cashu/nuts/nut14/

mod.rs

//! NUT-14: Hashed Time Lock Contacts (HTLC)
//!
//! <https://github.com/cashubtc/nuts/blob/main/14.md>

use std::str::FromStr;

use bitcoin::hashes::sha256::Hash as Sha256Hash;
use bitcoin::hashes::Hash;
use bitcoin::secp256k1::schnorr::Signature;
use serde::{Deserialize, Serialize};
use thiserror::Error;

use super::nut00::Witness;
use super::nut10::Secret;
use super::nut11::valid_signatures;
use super::{Conditions, Proof};
use crate::nut10::get_pubkeys_and_required_sigs;
use crate::nut11::extract_signatures_from_witness;
use crate::util::{hex, unix_time};
use crate::SpendingConditions;

pub mod serde_htlc_witness;

/// NUT14 Errors
#[derive(Debug, Error)]
pub enum Error {
    /// Incorrect secret kind
    #[error("Secret is not a HTLC secret")]
    IncorrectSecretKind,
    /// HTLC locktime has already passed
    #[error("Locktime in past")]
    LocktimeInPast,
    /// Witness signature is not valid
    #[error("Invalid signature")]
    InvalidSignature,
    /// Hash Required
    #[error("Hash required")]
    HashRequired,
    /// Hash is not valid
    #[error("Hash is not valid")]
    InvalidHash,
    /// Preimage does not match
    #[error("Preimage does not match")]
    Preimage,
    /// HTLC preimage must be valid hex encoding
    #[error("Preimage must be valid hex encoding")]
    InvalidHexPreimage,
    /// HTLC preimage must be exactly 32 bytes
    #[error("Preimage must be exactly 32 bytes (64 hex characters)")]
    PreimageInvalidSize,
    /// Witness Signatures not provided
    #[error("Witness did not provide signatures")]
    SignaturesNotProvided,
    /// SIG_ALL not supported in this context
    #[error("SIG_ALL proofs must be verified using a different method")]
    SigAllNotSupportedHere,
    /// HTLC Spend conditions not met
    #[error("HTLC spend conditions are not met")]
    SpendConditionsNotMet,
    /// From hex error
    #[error(transparent)]
    HexError(#[from] hex::Error),
    /// Secp256k1 error
    #[error(transparent)]
    Secp256k1(#[from] bitcoin::secp256k1::Error),
    /// NUT11 Error
    #[error(transparent)]
    NUT11(#[from] super::nut11::Error),
    #[error(transparent)]
    /// Serde Error
    Serde(#[from] serde_json::Error),
}

/// HTLC Witness
#[derive(Default, Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
pub struct HTLCWitness {
    /// Preimage
    pub preimage: String,
    /// Signatures
    #[serde(skip_serializing_if = "Option::is_none")]
    pub signatures: Option<Vec<String>>,
}

impl HTLCWitness {
    /// Decode the preimage from hex and verify it's exactly 32 bytes
    ///
    /// Returns the 32-byte preimage data if valid, or an error if:
    /// - The hex decoding fails
    /// - The decoded data is not exactly 32 bytes
    pub fn preimage_data(&self) -> Result<[u8; 32], Error> {
        const REQUIRED_PREIMAGE_BYTES: usize = 32;

        // Decode the 64-character hex string to bytes
        let preimage_bytes = hex::decode(&self.preimage).map_err(|_| Error::InvalidHexPreimage)?;

        // Verify the preimage is exactly 32 bytes
        if preimage_bytes.len() != REQUIRED_PREIMAGE_BYTES {
            return Err(Error::PreimageInvalidSize);
        }

