Of course it is possible to encrypt an unreliable unidirectional bitstream. (Tons of systems do this, eg. satellite video links.) Contrary to what other commen

Of course it is possible to encrypt an unreliable unidirectional bitstream. (Tons of systems do this, eg. satellite video links.) Contrary to what other commenters suggest, a proper stream cipher like ChaCha20 is not even needed. You could just use CTR mode, which turns any block cipher (like AES) in a stream cipher and prevents ciphertext bit errors from creating more plaintext bit errors. Also, transmit the counter every once in a while so that dropped packets don't prevent you from decrypting subsequent packets.