Merge rust-bitcoin/rust-bitcoin#994: Add a `LockTime` type

0ed78e543b Add lock time types (Tobin C. Harding)
1390ee12ec Add a max scriptnum constant (Tobin C. Harding)

Pull request description:

  Implement a `LockTime` type that adds support for lock time values based on nLockTime and OP_CHECKLOCKTIMEVERIFY.

  For example usage in `rust-miniscript` please see https://github.com/rust-bitcoin/rust-miniscript/pull/408.

  ### Notes:

  I probably should have opened a new PR, this is a total re-write and very different from what is being discussed below, sorry, my bad.

  This is just half of the 'timelock' story, its the easier half. The reason I switched terminology from timelock to locktime is that we have to compare two u32s so it does not make sense to call them _both_ timelocks.

  If I have missed, or apparently ignored, anything you said reviewers please accept my apology in advance, it was not intentional. The thread of discussion is long and this topic is complex. Please do restate your views liberally :)

  Here is a useful blog post I used while touching up on timelock specifics: https://medium.com/summa-technology/bitcoins-time-locks-27e0c362d7a1. It links to all the relevant bips too.

ACKs for top commit:
  Kixunil:
    ACK 0ed78e543b
  apoelstra:
    ACK 0ed78e543b

Tree-SHA512: 486fcce859b38fa1e8e6b11cd2f494462d6d7d1d9030d096ce6b260f6c9d0342b8952afe35152bdf3afe323a234a8165ac3d6c946304afcc13406d7a0489d75a

examples/ecdsa-psbt.rs

@@ -43,8 +43,8 @@ use bitcoin::util::bip32::{
 };
 use bitcoin::util::psbt::{self, Input, Psbt, PsbtSighashType};
 use bitcoin::{
-    Address, Amount, Network, OutPoint, PrivateKey, PublicKey, Script, Sequence, Transaction, TxIn,
+    Address, Amount, Network, OutPoint, PackedLockTime, PrivateKey, PublicKey, Script, Sequence,
-    TxOut, Txid, Witness,
+    Transaction, TxIn, TxOut, Txid, Witness,
 };
 
 use self::psbt_sign::*;
@@ -207,7 +207,7 @@ impl WatchOnly {
 
         let tx = Transaction {
             version: 2,
-            lock_time: 0,
+            lock_time: PackedLockTime::ZERO,
             input: vec![TxIn {
                 previous_output: OutPoint {
                     txid: Txid::from_hex(INPUT_UTXO_TXID)?,

src/blockdata/constants.rs

@@ -16,6 +16,7 @@ use crate::hashes::hex::{self, HexIterator};
 use crate::hashes::{Hash, sha256d};
 use crate::blockdata::opcodes;
 use crate::blockdata::script;
+use crate::blockdata::locktime::PackedLockTime;
 use crate::blockdata::transaction::{OutPoint, Transaction, TxOut, TxIn, Sequence};
 use crate::blockdata::block::{Block, BlockHeader};
 use crate::blockdata::witness::Witness;
@@ -53,6 +54,8 @@ pub const SCRIPT_ADDRESS_PREFIX_TEST: u8 = 196; // 0xc4
 pub const MAX_SCRIPT_ELEMENT_SIZE: usize = 520;
 /// How may blocks between halvings.
 pub const SUBSIDY_HALVING_INTERVAL: u32 = 210_000;
+/// Maximum allowed value for an integer in Script.
+pub const MAX_SCRIPTNUM_VALUE: u32 = 0x80000000; // 2^31
 
 /// In Bitcoind this is insanely described as ~((u256)0 >> 32)
 pub fn max_target(_: Network) -> Uint256 {
@@ -71,7 +74,7 @@ fn bitcoin_genesis_tx() -> Transaction {
     // Base
     let mut ret = Transaction {
         version: 1,
-        lock_time: 0,
+        lock_time: PackedLockTime::ZERO,
         input: vec![],
         output: vec![],
     };
@@ -199,6 +202,7 @@ mod test {
     use crate::hashes::hex::{ToHex, FromHex};
     use crate::network::constants::Network;
     use crate::consensus::encode::serialize;
+    use crate::blockdata::locktime::PackedLockTime;
 
     #[test]
     fn bitcoin_genesis_first_transaction() {
@@ -216,7 +220,7 @@ mod test {
         assert_eq!(serialize(&gen.output[0].script_pubkey),
                    Vec::from_hex("434104678afdb0fe5548271967f1a67130b7105cd6a828e03909a67962e0ea1f61deb649f6bc3f4cef38c4f35504e51ec112de5c384df7ba0b8d578a4c702b6bf11d5fac").unwrap());
         assert_eq!(gen.output[0].value, 50 * COIN_VALUE);
-        assert_eq!(gen.lock_time, 0);
+        assert_eq!(gen.lock_time, PackedLockTime::ZERO);
 
         assert_eq!(gen.wtxid().to_hex(), "4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b");
     }

