Key types are now encoded through a category in the upper 4 bits (bits
28-31) and a type-within-category in the next 11 bits (bits 17-27),
with bit 16 unused and bits 0-15 only used for the EC curve or DH
group.
For symmetric keys, bits 20-22 encode the block size (0x0=stream,
0x3=8B, 0x4=16B).