tor_netdir/

hsdir_params.rs

//! Compute which time period and shared random value from a consensus to use at
//! any given time.
//!
//! This is, unfortunately, a bit complex.  It works as follows:
//!
//!   * The _current_ time period is the one that contains the valid-after time
//!     for the consensus...
//!      * but to compute the time period interval, you need to look at the
//!        consensus parameters,
//!      * and to compute the time period offset, you need to know the consensus
//!        voting interval.
//!
//!   * The SRV for any given time period is the one that that was the most
//!     recent at the _start_ of the time period...
//!      * but to know when an SRV was most recent, you need to read a timestamp
//!        from it that won't be there until proposal 342 is implemented...
//!      * and until then, you have to compute the start of the UTC day when the
//!        consensus became valid.
//!
//! This module could conceivably be part of `tor-netdoc`, but it seems better
//! to make it part of `tor-netdir`: this is where we put our complexity.
///
/// (Here in Arti we use the word "ring" in types and variable names only
/// to refer to the actual actual reified ring, not to HSDir parameters, or
/// or other aspects of the HSDir ring structure.)
use std::time::{Duration, SystemTime};

use crate::{Error, HsDirs, Result, params::NetParameters};
use tor_hscrypto::time::TimePeriod;
use tor_netdoc::doc::netstatus::{MdConsensus, SharedRandVal};

#[cfg(feature = "hs-service")]
use tor_hscrypto::ope::SrvPeriodOffset;

/// Parameters for generating and using an HsDir ring.
///
/// These parameters are derived from the shared random values and time
/// parameters in the consensus, and are used to determine the
/// position of each HsDir within the ring.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct HsDirParams {
    /// The time period for this ring.  It's used to ensure that blinded onion
    /// keys rotate in a _predictable_ way over time.
    pub(crate) time_period: TimePeriod,
    /// The SharedRandVal for this ring.  It's used to ensure that the position
    /// of each HsDir within the ring rotates _unpredictably_ over time.
    pub(crate) shared_rand: SharedRandVal,
    /// The range of times over which the srv is most current.
    pub(crate) srv_lifespan: std::ops::Range<SystemTime>,
}

/// By how many voting periods do we offset the beginning of our first time
/// period from the epoch?
///
/// We do this so that each of our time periods begins at a time when the SRV is
/// not rotating.
const VOTING_PERIODS_IN_OFFSET: u32 = 12;

/// How many voting periods make up an entire round of the shared random value
/// commit-and-reveal protocol?
///
/// We use this to compute an SRV lifetime if one of the SRV values is missing.
const VOTING_PERIODS_IN_SRV_ROUND: u32 = 24;

impl HsDirParams {
    /// Return the time period for which these parameters are valid.
    ///
    /// The `hs_blind_id` for an onion service changes every time period: when
    /// uploading, callers should use this time period to determine which
    /// `hs_blind_id`'s descriptor should be sent to which directory.
    pub fn time_period(&self) -> TimePeriod {
        self.time_period
    }

    /// Return the starting time for the shared-random-value protocol that
    /// produced the SRV for this time period.
    pub fn start_of_shard_rand_period(&self) -> SystemTime {
        self.srv_lifespan.start
    }

    /// Return an opaque offset for `when` from the start of the shared-random-value protocol
    /// period corresponding to the SRV for this time period.
    ///
    /// When uploading, callers should this offset to determine
    /// the revision counter for their descriptors.
    ///
    /// Returns `None` if when is after the start of the SRV period.
    #[cfg(feature = "hs-service")]
    pub fn offset_within_srv_period(&self, when: SystemTime) -> Option<SrvPeriodOffset> {
        if when >= self.srv_lifespan.start {
            let d = when
                .duration_since(self.srv_lifespan.start)
                .expect("Somehow, range comparison was not reliable!");
            return Some(SrvPeriodOffset::from(d.as_secs() as u32));
        }

