Yes, for an Indian citizen, it would generally be illegal to sign a bilateral treaty without permission from the Indian government. In India, the authority to negotiate and sign treaties rests with the government, specifically with the executive branch. The President of India, the Prime Minister, or other designated government officials, typically the Minister of External Affairs, are authorized to represent the country in international relations and enter into treaties on behalf of India.
India's constitutional framework and legal system establish procedures for the negotiation, conclusion, and ratification of treaties. The Ministry of External Affairs, along with other relevant government departments and agencies, is responsible for overseeing the treaty-making process. Treaties must be ratified by the President of India before they become legally binding on the country.
Individuals, including Indian citizens, do not have the authority to enter into treaties on behalf of the Indian government without proper authorization. Unauthorized treaty-making by individuals would likely be considered invalid and may have legal consequences under Indian law. It could also have diplomatic implications and affect India's international relations.
Therefore, if an Indian citizen wishes to negotiate or sign a bilateral treaty on behalf of India, they would need to obtain permission and authorization from the Indian government, typically through the Ministry of External Affairs or other relevant government authorities.