diff --git a/README.md b/README.md
index 73915cc..ab9daaa 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,7 @@ It provides:
 - a versioned schema registry
 - append-only SQLite ledger storage
 - tamper-evident hash chaining
+- detached Ed25519 signing for build contracts
 - delta-sync packets for partial connectivity
 - anchor records for external attestation
 - CI context adapters for GitHub Actions, GitLab CI, and CircleCI
@@ -49,6 +50,8 @@ python -m buildledger schema-manifest
 python -m buildledger init-ledger buildledger.sqlite
 python -m buildledger append-sample buildledger.sqlite
 python -m buildledger verify buildledger.sqlite
+python -m buildledger sign-contract contract.json private-key.pem --output signed-contract.json
+python -m buildledger verify-contract signed-contract.json public-key.pem
 ```
 
 ## Testing
@@ -59,4 +62,4 @@ python -m buildledger verify buildledger.sqlite
 
 ## Status
 
-This repository currently implements the core ledger, schema, sync, anchor, and CI-adapter foundation. Registry publishing adapters and full distributed integrations can be added on top of this base.
+This repository currently implements the core ledger, schema, sync, anchor, CI-adapter, and contract-signing foundation. Registry publishing adapters and full distributed integrations can be added on top of this base.
diff --git a/buildledger/cli.py b/buildledger/cli.py
index 69c292d..3f78c48 100644
--- a/buildledger/cli.py
+++ b/buildledger/cli.py
@@ -39,6 +39,15 @@ def build_parser() -> argparse.ArgumentParser:
     gh = sub.add_parser("from-github")
     gh.add_argument("context_json")
 
+    sign = sub.add_parser("sign-contract")
+    sign.add_argument("contract_json")
+    sign.add_argument("private_key_pem")
+    sign.add_argument("--output")
+
+    verify_contract = sub.add_parser("verify-contract")
+    verify_contract.add_argument("contract_json")
+    verify_contract.add_argument("public_key_pem")
+
     return parser
 
 
@@ -87,6 +96,26 @@ def main(argv: list[str] | None = None) -> int:
         print(contract.model_dump_json(indent=2))
         return 0
 
+    if args.command == "sign-contract":
+        from .models import BuildContract
+
+        contract = BuildContract.model_validate(json.loads(Path(args.contract_json).read_text(encoding="utf-8")))
+        signed = contract.sign(Path(args.private_key_pem).read_text(encoding="utf-8"))
+        output = signed.model_dump_json(indent=2)
+        if args.output:
+            Path(args.output).write_text(output + "\n", encoding="utf-8")
+        else:
+            print(output)
+        return 0
+
+    if args.command == "verify-contract":
+        from .models import BuildContract
+
+        contract = BuildContract.model_validate(json.loads(Path(args.contract_json).read_text(encoding="utf-8")))
+        ok = contract.verify_signature(Path(args.public_key_pem).read_text(encoding="utf-8"))
+        print(json.dumps({"ok": ok}, indent=2))
+        return 0 if ok else 1
+
     return 1
 
 
diff --git a/buildledger/models.py b/buildledger/models.py
index 04f812e..8faa3b7 100644
--- a/buildledger/models.py
+++ b/buildledger/models.py
@@ -1,11 +1,16 @@
 from __future__ import annotations
 
 import hashlib
+import base64
+import binascii
 import json
 from datetime import datetime, timezone
 from enum import Enum
 from typing import Any, Literal
 
+from cryptography.exceptions import InvalidSignature
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey, Ed25519PublicKey
+from cryptography.hazmat.primitives.serialization import load_pem_private_key, load_pem_public_key
 from pydantic import BaseModel, ConfigDict, Field, field_validator
 
 
@@ -149,8 +154,54 @@ class BuildContract(StrictModel):
     signature: str | None = None
     notes: list[str] = Field(default_factory=list)
 
-    def canonical_payload(self) -> dict[str, Any]:
-        return self.model_dump(mode="python")
+    def canonical_payload(self, *, include_signature: bool = False) -> dict[str, Any]:
+        payload = self.model_dump(mode="python")
+        if not include_signature:
+            payload.pop("signature", None)
+        return payload
+
+    def signing_bytes(self) -> bytes:
+        return canonical_json(self.canonical_payload()).encode("utf-8")
+
+    def sign(
+        self,
+        private_key: Ed25519PrivateKey | bytes | str,
+    ) -> "BuildContract":
+        key = _load_private_key(private_key)
+        signature = base64.b64encode(key.sign(self.signing_bytes())).decode("ascii")
+        return self.model_copy(update={"signature": signature})
+
+    def verify_signature(self, public_key: Ed25519PublicKey | bytes | str) -> bool:
+        if self.signature is None:
+            return False
+        key = _load_public_key(public_key)
+        try:
+            key.verify(base64.b64decode(self.signature), self.signing_bytes())
+        except (ValueError, TypeError, binascii.Error, InvalidSignature):
+            return False
+        return True
+
+
+def _load_private_key(private_key: Ed25519PrivateKey | bytes | str) -> Ed25519PrivateKey:
+    if isinstance(private_key, Ed25519PrivateKey):
+        return private_key
+    if isinstance(private_key, str):
+        private_key = private_key.encode("utf-8")
+    key = load_pem_private_key(private_key, password=None)
+    if not isinstance(key, Ed25519PrivateKey):
+        raise TypeError("expected an Ed25519 private key")
+    return key
+
+
+def _load_public_key(public_key: Ed25519PublicKey | bytes | str) -> Ed25519PublicKey:
+    if isinstance(public_key, Ed25519PublicKey):
+        return public_key
+    if isinstance(public_key, str):
+        public_key = public_key.encode("utf-8")
+    key = load_pem_public_key(public_key)
+    if not isinstance(key, Ed25519PublicKey):
+        raise TypeError("expected an Ed25519 public key")
+    return key
 
 
 class LedgerEntry(StrictModel):
diff --git a/pyproject.toml b/pyproject.toml
index a2aea88..4713498 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -8,7 +8,7 @@ version = "0.1.0"
 description = "BuildLedger captures, verifies, and syncs reproducible build provenance across registries."
