look like a GH action; giving up"),
+    ]
+
+
+def test_detect_github_bad_request_token(monkeypatch):
+    monkeypatch.setenv("GITHUB_ACTIONS", "true")
+    monkeypatch.delenv("ACTIONS_ID_TOKEN_REQUEST_TOKEN", raising=False)
+    monkeypatch.setenv("ACTIONS_ID_TOKEN_REQUEST_URL", "fakeurl")
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match="GitHub: missing or insufficient OIDC token permissions?",
+    ):
+        ambient.detect_github("some-audience")
+    assert logger.debug.calls == [
+        pretend.call("GitHub: looking for OIDC credentials"),
+    ]
+
+
+def test_detect_github_bad_request_url(monkeypatch):
+    monkeypatch.setenv("GITHUB_ACTIONS", "true")
+    monkeypatch.setenv("ACTIONS_ID_TOKEN_REQUEST_TOKEN", "faketoken")
+    monkeypatch.delenv("ACTIONS_ID_TOKEN_REQUEST_URL", raising=False)
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match="GitHub: missing or insufficient OIDC token permissions?",
+    ):
+        ambient.detect_github("some-audience")
+    assert logger.debug.calls == [
+        pretend.call("GitHub: looking for OIDC credentials"),
+    ]
+
+
+def test_detect_github_request_fails(monkeypatch):
+    monkeypatch.setenv("GITHUB_ACTIONS", "true")
+    monkeypatch.setenv("ACTIONS_ID_TOKEN_REQUEST_TOKEN", "faketoken")
+    monkeypatch.setenv("ACTIONS_ID_TOKEN_REQUEST_URL", "fakeurl")
+
+    resp = pretend.stub(raise_for_status=pretend.raiser(HTTPError), status_code=999)
+    requests = pretend.stub(
+        get=pretend.call_recorder(lambda url, **kw: resp), HTTPError=HTTPError
+    )
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match=r"GitHub: OIDC token request failed \(code=999\)",
+    ):
+        ambient.detect_github("some-audience")
+    assert requests.get.calls == [
+        pretend.call(
+            "fakeurl",
+            params={"audience": "some-audience"},
+            headers={"Authorization": "bearer faketoken"},
+        )
+    ]
+
+
+def test_detect_github_bad_payload(monkeypatch):
+    monkeypatch.setenv("GITHUB_ACTIONS", "true")
+    monkeypatch.setenv("ACTIONS_ID_TOKEN_REQUEST_TOKEN", "faketoken")
+    monkeypatch.setenv("ACTIONS_ID_TOKEN_REQUEST_URL", "fakeurl")
+
+    resp = pretend.stub(
+        raise_for_status=lambda: None, json=pretend.call_recorder(lambda: {})
+    )
+    requests = pretend.stub(get=pretend.call_recorder(lambda url, **kw: resp))
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match="GitHub: malformed or incomplete JSON",
+    ):
+        ambient.detect_github("some-audience")
+    assert requests.get.calls == [
+        pretend.call(
+            "fakeurl",
+            params={"audience": "some-audience"},
+            headers={"Authorization": "bearer faketoken"},
+        )
+    ]
+    assert resp.json.calls == [pretend.call()]
+
+
+def test_detect_github(monkeypatch):
+    monkeypatch.setenv("GITHUB_ACTIONS", "true")
+    monkeypatch.setenv("ACTIONS_ID_TOKEN_REQUEST_TOKEN", "faketoken")
+    monkeypatch.setenv("ACTIONS_ID_TOKEN_REQUEST_URL", "fakeurl")
+
+    resp = pretend.stub(
+        raise_for_status=lambda: None,
+        json=pretend.call_recorder(lambda: {"value": "fakejwt"}),
+    )
+    requests = pretend.stub(get=pretend.call_recorder(lambda url, **kw: resp))
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    assert ambient.detect_github("some-audience") == "fakejwt"
+    assert requests.get.calls == [
+        pretend.call(
+            "fakeurl",
+            params={"audience": "some-audience"},
+            headers={"Authorization": "bearer faketoken"},
+        )
+    ]
+    assert resp.json.calls == [pretend.call()]
+
+
+def test_gcp_impersonation_access_token_request_fail(monkeypatch):
+    monkeypatch.setenv(
+        "GOOGLE_SERVICE_ACCOUNT_NAME", "identity@project.iam.gserviceaccount.com"
+    )
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    resp = pretend.stub(raise_for_status=pretend.raiser(HTTPError), status_code=999)
+    requests = pretend.stub(
+        get=pretend.call_recorder(lambda url, **kw: resp), HTTPError=HTTPError
+    )
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match=r"GCP: access token request failed \(code=999\)",
