sops-mcp

<!-- mcp-name: io.github.privacyplaybook/sops-mcp -->

MCP server for creating and managing SOPS-encrypted secret files using age encryption.

Designed for Claude Code (or any MCP client) to produce encrypted secrets.enc.yaml files without the model ever seeing plaintext values. All file content is passed as text parameters and returned as text — the server has no filesystem access to the client.

Why

Two goals drive the design:

1. Keep secrets in your source tree without leaking them. For a small project, running a full secrets manager (Vault, AWS Secrets Manager, etc.) is overkill for a handful of credentials. Encrypting secrets at rest in git and decrypting them in your CI/CD pipeline at deploy time is much cheaper:

1. Create secrets.enc.yaml via this server — age-encrypted against your public key, safe to commit.
2. Commit it alongside your code.
3. Your CI/CD pipeline holds the age private key, decrypts at deploy time, and injects plaintext as environment variables into your container orchestrator.

The age private key lives in exactly one place: your CI/CD secrets store. Everywhere else — your laptop, your git remote, your container images — sees only ciphertext. See the worked example below.

2. Let an AI coding agent generate secrets it can never read. Claude (or any MCP client) can create passwords, rotate them, derive hashes, rename and delete them — but plaintext values never cross the MCP boundary back to the model. The server holds the encryption key; the client only submits requests and receives metadata. There is deliberately no "decrypt this one secret" tool. If a prompt injection or a misbehaving agent tried to exfiltrate a secret via tool output, there is no tool output to exfiltrate.

This pattern assumes a single age recipient (the one CI private key). For multi-recipient / team key management, use the sops CLI directly for recipient rotations and this server for content management.

Design

Three ideas shape the tool surface:

1. No plaintext crosses the MCP boundary. Generated secret values are never returned to the client. There is deliberately no "decrypt this one key" tool. If you need plaintext, run sops decrypt yourself with the age private key.
2. Metadata in plaintext. A _meta_unencrypted block sits alongside the encrypted values (using SOPS's unencrypted_suffix feature) and records each secret's source, how it was generated, and when it was last rotated. This lets the server list and rotate secrets without decrypting.
3. No in-place value update for generated or derived secrets. Those change only via rotation — the mutation model is deliberate, not accidental. External secrets (e.g. an upstream API key the user controls) can be updated with sops_update_external.

Secret sources

Every secret is one of three sources, recorded in _meta_unencrypted:

• generated — Cryptographically random values (Python secrets / OS CSPRNG). You specify length and charset; the server stores both so it can regenerate on rotation.
• external — User-provided values encrypted as-is (SMTP credentials, third-party API keys, etc.). Preserved across rotation. Updated via sops_update_external.
• derived — Computed from another key in the same file via a named transform. When the source is rotated (or an external source is updated), the derived value is automatically recomputed in topological order. Useful for things like Authelia's PBKDF2 hashes of OIDC client secrets.

Transforms (for derived secrets)

Transform	Purpose	Deterministic
pbkdf2_sha512_authelia	PBKDF2-SHA512 hash in Authelia's configuration.yml format ($pbkdf2-sha512$310000$...)	No — random salt per call
sha256_hex	Hex-encoded SHA-256 digest	Yes

Tools

Creation and listing

Tool	What it does
sops_create_secrets	Create a new encrypted file with one or more secrets (any mix of sources).
sops_list_secrets	List keys, sources, and descriptions from a file without decrypting.
sops_create_oidc_secret	Convenience: create an Authelia OIDC client secret as a generated + derived (pbkdf2_sha512_authelia) pair in one call. The hash is returned in the response for pasting into configuration.yml.

Mutation (require SOPS_AGE_KEY)

Tool	What it does
sops_rotate_generated	Regenerate all generated secrets. Derived secrets whose source was rotated are recomputed; others are preserved. External secrets are preserved.
sops_add_secrets	Add new secrets to an existing file. Supports all three sources. Rejects collisions with existing keys.
sops_update_external	Replace the value of an external secret. Cascades to any derived secrets that reference it. Rejects attempts to update generated or derived.
sops_rename_secret	Rename a key, preserving its value and metadata. Updates from: references in any derived secrets.
sops_delete_secrets	Remove one or more keys. Rejects deleting a secret that another derived secret still references (unless the dependent is deleted in the same call).
sops_add_metadata	Retrofit _meta_unencrypted onto a legacy SOPS file that lacks it. Supports generated, external, and derived entries.

