  We present analytically and numerically the spectrum of high harmonic
emission generated by twisted electrons in the presence of linearly polarized
light. Ensuing transitions from electronic continuum states with orbital
angular momentum to bound states give rise to circularly polarized attosecond
pulses. For central collisions with twisted wavepackets continuum-bound
transitions are subject to dipole selection rules. For non-central collisions a
crossover from circularly to linearly polarized emission occurs for increasing
impact parameter, due to the transverse topology of twisted wavepackets.