+    ):
+        ambient.detect_gcp("some-audience")
+
+    assert logger.debug.calls == [
+        pretend.call("GCP: looking for OIDC credentials"),
+        pretend.call("GCP: GOOGLE_SERVICE_ACCOUNT_NAME set; attempting impersonation"),
+        pretend.call("GCP: requesting access token"),
+    ]
+
+
+def test_gcp_impersonation_access_token_missing(monkeypatch):
+    monkeypatch.setenv(
+        "GOOGLE_SERVICE_ACCOUNT_NAME", "identity@project.iam.gserviceaccount.com"
+    )
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    resp = pretend.stub(raise_for_status=lambda: None, json=lambda: {})
+    requests = pretend.stub(get=pretend.call_recorder(lambda url, **kw: resp))
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match=r"GCP: access token missing from response",
+    ):
+        ambient.detect_gcp("some-audience")
+
+    assert logger.debug.calls == [
+        pretend.call("GCP: looking for OIDC credentials"),
+        pretend.call("GCP: GOOGLE_SERVICE_ACCOUNT_NAME set; attempting impersonation"),
+        pretend.call("GCP: requesting access token"),
+    ]
+
+
+def test_gcp_impersonation_identity_token_request_fail(monkeypatch):
+    monkeypatch.setenv(
+        "GOOGLE_SERVICE_ACCOUNT_NAME", "identity@project.iam.gserviceaccount.com"
+    )
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    access_token = pretend.stub()
+    get_resp = pretend.stub(
+        raise_for_status=lambda: None, json=lambda: {"access_token": access_token}
+    )
+    post_resp = pretend.stub(
+        raise_for_status=pretend.raiser(HTTPError), status_code=999
+    )
+    requests = pretend.stub(
+        get=pretend.call_recorder(lambda url, **kw: get_resp),
+        post=pretend.call_recorder(lambda url, **kw: post_resp),
+        HTTPError=HTTPError,
+    )
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match=r"GCP: OIDC token request failed \(code=999\)",
+    ):
+        ambient.detect_gcp("some-audience")
+
+    assert logger.debug.calls == [
+        pretend.call("GCP: looking for OIDC credentials"),
+        pretend.call("GCP: GOOGLE_SERVICE_ACCOUNT_NAME set; attempting impersonation"),
+        pretend.call("GCP: requesting access token"),
+        pretend.call("GCP: requesting OIDC token"),
+    ]
+
+
+def test_gcp_impersonation_identity_token_missing(monkeypatch):
+    monkeypatch.setenv(
+        "GOOGLE_SERVICE_ACCOUNT_NAME", "identity@project.iam.gserviceaccount.com"
+    )
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    access_token = pretend.stub()
+    get_resp = pretend.stub(
+        raise_for_status=lambda: None, json=lambda: {"access_token": access_token}
+    )
+    post_resp = pretend.stub(raise_for_status=lambda: None, json=lambda: {})
+    requests = pretend.stub(
+        get=pretend.call_recorder(lambda url, **kw: get_resp),
+        post=pretend.call_recorder(lambda url, **kw: post_resp),
+        HTTPError=HTTPError,
+    )
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match=r"GCP: OIDC token missing from response",
+    ):
+        ambient.detect_gcp("some-audience")
+
+    assert logger.debug.calls == [
+        pretend.call("GCP: looking for OIDC credentials"),
+        pretend.call("GCP: GOOGLE_SERVICE_ACCOUNT_NAME set; attempting impersonation"),
+        pretend.call("GCP: requesting access token"),
+        pretend.call("GCP: requesting OIDC token"),
+    ]
+
+
+def test_gcp_impersonation_succeeds(monkeypatch):
+    monkeypatch.setenv(
+        "GOOGLE_SERVICE_ACCOUNT_NAME", "identity@project.iam.gserviceaccount.com"
+    )
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    access_token = pretend.stub()
+    oidc_token = pretend.stub()
+    get_resp = pretend.stub(
+        raise_for_status=lambda: None, json=lambda: {"access_token": access_token}
+    )
+    post_resp = pretend.stub(
+        raise_for_status=lambda: None, json=lambda: {"token": oidc_token}
+    )
+    requests = pretend.stub(
+        get=pretend.call_recorder(lambda url, **kw: get_resp),
+        post=pretend.call_recorder(lambda url, **kw: post_resp),
+        HTTPError=HTTPError,
+    )