        // Convert to fixed-size array
        let mut array = [0u8; 32];
        array.copy_from_slice(&preimage_bytes);
        Ok(array)
    }
}

impl Proof {
    /// Verify HTLC
    ///
    /// Per NUT-14, there are two spending pathways:
    /// 1. Receiver path (preimage + pubkeys): ALWAYS available
    /// 2. Sender/Refund path (refund keys, no preimage): available AFTER locktime
    ///
    /// The verification tries to determine which path is being used based on
    /// the witness provided, then validates accordingly.
    pub fn verify_htlc(&self) -> Result<(), Error> {
        let secret: Secret = self.secret.clone().try_into()?;
        let spending_conditions: Conditions = secret
            .secret_data()
            .tags()
            .cloned()
            .unwrap_or_default()
            .try_into()
            .map_err(|_| Error::SpendConditionsNotMet)?;

        if spending_conditions.sig_flag == super::SigFlag::SigAll {
            return Err(Error::SigAllNotSupportedHere);
        }

        if secret.kind() != super::Kind::HTLC {
            return Err(Error::IncorrectSecretKind);
        }

        // Get the spending requirements (includes both receiver and refund paths)
        let now = unix_time();
        let requirements =
            super::nut10::get_pubkeys_and_required_sigs(&secret, now).map_err(|err| match err {
                super::nut10::Error::NUT14(nut14_err) => nut14_err,
                _ => Error::SpendConditionsNotMet,
            })?;

        // Try to extract HTLC witness - must be correct type
        let htlc_witness = match &self.witness {
            Some(Witness::HTLCWitness(witness)) => witness,
            _ => {
                // Wrong witness type or no witness
                // If refund path is available with 0 required sigs, anyone can spend
                if let Some(refund_path) = &requirements.refund_path {
                    if refund_path.required_sigs == 0 {
                        return Ok(());
                    }
                }
                return Err(Error::IncorrectSecretKind);
            }
        };

        // Try to verify the preimage and capture the specific error if it fails
        let preimage_result = verify_htlc_preimage(htlc_witness, &secret);

        // Determine which path to use:
        // - If preimage is valid → use receiver path (always available)
        // - If preimage is invalid/missing → try refund path (if available)
        if preimage_result.is_ok() {
            // Receiver path: preimage valid, now check signatures against pubkeys
            if requirements.required_sigs == 0 {
                return Ok(());
            }

            let witness_signatures = htlc_witness
                .signatures
                .as_ref()
                .ok_or(Error::SignaturesNotProvided)?;

            let signatures: Vec<Signature> = witness_signatures
                .iter()
                .map(|s| Signature::from_str(s))
                .collect::<Result<Vec<_>, _>>()?;

            let msg: &[u8] = self.secret.as_bytes();
            let valid_sig_count = valid_signatures(msg, &requirements.pubkeys, &signatures)?;

            if valid_sig_count >= requirements.required_sigs {
                Ok(())
            } else {
                Err(Error::NUT11(super::nut11::Error::SpendConditionsNotMet))
            }
        } else if let Some(refund_path) = &requirements.refund_path {
            // Refund path: preimage not valid/provided, but locktime has passed
            // Check signatures against refund keys
            if refund_path.required_sigs == 0 {
                // Anyone can spend (locktime passed, no refund keys)
                return Ok(());
            }

            let witness_signatures = htlc_witness
                .signatures
                .as_ref()
                .ok_or(Error::SignaturesNotProvided)?;

            let signatures: Vec<Signature> = witness_signatures
                .iter()
                .map(|s| Signature::from_str(s))
                .collect::<Result<Vec<_>, _>>()?;

            let msg: &[u8] = self.secret.as_bytes();
            let valid_sig_count = valid_signatures(msg, &refund_path.pubkeys, &signatures)?;

            if valid_sig_count >= refund_path.required_sigs {
                Ok(())
            } else {
                Err(Error::NUT11(super::nut11::Error::SpendConditionsNotMet))
            }
        } else {
            // No valid preimage and refund path not available (locktime not passed)
            // Return the specific error from preimage verification
            preimage_result
        }
    }

