Scale a ciphertext down from extended CRT basis to Q

Brings a ciphertext that lives in the extended PQ basis (for example, the output of eval_fast_rotation_ext() with hybrid key switching) back to the standard Q basis. Only supported when the scheme is configured with hybrid key switching — other key-switching techniques have no round-trip to extended PQ and therefore nothing to scale back from.

Usage

key_switch_down(ct)

Arguments

ct

A Ciphertext in the extended P*Q basis.

Value

A Ciphertext in the Q basis.

Details

R-first binding: openfhe-python v1.5.1.0 does not expose KeySwitchDown at all. See notes/upstream-defects.md for the R-only surface tracking.

See also

eval_fast_rotation_ext()