src/blockdata/locktime.rs

@@ -0,0 +1,762 @@
+// Rust Bitcoin Library
+// Written in 2022 by
+//     Tobin C. Harding <me@tobin.cc>
+// To the extent possible under law, the author(s) have dedicated all
+// copyright and related and neighboring rights to this software to
+// the public domain worldwide. This software is distributed without
+// any warranty.
+//
+// You should have received a copy of the CC0 Public Domain Dedication
+// along with this software.
+// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
+//
+
+//! Provides type [`LockTime`] that implements the logic around nLockTime/OP_CHECKLOCKTIMEVERIFY.
+//!
+//! There are two types of lock time: lock-by-blockheight and lock-by-blocktime, distinguished by
+//! whether `LockTime < LOCKTIME_THRESHOLD`.
+//!
+
+use core::{mem, fmt};
+use core::cmp::{PartialOrd, Ordering};
+use core::convert::TryFrom;
+use core::str::FromStr;
+use core::num::ParseIntError;
+
+use crate::consensus::encode::{self, Decodable, Encodable};
+use crate::io::{self, Read, Write};
+use crate::prelude::*;
+use crate::internal_macros::write_err;
+
+/// The Threshold for deciding whether a lock time value is a height or a time (see [Bitcoin Core]).
+///
+/// `LockTime` values _below_ the threshold are interpreted as block heights, values _above_ (or
+/// equal to) the threshold are interpreted as block times (UNIX timestamp, seconds since epoch).
+///
+/// Bitcoin is able to safely use this value because a block height greater than 500,000,000 would
+/// never occur because it would represent a height in approximately 9500 years. Conversely, block
+/// times under 500,000,000 will never happen because they would represent times before 1986 which
+/// are, for obvious reasons, not useful within the Bitcoin network.
+///
+/// [Bitcoin Core]: https://github.com/bitcoin/bitcoin/blob/9ccaee1d5e2e4b79b0a7c29aadb41b97e4741332/src/script/script.h#L39
+pub const LOCK_TIME_THRESHOLD: u32 = 500_000_000;
+
+/// Packed lock time wraps a [`LockTime`] consensus value i.e., the raw `u32` used by the network.
+///
+/// This struct may be preferred in performance-critical applications because it's slightly smaller
+/// than [`LockTime`] and has a bit more performant (de)serialization. In particular, this may be
+/// relevant when the value is not processed, just passed around. Note however that the difference
+/// is super-small, so unless you do something extreme you shouldn't worry about it.
+///
+/// This type implements a naive ordering based on the `u32`, this is _not_ a semantically correct
+/// ordering for a lock time, hence [`LockTime`] does not implement `Ord`. This type is useful if
+/// you want to use a lock time as part of a struct and wish to derive `Ord`. For all other uses,
+/// consider using [`LockTime`] directly.
+///
+/// # Examples
+/// ```
+/// # use bitcoin::{Amount, PackedLockTime, LockTime};
+/// #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+/// struct S {
+///     lock_time: PackedLockTime,
+///     amount: Amount,
+/// }
+///
+/// let _ = S {
+///     lock_time: LockTime::from_consensus(741521).into(),
+///     amount: Amount::from_sat(10_000_000),
+/// };
+/// ```
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "serde", serde(crate = "actual_serde"))]
+pub struct PackedLockTime(pub u32);
+
+impl PackedLockTime {
+    /// If [`crate::Transaction::lock_time`] is set to zero it is ignored, in other words a
+    /// transaction with nLocktime==0 is able to be included immediately in any block.
+    pub const ZERO: PackedLockTime = PackedLockTime(0);
+
+    /// Returns the inner `u32`.
+    #[inline]
+    pub fn to_u32(&self) -> u32 {
+        self.0
+    }
+}
+
+impl fmt::Display for PackedLockTime {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(&self.0, f)
+    }
+}
+
+impl Encodable for PackedLockTime {
+    #[inline]
+    fn consensus_encode<W: Write + ?Sized>(&self, w: &mut W) -> Result<usize, io::Error> {
+        self.0.consensus_encode(w)
+    }
+}
+
+impl Decodable for PackedLockTime {
+    #[inline]
+    fn consensus_decode<R: Read + ?Sized>(r: &mut R) -> Result<Self, encode::Error> {
+        u32::consensus_decode(r).map(PackedLockTime)
+    }
+}
+
+impl From<LockTime> for PackedLockTime {
+    fn from(n: LockTime) -> Self {
+        PackedLockTime(n.to_consensus_u32())
+    }
+}
+
+impl From<PackedLockTime> for LockTime {
+    fn from(n: PackedLockTime) -> Self {
+        LockTime::from_consensus(n.0)
+    }
+}
+
+impl From<&LockTime> for PackedLockTime {
+    fn from(n: &LockTime) -> Self {
+        PackedLockTime(n.to_consensus_u32())
+    }
+}
+
+impl From<&PackedLockTime> for LockTime {
+    fn from(n: &PackedLockTime) -> Self {
+        LockTime::from_consensus(n.0)
+    }
+}
+
+impl From<PackedLockTime> for u32 {
+    fn from(p: PackedLockTime) -> Self {
+        p.0
+    }
+}
+
+impl FromStr for PackedLockTime {
+    type Err = ParseIntError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        s.parse().map(PackedLockTime)
+    }
+}
+
+impl TryFrom<&str> for PackedLockTime {
+    type Error = ParseIntError;
+
+    fn try_from(s: &str) -> Result<Self, Self::Error> {
+        PackedLockTime::from_str(s)
+    }
+}
+
+impl TryFrom<String> for PackedLockTime {
+    type Error = ParseIntError;
+
+    fn try_from(s: String) -> Result<Self, Self::Error> {
+        PackedLockTime::from_str(&s)
+    }
+}
+
+impl fmt::LowerHex for PackedLockTime {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{:x}", self.0)
+    }
+}
+
+impl fmt::UpperHex for PackedLockTime {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{:X}", self.0)
+    }
+}
+
+/// A lock time value, representing either a block height or a UNIX timestamp (seconds since epoch).
+///
+/// Used for transaction lock time (`nLockTime` in Bitcoin Core and [`crate::Transaction::lock_time`]
+/// in this library) and also for the argument to opcode 'OP_CHECKLOCKTIMEVERIFY`.
