IIRC these messages usually have their dimensions equal to primes, to help the aliens a little bit - for example the Arecibo message was 1679 = 73 * 23 bits. W

IIRC these messages usually have their dimensions equal to primes, to help the aliens a little bit - for example the Arecibo message was 1679 = 73 * 23 bits. We're hoping the aliens have the sense to recognize a product of two primes.