Setup

Prerequisites

• Python 3.11+
• sops CLI binary
• An age keypair (see below)

Generating an age keypair

If you don't already have one, install age and run:

age-keygen -o age-key.txt

The file looks like:

# created: 2026-04-22T12:34:56Z
# public key: age1abc...xyz
AGE-SECRET-KEY-1HH...

• Public key (age1...) — pass to this server as SOPS_MCP_AGE_PUBLIC_KEY. Safe to share anywhere.
• Private key (AGE-SECRET-KEY-...) — store as a CI/CD secret (commonly named SOPS_AGE_KEY). Never commit to source control. Anyone with this key can decrypt every secrets.enc.yaml encrypted to the matching public key.

Back up the private key somewhere safe (password manager, hardware token). Losing it means losing access to every secret you've encrypted.

Installation

git clone <repo-url>
cd sops-mcp
python3 -m venv .venv
.venv/bin/pip install -e .

Claude Code configuration

Add to your project's .mcp.json:

{
  "mcpServers": {
    "sops-mcp": {
      "command": "/path/to/sops-mcp/.venv/bin/python",
      "args": ["-m", "sops_mcp"],
      "env": {
        "SOPS_MCP_SOPS_BINARY": "/path/to/sops",
        "SOPS_MCP_AGE_PUBLIC_KEY": "<your-age-public-key>"
      }
    }
  }
}

Environment variables

Variable	Required	Purpose
SOPS_MCP_AGE_PUBLIC_KEY	Yes*	Age public key for encryption
SOPS_AGE_RECIPIENTS	Yes*	Alternative to SOPS_MCP_AGE_PUBLIC_KEY
SOPS_MCP_SOPS_BINARY	No	Path to sops binary (default: sops)
SOPS_MCP_LOG_LEVEL	No	Log level (default: WARNING)
SOPS_AGE_KEY	Sometimes	Age private key — required for any mutation tool (rotate, add, update, rename, delete)
SOPS_MCP_TRANSPORT	No	stdio (default) or sse
SOPS_MCP_HOST / SOPS_MCP_PORT	No	Bind host/port for SSE transport (default: 127.0.0.1:55090). Binding to 0.0.0.0 requires SOPS_MCP_API_TOKEN — the server refuses to start otherwise.
SOPS_MCP_ALLOWED_HOSTS	No	Comma-separated allowlist for the SSE Host header (DNS rebinding protection). Default: 127.0.0.1,127.0.0.1:*,localhost,localhost:*. Set explicitly when binding to a non-loopback address — e.g. mcp.example.com,mcp.example.com:*.
SOPS_MCP_API_TOKEN	Sometimes	Required when SSE transport binds to 0.0.0.0; otherwise optional. When set, SSE requires Authorization: Bearer <token>.

* One of SOPS_MCP_AGE_PUBLIC_KEY or SOPS_AGE_RECIPIENTS must be set.

Security

• No filesystem access — The server never reads from or writes to the client filesystem. All content is passed as text parameters and returned as text.
• No private key for normal use — Encryption uses only the public key. The private key is needed only for mutation tools.
• No plaintext in responses — Generated secret values are never returned. Derived values are returned because they're meant to be pasted into config files (e.g. an Authelia PBKDF2 hash) — don't derive values you don't intend to publish.
• Secure temp files — Used only for sops CLI invocation. Created with 0600 permissions, overwritten with zeros before deletion, cleanup guaranteed by finally block.
• OS-level entropy — Secret generation uses Python's secrets module (backed by /dev/urandom).
• stdio transport by default — No network exposure; runs as a client child process.
• Input validation — Key names must match ^[A-Z][A-Z0-9_]*$.

Encrypted file format

DB_PASSWORD: ENC[AES256_GCM,data:...,tag:...,type:str]
SMTP_USER: ENC[AES256_GCM,data:...,tag:...,type:str]
DB_PASSWORD_HASH: ENC[AES256_GCM,data:...,tag:...,type:str]
_meta_unencrypted:
    version: 1
    secrets:
        DB_PASSWORD:
            source: generated
            description: Database password
            generation:
                length: 32
                charset: alphanumeric
            last_rotated: "2026-04-21T15:30:00Z"
        SMTP_USER:
            source: external
            description: SMTP username
        DB_PASSWORD_HASH:
            source: derived
            derivation:
                transform: sha256_hex
                from: DB_PASSWORD
            last_rotated: "2026-04-21T15:30:00Z"
sops:
    age:
        - recipient: age1...
          enc: |
            -----BEGIN AGE ENCRYPTED FILE-----
            ...
    unencrypted_suffix: _unencrypted

Secret values are AES-256-GCM encrypted. The _meta_unencrypted block is stored in plaintext (using SOPS's unencrypted_suffix feature) so metadata is readable without decryption.