 readme = "README.md"
 requires-python = ">=3.11"
-dependencies = ["pydantic>=2.7,<3"]
+dependencies = ["pydantic>=2.7,<3", "cryptography>=42,<45"]
 
 [project.optional-dependencies]
 test = ["pytest>=8", "build>=1.2"]
diff --git a/tests/test_buildledger.py b/tests/test_buildledger.py
index c5fcdc8..1a9164a 100644
--- a/tests/test_buildledger.py
+++ b/tests/test_buildledger.py
@@ -4,9 +4,12 @@ import json
 from pathlib import Path
 
 import pytest
+from cryptography.hazmat.primitives import serialization
+from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
 
 from buildledger.adapters import GitHubActionsAdapter
 from buildledger.anchors import AnchorStore
+from buildledger.cli import main as cli_main
 from buildledger.ledger import SQLiteLedger
 from buildledger.models import Artifact, BuildContract, BuildEnvironment, BuildLog, BuildObject, DependencyGraph, DependencyNode, ReproManifest
 from buildledger.schemas import SchemaRegistry
@@ -51,10 +54,34 @@ def make_contract() -> BuildContract:
     )
 
 
+def write_ed25519_keypair(tmp_path: Path) -> tuple[Path, Path]:
+    private_key = Ed25519PrivateKey.generate()
+    public_key = private_key.public_key()
+    private_key_path = tmp_path / "private.pem"
+    public_key_path = tmp_path / "public.pem"
+    private_key_path.write_text(
+        private_key.private_bytes(
+            encoding=serialization.Encoding.PEM,
+            format=serialization.PrivateFormat.PKCS8,
+            encryption_algorithm=serialization.NoEncryption(),
+        ).decode("utf-8"),
+        encoding="utf-8",
+    )
+    public_key_path.write_text(
+        public_key.public_bytes(
+            encoding=serialization.Encoding.PEM,
+            format=serialization.PublicFormat.SubjectPublicKeyInfo,
+        ).decode("utf-8"),
+        encoding="utf-8",
+    )
+    return private_key_path, public_key_path
+
+
 def test_schema_registry_validates_contract() -> None:
     registry = SchemaRegistry.with_defaults()
     contract = make_contract()
     loaded = registry.validate("BuildContract", "1.0", contract.model_dump(mode="python"))
+    assert isinstance(loaded, BuildContract)
     assert loaded.contract_id == "contract-1"
     assert registry.manifest()["BuildContract"] == ["1.0"]
 
@@ -108,3 +135,25 @@ def test_ci_adapter_generates_contract() -> None:
     assert contract.build_object.commit == "abc123"
     assert contract.environment.variables["env.PYTHONHASHSEED"] == "0"
     assert contract.manifest.run_count == 2
+
+
+def test_contract_signing_and_cli_round_trip(tmp_path: Path, capsys: pytest.CaptureFixture[str]) -> None:
+    private_key_path, public_key_path = write_ed25519_keypair(tmp_path)
+    contract = make_contract()
+
+    signed = contract.sign(private_key_path.read_text(encoding="utf-8"))
+    assert signed.signature is not None
+    assert signed.verify_signature(public_key_path.read_text(encoding="utf-8"))
+
+    tampered = signed.model_copy(update={"notes": ["tampered"]})
+    assert not tampered.verify_signature(public_key_path.read_text(encoding="utf-8"))
+
+    contract_path = tmp_path / "contract.json"
+    signed_path = tmp_path / "signed.json"
+    contract_path.write_text(contract.model_dump_json(indent=2) + "\n", encoding="utf-8")
+
+    assert cli_main(["sign-contract", str(contract_path), str(private_key_path), "--output", str(signed_path)]) == 0
+    assert cli_main(["verify-contract", str(signed_path), str(public_key_path)]) == 0
+
+    out = capsys.readouterr().out
+    assert json.loads(out)["ok"] is True