From 23e07a94cf4c49c514cdb2d652730ed797f56597 Mon Sep 17 00:00:00 2001
From: teor <teor@riseup.net>
Date: Thu, 3 Dec 2020 14:28:42 +1000
Subject: [PATCH] Implement the block header time consensus rules

---
 zebra-state/src/error.rs                    |  8 +++++
 zebra-state/src/service/check.rs            | 40 ++++++++++++++-------
 zebra-state/src/service/check/difficulty.rs | 11 ++++++
 3 files changed, 47 insertions(+), 12 deletions(-)

diff --git a/zebra-state/src/error.rs b/zebra-state/src/error.rs
index 67d17f27..e7425bf9 100644
--- a/zebra-state/src/error.rs
+++ b/zebra-state/src/error.rs
@@ -43,6 +43,14 @@ pub enum ValidateContextError {
     #[non_exhaustive]
     NonSequentialBlock,
 
+    /// block.header.time is less than or equal to the median-time-past for the block
+    #[non_exhaustive]
+    TimeTooEarly,
+
+    /// block.header.time is greater than the median-time-past for the block plus 90 minutes
+    #[non_exhaustive]
+    TimeTooLate,
+
     /// block.header.difficulty_threshold is not equal to the adjusted difficulty for the block
     #[non_exhaustive]
     InvalidDifficultyThreshold,
diff --git a/zebra-state/src/service/check.rs b/zebra-state/src/service/check.rs
index 103ae1c7..1e6b2225 100644
--- a/zebra-state/src/service/check.rs
+++ b/zebra-state/src/service/check.rs
@@ -2,6 +2,7 @@
 
 use std::borrow::Borrow;
 
+use chrono::Duration;
 use zebra_chain::{
     block::{self, Block},
     parameters::Network,
@@ -74,11 +75,11 @@ where
             block.borrow().header.time,
         )
     });
-    let expected_difficulty =
+    let difficulty_adjustment =
         AdjustedDifficulty::new_from_block(&prepared.block, network, relevant_data);
     check::difficulty_threshold_is_valid(
         prepared.block.header.difficulty_threshold,
-        expected_difficulty,
+        difficulty_adjustment,
     )?;
 
     // TODO: other contextual validation design and implementation
@@ -111,22 +112,37 @@ fn height_one_more_than_parent_height(
     }
 }
 
-/// Validate the `difficulty_threshold` from a candidate block's header, based
-/// on an `expected_difficulty` for that block.
+/// Validate the `time` and `difficulty_threshold` from a candidate block's
+/// header.
 ///
-/// Uses `expected_difficulty` to calculate the expected difficulty value, then
-/// compares that value to the `difficulty_threshold`.
+/// Uses the `difficulty_adjustment` context for the block to:
+///   * check that the the `time` field is within the valid range, and
+///   * check that the expected difficulty is equal to the block's
+///     `difficulty_threshold`.
 ///
-/// The check passes if the values are equal.
+/// These checks are performed together, because the time field is used to
+/// calculate the expected difficulty adjustment.
 fn difficulty_threshold_is_valid(
     difficulty_threshold: CompactDifficulty,
-    expected_difficulty: AdjustedDifficulty,
+    difficulty_adjustment: AdjustedDifficulty,
 ) -> Result<(), ValidateContextError> {
-    if difficulty_threshold == expected_difficulty.expected_difficulty_threshold() {
-        Ok(())
-    } else {
-        Err(ValidateContextError::InvalidDifficultyThreshold)
+    // Check the block header time consensus rules from the Zcash specification
+    let median_time_past = difficulty_adjustment.median_time_past();
+    let block_max_time_since_median = Duration::seconds(difficulty::BLOCK_MAX_TIME_SINCE_MEDIAN);
+    if difficulty_adjustment.candidate_time() <= median_time_past {
+        Err(ValidateContextError::TimeTooEarly)?
+    } else if difficulty_adjustment.candidate_time()
+        > median_time_past + block_max_time_since_median
+    {
+        Err(ValidateContextError::TimeTooLate)?
     }
+
+    let expected_difficulty = difficulty_adjustment.expected_difficulty_threshold();
+    if difficulty_threshold != expected_difficulty {
+        Err(ValidateContextError::InvalidDifficultyThreshold)?
+    }
+
+    Ok(())
 }
 
 #[cfg(test)]
diff --git a/zebra-state/src/service/check/difficulty.rs b/zebra-state/src/service/check/difficulty.rs
index 94875450..617acfec 100644
--- a/zebra-state/src/service/check/difficulty.rs
+++ b/zebra-state/src/service/check/difficulty.rs
@@ -36,6 +36,12 @@ pub const POW_MAX_ADJUST_UP_PERCENT: i32 = 16;
 /// `PoWMaxAdjustDown * 100` in the Zcash specification.
 pub const POW_MAX_ADJUST_DOWN_PERCENT: i32 = 32;
 
+/// The maximum number of seconds between the `meadian-time-past` of a block,
+/// and the block's `time` field.
+///
+/// Part of the block header consensus rules in the Zcash specification.
+pub const BLOCK_MAX_TIME_SINCE_MEDIAN: i64 = 90 * 60;
+
 /// Contains the context needed to calculate the adjusted difficulty for a block.
 pub(super) struct AdjustedDifficulty {
     /// The `header.time` field from the candidate block
@@ -141,6 +147,11 @@ impl AdjustedDifficulty {
         }
     }
 
+    /// Returns the candidate block's time field.
+    pub fn candidate_time(&self) -> DateTime<Utc> {
+        self.candidate_time
+    }
+
     /// Calculate the expected `difficulty_threshold` for a candidate block, based
     /// on the `candidate_time`, `candidate_height`, `network`, and the
     /// `difficulty_threshold`s and `time`s from the previous