Why these tools and not others

Per-key read (decrypt-one-secret): intentionally absent. Returning plaintext over the MCP boundary would give the model access to secret material during tool calls — an accidental exfiltration vector. If you need a plaintext value, run sops decrypt yourself with the age private key.

Per-key in-place update for generated/derived secrets: intentionally absent. sops_rotate_generated is the one path that changes those values, so rotations leave an audit trail (last_rotated timestamp) and cascade cleanly to derived secrets.

Multi-recipient / team key management (.sops.yaml, updatekeys): planned for a future release. v1 assumes a single age recipient. For multi-recipient setups, use the sops CLI directly for recipient rotations and this server for content management.

Other source types (imported, templated): out of scope. Those are orchestration concerns — fetch values from Vault or compose URLs in your deployment templating layer, then pass the result here as an external secret.

Supply chain integrity

The Docker build is hardened with three layers of verification, enforced by a CI gate.

Base image digest pinning

The Dockerfile pins python:3.12-slim by SHA-256 digest (@sha256:...) so Docker always pulls the exact image that was audited, not whatever the slim tag currently points to. The digest and cosign signature status are tracked in base-images.lock.json.

Update the base image (when upstream publishes security patches):

pip install requests  # one-time
python3 lib/pin_base_images.py

Binary checksum verification

The sops and age binaries downloaded in the Dockerfile are verified with sha256sum -c against checksums from the official release pages. A tampered binary fails the build.

Python dependency hash pinning

Runtime dependencies are installed from requirements.lock.txt with pip install --require-hashes, which rejects any package whose content doesn't match the recorded SHA-256 hashes. This prevents dependency hijacking and typosquatting.

Update dependencies after editing requirements.in:

pip install pip-tools  # one-time
lib/compile_requirements.sh

CI verification

The supply-chain.yml workflow runs lib/verify_requirements.py and lib/verify_base_images.py on every push and PR. It checks that all lockfiles are well-formed and all Dockerfile FROM lines are digest-pinned.

Verifying a published release

Every container image published to GHCR is signed by this repo's publish workflow using keyless cosign via sigstore, and ships with SLSA v1.0 build provenance and an SPDX SBOM attached as OCI artifacts. Every commit on main and every release tag is SSH-signed. You can verify all of this without holding any long-lived key material — the signatures are anchored in sigstore's public transparency log.

Install cosign: https://docs.sigstore.dev/system_config/installation

Verify the image signature. A successful verification proves the image was built by this repository's publish.yml workflow, not just that its digest matches a reference someone sent you.

cosign verify \
  --certificate-identity-regexp 'https://github.com/privacyplaybook/sops-mcp/\.github/workflows/publish\.yml@.*' \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  ghcr.io/privacyplaybook/sops-mcp:<tag>

Inspect the attestations.

# List all artifacts attached to the image
cosign tree ghcr.io/privacyplaybook/sops-mcp:<tag>

# Download the SBOM (SPDX JSON)
cosign download sbom ghcr.io/privacyplaybook/sops-mcp:<tag>

# Verify the SLSA build provenance
cosign verify-attestation --type slsaprovenance1 \
  --certificate-identity-regexp 'https://github.com/privacyplaybook/sops-mcp/\.github/workflows/publish\.yml@.*' \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  ghcr.io/privacyplaybook/sops-mcp:<tag>

Verify git tags and commits. The simplest check is the green Verified badge on the github.com commits and tags pages.

Development

python3 -m venv .venv
.venv/bin/pip install -e ".[dev]"
pytest tests/ -v  # 29 tests including end-to-end sops round-trip
ruff check src/ tests/

Example: integrating with a CI/CD deployment pipeline

A common pattern: use this server to produce secrets.enc.yaml files committed to your infrastructure repo, then decrypt them in CI and inject the plaintext values as environment variables to a container orchestrator (Portainer, Kubernetes, Nomad).

1. Generate an age keypair once. Give the private key to your CI as a secret (e.g. SOPS_AGE_KEY), the public key to Claude Code as SOPS_MCP_AGE_PUBLIC_KEY.
2. Ask the model to produce a secrets.enc.yaml with sops_create_secrets.
3. Commit the encrypted file.
4. In your deploy workflow, run sops decrypt secrets.enc.yaml > .env (or equivalent) and pass the result to your orchestrator.
5. When secrets need rotating, ask the model to run sops_rotate_generated or sops_update_external; commit and redeploy.

The _meta_unencrypted block lets your tools filter out the metadata (keys starting with _) when pushing values to an orchestrator, so metadata never leaks into environment variables.

License

Apache-2.0. See LICENSE.