    /// Add Preimage
    #[inline]
    pub fn add_preimage(&mut self, preimage: String) {
        let signatures = self
            .witness
            .as_ref()
            .map(super::nut00::Witness::signatures)
            .unwrap_or_default();

        self.witness = Some(Witness::HTLCWitness(HTLCWitness {
            preimage,
            signatures,
        }))
    }
}

impl SpendingConditions {
    /// New HTLC [SpendingConditions]
    pub fn new_htlc(preimage: String, conditions: Option<Conditions>) -> Result<Self, Error> {
        const MAX_PREIMAGE_BYTES: usize = 32;

        let preimage_bytes = hex::decode(preimage)?;

        if preimage_bytes.len() != MAX_PREIMAGE_BYTES {
            return Err(Error::PreimageInvalidSize);
        }

        let htlc = Sha256Hash::hash(&preimage_bytes);

        Ok(Self::HTLCConditions {
            data: htlc,
            conditions,
        })
    }

    /// New HTLC [SpendingConditions] from a hash directly instead of preimage
    pub fn new_htlc_hash(hash: &str, conditions: Option<Conditions>) -> Result<Self, Error> {
        let hash = Sha256Hash::from_str(hash).map_err(|_| Error::InvalidHash)?;

        Ok(Self::HTLCConditions {
            data: hash,
            conditions,
        })
    }
}

/// Verify that a preimage matches the hash in the secret data
///
/// The preimage should be a 64-character hex string representing 32 bytes.
/// We decode it from hex, hash it with SHA256, and compare to the hash in secret.data
fn verify_htlc_preimage(witness: &HTLCWitness, secret: &Secret) -> Result<(), Error> {
    use bitcoin::hashes::sha256::Hash as Sha256Hash;
    use bitcoin::hashes::Hash;

    // Get the hash lock from the secret data
    let hash_lock =
        Sha256Hash::from_str(secret.secret_data().data()).map_err(|_| Error::InvalidHash)?;

    // Decode and validate the preimage (returns [u8; 32])
    let preimage_bytes = witness.preimage_data()?;

    // Hash the 32-byte preimage
    let preimage_hash = Sha256Hash::hash(&preimage_bytes);

    // Compare with the hash lock
    if hash_lock.ne(&preimage_hash) {
        return Err(Error::Preimage);
    }

    Ok(())
}

/// Verify HTLC SIG_ALL signatures
///
/// Do NOT call this directly. This is called only from 'verify_full_sig_all_check',
/// which has already done many important SIG_ALL checks. This performs the final
/// signature verification for SIG_ALL+HTLC transactions.
///
/// Per NUT-14, there are two spending pathways:
/// 1. Receiver path (preimage + pubkeys): ALWAYS available
/// 2. Sender/Refund path (refund keys, no preimage): available AFTER locktime
pub(crate) fn verify_sig_all_htlc(first_input: &Proof, msg_to_sign: String) -> Result<(), Error> {
    // Get the first input, as it's the one with the signatures
    let first_secret =
        Secret::try_from(&first_input.secret).map_err(|_| Error::IncorrectSecretKind)?;

    // Record current time for locktime evaluation
    let current_time = crate::util::unix_time();

    // Get the spending requirements (includes both receiver and refund paths)
    let requirements = get_pubkeys_and_required_sigs(&first_secret, current_time)
        .map_err(|_| Error::SpendConditionsNotMet)?;

    // Try to extract HTLC witness and check if preimage is valid
    let htlc_witness = match first_input.witness.as_ref() {
        Some(super::Witness::HTLCWitness(witness)) => Some(witness),
        _ => None,
    };

    // Check if a valid preimage is provided
    let preimage_valid = htlc_witness
        .map(|w| verify_htlc_preimage(w, &first_secret).is_ok())
        .unwrap_or(false);

    // Check for "anyone can spend" case first (preimage invalid, locktime passed, no refund keys)
    // This doesn't require any signatures
    if !preimage_valid {
        if let Some(refund_path) = &requirements.refund_path {
            if refund_path.required_sigs == 0 {
                return Ok(());
            }
        }
    }

    // Get the witness (needed for signature extraction)
    let first_witness = first_input
        .witness
        .as_ref()
        .ok_or(Error::SignaturesNotProvided)?;