        None
    }

    /// Compute the `HsDirParams` for the current time period, according to a given
    /// consensus.
    ///
    /// rend-spec-v3 section 2.2.1 et seq
    ///
    /// Return the ring parameters for the current period (which clients use when
    /// fetching onion service descriptors), along with a Vec of ring
    /// parameters for any secondary periods that onion services should additionally
    /// use when publishing their descriptors.
    ///
    /// Note that "current" here is always relative to a given consensus, not the
    /// current wall-clock time.
    ///
    /// (This function's return type is a bit cumbersome; these parameters are
    /// bundled together because it is efficient to compute them all at once.)
    ///
    /// Note that this function will only return an error if something is
    /// _extremely_ wrong with the provided consensus: for other error cases, it
    /// returns a "disaster fallback".
    pub(crate) fn compute(
        consensus: &MdConsensus,
        params: &NetParameters,
    ) -> Result<HsDirs<HsDirParams>> {
        let srvs = extract_srvs(consensus)?;
        let tp_length: Duration = params.hsdir_timeperiod_length.try_into().map_err(|_| {
            // Note that this error should be impossible:
            // The type of hsdir_timeperiod_length() is IntegerMinutes<BoundedInt32<30, 14400>>...
            // It should be at most 10 days, which _definitely_ fits into a Duration.
            Error::InvalidConsensus(
                "Minutes in hsdir timeperiod could not be converted to a Duration",
            )
        })?;
        let offset = consensus.lifetime().voting_period() * VOTING_PERIODS_IN_OFFSET;
        let cur_period = TimePeriod::new(tp_length, consensus.lifetime().valid_after(), offset)
            .map_err(|_| {
                // This error should be nearly impossible too:
                // - It can occur if the time period length is not an integer
                //   number of minutes--but we took it from an IntegerMinutes,
                //   so that's unlikely.
                // - It can occur if the time period length or the offset is
                //   greater than can be represented in u32 seconds.
                // - It can occur if the valid_after time is so far from the
                //   epoch that we can't represent the distance as a Duration.
                Error::InvalidConsensus("Consensus valid-after did not fall in a time period")
            })?;

        let current = find_params_for_time(&srvs[..], cur_period)?.unwrap_or_else(|| {
            tracing::debug!("No SRV params for {cur_period:?}; falling back to disaster params");
            disaster_params(cur_period)
        });

        // When computing secondary rings, we don't try so many fallback operations:
        // if they aren't available, they aren't available.
        #[cfg(feature = "hs-service")]
        let secondary = [cur_period.prev(), cur_period.next()]
            .iter()
            .flatten()
            .flat_map(|period| find_params_for_time(&srvs[..], *period).ok().flatten())
            .collect();

        Ok(HsDirs {
            current,
            #[cfg(feature = "hs-service")]
            secondary,
        })
    }
}

/// Compute ring parameters using a Disaster SRV for this period.
fn disaster_params(period: TimePeriod) -> HsDirParams {
    HsDirParams {
        time_period: period,
        shared_rand: disaster_srv(period),
        srv_lifespan: period
            .range()
            .expect("Time period cannot be represented as SystemTime"),
    }
}

/// Compute the "Disaster SRV" for a given time period.
///
/// This SRV is used if the authorities do not list any shared random value for
/// that time period, but we need to compute an HsDir ring for it anyway.
fn disaster_srv(period: TimePeriod) -> SharedRandVal {
    use digest::Digest;
    let mut d = tor_llcrypto::d::Sha3_256::new();
    d.update(b"shared-random-disaster");
    d.update(u64::from(period.length().as_minutes()).to_be_bytes());
    d.update(period.interval_num().to_be_bytes());

    let v: [u8; 32] = d.finalize().into();
    v.into()
}

/// Helper type: A `SharedRandVal`, and the time range over which it is the most
/// recent.
type SrvInfo = (SharedRandVal, std::ops::Range<SystemTime>);

/// Given a list of SrvInfo, return an HsRingParams instance for a given time
/// period, if possible.
fn find_params_for_time(info: &[SrvInfo], period: TimePeriod) -> Result<Option<HsDirParams>> {
    let start = period
        .range()
        .map_err(|_| {
            Error::InvalidConsensus(
                "HsDir time period in consensus could not be represented as a SystemTime range.",
            )
        })?
        .start;

    Ok(find_srv_for_time(info, start).map(|srv| HsDirParams {
        time_period: period,
        shared_rand: srv.0,
        srv_lifespan: srv.1.clone(),
    }))
}

/// Given a list of SrvInfo, return the SrvInfo (if any) that is the most
/// recent SRV at `when`.
fn find_srv_for_time(info: &[SrvInfo], when: SystemTime) -> Option<&SrvInfo> {
    info.iter().find(|(_, range)| range.contains(&when))
}