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    assert ambient.detect_gcp("some-audience") == oidc_token
+
+    assert logger.debug.calls == [
+        pretend.call("GCP: looking for OIDC credentials"),
+        pretend.call("GCP: GOOGLE_SERVICE_ACCOUNT_NAME set; attempting impersonation"),
+        pretend.call("GCP: requesting access token"),
+        pretend.call("GCP: requesting OIDC token"),
+        pretend.call("GCP: successfully requested OIDC token"),
+    ]
+
+
+def test_gcp_bad_env(monkeypatch):
+    oserror = pretend.raiser(OSError)
+    monkeypatch.setitem(ambient.__builtins__, "open", oserror)  # type: ignore
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    assert ambient.detect_gcp("some-audience") is None
+    assert logger.debug.calls == [
+        pretend.call("GCP: looking for OIDC credentials"),
+        pretend.call(
+            "GCP: GOOGLE_SERVICE_ACCOUNT_NAME not set; skipping impersonation"
+        ),
+        pretend.call("GCP: environment doesn't have GCP product name file; giving up"),
+    ]
+
+
+def test_gcp_wrong_product(monkeypatch):
+    stub_file = pretend.stub(
+        __enter__=lambda *a: pretend.stub(read=lambda: "Unsupported Product"),
+        __exit__=lambda *a: None,
+    )
+    monkeypatch.setitem(ambient.__builtins__, "open", lambda fn: stub_file)  # type: ignore
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    assert ambient.detect_gcp("some-audience") is None
+
+    assert logger.debug.calls == [
+        pretend.call("GCP: looking for OIDC credentials"),
+        pretend.call(
+            "GCP: GOOGLE_SERVICE_ACCOUNT_NAME not set; skipping impersonation"
+        ),
+        pretend.call(
+            "GCP: product name file exists, but product name is 'Unsupported Product'; giving up"
+        ),
+    ]
+
+
+def test_detect_gcp_request_fails(monkeypatch):
+    stub_file = pretend.stub(
+        __enter__=lambda *a: pretend.stub(read=lambda: "Google"),
+        __exit__=lambda *a: None,
+    )
+    monkeypatch.setitem(ambient.__builtins__, "open", lambda fn: stub_file)  # type: ignore
+
+    resp = pretend.stub(raise_for_status=pretend.raiser(HTTPError), status_code=999)
+    requests = pretend.stub(
+        get=pretend.call_recorder(lambda url, **kw: resp), HTTPError=HTTPError
+    )
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    with pytest.raises(
+        ambient.AmbientCredentialError,
+        match=r"GCP: OIDC token request failed \(code=999\)",
+    ):
+        ambient.detect_gcp("some-audience")
+    assert requests.get.calls == [
+        pretend.call(
+            ambient._GCP_IDENTITY_REQUEST_URL,
+            params={"audience": "some-audience", "format": "full"},
+            headers={"Metadata-Flavor": "Google"},
+        )
+    ]
+
+
+@pytest.mark.parametrize("product_name", ("Google", "Google Compute Engine"))
+def test_detect_gcp(monkeypatch, product_name):
+    stub_file = pretend.stub(
+        __enter__=lambda *a: pretend.stub(read=lambda: product_name),
+        __exit__=lambda *a: None,
+    )
+    monkeypatch.setitem(ambient.__builtins__, "open", lambda fn: stub_file)  # type: ignore
+
+    logger = pretend.stub(debug=pretend.call_recorder(lambda s: None))
+    monkeypatch.setattr(ambient, "logger", logger)
+
+    resp = pretend.stub(
+        raise_for_status=lambda: None,
+        text="fakejwt",
+    )
+    requests = pretend.stub(get=pretend.call_recorder(lambda url, **kw: resp))
+    monkeypatch.setattr(ambient, "requests", requests)
+
+    assert ambient.detect_gcp("some-audience") == "fakejwt"
+    assert requests.get.calls == [
+        pretend.call(
+            ambient._GCP_IDENTITY_REQUEST_URL,
+            params={"audience": "some-audience", "format": "full"},
+            headers={"Metadata-Flavor": "Google"},
+        )
+    ]
+    assert logger.debug.calls == [
+        pretend.call("GCP: looking for OIDC credentials"),
+        pretend.call(
+            "GCP: GOOGLE_SERVICE_ACCOUNT_NAME not set; skipping impersonation"
+        ),
+        pretend.call("GCP: requesting OIDC token"),
+        pretend.call("GCP: successfully requested OIDC token"),
+    ]