diff --git a/Blockchain/Sources/Blockchain/BlockchainDataProvider/InMemoryDataProvider.swift b/Blockchain/Sources/Blockchain/BlockchainDataProvider/InMemoryDataProvider.swift
index 8f43b916..fadcba39 100644
--- a/Blockchain/Sources/Blockchain/BlockchainDataProvider/InMemoryDataProvider.swift
+++ b/Blockchain/Sources/Blockchain/BlockchainDataProvider/InMemoryDataProvider.swift
@@ -70,7 +70,7 @@ extension InMemoryDataProvider: BlockchainDataProvider {
 
     public func add(block: BlockRef) {
         blockByHash[block.hash] = block
-        hashByTimeslot[block.header.timeslotIndex, default: Set()].insert(block.hash)
+        hashByTimeslot[block.header.timeslot, default: Set()].insert(block.hash)
     }
 
     public func setFinalizedHead(hash: Data32) {
@@ -85,7 +85,7 @@ extension InMemoryDataProvider: BlockchainDataProvider {
     }
 
     public func remove(hash: Data32) {
-        let timeslot = blockByHash[hash]?.header.timeslotIndex ?? stateByBlockHash[hash]?.value.timeslot
+        let timeslot = blockByHash[hash]?.header.timeslot ?? stateByBlockHash[hash]?.value.timeslot
         stateByBlockHash.removeValue(forKey: hash)
 
         if let timeslot {
diff --git a/Blockchain/Sources/Blockchain/Runtime.swift b/Blockchain/Sources/Blockchain/Runtime.swift
index 14e08be7..58efb687 100644
--- a/Blockchain/Sources/Blockchain/Runtime.swift
+++ b/Blockchain/Sources/Blockchain/Runtime.swift
@@ -1,3 +1,4 @@
+import Codec
 import Utils
 
 // the STF
@@ -6,6 +7,12 @@ public final class Runtime {
         case safroleError(SafroleError)
         case invalidTimeslot
         case invalidReportAuthorizer
+        case unableToComputeExtrinsicHash(any Swift.Error)
+        case invalidExtrinsicHash
+        case invalidParentHash
+        case invalidHeaderStateRoot
+        case invalidHeaderEpochMarker
+        case invalidHeaderWinningTickets
         case other(any Swift.Error)
     }
 
@@ -23,12 +30,39 @@ public final class Runtime {
         self.config = config
     }
 
-    public func validate(block: BlockRef, state _: StateRef, context: ApplyContext) throws(Error) {
-        guard context.timeslot >= block.header.timeslotIndex else {
+    public func validateHeader(block: BlockRef, state: StateRef, context: ApplyContext) throws(Error) {
+        guard block.header.parentHash == state.value.lastBlockHash else {
+            throw Error.invalidParentHash
+        }
+
+        guard block.header.priorStateRoot == state.stateRoot else {
+            throw Error.invalidHeaderStateRoot
+        }
+
+        let expectedExtrinsicHash = try Result { try blake2b256(JamEncoder.encode(block.extrinsic)) }
+            .mapError(Error.unableToComputeExtrinsicHash).get()
+
+        guard block.header.extrinsicsHash == expectedExtrinsicHash else {
+            throw Error.invalidExtrinsicHash
+        }
+
+        guard block.header.timeslot <= context.timeslot else {
             throw Error.invalidTimeslot
         }
 
+        // epoch is validated at apply time
+
+        // winning tickets is validated at apply time
+
+        // TODO: validate judgementsMarkers
+        // TODO: validate offendersMarkers
+
         // TODO: validate block.header.seal
+    }
+
+    public func validate(block: BlockRef, state: StateRef, context: ApplyContext) throws(Error) {
+        try validateHeader(block: block, state: state, context: context)
+
         // TODO: abstract input validation logic from Safrole state update function and call it here
         // TODO: validate other things
     }
@@ -41,10 +75,18 @@ public final class Runtime {
         do {
             newState.recentHistory = try updateRecentHistory(block: block, state: prevState)
 