+///
+/// ### Relevant BIPs
+///
+/// * [BIP-65 OP_CHECKLOCKTIMEVERIFY](https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki)
+/// * [BIP-113 Median time-past as endpoint for lock-time calculations](https://github.com/bitcoin/bips/blob/master/bip-0113.mediawiki)
+///
+/// # Examples
+/// ```
+/// # use bitcoin::{LockTime, LockTime::*};
+/// # let n = LockTime::from_consensus(100);          // n OP_CHECKLOCKTIMEVERIFY
+/// # let lock_time = LockTime::from_consensus(100);  // nLockTime
+/// // To compare lock times there are various `is_satisfied_*` methods, you may also use:
+/// let is_satisfied = match (n, lock_time) {
+///     (Blocks(n), Blocks(lock_time)) => n <= lock_time,
+///     (Seconds(n), Seconds(lock_time)) => n <= lock_time,
+///     _ => panic!("handle invalid comparison error"),
+/// };
+/// ```
+#[allow(clippy::derive_ord_xor_partial_ord)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "serde", serde(crate = "actual_serde"))]
+pub enum LockTime {
+    /// A block height lock time value.
+    ///
+    /// # Examples
+    /// ```rust
+    /// use bitcoin::LockTime;
+    ///
+    /// let block: u32 = 741521;
+    /// let n = LockTime::from_height(block).expect("valid height");
+    /// assert!(n.is_block_height());
+    /// assert_eq!(n.to_consensus_u32(), block);
+    /// ```
+    Blocks(Height),
+    /// A UNIX timestamp lock time value.
+    ///
+    /// # Examples
+    /// ```rust
+    /// use bitcoin::LockTime;
+    ///
+    /// let seconds: u32 = 1653195600; // May 22nd, 5am UTC.
+    /// let n = LockTime::from_time(seconds).expect("valid time");
+    /// assert!(n.is_block_time());
+    /// assert_eq!(n.to_consensus_u32(), seconds);
+    /// ```
+    Seconds(Time),
+}
+
+impl LockTime {
+    /// If [`crate::Transaction::lock_time`] is set to zero it is ignored, in other words a
+    /// transaction with nLocktime==0 is able to be included immediately in any block.
+    pub const ZERO: LockTime = LockTime::Blocks(Height(0));
+
+    /// Constructs a `LockTime` from an nLockTime value or the argument to OP_CHEKCLOCKTIMEVERIFY.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use bitcoin::LockTime;
+    /// # let n = LockTime::from_consensus(741521); // n OP_CHECKLOCKTIMEVERIFY
+    ///
+    /// // `from_consensus` roundtrips as expected with `to_consensus_u32`.
+    /// let n_lock_time: u32 = 741521;
+    /// let lock_time = LockTime::from_consensus(n_lock_time);
+    /// assert_eq!(lock_time.to_consensus_u32(), n_lock_time);
+    #[inline]
+    pub fn from_consensus(n: u32) -> Self {
+        if is_block_height(n) {
+            Self::Blocks(Height::from_consensus(n).expect("n is valid"))
+        } else {
+            Self::Seconds(Time::from_consensus(n).expect("n is valid"))
+        }
+    }
+
+    /// Constructs a `LockTime` from `n`, expecting `n` to be a valid block height.
+    ///
+    /// See [`LOCK_TIME_THRESHOLD`] for definition of a valid height value.
+    ///
+    /// # Examples
+    /// ```rust
+    /// # use bitcoin::LockTime;
+    /// assert!(LockTime::from_height(741521).is_ok());
+    /// assert!(LockTime::from_height(1653195600).is_err());
+    /// ```
+    #[inline]
+    pub fn from_height(n: u32) -> Result<Self, Error> {
+        let height = Height::from_consensus(n)?;
+        Ok(LockTime::Blocks(height))
+    }
+
+    /// Constructs a `LockTime` from `n`, expecting `n` to be a valid block time.
+    ///
+    /// See [`LOCK_TIME_THRESHOLD`] for definition of a valid time value.
+    ///
+    /// # Examples
+    /// ```rust
+    /// # use bitcoin::LockTime;
+    /// assert!(LockTime::from_time(1653195600).is_ok());
+    /// assert!(LockTime::from_time(741521).is_err());
+    /// ```
+    #[inline]
+    pub fn from_time(n: u32) -> Result<Self, Error> {
+        let time = Time::from_consensus(n)?;
+        Ok(LockTime::Seconds(time))
+    }
+
+    /// Returns true if both lock times use the same unit i.e., both height based or both time based.
+    #[inline]
+    pub fn is_same_unit(&self, other: LockTime) -> bool {
+        mem::discriminant(self) == mem::discriminant(&other)
+    }
+
+    /// Returns true if this lock time value is a block height.
+    #[inline]
+    pub fn is_block_height(&self) -> bool {
+        match *self {
+            LockTime::Blocks(_) => true,
+            LockTime::Seconds(_) => false,
+        }
+    }
+
+    /// Returns true if this lock time value is a block time (UNIX timestamp).
+    #[inline]
+    pub fn is_block_time(&self) -> bool {
+        !self.is_block_height()
+    }
+
+    /// Returns true if this timelock constraint is satisfied by the respective `height`/`time`.
+    ///
+    /// If `self` is a blockheight based lock then it is checked against `height` and if `self` is a
+    /// blocktime based lock it is checked against `time`.
+    ///
+    /// A 'timelock constraint' refers to the `n` from `n OP_CHEKCLOCKTIMEVERIFY`, this constraint
+    /// is satisfied if a transaction with nLockTime ([`crate::Transaction::lock_time`]) set to
+    /// `height`/`time` is valid.
+    ///
+    /// # Examples
+    /// ```no_run
+    /// # use bitcoin::blockdata::locktime::{LockTime, Height, Time};
+    /// // Can be implemented if block chain data is available.
+    /// fn get_height() -> Height { todo!("return the current block height") }
+    /// fn get_time() -> Time { todo!("return the current block time") }
+    ///
+    /// let n = LockTime::from_consensus(741521); // `n OP_CHEKCLOCKTIMEVERIFY`.
+    /// if n.is_satisfied_by(get_height(), get_time()) {
+    ///     // Can create and mine a transaction that satisfies the OP_CLTV timelock constraint.
+    /// }
+    /// ````
+    #[inline]
+    pub fn is_satisfied_by(&self, height: Height, time: Time) -> bool {
+        use LockTime::*;
+
+        match *self {
+            Blocks(n) => n <= height,
