Key ID from DNSKEY - how?

[Phil Mayers]

On 10/27/2010 06:46 PM, Mark Elkins wrote:
> I would like to calculate the Key-ID from a DNSKEY record. I'd prefer to
> do this in PHP as this is inside some existing PHP (Web) scripts but I
> guess calling a C program would not be too inconvenient.

I use some Python code to do this in our debugging/management tools, 
translated straight from the RFC; it might convert pretty easily into 
PHP, although in my experience language number/bit-shift/overflow 
behaviour can be a bit... odd.

def key2keytag(flags, alg1, alg2, keydata):
     data = struct.pack('!HBB', flags, alg1, alg2)
     data += keydata.decode('base64')
     v = 0
     for i in range(len(data)):
         if i & 1:
             v += ord(data[i])
         else:
             v += ord(data[i]) << 8
     v += (v >> 16) & 0xffff
     return v & 0xffff

Called like so:

tag = key2tag(257, 3, 5, 'AwEAA...')

Very handy during testing is:

dig +multi domain.com DNSKEY

...which displays the tag as a comment. HTH