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How would two clients communicate over IPv6 without a third-party knowing which addresses are used? This is one of the abstract problems I tried to solve back in 2013, when developing the idea of a secure messaging client that makes use of certain features associated with IPv6 (many thanks to Sam Bowne and Chris Tubb for the inspiration). It was based on two assumptions: a) both parties are assigned a block of IPv6 addresses rather than a single address, and b) communicating parties are able to arbitrarily select addresses from within their address ranges.

Address Spaces and Allocations
Given the number of possible IPv6 addresses (2^38 minus a few reserved address ranges), it’s possible that a person would be assigned a sizeable block of addresses from this, such as a 32-bit address space with 4294967296 possible addresses.

I’ve done a bit of research to determine the likely address space a person would typically be assigned. RFC 6177 reccommends allocating /48 blocks to each individual ISP customer. Whether this would actually happen in the real world remains to be seen – it’s also strongly recommended because IPv6 removes the requirement for Network Address Translation, which in turns means that an ideal allocation for a home network would be large enough to make network enumeration a little more time consuming.
IPv6 also allows for stateless address configuration, which should enable clients to select their own addresses, although this depends on how the local router is configured.

The Address Generation Algorithm
My solution is something like:

The session key is secret between two clients – how they share this is another problem which might require out-of-band communication using a public key system. Actually my proposal would be a good candidate for an instant messaging system or social network that works alongside Dark Mail.

The second parameter is the system time, in ‘HHMM’ format, because the algorithm should generate a different IPv6 address every x number of minutes, and HHMM should also be the same for both communicating clients. With a little more coding later, two clients might get this value from a shared source, perhaps over NTP.

Python Implementation
The following imports are required for implementing the concept as a Python script:
* string
* hashlib
* netaddr
* pprint
* time.gmtime and time.strftime

New addresses are generated from a current IPv6 address and a session key that might be shared between peers. These might be read from an application database and/or network interface.

selfAddress = '3ffe:1900:4545:0003:0200:f8ff:fe21:67cf'
selfKey = 'mypassword123'
peerAddress = ' '
peerKey = ' '
currentTime = strftime("%H%M")

In order to get the current address, we require a networking/NIC module that enables us to select the network interface to read from. I’m most of the way through coding a C# version of the client, using System.Net.NetworkInformation to populate a drop-down list of interfaces.

Using the netaddr and pprint modules, an address can be formatted as a hexadecimal string – basically to get the digits without the octet delimiters. The line ‘selfAddressToHex[2:]‘ removes the ‘0x‘ characters from the output.

ip = IPAddress(0, 6)
ip = IPNetwork(selfAddress)
selfAddressToHex = hex(ip.ip)
selfAddressString = selfAddressToHex[2:]

Then a SHA256 fingerprint is generated with [sessionKey+HHMM] as inputs.

hashInput = (selfKey + currentTime)
print('Hash Input: ' + hashInput)
hashedValue = hashlib.sha256(hashInput)
hashedValueString = (hashedValue).hexdigest()
print('SHA256 Fingerprint: ' + hashedValueString)

Now we can substitute the last eight bytes of the current IP address with the last eight bytes of the SHA256 value to generate a new address:

final32 = hashedValueString[56:64]
print('New Suffix: ' + final32)
newAddressString = selfAddressString.replace(selfAddressString[24:32], final32)

Finally, reformat the hex string as a valid IPv6 address by adding the delimiters between octets:

newAddress = ':'.join([newAddressString[i:i+4] for i in range(0, len(newAddressString), 4)])
print('New Address: ' + newAddress)

The running script will produce something like:

We can later write newAddress back to the application database as ‘currentAddress’, and have something that triggers this part of the application every 15 minutes.
There are other things I’d like to build on this, namely components for setting newAddress as the local IP address, and messaging between two clients running the script.