convert Knot DNS sigantures certs to BIND format.

[Tony Finch]

Milan Jeskynka Kazatel <KazatelM at seznam.cz> wrote:
>
> I received a hint for a tool which allows converting .pem format used in
> Knot to .key and .private used in BIND, but it, unfortunately, does not
> support ECDSAP256SHA256 algorithm which I used.

Ah, sounds like Knot uses a relatively familiar key format, so we can hack
around with OpenSSL command line tools.

Unless I have missed something, BIND doesn't have any support for non-BIND
key files: it has its own code for reading and writing keys, which knows
about OpenSSL's in-memory key format. (I think this is related to support
for multiple crypto providers, and the fact that supporting PEM implies
supporting ASN.1 which is not a task any wise programmer would take on.)

So I think you'll have to get dirty with the key internals; fortunately
the modern key types handle the private material as a blob so you don't
have to fiddle around with half a dozen parameters.

If you have an ECDSA key in PEM format, you can break it open like
this. The short blob is the private key and the long one is the public
key.

$ openssl ec </var/lib/knot/keys/keys/c3e8539dc582bb2ceeca0ab9fb7b89d521a4f658.pem |
  openssl asn1parse -dump
read EC key
writing EC key
    0:d=0  hl=2 l= 119 cons: SEQUENCE
    2:d=1  hl=2 l=   1 prim: INTEGER           :01
    5:d=1  hl=2 l=  32 prim: OCTET STRING
      0000 - f5 60 92 ac fe 6f 49 3a-cf 32 b3 16 21 2c f7 37   .`...oI:.2..!,.7
      0010 - 46 94 eb 06 4f 71 11 f1-71 92 84 f6 0d 16 73 de   F...Oq..q.....s.
   39:d=1  hl=2 l=  10 cons: cont [ 0 ]
   41:d=2  hl=2 l=   8 prim: OBJECT            :prime256v1
   51:d=1  hl=2 l=  68 cons: cont [ 1 ]
   53:d=2  hl=2 l=  66 prim: BIT STRING
      0000 - 00 04 87 d7 36 06 dc d7-86 36 07 49 d2 c2 f9 7b   ....6....6.I...{
      0010 - 2d 30 64 3a 1c 12 e0 a1-ea dc cd 1f be a4 0f e8   -0d:............
      0020 - c2 d5 af fe 30 71 be 12-62 60 ba 07 ea 07 17 28   ....0q..b`.....(
      0030 - 97 5d 08 cd c4 55 c1 88-bf db b6 e5 34 12 1d 0e   .]...U......4...
      0040 - d2 ac                                             ..

BIND wants these in base64. A not completely impossible way to do this is
to feed the binary (DER) form of the key to a bit of perl. (PEM is base64
encoded DER.) This involves some magic numbers for the offsets of the
blobs derived from the asn1 dump above.

$ openssl ec -outform der </var/lib/knot/keys/keys/c3e8539dc582bb2ceeca0ab9fb7b89d521a4f658.pem |
  perl -Mv5.10 -MMIME::Base64 -e '
    undef $/; my $k = <STDIN>;
    print encode_base64 substr $k, 7, 32;
    print encode_base64 substr $k, -64;'
read EC key
writing EC key
9WCSrP5vSTrPMrMWISz3N0aU6wZPcRHxcZKE9g0Wc94=
h9c2BtzXhjYHSdLC+XstMGQ6HBLgoerczR++pA/owtWv/jBxvhJiYLoH6gcXKJddCM3EVcGIv9u2
5TQSHQ7SrA==

The first line is the private key; the second and third lines are the
public key. We can check it matches:

$ cat /var/lib/knot/keys/zone_example.com.json
{
  "policy": "\u0006policy",
  "nsec3_salt": null,
  "nsec3_salt_created": null,
  "keys": [
    {
      "id": "c3e8539dc582bb2ceeca0ab9fb7b89d521a4f658",
      "keytag": 19633,
      "algorithm": 13,
      "public_key": "h9c2BtzXhjYHSdLC+XstMGQ6HBLgoerczR++pA/owtWv/jBxvhJiYLoH6gcXKJddCM3EVcGIv9u25TQSHQ7SrA==",
      "ksk": false,
      "created": "2019-03-12T15:44:02+0000"
    }
  ]
}

Probably the easiest way to turn this into BIND key files is to run
`dnssec-keygen -a ecdsa256 example.com` and edit the output to insert the
short private and long public base64 blobs emitted by the perl. You will
also need to rename the files to match the keytag in knot's zone_*.json
file.

Tony.
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