Forwarding key rollover design

This document describes an implementation for key rollover in a SCION AS.

Because key rollover is an AS-local concept (the same AS does both MAC computation and verification), it is not a part of the SCION protocol. Every AS is free to implement these aspects however it sees fit, and this document presents one possible implementation.

Notation

ExpirationField: The value of the 1-byte HopField ExpTime field inserted by the local beacon server. The linear scale as defined in the SCION book is used. This is unknown, and must be computed by the beacon server during beaconing (subject to AS policy).

MACField: The value of the 3-byte HopField MAC field inserted by the local beacon server. This is unknown, and must be computed by the beacon server during beaconing.

OriginationTime: The timestamp when a beacon is created. This is the timestamp that is included in the Info Field of the segment. This is hardcoded in the segment and cannot be changed.

ExpirationTime: The absolute point in time when the hop field expires. This is defined as OriginationTime + (1 + ExpirationField) * 337s, where s refers to seconds.

MasterKey: The key used to derive forwarding keys.

This document explains how, given OriginationTime, the CS should compute ExpirationField and MACField to maximize the lifetime of the segment, in a way that the border router can later successfully verify the MAC if and only if it was created by the CS.

Design

Properties

Each key has two attributes:

  • NotValidBefore: the absolute time before which the key MUST NOT be used to compute or verify MACs;
  • NotValidAfter: the absolute time after which the key MUST NOT be used to compute or verify MACs;

If at a point of reference the current time is greater or equal to NotValidBefore and less than NotValidAfter, a key is said to be valid.

Property 1: A constant number of keys must be valid at any time.

Let KeyLifetime be defined as NotValidAfter - NotValidBefore.

Property 2: KeyLifetime must be constant for all keys.

Property 3: A key rollover sequence is defined as a sequence of keys with the following constraints:

  • Properties 1 and 2 must hold
  • NotValidBefore values need to be monotonically increasing and equally spaced in time

The diagram below shows a well formed key rollover sequence of size 4, with 2 keys valid at the same time: K_n and K_(n+1).

../_images/rollover-4-slot-rollover.png

For simplicity, for the rest of this document we assume the following sizes:

  • exactly 2 keys are valid at any time
  • the rollover sequences size is 4

OriginationTime is problematic because it is rarely decided by the local AS. Depending on how beaconing happens upstream, OriginationTime can be so far in the past that no forwarding key can be valid for the whole lifetime of the path. However, assuming somewhat comparable clocks between CSes and routers, it is irrelevant if the key is valid for any point in time prior to the moment when the MAC is computed.

Refer to the diagram below. In this diagram, a CS must propagate an old beacon, and must thus choose which of the two currently valid keys, K_n and K_(n+1), to use for the computation of the MAC. The beacon is still valid, but its OriginationTime is very far in the past, so far that no currently valid key is valid for both. The question the CS must ask itself is: what is the remaining lifetime of this path going to be, according to my security policy? OriginationTime is irrelevant to the answer, which leads to the next property.

../_images/rollover-origination-time.png

Property 4: OriginationTime must not be used directly for key selection; ExpirationTime should be used instead.

If OriginationTime is not used by the CS when deciding to compute the MAC, it also cannot be used by the router when verifying the MAC (note that it will be used by the router when deciding if the HF is still within its predefined lifetime).

The CS must choose between K_n and K_(n+1). The only value it can publish about its decision is the ExpirationTime (which it does via the ExpirationField in the HF). The router will later need to decide which key to use based only on the ExpirationTime; therefore, the choice must be unambiguous.

We remove the ambiguity by always choosing the key that is valid at ExpirationTime and expires last. We showcase this with two examples.

In the diagram below, a beacon must be constructed, and at CurrentTime keys K_n and K_(n+1) are valid. The CS wants to build a HF with a desired remaining lifetime of half of KeyLifetime. Because K_n is not valid for the desired remaining lifetime, K_(n+1) must be chosen.

../_images/rollover-key-choice.png

In the diagram below, the CS wants to construct a HF with a much shorter lifetime. Both keys are valid for the remaining lifetime. For the router to later choose a key unambiguously, it must select K_n.

../_images/rollover-two-key-choices.png

The router does the same computation to determine the key.

Property 5: Given the restrictions established in this section (exactly 2 keys valid), MAC computation is always done by selecting the key with the smallest NotValidAfter value that is strictly greater than the desired ExpirationTime (the strict here is important, because key lifetime is an open interval on right).

There are situations where the desired remaining lifetime is impossible to satisfy. This might be always incompatible with key policy (the desired remaining lifetime is greater than key lifetime), or no valid key might exist at current time.

The latter situation is described in the diagram below. Desired remaining lifetime is 3/4 of KeyLifetime. Thus, no valid key can be found.