/// Return every SRV from a consensus, along with a duration over which it is
/// most recent SRV.
fn extract_srvs(consensus: &MdConsensus) -> Result<Vec<SrvInfo>> {
    let mut v = Vec::new();
    let srv_interval = srv_interval(consensus);

    if let Some(cur) = consensus.shared_rand_cur() {
        let ts_begin = cur
            .timestamp()
            .map(Ok)
            .unwrap_or_else(|| start_of_sr_round(consensus))?;
        let ts_end = ts_begin + srv_interval;
        v.push((*cur.value(), ts_begin..ts_end));
    }
    if let Some(prev) = consensus.shared_rand_prev() {
        let ts_begin = prev
            .timestamp()
            .map(Ok)
            .unwrap_or_else(|| start_of_sr_round(consensus).map(|t| t - srv_interval))?;
        let ts_end = ts_begin + srv_interval;
        v.push((*prev.value(), ts_begin..ts_end));
    }

    Ok(v)
}

/// Return the length of time for which a single SRV value is valid.
fn srv_interval(consensus: &MdConsensus) -> Duration {
    // What we _want_ to do, ideally, is is to learn the duration from the
    // difference between the declared time for the previous value and the
    // declared time for the current one.
    //
    // (This assumes that proposal 342 is implemented.)
    if let (Some(cur), Some(prev)) = (consensus.shared_rand_cur(), consensus.shared_rand_prev()) {
        if let (Some(cur_ts), Some(prev_ts)) = (cur.timestamp(), prev.timestamp()) {
            if let Ok(d) = cur_ts.duration_since(prev_ts) {
                return d;
            }
        }
    }

    // But if one of those values is missing, or if it has no timestamp, we have
    // to fall back to admitting that we know the schedule for the voting
    // algorithm.
    consensus.lifetime().voting_period() * VOTING_PERIODS_IN_SRV_ROUND
}

/// Return the start time of the current SR protocol run
/// of the specified `consensus`.
///
/// If a full SR protocol run is 24 hours, this function returns
/// the start of the UTC day containing the `valid-after` timestamp
/// of the consensus.
///
/// Corresponds to C Tor's `sr_state_get_start_time_of_current_protocol_run()`.
fn start_of_sr_round(consensus: &MdConsensus) -> Result<SystemTime> {
    let t = consensus.lifetime().valid_after();
    let beginning_of_curr_round = t
        .duration_since(SystemTime::UNIX_EPOCH)
        .map_err(|_| Error::InvalidConsensus("consensus valid-after is before Unix epoch?!"))?
        .as_secs();
    let voting_interval = consensus.lifetime().voting_period().as_secs();

    // The voting_interval is always going to be greater than 0,
    // because the Lifetime's constructor enforces that
    // valid_after < fresh_until < valid_until.
    debug_assert!(voting_interval > 0);

    let curr_round_slot =
        (beginning_of_curr_round / voting_interval) % u64::from(VOTING_PERIODS_IN_SRV_ROUND);
    let time_elapsed_since_start_of_run = curr_round_slot * voting_interval;

    let offset = Duration::from_secs(beginning_of_curr_round - time_elapsed_since_start_of_run);
    Ok(SystemTime::UNIX_EPOCH + offset)
}

#[cfg(test)]
mod test {
    // @@ begin test lint list maintained by maint/add_warning @@
    #![allow(clippy::bool_assert_comparison)]
    #![allow(clippy::clone_on_copy)]
    #![allow(clippy::dbg_macro)]
    #![allow(clippy::mixed_attributes_style)]
    #![allow(clippy::print_stderr)]
    #![allow(clippy::print_stdout)]
    #![allow(clippy::single_char_pattern)]
    #![allow(clippy::unwrap_used)]
    #![allow(clippy::unchecked_time_subtraction)]
    #![allow(clippy::useless_vec)]
    #![allow(clippy::needless_pass_by_value)]
    #![allow(clippy::string_slice)] // See arti#2571
    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->
    use super::*;
    use hex_literal::hex;
    use tor_netdoc::doc::netstatus::{Lifetime, MdConsensusBuilder};

