Comment # 4 on bug 1019412 from Jan Engelhardt

I hope I got this right.. you execute

  ... "hmac(sha256)" 0xsomevalue 128

but get back

  ... "hmac(sha256)" 0xsomevalue 96?

That might be a kernel issue. On 4.8.X (yes, it's not the default openSUSE), I
observe that 128 stays 128:

# ip x s a src ::2 dst ::3 proto ah spi 0xa auth-trunc "hmac(sha256)" 0 128
# ip x s
 ...
 auth-trunc hmac(sha256) 0x30 128

(0 -> 0x30 because '0' has ASCII code 0x30)