feat: add SSH key payload signing and verification endpoints

- POST /ssh-keys/{id}/sign - sign payload with Ed25519 private key
- POST /ssh-keys/{id}/verify - verify signature with public key
- Returns base64-encoded signatures

apps/api/src/api/ssh_keys.py

@@ -1,3 +1,4 @@
+import base64
 import uuid
 from datetime import datetime
 
@@ -74,6 +75,23 @@ class SSHKeyResponse(BaseModel):
     created_at: datetime
 
 
+class SignPayloadRequest(BaseModel):
+    payload: str
+
+
+class SignatureResponse(BaseModel):
+    signature: str
+
+
+class VerifySignatureRequest(BaseModel):
+    payload: str
+    signature: str
+
+
+class VerifySignatureResponse(BaseModel):
+    valid: bool
+
+
 @router.post(
     "",
     response_model=SSHKeyResponse,
@@ -166,3 +184,80 @@ async def delete_ssh_key(
 
     await session.delete(ssh_key)
     await session.commit()
+
+
+@router.post(
+    "/{key_id}/sign",
+    response_model=SignatureResponse,
+    summary="Sign payload",
+    description="Sign a payload using the SSH private key.",
+)
+async def sign_payload(
+    key_id: uuid.UUID,
+    data: SignPayloadRequest,
+    user_id: uuid.UUID = Depends(get_current_user_id),
+    session: AsyncSession = Depends(get_db_session),
+) -> SignatureResponse:
+    """Sign a payload with an SSH key.
+
+    Args:
+        key_id: UUID of the SSH key to use for signing.
+        data: Sign request containing the payload string.
+        user_id: ID of the authenticated user.
+        session: Database session.
+
+    Returns:
+        Base64-encoded Ed25519 signature.
+    """
+    user = await _get_user(session, user_id)
+    ssh_key = await session.get(SSHKey, key_id)
+    if ssh_key is None or ssh_key.user_id != user.id:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="ssh key not found")
+
+    fernet = _get_fernet()
+    private_key_pem = fernet.decrypt(ssh_key.private_key_encrypted.encode()).decode()
+
+    private_key = serialization.load_ssh_private_key(
+        private_key_pem.encode(), password=None
+    )
+
+    signature = private_key.sign(data.payload.encode())
+    return SignatureResponse(signature=base64.b64encode(signature).decode())
+
+
+@router.post(
+    "/{key_id}/verify",
+    response_model=VerifySignatureResponse,
+    summary="Verify signature",
+    description="Verify a signature against a payload using the SSH public key.",
+)
+async def verify_signature(
+    key_id: uuid.UUID,
+    data: VerifySignatureRequest,
+    user_id: uuid.UUID = Depends(get_current_user_id),
+    session: AsyncSession = Depends(get_db_session),
+) -> VerifySignatureResponse:
+    """Verify a signature with an SSH key's public key.
+
+    Args:
+        key_id: UUID of the SSH key to use for verification.
+        data: Verify request containing payload and base64-encoded signature.
+        user_id: ID of the authenticated user.
+        session: Database session.
+
+    Returns:
+        Whether the signature is valid.
+    """
+    user = await _get_user(session, user_id)
+    ssh_key = await session.get(SSHKey, key_id)
+    if ssh_key is None or ssh_key.user_id != user.id:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="ssh key not found")
+
+    public_key = serialization.load_ssh_public_key(ssh_key.public_key.encode())
+
+    try:
+        signature = base64.b64decode(data.signature)
+        public_key.verify(signature, data.payload.encode())
+        return VerifySignatureResponse(valid=True)
+    except Exception:
+        return VerifySignatureResponse(valid=False)