    // Determine which path to use:
    // - If preimage is valid → use receiver path (always available)
    // - If preimage is invalid/missing → try refund path (if available)
    if preimage_valid {
        // Receiver path: preimage valid, now check SIG_ALL signatures against pubkeys
        if requirements.required_sigs == 0 {
            return Ok(());
        }

        let signatures = extract_signatures_from_witness(first_witness)?;
        let valid_sig_count = super::nut11::valid_signatures(
            msg_to_sign.as_bytes(),
            &requirements.pubkeys,
            &signatures,
        )
        .map_err(|_| Error::InvalidSignature)?;

        if valid_sig_count >= requirements.required_sigs {
            Ok(())
        } else {
            Err(Error::SpendConditionsNotMet)
        }
    } else if let Some(refund_path) = &requirements.refund_path {
        // Refund path: preimage not valid/provided, but locktime has passed
        // Check SIG_ALL signatures against refund keys
        let signatures = extract_signatures_from_witness(first_witness)?;
        let valid_sig_count = super::nut11::valid_signatures(
            msg_to_sign.as_bytes(),
            &refund_path.pubkeys,
            &signatures,
        )
        .map_err(|_| Error::InvalidSignature)?;

        if valid_sig_count >= refund_path.required_sigs {
            Ok(())
        } else {
            Err(Error::SpendConditionsNotMet)
        }
    } else {
        // No valid preimage and refund path not available (locktime not passed)
        Err(Error::SpendConditionsNotMet)
    }
}

#[cfg(test)]
mod tests {
    use bitcoin::hashes::sha256::Hash as Sha256Hash;
    use bitcoin::hashes::Hash;

    use super::*;
    use crate::nuts::nut00::Witness;
    use crate::nuts::nut10::Kind;
    use crate::nuts::Nut10Secret;
    use crate::secret::Secret as SecretString;
    use crate::SecretData;

    /// Tests that verify_htlc correctly accepts a valid HTLC with the correct preimage.
    ///
    /// This test ensures that a properly formed HTLC proof with the correct preimage
    /// passes verification.
    ///
    /// Mutant testing: Combined with negative tests, this catches mutations that
    /// replace verify_htlc with Ok(()) since the negative tests will fail.
    #[test]
    fn test_verify_htlc_valid() {
        // Create a valid HTLC secret with a known preimage (32 bytes)
        let preimage_bytes = [42u8; 32]; // 32-byte preimage
        let hash = Sha256Hash::hash(&preimage_bytes);
        let hash_str = hash.to_string();

        let nut10_secret = Nut10Secret::new(
            Kind::HTLC,
            SecretData::new(hash_str, None::<Vec<Vec<String>>>),
        );
        let secret: SecretString = nut10_secret.try_into().unwrap();

        let htlc_witness = HTLCWitness {
            preimage: hex::encode(preimage_bytes),
            signatures: None,
        };

        let proof = Proof {
            amount: crate::Amount::from(1),
            keyset_id: crate::nuts::nut02::Id::from_str("00deadbeef123456").unwrap(),
            secret,
            c: crate::nuts::nut01::PublicKey::from_hex(
                "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
            )
            .unwrap(),
            witness: Some(Witness::HTLCWitness(htlc_witness)),
            dleq: None,
            p2pk_e: None,
        };

        // Valid HTLC should verify successfully
        assert!(proof.verify_htlc().is_ok());
    }

    /// Tests that verify_htlc correctly rejects an HTLC with a wrong preimage.
    ///
    /// This test is critical for security - if the verification function doesn't properly
    /// check the preimage against the hash, an attacker could spend HTLC-locked funds
    /// without knowing the correct preimage.
    ///
    /// Mutant testing: Catches mutations that replace verify_htlc with Ok(()) or remove
    /// the preimage verification logic.
    #[test]
    fn test_verify_htlc_wrong_preimage() {
        // Create an HTLC secret with a specific hash (32 bytes)
        let correct_preimage_bytes = [42u8; 32];
        let hash = Sha256Hash::hash(&correct_preimage_bytes);
        let hash_str = hash.to_string();

        let nut10_secret = Nut10Secret::new(
            Kind::HTLC,
            SecretData::new(hash_str, None::<Vec<Vec<String>>>),
        );
        let secret: SecretString = nut10_secret.try_into().unwrap();