+            Seconds(n) => n <= time,
+        }
+    }
+
+    /// Returns the inner `u32` value. This is the value used when creating this `LockTime`
+    /// i.e., `n OP_CHECKLOCKTIMEVERIFY` or nLockTime.
+    ///
+    /// # Warning
+    ///
+    /// Do not compare values return by this method. The whole point of the `LockTime` type is to
+    /// assist in doing correct comparisons. Either use `is_satisfied_by` or use the pattern below:
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use bitcoin::{LockTime, LockTime::*};
+    /// # let n = LockTime::from_consensus(100);          // n OP_CHECKLOCKTIMEVERIFY
+    /// # let lock_time = LockTime::from_consensus(100);  // nLockTime
+    ///
+    /// let is_satisfied = match (n, lock_time) {
+    ///     (Blocks(n), Blocks(lock_time)) => n <= lock_time,
+    ///     (Seconds(n), Seconds(lock_time)) => n <= lock_time,
+    ///     _ => panic!("invalid comparison"),
+    /// };
+    ///
+    /// // Or, if you have Rust 1.53 or greater
+    /// // let is_satisfied = n.partial_cmp(&lock_time).expect("invalid comparison").is_le();
+    /// ```
+    #[inline]
+    pub fn to_consensus_u32(&self) -> u32 {
+        match *self {
+            LockTime::Blocks(ref h) => h.to_consensus_u32(),
+            LockTime::Seconds(ref t) => t.to_consensus_u32(),
+        }
+    }
+}
+
+impl FromStr for LockTime {
+    type Err = ParseIntError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        s.parse().map(LockTime::from_consensus)
+    }
+}
+
+impl TryFrom<&str> for LockTime {
+    type Error = ParseIntError;
+
+    fn try_from(s: &str) -> Result<Self, Self::Error> {
+        LockTime::from_str(s)
+    }
+}
+
+impl TryFrom<String> for LockTime {
+    type Error = ParseIntError;
+
+    fn try_from(s: String) -> Result<Self, Self::Error> {
+        LockTime::from_str(&s)
+    }
+}
+
+impl From<Height> for LockTime {
+    fn from(h: Height) -> Self {
+        LockTime::Blocks(h)
+    }
+}
+
+impl From<Time> for LockTime {
+    fn from(t: Time) -> Self {
+        LockTime::Seconds(t)
+    }
+}
+
+impl PartialOrd for LockTime {
+    fn partial_cmp(&self, other: &LockTime) -> Option<Ordering> {
+        use LockTime::*;
+
+        match (*self, *other) {
+            (Blocks(ref a), Blocks(ref b)) => a.partial_cmp(b),
+            (Seconds(ref a), Seconds(ref b)) => a.partial_cmp(b),
+            (_, _) => None,
+        }
+    }
+}
+
+impl fmt::Display for LockTime {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use LockTime::*;
+
+        if f.alternate() {
+            match *self {
+                Blocks(ref h) => write!(f, "block-height {}", h),
+                Seconds(ref t) => write!(f, "block-time {} (seconds since epoch)", t),
+            }
+        } else {
+            match *self {
+                Blocks(ref h) => fmt::Display::fmt(h, f),
+                Seconds(ref t) => fmt::Display::fmt(t, f),
+            }
+        }
+    }
+}
+
+impl Encodable for LockTime {
+    #[inline]
+    fn consensus_encode<W: Write + ?Sized>(&self, w: &mut W) -> Result<usize, io::Error> {
+        let v = self.to_consensus_u32();
+        v.consensus_encode(w)
+    }
+}
+
+impl Decodable for LockTime {
+    #[inline]
+    fn consensus_decode<R: Read + ?Sized>(r: &mut R) -> Result<Self, encode::Error> {
+        u32::consensus_decode(r).map(LockTime::from_consensus)
+    }
+}
+
+/// An absolute block height, guaranteed to always contain a valid height value.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "serde", serde(crate = "actual_serde"))]
+pub struct Height(u32);
+
+impl Height {
+    /// Constructs a new block height.
+    ///
+    /// # Errors
+    ///
+    /// If `n` does not represent a block height value (see documentation on [`LockTime`]).
+    ///
+    /// # Examples
+    /// ```rust
+    /// use bitcoin::blockdata::locktime::Height;
+    ///
+    /// let h: u32 = 741521;
+    /// let height = Height::from_consensus(h).expect("invalid height value");
+    /// assert_eq!(height.to_consensus_u32(), h);
+    /// ```
+    #[inline]
+    pub fn from_consensus(n: u32) -> Result<Height, Error> {
+        if is_block_height(n) {
+            Ok(Self(n))
+        } else {
+            Err(ConversionError::invalid_height(n).into())
+        }
+    }
+
+    /// Converts this `Height` to its inner `u32` value.
+    ///
+    /// # Examples
+    /// ```rust
+    /// use bitcoin::LockTime;
+    ///
+    /// let n_lock_time: u32 = 741521;
+    /// let lock_time = LockTime::from_consensus(n_lock_time);
+    /// assert!(lock_time.is_block_height());
+    /// assert_eq!(lock_time.to_consensus_u32(), n_lock_time);
+    #[inline]
+    pub fn to_consensus_u32(&self) -> u32 {
+        self.0
+    }
+}
+
+impl fmt::Display for Height {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(&self.0, f)
+    }
+}
+
+impl FromStr for Height {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let n = s.parse::<u32>().map_err(|e| Error::Parse(e, s.to_owned()))?;
+        Height::from_consensus(n)
+    }
+}
+
+impl TryFrom<&str> for Height {
+    type Error = Error;
+
+    fn try_from(s: &str) -> Result<Self, Self::Error> {
+        Height::from_str(s)
+    }
+}
+
+impl TryFrom<String> for Height {
+    type Error = Error;
+
+    fn try_from(s: String) -> Result<Self, Self::Error> {
+        let n = s.parse::<u32>().map_err(|e| Error::Parse(e, s))?;
+        Height::from_consensus(n)
+    }
+}
+
+/// A UNIX timestamp, seconds since epoch, guaranteed to always contain a valid time value.
+///
+/// Note that there is no manipulation of the inner value during construction or when using
+/// `to_consensus_u32()`. Said another way, `Time(x)` means 'x seconds since epoch' _not_ '(x -
+/// threshold) seconds since epoch'.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "serde", serde(crate = "actual_serde"))]
+pub struct Time(u32);
+
+impl Time {
+    /// Constructs a new block time.
