xbytes | Parse bytes to human readable sizes | Parser library

Remove the root level tag:

(More or less) analogous to how the int type knows how to convert an integer instance to bytes, it is int again who knows how to convert bytes back to an integer.

Your xbytes variable is a bytes object, so it doesn't know how to convert itself to an integer.

Instead you do it like this:

secp521r1 uses a 521-bit prime field. So the X or Y coordinates are represented as 521-bit numbers. Yes, 521, not 512.

521 bits is 65 full bytes and one bit left over. In a fixed-size encoding of the public key the most significant byte would always have the 7 most significant bits set to 0, and the 8th bit will be 1 or 0 (so the whole byte is 0x00 or 0x01).

Since the coordinate space isn't quite all of the 521-bit numbers, you have a slightly less than 50% chance of the high bit being set for any particular point.

Go's method seems to be returning the value using a minimum-byte representation. So you should see something like

• 66 bytes: 49% of the time
• 65 bytes: 50% of the time
• 64 bytes (top 9 bits are all 0): 0.2%
• (63 bytes and below are possible, but with vanishingly small percentages)