[cabfpub] Thoughts on reducing SCT sizes (was Re: Updated Certificate Transparency + Extended Validation plan)
jeremy.rowley at digicert.com
Wed Feb 5 16:19:07 UTC 2014
Table 1 of the plan document said both 3 SCTs and 4 SCTs for 27 months.
Until there is clarification on which is required, 3-4 is the best
representation of the requirement. I'm hoping Ben meant 15-27 months = 3 and
>27 = 4, but it's not clear from the document.
From: public-bounces at cabforum.org [mailto:public-bounces at cabforum.org] On
Behalf Of Rob Stradling
Sent: Wednesday, February 05, 2014 5:37 AM
To: certificate-transparency at googlegroups.com
Cc: therightkey at ietf.org; CABFPub
Subject: [cabfpub] Thoughts on reducing SCT sizes (was Re: Updated
Certificate Transparency + Extended Validation plan)
On Tue, Feb 4, 2014 at 12:33 PM, Jeremy Rowley wrote:
> Three or four proofs for a 27 month certificate is way too many.
> Adding 400 bytes per certificate will make EV certificates unusable by
entities concerned with performance.
The updated CT+EV plan requires three SCTs for a (maximum length) 27-month
EV certificate, not four. 400 bytes for three SCTs is about right though.
Assuming RFC6962-compliant v1 SCTs that contain no SCT extensions and are
signed using ECDSA and a P-256 private key, then, including all of the ASN.1
fluff for the SCT List certificate extension, I calculate that it'll be...
140 or 141 bytes to embed 1 SCT
261 to 263 bytes to embed 2 SCTs
380 to 383 bytes to embed 3 SCTs
For (non-EV) validity periods between 27 and 39 months:
499 to 503 bytes to embed 4 SCTs
On 04/02/14 17:52, Adam Langley wrote:
> We should make the SCTs as small as possible
Agreed. Time for some back-of-an-envelope sums. For SCT v2, if we were to
pack in the data as tightly as possible I reckon we could cut it down to as
84 bytes to embed 1 SCT
159 bytes to embed 2 SCTs
231 bytes to embed 3 SCTs
303 bytes to embed 4 SCTs
1. Use a shorter OID for the SCT List extension. Perhaps CABForum could
define 2.23.140.n (with n < 128). Save 6 bytes.
2. The first 2 bytes of the SignedCertificateTimestampList structure are its
total length. Since this can be calculated from the OCTET STRING length,
these 2 bytes could be omitted. Save 2 bytes.
3. Pack the SCT fields into as few bytes as possible for the common case,
whilst retaining options for future expansion. Save 37 bytes per SCT.
(1 byte) Version sct_version;
(32 bytes) LogID id;
(8 bytes) uint64 timestamp;
(2+? bytes) CtExtensions extensions;
(2 bits) sct_version (00=v1; 01=v2; 10,11=unassigned)
(2 bits) log_id_type (00=SHA-256(log_public_key);
01=1-byte Registered Log ID;
10=2-byte Registered Log ID;
11=4-byte Registered Log ID)
(2 bits) timestamp_size (00=8-bytes;
(1 bit) extensions (0=CtExtensions is present;
1=CtExtensions is absent)
(1 bit) signature_type (0=digitally-signed struct;
1=raw Ed25519 signature)
For the common case:
(1 byte) Registered Log ID
(4 bytes) Timestamp (seconds, not milliseconds)
4. Use the Ed25519 signature scheme instead of ECDSA. ECDSA signatures
using a P-256 key seem to be 72 or 73 bytes, whereas Ed25519 signatures are
64 bytes. Save 8 or 9 bytes per SCT.
Also, for Ed25519, omit the 2 bytes containing the hash algorithm and
signature algorithm from the "digitally-signed struct" header. Save 2 bytes
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