+    ///
+    /// # Errors
+    ///
+    /// If `n` does not encode a UNIX time stamp (see documentation on [`LockTime`]).
+    ///
+    /// # Examples
+    /// ```rust
+    /// use bitcoin::blockdata::locktime::Time;
+    ///
+    /// let t: u32 = 1653195600; // May 22nd, 5am UTC.
+    /// let time = Time::from_consensus(t).expect("invalid time value");
+    /// assert_eq!(time.to_consensus_u32(), t);
+    /// ```
+    #[inline]
+    pub fn from_consensus(n: u32) -> Result<Time, Error> {
+        if is_block_time(n) {
+            Ok(Self(n))
+        } else {
+            Err(ConversionError::invalid_time(n).into())
+        }
+    }
+
+    /// Converts this `Time` to its inner `u32` value.
+    ///
+    /// # Examples
+    /// ```rust
+    /// use bitcoin::LockTime;
+    ///
+    /// let n_lock_time: u32 = 1653195600; // May 22nd, 5am UTC.
+    /// let lock_time = LockTime::from_consensus(n_lock_time);
+    /// assert_eq!(lock_time.to_consensus_u32(), n_lock_time);
+    /// ```
+    #[inline]
+    pub fn to_consensus_u32(&self) -> u32 {
+        self.0
+    }
+}
+
+impl fmt::Display for Time {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Display::fmt(&self.0, f)
+    }
+}
+
+impl FromStr for Time {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let n = s.parse::<u32>().map_err(|e| Error::Parse(e, s.to_owned()))?;
+        Time::from_consensus(n)
+    }
+}
+
+impl TryFrom<&str> for Time {
+    type Error = Error;
+
+    fn try_from(s: &str) -> Result<Self, Self::Error> {
+        Time::from_str(s)
+    }
+}
+
+impl TryFrom<String> for Time {
+    type Error = Error;
+
+    fn try_from(s: String) -> Result<Self, Self::Error> {
+        let n = s.parse::<u32>().map_err(|e| Error::Parse(e, s))?;
+        Time::from_consensus(n)
+    }
+}
+
+/// Returns true if `n` is a block height i.e., less than 500,000,000.
+fn is_block_height(n: u32) -> bool {
+    n < LOCK_TIME_THRESHOLD
+}
+
+/// Returns true if `n` is a UNIX timestamp i.e., greater than or equal to 500,000,000.
+fn is_block_time(n: u32) -> bool {
+    n >= LOCK_TIME_THRESHOLD
+}
+
+/// Catchall type for errors that relate to time locks.
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum Error {
+    /// An error occurred while converting a `u32` to a lock time variant.
+    Conversion(ConversionError),
+    /// An error occurred while operating on lock times.
+    Operation(OperationError),
+    /// An error occurred while parsing a string into an `u32`.
+    Parse(ParseIntError, String),
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use self::Error::*;
+
+        match *self {
+            Conversion(ref e) => write_err!(f, "error converting lock time value"; e),
+            Operation(ref e) => write_err!(f, "error during lock time operation"; e),
+            Parse(ref e, ref s) => write_err!(f, "error parsing string: {}", s; e),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use self::Error::*;
+
+        match *self {
+            Conversion(ref e) => Some(e),
+            Operation(ref e) => Some(e),
+            Parse(ref e, _) => Some(e),
+        }
+    }
+}
+
+impl From<ConversionError> for Error {
+    fn from(e: ConversionError) -> Self {
+        Error::Conversion(e)
+    }
+}
+
+impl From<OperationError> for Error {
+    fn from(e: OperationError) -> Self {
+        Error::Operation(e)
+    }
+}
+
+/// An error that occurs when converting a `u32` to a lock time variant.
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+pub struct ConversionError {
+    /// The expected timelock unit, height (blocks) or time (seconds).
+    unit: LockTimeUnit,
+    /// The invalid input value.
+    input: u32,
+}
+
+impl ConversionError {
+    /// Constructs a `ConversionError` from an invalid `n` when expecting a height value.
+    fn invalid_height(n: u32) -> Self {
+        Self {
+            unit: LockTimeUnit::Blocks,
+            input: n,
+        }
+    }
+
+    /// Constructs a `ConversionError` from an invalid `n` when expecting a time value.
+    fn invalid_time(n: u32) -> Self {
+        Self {
+            unit: LockTimeUnit::Seconds,
+            input: n,
+        }
+    }
+}
+
+impl fmt::Display for ConversionError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "invalid lock time value {}, {}", self.input, self.unit)
+    }
+}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for ConversionError {}
+
+/// Describes the two types of locking, lock-by-blockheight and lock-by-blocktime.
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
+enum LockTimeUnit {
+    /// Lock by blockheight.
+    Blocks,
+    /// Lock by blocktime.
+    Seconds,
+}
+
+impl fmt::Display for LockTimeUnit {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use LockTimeUnit::*;
+
+        match *self {
+            Blocks => write!(f, "expected lock-by-blockheight (must be < {})", LOCK_TIME_THRESHOLD),
+            Seconds => write!(f, "expected lock-by-blocktime (must be >= {})", LOCK_TIME_THRESHOLD),
+        }
+    }
+}
+
+/// Errors than occur when operating on lock times.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[non_exhaustive]
+pub enum OperationError {
+    /// Cannot compare different lock time units (height vs time).
+    InvalidComparison,
+}
+
+impl fmt::Display for OperationError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use self::OperationError::*;
+
+        match *self {
+            InvalidComparison => f.write_str("cannot compare different lock units (height vs time)"),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl std::error::Error for OperationError {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn display_and_alternate() {
+        let n = LockTime::from_consensus(100);
+        let s = format!("{}", n);
+        assert_eq!(&s, "100");
+
+        let got = format!("{:#}", n);
+        assert_eq!(got, "block-height 100");
+    }
+}

