[ntp:questions] NTP packets with MACs longer than SHA1
mlichvar at redhat.com
Wed Mar 13 10:32:29 UTC 2019
On Tue, Mar 12, 2019 at 01:24:52PM -0400, Danny Mayer wrote:
> On 3/12/19 4:22 AM, Miroslav Lichvar wrote:
> > On 2019-03-11, Nelson Bolyard <nbolyard at silverspringnet.com> wrote:
> >> NTPv3 supported MD5 and SHA1 Message Authentication Code (MACs) of
> >> length 16 and 20 bytes respectively. RFC 5906 says that NTP V4
> >> supports any MAC, but offers no advice about how to send MACs that are
> >> longer than 20 bytes, such as SHA256 MACs.
> >> Are longer MACs sent in their entirety?
> >> Are they truncated to 20 bytes? or to 16 bytes?
> > The digests are truncated to 20 bytes in order to follow RFC 7822.
> Actually it says that they can be no longer than 24 unless otherwise
> negotiated by client and server. See Section 126.96.36.199. In the
> introduction it says it can be 20 or 24 bytes.
Right. The MAC includes a 32-bit key ID, so the digest has to be
shorter by 4 bytes, which is what the OP seemed to be interested in
truncating (it may depend on the actual implementation).
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