    /// Helper: parse an rfc3339 time.
    ///
    /// # Panics
    ///
    /// Panics if the time is invalid.
    fn t(s: &str) -> SystemTime {
        humantime::parse_rfc3339(s).unwrap()
    }
    /// Helper: parse a duration.
    ///
    /// # Panics
    ///
    /// Panics if the time is invalid.
    fn d(s: &str) -> Duration {
        humantime::parse_duration(s).unwrap()
    }

    fn example_lifetime() -> Lifetime {
        Lifetime::new(
            t("1985-10-25T07:00:00Z"),
            t("1985-10-25T08:00:00Z"),
            t("1985-10-25T10:00:00Z"),
        )
        .unwrap()
    }

    const SRV1: [u8; 32] = *b"next saturday night were sending";
    const SRV2: [u8; 32] = *b"you......... back to the future!";

    fn example_consensus_builder() -> MdConsensusBuilder {
        let mut bld = MdConsensus::builder();

        bld.consensus_method(34)
            .lifetime(example_lifetime())
            .param("bwweightscale", 1)
            .param("hsdir_interval", 1440)
            .weights("".parse().unwrap())
            .shared_rand_prev(7, SRV1.into(), None)
            .shared_rand_cur(7, SRV2.into(), None);

        bld
    }

    #[test]
    fn invalid_lifetime() {
        let lifetimes = [
            // (valid_after, fresh_until, valid_until)
            //
            // Invalid because valid_after >= fresh_until
            (
                "2015-04-20T00:00:00Z",
                "2015-04-20T00:00:00Z",
                "2015-04-22T00:00:00Z",
            ),
            (
                "2015-04-21T00:00:00Z",
                "2015-04-20T00:00:00Z",
                "2015-04-22T00:00:00Z",
            ),
            // Invalid because fresh_until >= valid_until
            (
                "2015-04-20T00:00:00Z",
                "2015-04-22T00:00:00Z",
                "2015-04-22T00:00:00Z",
            ),
            (
                "2015-04-20T00:00:00Z",
                "2015-04-23T00:00:00Z",
                "2015-04-22T00:00:00Z",
            ),
        ];

        for (valid_after, fresh_until, valid_until) in lifetimes {
            let err = Lifetime::new(t(valid_after), t(fresh_until), t(valid_until))
                .unwrap_err()
                .netdoc_error_kind();

            assert_eq!(err, tor_netdoc::NetdocErrorKind::InvalidLifetime);
        }
    }

    #[test]
    fn vote_period() {
        assert_eq!(example_lifetime().voting_period(), d("1 hour"));

        let lt2 = Lifetime::new(
            t("1985-10-25T07:00:00Z"),
            t("1985-10-25T07:22:00Z"),
            t("1985-10-25T07:59:00Z"),
        )
        .unwrap();

        assert_eq!(lt2.voting_period(), d("22 min"));
    }

    #[test]
    fn srv_period() {
        // In a basic consensus with no SRV timestamps, we'll assume 24 voting periods.
        let consensus = example_consensus_builder().testing_consensus().unwrap();
        assert_eq!(srv_interval(&consensus), d("1 day"));

        // If there are timestamps, we look at the difference between them.
        let consensus = example_consensus_builder()
            .shared_rand_prev(7, SRV1.into(), Some(t("1985-10-25T00:00:00Z")))
            .shared_rand_cur(7, SRV2.into(), Some(t("1985-10-25T06:00:05Z")))
            .testing_consensus()
            .unwrap();
        assert_eq!(srv_interval(&consensus), d("6 hours 5 sec"));

        // Note that if the timestamps are in reversed order, we fall back to 24 hours.
        let consensus = example_consensus_builder()
            .shared_rand_cur(7, SRV1.into(), Some(t("1985-10-25T00:00:00Z")))
            .shared_rand_prev(7, SRV2.into(), Some(t("1985-10-25T06:00:05Z")))
            .testing_consensus()
            .unwrap();
        assert_eq!(srv_interval(&consensus), d("1 day"));
    }

    #[test]
    fn srvs_extract_and_find() {
        let consensus = example_consensus_builder().testing_consensus().unwrap();
        let srvs = extract_srvs(&consensus).unwrap();
        assert_eq!(
            srvs,
            vec![
                // Since no timestamps are given in the example, the current srv
                // is valid from midnight to midnight...
                (
                    SRV2.into(),
                    t("1985-10-25T00:00:00Z")..t("1985-10-26T00:00:00Z")
                ),
                // ...and the previous SRV is valid midnight-to-midnight on the
                // previous day.
                (
                    SRV1.into(),
                    t("1985-10-24T00:00:00Z")..t("1985-10-25T00:00:00Z")
                )
            ]
        );