src/blockdata/mod.rs

@@ -8,6 +8,7 @@
 //!
 
 pub mod constants;
+pub mod locktime;
 pub mod opcodes;
 pub mod script;
 pub mod transaction;

src/blockdata/transaction.rs

@@ -26,6 +26,7 @@ use crate::blockdata::constants::{WITNESS_SCALE_FACTOR, MAX_SEQUENCE};
 #[cfg(feature="bitcoinconsensus")] use crate::blockdata::script;
 use crate::blockdata::script::Script;
 use crate::blockdata::witness::Witness;
+use crate::blockdata::locktime::{LockTime, PackedLockTime, Height, Time};
 use crate::consensus::{encode, Decodable, Encodable};
 use crate::hash_types::{Sighash, Txid, Wtxid};
 use crate::VarInt;
@@ -206,6 +207,20 @@ pub struct TxIn {
     pub witness: Witness
 }
 
+impl TxIn {
+    /// Returns true if this input enables the [`LockTime`]  (aka `nLockTime`) of its [`Transaction`].
+    ///
+    /// `nLockTime` is enabled if *any* input enables it. See [`Transaction::is_lock_time_enabled`]
+    ///  to check the overall state. If none of the inputs enables it, the lock time value is simply
+    ///  ignored. If this returns false and OP_CHECKLOCKTIMEVERIFY is used in the redeem script with
+    ///  this input then the script execution will fail [BIP-0065].
+    ///
+    /// [BIP-65](https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki)
+    pub fn enables_lock_time(&self) -> bool {
+        self.sequence != Sequence::MAX
+    }
+}
+
 impl Default for TxIn {
     fn default() -> TxIn {
         TxIn {
@@ -554,8 +569,13 @@ impl<E> EncodeSigningDataResult<E> {
 pub struct Transaction {
     /// The protocol version, is currently expected to be 1 or 2 (BIP 68).
     pub version: i32,
-    /// Block number before which this transaction is valid, or 0 for valid immediately.
+    /// Block height or timestamp. Transaction cannot be included in a block until this height/time.
-    pub lock_time: u32,
+    ///
+    /// ### Relevant BIPs
+    ///
+    /// * [BIP-65 OP_CHECKLOCKTIMEVERIFY](https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki)
+    /// * [BIP-113 Median time-past as endpoint for lock-time calculations](https://github.com/bitcoin/bips/blob/master/bip-0113.mediawiki)
+    pub lock_time: PackedLockTime,
     /// List of transaction inputs.
     pub input: Vec<TxIn>,
     /// List of transaction outputs.
@@ -916,6 +936,27 @@ impl Transaction {
     pub fn is_explicitly_rbf(&self) -> bool {
         self.input.iter().any(|input| input.sequence.is_rbf())
     }
+
+    /// Returns true if this [`Transaction`]'s absolute timelock is satisfied at `height`/`time`.
+    ///
+    /// # Returns
+    ///
+    /// By definition if the lock time is not enabled the transaction's absolute timelock is
+    /// considered to be satisfied i.e., there are no timelock constraints restricting this
+    /// transaction from being mined immediately.
+    pub fn is_absolute_timelock_satisfied(&self, height: Height, time: Time) -> bool {
+        if !self.is_lock_time_enabled() {
+            return true;
+        }
+        LockTime::from(self.lock_time).is_satisfied_by(height, time)
+    }
+
+    /// Returns `true` if this transactions nLockTime is enabled ([BIP-65]).
+    ///
+    /// [BIP-65]: https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki
+    pub fn is_lock_time_enabled(&self) -> bool {
+        self.input.iter().any(|i| i.enables_lock_time())
+    }
 }
 
 impl_consensus_encoding!(TxOut, value, script_pubkey);
@@ -1224,6 +1265,7 @@ mod tests {
     use core::str::FromStr;
     use crate::blockdata::constants::WITNESS_SCALE_FACTOR;
     use crate::blockdata::script::Script;
+    use crate::blockdata::locktime::PackedLockTime;
     use crate::consensus::encode::serialize;
     use crate::consensus::encode::deserialize;
 