-            let res = try newState.updateSafrole(
-                config: config, slot: block.header.timeslotIndex, entropy: newState.entropyPool.t0, extrinsics: block.extrinsic.tickets
+            let safroleResult = try newState.updateSafrole(
+                config: config, slot: block.header.timeslot, entropy: newState.entropyPool.t0, extrinsics: block.extrinsic.tickets
             )
-            newState.mergeWith(postState: res.state)
+            newState.mergeWith(postState: safroleResult.state)
+
+            guard safroleResult.epochMark == block.header.epoch else {
+                throw Error.invalidHeaderEpochMarker
+            }
+
+            guard safroleResult.ticketsMark == block.header.winningTickets else {
+                throw Error.invalidHeaderWinningTickets
+            }
 
             newState.coreAuthorizationPool = try updateAuthorizationPool(
                 block: block, state: prevState
@@ -102,7 +144,7 @@ public final class Runtime {
             if coreQueue.count == 0 {
                 continue
             }
-            let newItem = coreQueue[Int(block.header.timeslotIndex) % coreQueue.count]
+            let newItem = coreQueue[Int(block.header.timeslot) % coreQueue.count]
 
             // remove used authorizers from pool
             for report in block.extrinsic.reports.guarantees {
@@ -126,7 +168,7 @@ public final class Runtime {
     public func updateValidatorActivityStatistics(block: BlockRef, state: StateRef) throws -> ValidatorActivityStatistics {
         let epochLength = UInt32(config.value.epochLength)
         let currentEpoch = state.value.timeslot / epochLength
-        let newEpoch = block.header.timeslotIndex / epochLength
+        let newEpoch = block.header.timeslot / epochLength
         let isEpochChange = currentEpoch != newEpoch
 
         var acc = try isEpochChange
diff --git a/Blockchain/Sources/Blockchain/Types/Header.swift b/Blockchain/Sources/Blockchain/Types/Header.swift
index db799f21..ab50c5aa 100644
--- a/Blockchain/Sources/Blockchain/Types/Header.swift
+++ b/Blockchain/Sources/Blockchain/Types/Header.swift
@@ -10,10 +10,10 @@ public struct Header: Sendable, Equatable, Codable {
         public var priorStateRoot: Data32 // state root of the after parent block execution
 
         // Hx: extrinsic hash
-        public var extrinsicsRoot: Data32
+        public var extrinsicsHash: Data32
 
         // Ht: timeslot index
-        public var timeslotIndex: TimeslotIndex
+        public var timeslot: TimeslotIndex
 
         // He: the epoch
         // the header’s epoch marker He is either empty or, if the block is the first in a new epoch,
@@ -47,8 +47,8 @@ public struct Header: Sendable, Equatable, Codable {
         public init(
             parentHash: Data32,
             priorStateRoot: Data32,
-            extrinsicsRoot: Data32,
-            timeslotIndex: TimeslotIndex,
+            extrinsicsHash: Data32,
+            timeslot: TimeslotIndex,
             epoch: EpochMarker?,
             winningTickets: ConfigFixedSizeArray<
                 Ticket,
@@ -61,8 +61,8 @@ public struct Header: Sendable, Equatable, Codable {
         ) {
             self.parentHash = parentHash
             self.priorStateRoot = priorStateRoot
-            self.extrinsicsRoot = extrinsicsRoot
-            self.timeslotIndex = timeslotIndex
+            self.extrinsicsHash = extrinsicsHash
+            self.timeslot = timeslot
             self.epoch = epoch
             self.winningTickets = winningTickets
             self.judgementsMarkers = judgementsMarkers
@@ -97,8 +97,8 @@ extension Header.Unsigned: Dummy {
         Header.Unsigned(
             parentHash: Data32(),
             priorStateRoot: Data32(),
-            extrinsicsRoot: Data32(),
-            timeslotIndex: 0,
+            extrinsicsHash: Data32(),
+            timeslot: 0,
             epoch: nil,
             winningTickets: nil,
             judgementsMarkers: [],
@@ -130,8 +130,8 @@ extension Header {
 
     public var parentHash: Data32 { unsigned.parentHash }
     public var priorStateRoot: Data32 { unsigned.priorStateRoot }
-    public var extrinsicsRoot: Data32 { unsigned.extrinsicsRoot }
-    public var timeslotIndex: TimeslotIndex { unsigned.timeslotIndex }
+    public var extrinsicsHash: Data32 { unsigned.extrinsicsHash }
+    public var timeslot: TimeslotIndex { unsigned.timeslot }
     public var epoch: EpochMarker? { unsigned.epoch }
     public var winningTickets: ConfigFixedSizeArray<Ticket, ProtocolConfig.EpochLength>? { unsigned.winningTickets }
     public var judgementsMarkers: [Data32] { unsigned.judgementsMarkers }