        // Now try with explicit timestamps on the SRVs.
        let consensus = example_consensus_builder()
            .shared_rand_prev(7, SRV1.into(), Some(t("1985-10-25T00:00:00Z")))
            .shared_rand_cur(7, SRV2.into(), Some(t("1985-10-25T06:00:05Z")))
            .testing_consensus()
            .unwrap();
        let srvs = extract_srvs(&consensus).unwrap();
        assert_eq!(
            srvs,
            vec![
                (
                    SRV2.into(),
                    t("1985-10-25T06:00:05Z")..t("1985-10-25T12:00:10Z")
                ),
                (
                    SRV1.into(),
                    t("1985-10-25T00:00:00Z")..t("1985-10-25T06:00:05Z")
                )
            ]
        );

        // See if we can look up SRVs in that period.
        assert_eq!(None, find_srv_for_time(&srvs, t("1985-10-24T23:59:00Z")));
        assert_eq!(
            Some(&srvs[1]),
            find_srv_for_time(&srvs, t("1985-10-25T00:00:00Z"))
        );
        assert_eq!(
            Some(&srvs[1]),
            find_srv_for_time(&srvs, t("1985-10-25T03:59:00Z"))
        );
        assert_eq!(
            Some(&srvs[1]),
            find_srv_for_time(&srvs, t("1985-10-25T00:00:00Z"))
        );
        assert_eq!(
            Some(&srvs[0]),
            find_srv_for_time(&srvs, t("1985-10-25T06:00:05Z"))
        );
        assert_eq!(
            Some(&srvs[0]),
            find_srv_for_time(&srvs, t("1985-10-25T12:00:00Z"))
        );
        assert_eq!(None, find_srv_for_time(&srvs, t("1985-10-25T12:00:30Z")));
    }

    #[test]
    fn disaster() {
        use digest::Digest;
        use tor_llcrypto::d::Sha3_256;
        let period = TimePeriod::new(d("1 day"), t("1970-01-02T17:33:00Z"), d("12 hours")).unwrap();
        assert_eq!(period.length().as_minutes(), 86400 / 60);
        assert_eq!(period.interval_num(), 1);

        let dsrv = disaster_srv(period);
        assert_eq!(
            dsrv.as_ref(),
            &hex!("F8A4948707653837FA44ABB5BBC75A12F6F101E7F8FAF699B9715F4965D3507D")
        );
        assert_eq!(
            &dsrv.as_ref()[..],
            &Sha3_256::digest(b"shared-random-disaster\0\0\0\0\0\0\x05\xA0\0\0\0\0\0\0\0\x01")[..]
        );
    }

    #[test]
    #[cfg(feature = "hs-service")]
    fn ring_params_simple() {
        // Compute ring parameters in a legacy environment, where the time
        // period and the SRV lifetime are one day long, and they are offset by
        // 12 hours.
        let consensus = example_consensus_builder().testing_consensus().unwrap();
        let netparams = NetParameters::from_map(consensus.params());
        let HsDirs { current, secondary } = HsDirParams::compute(&consensus, &netparams).unwrap();

        assert_eq!(
            current.time_period,
            TimePeriod::new(d("1 day"), t("1985-10-25T07:00:00Z"), d("12 hours")).unwrap()
        );
        // We use the "previous" SRV since the start of this time period was 12:00 on the 24th.
        assert_eq!(current.shared_rand.as_ref(), &SRV1);

        // Our secondary SRV will be the one that starts when we move into the
        // next time period.
        assert_eq!(secondary.len(), 1);
        assert_eq!(
            secondary[0].time_period,
            TimePeriod::new(d("1 day"), t("1985-10-25T12:00:00Z"), d("12 hours")).unwrap(),
        );
        assert_eq!(secondary[0].shared_rand.as_ref(), &SRV2);
    }

    #[test]
    #[cfg(feature = "hs-service")]
    fn ring_params_tricky() {
        // In this case we give the SRVs timestamps and we choose an odd hsdir_interval.
        let consensus = example_consensus_builder()
            .shared_rand_prev(7, SRV1.into(), Some(t("1985-10-25T00:00:00Z")))
            .shared_rand_cur(7, SRV2.into(), Some(t("1985-10-25T05:00:00Z")))
            .param("hsdir_interval", 120) // 2 hours
            .testing_consensus()
            .unwrap();
        let netparams = NetParameters::from_map(consensus.params());
        let HsDirs { current, secondary } = HsDirParams::compute(&consensus, &netparams).unwrap();

        assert_eq!(
            current.time_period,
            TimePeriod::new(d("2 hours"), t("1985-10-25T07:00:00Z"), d("12 hours")).unwrap()
        );
        assert_eq!(current.shared_rand.as_ref(), &SRV2);

        assert_eq!(secondary.len(), 2);
        assert_eq!(
            secondary[0].time_period,
            TimePeriod::new(d("2 hours"), t("1985-10-25T05:00:00Z"), d("12 hours")).unwrap()
        );
        assert_eq!(secondary[0].shared_rand.as_ref(), &SRV1);
        assert_eq!(
            secondary[1].time_period,
            TimePeriod::new(d("2 hours"), t("1985-10-25T09:00:00Z"), d("12 hours")).unwrap()
        );
        assert_eq!(secondary[1].shared_rand.as_ref(), &SRV2);
    }