@@ -1324,7 +1366,7 @@ mod tests {
                    "ce9ea9f6f5e422c6a9dbcddb3b9a14d1c78fab9ab520cb281aa2a74a09575da1".to_string());
         assert_eq!(realtx.input[0].previous_output.vout, 1);
         assert_eq!(realtx.output.len(), 1);
-        assert_eq!(realtx.lock_time, 0);
+        assert_eq!(realtx.lock_time, PackedLockTime::ZERO);
 
         assert_eq!(format!("{:x}", realtx.txid()),
                    "a6eab3c14ab5272a58a5ba91505ba1a4b6d7a3a9fcbd187b6cd99a7b6d548cb7".to_string());
@@ -1358,7 +1400,7 @@ mod tests {
                    "7cac3cf9a112cf04901a51d605058615d56ffe6d04b45270e89d1720ea955859".to_string());
         assert_eq!(realtx.input[0].previous_output.vout, 1);
         assert_eq!(realtx.output.len(), 1);
-        assert_eq!(realtx.lock_time, 0);
+        assert_eq!(realtx.lock_time, PackedLockTime::ZERO);
 
         assert_eq!(format!("{:x}", realtx.txid()),
                    "f5864806e3565c34d1b41e716f72609d00b55ea5eac5b924c9719a842ef42206".to_string());
@@ -1552,7 +1594,7 @@ mod tests {
         // We need a tx with more inputs than outputs.
         let tx = Transaction {
             version: 1,
-            lock_time: 0,
+            lock_time: PackedLockTime::ZERO,
             input: vec![TxIn::default(), TxIn::default()],
             output: vec![TxOut::default()],
         };

src/lib.rs

@@ -94,6 +94,7 @@ use std::io;
 use core2::io;
 
 pub use crate::blockdata::block::{Block, BlockHeader};
+pub use crate::blockdata::locktime::{self, LockTime, PackedLockTime};
 pub use crate::blockdata::script::Script;
 #[allow(deprecated)]
 pub use crate::blockdata::transaction::SigHashType;

src/util/bip143.rs

@@ -14,6 +14,7 @@ use crate::hash_types::Sighash;
 use crate::blockdata::script::Script;
 use crate::blockdata::witness::Witness;
 use crate::blockdata::transaction::{Transaction, TxIn, EcdsaSighashType};
+use crate::blockdata::locktime::LockTime;
 use crate::consensus::{encode, Encodable};
 
 use crate::io;
@@ -26,7 +27,7 @@ use crate::util::sighash;
 #[deprecated(since = "0.24.0", note = "please use [sighash::SighashCache] instead")]
 pub struct SighashComponents {
     tx_version: i32,
-    tx_locktime: u32,
+    tx_locktime: LockTime,
     /// Hash of all the previous outputs
     pub hash_prevouts: Sighash,
     /// Hash of all the input sequence nos
@@ -68,7 +69,7 @@ impl SighashComponents {
 
         SighashComponents {
             tx_version: tx.version,
-            tx_locktime: tx.lock_time,
+            tx_locktime: tx.lock_time.into(),
             hash_prevouts,
             hash_sequence,
             hash_outputs,
@@ -156,9 +157,9 @@ impl<R: DerefMut<Target = Transaction>> SigHashCache<R> {
     /// ```
     /// use bitcoin::blockdata::transaction::{Transaction, EcdsaSighashType};
     /// use bitcoin::util::bip143::SigHashCache;
-    /// use bitcoin::Script;
+    /// use bitcoin::{PackedLockTime, Script};
     ///
-    /// let mut tx_to_sign = Transaction { version: 2, lock_time: 0, input: Vec::new(), output: Vec::new() };
+    /// let mut tx_to_sign = Transaction { version: 2, lock_time: PackedLockTime::ZERO, input: Vec::new(), output: Vec::new() };
     /// let input_count = tx_to_sign.input.len();
     ///
     /// let mut sig_hasher = SigHashCache::new(&mut tx_to_sign);
@@ -181,6 +182,7 @@ mod tests {
     use crate::hash_types::Sighash;
     use crate::blockdata::script::Script;
     use crate::blockdata::transaction::Transaction;
+    use crate::blockdata::locktime::LockTime;
     use crate::consensus::encode::deserialize;
     use crate::network::constants::Network;
     use crate::util::address::Address;
@@ -225,7 +227,7 @@ mod tests {
             comp,
             SighashComponents {
                 tx_version: 1,
-                tx_locktime: 17,
+                tx_locktime: LockTime::from_consensus(17),
                 hash_prevouts: hex_hash!(
                     Sighash, "96b827c8483d4e9b96712b6713a7b68d6e8003a781feba36c31143470b4efd37"
                 ),
@@ -261,7 +263,7 @@ mod tests {
             comp,
             SighashComponents {
                 tx_version: 1,
-                tx_locktime: 1170,
+                tx_locktime: LockTime::from_consensus(1170),
                 hash_prevouts: hex_hash!(
                     Sighash, "b0287b4a252ac05af83d2dcef00ba313af78a3e9c329afa216eb3aa2a7b4613a"
                 ),
@@ -304,7 +306,7 @@ mod tests {
             comp,
             SighashComponents {
                 tx_version: 1,
-                tx_locktime: 0,
+                tx_locktime: LockTime::ZERO,
                 hash_prevouts: hex_hash!(
                     Sighash, "74afdc312af5183c4198a40ca3c1a275b485496dd3929bca388c4b5e31f7aaa0"
                 ),

