Answer: the sender's private key.
Explanation:
For messages sent through an insecure channel, a properly implemented digital signature gives the receiver reason to believe the message was sent by the claimed sender. While using a digital signature, the message digest is encrypted with the sender's private key. When the message is being sent through the sender computes a hash function of the message and encrypts it with the sender's private key . When the message reaches its intended recipient it is decrypted and the hash function is recomputed and compared to the digital signature to establish authenticity of the message and maintain its integrity.