        // Use a different preimage in the witness
        let wrong_preimage_bytes = [99u8; 32]; // Different from correct preimage
        let htlc_witness = HTLCWitness {
            preimage: hex::encode(wrong_preimage_bytes),
            signatures: None,
        };

        let proof = Proof {
            amount: crate::Amount::from(1),
            keyset_id: crate::nuts::nut02::Id::from_str("00deadbeef123456").unwrap(),
            secret,
            c: crate::nuts::nut01::PublicKey::from_hex(
                "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
            )
            .unwrap(),
            witness: Some(Witness::HTLCWitness(htlc_witness)),
            dleq: None,
            p2pk_e: None,
        };

        // Verification should fail with wrong preimage
        let result = proof.verify_htlc();
        assert!(result.is_err());
        assert!(matches!(result.unwrap_err(), Error::Preimage));
    }

    /// Tests that verify_htlc correctly rejects an HTLC with an invalid hash format.
    ///
    /// This test ensures that the verification function properly validates that the
    /// hash in the secret data is a valid SHA256 hash.
    ///
    /// Mutant testing: Catches mutations that replace verify_htlc with Ok(()) or
    /// remove the hash validation logic.
    #[test]
    fn test_verify_htlc_invalid_hash() {
        // Create an HTLC secret with an invalid hash (not a valid hex string)
        let invalid_hash = "not_a_valid_hash";

        let nut10_secret = Nut10Secret::new(
            Kind::HTLC,
            SecretData::new(invalid_hash.to_string(), None::<Vec<Vec<String>>>),
        );
        let secret: SecretString = nut10_secret.try_into().unwrap();

        let preimage_bytes = [42u8; 32]; // Valid 32-byte preimage
        let htlc_witness = HTLCWitness {
            preimage: hex::encode(preimage_bytes),
            signatures: None,
        };

        let proof = Proof {
            amount: crate::Amount::from(1),
            keyset_id: crate::nuts::nut02::Id::from_str("00deadbeef123456").unwrap(),
            secret,
            c: crate::nuts::nut01::PublicKey::from_hex(
                "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
            )
            .unwrap(),
            witness: Some(Witness::HTLCWitness(htlc_witness)),
            dleq: None,
            p2pk_e: None,
        };

        // Verification should fail with invalid hash
        let result = proof.verify_htlc();
        assert!(result.is_err());
        assert!(matches!(result.unwrap_err(), Error::InvalidHash));
    }

    #[test]
    fn test_htlc_num_sigs_zero_bypasses_signature_requirement() {
        let pubkey = crate::nuts::nut01::PublicKey::from_hex(
            "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
        )
        .unwrap();

        let preimage_bytes = [42u8; 32];
        let hash = Sha256Hash::hash(&preimage_bytes);
        let hash_str = hash.to_string();

        let tags = vec![
            vec!["pubkeys".to_string(), pubkey.to_string()],
            vec!["n_sigs".to_string(), "0".to_string()],
        ];

        let nut10_secret = Nut10Secret::new(Kind::HTLC, SecretData::new(hash_str, Some(tags)));
        // Since we patched deserialization itself (TryFrom<Secret>), let's verify that
        // extracting the conditions out of a constructed secret will error
        let conditions_res = crate::nuts::nut10::Conditions::try_from(
            nut10_secret.secret_data().tags().cloned().unwrap(),
        );
        assert!(
            conditions_res.is_err(),
            "Conditions should fail to parse due to n_sigs=0"
        );
    }