src/util/bip152.rs

@@ -366,13 +366,13 @@ mod test {
     use crate::hashes::hex::FromHex;
     use crate::{
         Block, BlockHash, BlockHeader, OutPoint, Script, Sequence, Transaction, TxIn, TxMerkleNode,
-        TxOut, Txid, Witness,
+        TxOut, Txid, Witness, LockTime,
     };
 
     fn dummy_tx(nonce: &[u8]) -> Transaction {
         Transaction {
             version: 1,
-            lock_time: 2,
+            lock_time: LockTime::from_consensus(2).into(),
             input: vec![TxIn {
                 previous_output: OutPoint::new(Txid::hash(nonce), 0),
                 script_sig: Script::new(),

src/util/psbt/mod.rs

@@ -335,6 +335,7 @@ impl Decodable for PartiallySignedTransaction {
 mod tests {
     use super::*;
 
+    use crate::blockdata::locktime::PackedLockTime;
     use crate::hashes::hex::FromHex;
     use crate::hashes::{sha256, hash160, Hash, ripemd160};
     use crate::hash_types::Txid;
@@ -358,7 +359,7 @@ mod tests {
         let psbt = PartiallySignedTransaction {
             unsigned_tx: Transaction {
                 version: 2,
-                lock_time: 0,
+                lock_time: PackedLockTime::ZERO,
                 input: vec![],
                 output: vec![],
             },
@@ -428,7 +429,7 @@ mod tests {
         let expected = PartiallySignedTransaction {
             unsigned_tx: Transaction {
                 version: 2,
-                lock_time: 1257139,
+                lock_time: PackedLockTime(1257139),
                 input: vec![TxIn {
                     previous_output: OutPoint {
                         txid: Txid::from_hex(
@@ -501,7 +502,7 @@ mod tests {
         // create some values to use in the PSBT
         let tx = Transaction {
             version: 1,
-            lock_time: 0,
+            lock_time: PackedLockTime::ZERO,
             input: vec![TxIn {
                 previous_output: OutPoint {
                     txid: Txid::from_hex("e567952fb6cc33857f392efa3a46c995a28f69cca4bb1b37e0204dab1ec7a389").unwrap(),
@@ -599,6 +600,7 @@ mod tests {
         use crate::blockdata::script::Script;
         use crate::blockdata::transaction::{EcdsaSighashType, Transaction, TxIn, TxOut, OutPoint, Sequence};
         use crate::consensus::encode::serialize_hex;
+        use crate::blockdata::locktime::PackedLockTime;
         use crate::util::psbt::map::{Map, Input, Output};
         use crate::util::psbt::raw;
         use crate::util::psbt::{PartiallySignedTransaction, Error};
@@ -687,7 +689,7 @@ mod tests {
             let unserialized = PartiallySignedTransaction {
                 unsigned_tx: Transaction {
                     version: 2,
-                    lock_time: 1257139,
+                    lock_time: PackedLockTime(1257139),
                     input: vec![TxIn {
                         previous_output: OutPoint {
                             txid: Txid::from_hex(
@@ -718,7 +720,7 @@ mod tests {
                 inputs: vec![Input {
                     non_witness_utxo: Some(Transaction {
                         version: 1,
-                        lock_time: 0,
+                        lock_time: PackedLockTime::ZERO,
                         input: vec![TxIn {
                             previous_output: OutPoint {
                                 txid: Txid::from_hex(
@@ -999,7 +1001,7 @@ mod tests {
         let mut unserialized = PartiallySignedTransaction {
             unsigned_tx: Transaction {
                 version: 2,
-                lock_time: 1257139,
+                lock_time: PackedLockTime(1257139),
                 input: vec![TxIn {
                     previous_output: OutPoint {
                         txid: Txid::from_hex(
@@ -1030,7 +1032,7 @@ mod tests {
             inputs: vec![Input {
                 non_witness_utxo: Some(Transaction {
                     version: 1,
-                    lock_time: 0,
+                    lock_time: PackedLockTime::ZERO,
                     input: vec![TxIn {
                         previous_output: OutPoint {
                             txid: Txid::from_hex(

src/util/sighash.rs

@@ -759,9 +759,9 @@ impl<R: DerefMut<Target=Transaction>> SighashCache<R> {
     /// ```
     /// use bitcoin::blockdata::transaction::{Transaction, EcdsaSighashType};
     /// use bitcoin::util::sighash::SighashCache;
-    /// use bitcoin::Script;
+    /// use bitcoin::{PackedLockTime, Script};
     ///
-    /// let mut tx_to_sign = Transaction { version: 2, lock_time: 0, input: Vec::new(), output: Vec::new() };
+    /// let mut tx_to_sign = Transaction { version: 2, lock_time: PackedLockTime::ZERO, input: Vec::new(), output: Vec::new() };
     /// let input_count = tx_to_sign.input.len();
     ///
     /// let mut sig_hasher = SighashCache::new(&mut tx_to_sign);
@@ -812,6 +812,7 @@ impl<'a> Encodable for Annex<'a> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::blockdata::locktime::PackedLockTime;
     use crate::consensus::deserialize;
     use crate::hashes::hex::FromHex;
     use crate::hashes::{Hash, HashEngine};
@@ -958,7 +959,7 @@ mod tests {
     fn test_sighash_errors() {
         let dumb_tx = Transaction {
             version: 0,
-            lock_time: 0,
+            lock_time: PackedLockTime::ZERO,
             input: vec![TxIn::default()],
             output: vec![],
         };