../_images/rollover-invalid-lifetime.png

To avoid this, we add the constraint that desired remaining lifetime be at smaller or equal to 1/2 of KeyLifetime. If an AS wants paths to have maximum lifetime while having constant remaining lifetime, then desired remaining lifetime can be set to 1/2 KeyLifetime.

Implementing the CS

The CS needs to compute two values:

  • ExpirationField
  • MACField

ExpirationField

The ExpirationField depends on the policy of desired remaining HF lifetime. Taking into account Property 5, we select a constant desired remaining HF lifetime of half of KeyLifetime.

Let DesiredRemainingLifetime be the time the HF is going to be valid for, relative to current time. This is a value chosen by AS security policy; the policy will usually specify a MaxRemainingLifetime, which is the upper bound on acceptable DesiredRemainingLifetime.

We add the constraint that the HF MUST NOT be valid for longer than a DesiredRemainingLifetime, but CAN be valid for less. This leads to a cleaner intuition of the security offered by the parameter, where stricter is better.

ExpirationField is computed as follows:

ExpirationField = floor((CurrentTime - OriginationTime + DesiredRemainingLifetime) / 337s) - 1

If ExpirationField is negative, then the DesiredRemainingLifetime is too short and not supported by the SCION protocol.

If ExpirationField is more than 255, then the DesiredRemainingLifetime is too long and not supported by the SCION protocol for this specific beaconing attempt. Because the value depends on CurrentTime, and can be the case that the DesiredRemainingLifetime can be satisfied for only certain beacons.

Note that an increase of 1 in the value of ExpirationField leads to an increase of 337 seconds in the lifetime of the HF. This means that the actual remaining lifetime of the HF is an approximation of the initial DesiredRemainingLifetime.

For CSes that originate the beacon, CurrentTime - OriginationTime SHOULD be 0.

MACField

First, the secret key needs to be computed from the MasterKey. The function we recommend is:

Key = HKDF(pad128(2*ExpirationTime//KeyLifetime - 1) || MasterKey)

where pad128 is a left padding function to a bit-length of 128, || is the bit concatenation operator, // is integer division with integer quotient, and MasterKey is a shared secret of the AS. How to disseminate MasterKey is up for discussion, with the solution of storing it in a file on-disk being the simplest option for now. Other systems like Vault might be useful in this scenario, if storing it on disk is undesirable. MasterKey should be refreshed periodically (e.g., once every 6 months).

Then, the MACField is computed as defined in the SCION protocol.

Implementing the router

When the router receives a path for processing, it must verify the HF. The first step in this process is computing the MAC verification key. To compute the key, first the ExpirationTime must be computed:

ExpirationTime = OriginationTime + (1 + ExpirationField) * 337s

If ExpirationTime < CurrentTime, the router SHOULD return an error stating as such. If the check is successful, then the same function as in the CS can be used to compute the key:

Key = HKDF(pad128(2*ExpirationTime//KeyLifetime - 1) || MasterKey)

If Key is not valid at CurrentTime, then HF verification MUST fail.

Fast key lookup

It is expensive to compute the Key for every MAC. A key rollover sequence could be used instead. We show how a rollover sequence of size 2 can be used and updated. Let KeyContext be this rollover sequence. The sequence has the following fields:

  • Key0, the first valid key;
  • Key1, the second valid key;
  • ExpirationTimeKey0, the expiration time of the first key;
  • ExpirationTimeKey1, the expiration time of the second key;

Let K_n be the sequence of forwarding keys, with n an integer value greater or equal to 0. K_0 is active from moment 0 (beginning of unix time) to KeyLifetime (excluding KeyLifetime, so open interval on KeyLifetime), K_1 is active from KeyLifetime//2 to 3*KeyLifetime//2, and so on and so forth. Key K_n is thus valid from n*KeyLifetime//2 to (n+2)*KeyLifetime//2. Whenever a new key becomes valid, the following procedure runs:

set_new_key(NewKey, ExpTimeNewKey)
    NewKeyCtx.Key0 = CurrentKeyCtx.Key1
    NewKeyCtx.ExpirationTimeKey0 = CurrentKeyCtx.ExpirationTimeKey1
    NewKeyCtx.Key1 = NewKey
    NewKeyCtx.ExpirationTimeKey1 = ExpirationTimeNewKey
    update_key_ctx(NewKeyCtx)

For each packet, the router must first compute the expiration time of the HF, similarly to the CS:

ExpirationTime = OriginationTime + (1 + ExpirationField) * 337s
get_key(ExpirationTime)
    if ExpirationTime < CurrentKeyCtx.ExpirationTimeKey0
        return CurrentKeyCtx.Key0
    else
        return CurrentKeyCtx.Key1

The key can then be used by the router to verify the MAC.