DP3-T Implementation profile

Against version 2020/4/8 of the whitepaper

Design 2

General

Byte sequences are 8 bit octed strings.

In both designs - the CDN's webserver its Date: header set (RFC 2616, section 14.18) can be used, during the daily fetch, as the authoritative time specifier.

Generating Empheral IDs

The H is an SHA256 as per per RFC 6234

TRUNKCATE128() takes the first 32 bytes (of the 64 byte SHA256)

Test vector:

Seed:     0000000000000000000000000000000000000000000000000000000000000000 
			(i.e. 0x00, 0x00 .. 0x00 32 bytes)
H (seed): 66687aadf862bd776c8fc18b8e9f8e20089714856ee233b3902a591d0d5f2925
TRUNKCATE128(H(seed)): 
          66687aadf862bd776c8fc18b8e9f8e20

Local storare / handling of ‘t’

‘t’ is a network order (big endian) unsigned 32 bit number. I.e. the number 1 is encoded transmitted as 0x00, 0x00, 0x00, 0x01 on the wire.

‘t’ contains the unix UTC/Z timestamp as defined by RFC 3339.

So the H(EphID||t) stored is a SHA256 taken over 16 + 4 = 20 sequentiel bytes in that order (EphID, then time).

Test vector:

Time: 2020-4-10 00:00:00 UTC
T = 1586476800 
        5E8FB700 (4 bytes)
EphID || t = 
        66687aadf862bd776c8fc18b8e9f8e201586476800) (16+4 butes)
H(EphID || t)
        109708e29597623f56fd365ba92f1c717ca23994aabd7939822909c465cb10a5 (32 bytes)

Cuckoo filter and serialisation version 1.00

(Aligned with https://github.com/dirkx/DP-3T-Documents/tree/editable-version/impl/design-2-openssl-C)

The depth of the Cuckoo filter shall be 4.

The hash shall be a SHA256.

The key shall be 32 bytes which are used as follows:

byte  0..3		up to Z bits for the LSB of the Cuckoo hash
byte  4..7		up to Z bits for the MSB of the Cuckoo hash
byte  8..		up to `verifylimit' bytes.

Where is the number of bits needed for the number of buckets (e.g Z=19 if the buckets are is 524288).

Where verify limit is set as low as is feasible given the acceptable false positive rates.

The Cuckoo filter shall be serialised as:

Magic string 	4	bytes D3, D3, 3D, 3D
Major version	1	byte, currently 1 for this version
Minor version 	1	byte, currently 0
<Depth>			1	byte, fixed to 4
<verifylimit>	1	byte
<N buckets>		4	bytes, unsigned 32 bit integer, network order
<N slots>		4	bytes, unsigned 32 bit integer, network order

Followed by

<N buckets> with each
	<Depth> slots with each
		is occupied			1 bit
		partial hash		31 bits, network order, of which <Z> are relevant.
		verify hash			<verifylimit> bytes of the verify hash

With the partial hash being limited to the number of bits needed for N buckets.

And with the Hash

Cuckoo filter publication

The filter should be published prefixed by an RFC3161 timestamp.

Design 1

The PRF used is HMAC-SHA256 as per RFC 6234 and RFC 2104 - and and where Skt_ is used as the `key’ and the string “Decentralized Privacy-Preserving Proximity Tracing” (without trailing \0, i.e. exactly those 50 US-ASCII characters is the plaintext.

Test vectors:

SK: 
	0000000000000000000000000000000000000000000000000000000000000000
SK derivation:
	66687aadf862bd776c8fc18b8e9f8e20089714856ee233b3902a591d0d5f2925

The PRG used is AES128 in counter mode; with the IV set to a 128 bit unsigned number in network order (i.e the first IV is a byte array if [ 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01 ]) we start at 0, not 1 and the plaintext 128 bits of 0’s.

IV: 00000000000000000000000000000000        
eph: 0 - Ephemeral(day:0, token:c7044845a6a0da7a61687e1bb08afca4)

IV: 00000000000000000000000000000001
eph: 1 - Ephemeral(day:0, token:a747e729bf2e3de3ec6ecbdb0f889f5b)

IV: 00000000000000000000000000000002
eph: 2 - Ephemeral(day:0, token:034015608c5a55672315cb614f5a94a3)

3.7 KiB Raw Blame History Unescape Escape