  Satellite-based quantum communication for secure key distribution is becoming
a more demanding field of research due to its unbreakable security. Prepare and
measure protocols such as BB84 consider the satellite as a trusted device,
fraught with danger looking at the current trend for satellite-based optical
communication. Therefore, entanglement-based protocols must be preferred since,
along with overcoming the distance limitation, one can consider the satellite
as an untrusted device too. E91 protocol is a good candidate for
satellite-based quantum communication; but the key rate is low as most of the
measured qubits are utilized to verify a Bell-CHSH inequality to ensure
security against Eve. An entanglement-based protocol requires a maximally
entangled state for more secure key distribution. The current work discusses
the effect of non-maximality on secure key distribution. It establishes a lower
bound on the non-maximality condition below which no secure key can be
extracted. BBM92 protocol will be more beneficial for key distribution as we
found a linear connection between the extent of violation for Bell-CHSH
inequality and the quantum bit error rate for a given setup.