    #[test]
    fn test_verify_sig_all_htlc_nsigs_zero_bypasses_sig_check() {
        let preimage_bytes = [42u8; 32];
        let hash = Sha256Hash::hash(&preimage_bytes);
        let hash_str = hash.to_string();

        let required_pubkey = crate::nuts::nut01::PublicKey::from_hex(
            "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
        )
        .unwrap();

        // SIG_ALL HTLC with pubkeys but n_sigs=0
        let tags = vec![
            vec!["pubkeys".to_string(), required_pubkey.to_string()],
            vec!["n_sigs".to_string(), "0".to_string()],
            vec!["sigflag".to_string(), "SIG_ALL".to_string()],
        ];

        let nut10_secret = Nut10Secret::new(Kind::HTLC, SecretData::new(hash_str, Some(tags)));

        let conditions_res = crate::nuts::nut10::Conditions::try_from(
            nut10_secret.secret_data().tags().cloned().unwrap(),
        );
        assert!(
            conditions_res.is_err(),
            "Conditions should fail to parse due to n_sigs=0"
        );
    }

    /// Tests that verify_htlc correctly rejects an HTLC with the wrong witness type.
    ///
    /// This test ensures that the verification function checks that the witness is
    /// of the correct type (HTLCWitness) and not some other witness type.
    ///
    /// Mutant testing: Catches mutations that replace verify_htlc with Ok(()) or
    /// remove the witness type check.
    #[test]
    fn test_verify_htlc_wrong_witness_type() {
        // Create an HTLC secret
        let preimage = "test_preimage";
        let hash = Sha256Hash::hash(preimage.as_bytes());
        let hash_str = hash.to_string();

        let nut10_secret = Nut10Secret::new(
            Kind::HTLC,
            SecretData::new(hash_str, None::<Vec<Vec<String>>>),
        );
        let secret: SecretString = nut10_secret.try_into().unwrap();

        // Create proof with wrong witness type (P2PKWitness instead of HTLCWitness)
        let proof = Proof {
            amount: crate::Amount::from(1),
            keyset_id: crate::nuts::nut02::Id::from_str("00deadbeef123456").unwrap(),
            secret,
            c: crate::nuts::nut01::PublicKey::from_hex(
                "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
            )
            .unwrap(),
            witness: Some(Witness::P2PKWitness(super::super::nut11::P2PKWitness {
                signatures: vec![],
            })),
            dleq: None,
            p2pk_e: None,
        };

        // Verification should fail with wrong witness type
        let result = proof.verify_htlc();
        assert!(result.is_err());
        assert!(matches!(result.unwrap_err(), Error::IncorrectSecretKind));
    }

    /// Tests that add_preimage correctly adds a preimage to the proof.
    ///
    /// This test ensures that add_preimage actually modifies the witness and doesn't
    /// just return without doing anything.
    ///
    /// Mutant testing: Catches mutations that replace add_preimage with () without
    /// actually adding the preimage.
    #[test]
    fn test_add_preimage() {
        let preimage_bytes = [42u8; 32]; // 32-byte preimage
        let hash = Sha256Hash::hash(&preimage_bytes);
        let hash_str = hash.to_string();

        let nut10_secret = Nut10Secret::new(
            Kind::HTLC,
            SecretData::new(hash_str, None::<Vec<Vec<String>>>),
        );
        let secret: SecretString = nut10_secret.try_into().unwrap();

        let mut proof = Proof {
            amount: crate::Amount::from(1),
            keyset_id: crate::nuts::nut02::Id::from_str("00deadbeef123456").unwrap(),
            secret,
            c: crate::nuts::nut01::PublicKey::from_hex(
                "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
            )
            .unwrap(),
            witness: None,
            dleq: None,
            p2pk_e: None,
        };

        // Initially, witness should be None
        assert!(proof.witness.is_none());

        // Add preimage (hex-encoded)
        let preimage_hex = hex::encode(preimage_bytes);
        proof.add_preimage(preimage_hex.clone());