    #[test]
    #[cfg(feature = "hs-service")]
    fn offset_within_srv_period() {
        // This test doesn't actually use the time_period or shared_rand values, so their value is
        // arbitrary.
        let time_period =
            TimePeriod::new(d("2 hours"), t("1985-10-25T05:00:00Z"), d("12 hours")).unwrap();

        let srv_start = t("1985-10-25T09:00:00Z");
        let srv_end = t("1985-10-25T20:00:00Z");
        let srv_lifespan = srv_start..srv_end;

        let params = HsDirParams {
            time_period,
            shared_rand: SRV1.into(),
            srv_lifespan,
        };

        let before_srv_period = t("1985-10-25T08:59:00Z");
        let after_srv_period = t("1985-10-26T10:19:00Z");
        assert!(params.offset_within_srv_period(before_srv_period).is_none());
        assert_eq!(
            params.offset_within_srv_period(srv_start).unwrap(),
            SrvPeriodOffset::from(0)
        );
        // The period is 11h long
        assert_eq!(
            params.offset_within_srv_period(srv_end).unwrap(),
            SrvPeriodOffset::from(11 * 60 * 60)
        );
        // This timestamp is 1 day 1h 19m from the start of the SRV period
        assert_eq!(
            params.offset_within_srv_period(after_srv_period).unwrap(),
            SrvPeriodOffset::from((25 * 60 + 19) * 60)
        );
    }

    #[test]
    fn start_of_sr_protocol_run() {
        let test_cases = [
            // (valid_after, fresh_until, expected_sr_start, expected_srv_interval)
            //
            // Voting interval = 1h
            // The start of the current SR run is always the start
            // of the day of valid-after of the consensus.
            // An SRV protocol run takes 1h * 24 rounds = 24h
            (
                "2015-04-20T00:00:00Z",
                "2015-04-20T01:00:00Z",
                "2015-04-20T00:00:00Z",
                "24 hours",
            ),
            (
                "2015-04-20T22:00:00Z",
                "2015-04-20T23:00:00Z",
                "2015-04-20T00:00:00Z",
                "24 hours",
            ),
            (
                "2015-04-19T23:00:00Z",
                "2015-04-20T00:00:00Z",
                "2015-04-19T00:00:00Z",
                "24 hours",
            ),
            // Voting interval = 10s
            // An SRV protocol run takes 10s * 24 rounds = 4 mins
            (
                "2015-04-20T00:15:30Z",
                "2015-04-20T00:15:40Z",
                "2015-04-20T00:12:00Z",
                "4 minutes",
            ),
            // Voting interval = 20s
            // An SRV protocol run takes 20s * 24 rounds = 8 mins
            (
                "2015-04-20T00:15:00Z",
                "2015-04-20T00:15:20Z",
                "2015-04-20T00:08:00Z",
                "8 minutes",
            ),
            (
                "2015-04-20T00:15:30Z",
                "2015-04-20T00:15:50Z",
                "2015-04-20T00:08:10Z",
                "8 minutes",
            ),
        ];

        for (valid_after, fresh_until, expected_start, expected_interval) in test_cases {
            let lifetime = Lifetime::new(
                t(valid_after),
                t(fresh_until),
                // The valid-until doesn't matter for the purposes of this test
                t("2020-10-25T10:00:00Z"),
            )
            .unwrap();

            let consensus = example_consensus_builder()
                .lifetime(lifetime)
                .testing_consensus()
                .unwrap();
            let sr_start = start_of_sr_round(&consensus).unwrap();

            assert_eq!(
                t(expected_start),
                sr_start,
                "{expected_start} != {} (valid-after = {valid_after}, fresh-until = {fresh_until})",
                humantime::format_rfc3339(sr_start)
            );

            assert_eq!(d(expected_interval), srv_interval(&consensus));
        }
    }
}