        // After adding, witness should be Some with HTLCWitness
        assert!(proof.witness.is_some());
        if let Some(Witness::HTLCWitness(witness)) = &proof.witness {
            assert_eq!(witness.preimage, preimage_hex);
        } else {
            panic!("Expected HTLCWitness");
        }

        // The proof with added preimage should verify successfully
        assert!(proof.verify_htlc().is_ok());
    }

    /// Tests that verify_htlc requires BOTH locktime expired AND no refund keys for "anyone can spend".
    ///
    /// This test verifies that when locktime has passed and refund keys are present,
    /// a signature from the refund keys is required (not anyone-can-spend).
    ///
    /// Per NUT-14: After locktime, the refund path requires signatures from refund keys.
    /// The "anyone can spend" case only applies when locktime passed AND no refund keys.
    #[test]
    fn test_htlc_locktime_and_refund_keys_logic() {
        use crate::nuts::nut01::PublicKey;
        use crate::nuts::nut10::Conditions;

        let correct_preimage_bytes = [42u8; 32]; // 32-byte preimage
        let hash = Sha256Hash::hash(&correct_preimage_bytes);
        let hash_str = hash.to_string();

        // Use WRONG preimage to force using refund path (not receiver path)
        let wrong_preimage_bytes = [99u8; 32];

        // Test: Locktime has passed (locktime=1) but refund keys ARE present
        // Since we provide wrong preimage, receiver path fails, so we try refund path.
        // Refund path with refund keys present should require a signature.
        let refund_pubkey = PublicKey::from_hex(
            "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
        )
        .unwrap();

        let conditions_with_refund = Conditions {
            locktime: Some(1), // Locktime in past (current time is much larger)
            pubkeys: None,
            refund_keys: Some(vec![refund_pubkey]), // Refund key present
            num_sigs: None,
            sig_flag: crate::nuts::nut11::SigFlag::default(),
            num_sigs_refund: None,
        };

        let nut10_secret = Nut10Secret::new(
            Kind::HTLC,
            SecretData::new(hash_str, Some(conditions_with_refund)),
        );
        let secret: SecretString = nut10_secret.try_into().unwrap();

        let htlc_witness = HTLCWitness {
            preimage: hex::encode(wrong_preimage_bytes), // Wrong preimage!
            signatures: None,                            // No signature provided
        };

        let proof = Proof {
            amount: crate::Amount::from(1),
            keyset_id: crate::nuts::nut02::Id::from_str("00deadbeef123456").unwrap(),
            secret,
            c: crate::nuts::nut01::PublicKey::from_hex(
                "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
            )
            .unwrap(),
            witness: Some(Witness::HTLCWitness(htlc_witness)),
            dleq: None,
            p2pk_e: None,
        };

        // Should FAIL because:
        // 1. Wrong preimage means receiver path fails
        // 2. Falls back to refund path (locktime passed)
        // 3. Refund keys are present, so signature is required
        // 4. No signature provided
        let result = proof.verify_htlc();
        assert!(
            result.is_err(),
            "Should fail when using refund path with refund keys but no signature"
        );
    }

    #[test]
    fn test_htlc_generated_empty_refund_keys_are_omitted() {
        use crate::nuts::nut10::Conditions;

        let preimage_bytes = [42u8; 32];
        let hash = Sha256Hash::hash(&preimage_bytes);
        let hash_str = hash.to_string();

        let conditions = Conditions {
            locktime: Some(1),
            pubkeys: None,
            refund_keys: Some(vec![]),
            num_sigs: None,
            sig_flag: crate::nuts::nut11::SigFlag::default(),
            num_sigs_refund: None,
        };

        let nut10_secret =
            Nut10Secret::new(Kind::HTLC, SecretData::new(hash_str, Some(conditions)));
        let secret: SecretString = nut10_secret.try_into().unwrap();

        let htlc_witness = HTLCWitness {
            preimage: hex::encode([0xffu8; 32]),
            signatures: None,
        };

        let proof = Proof {
            amount: crate::Amount::from(1),
            keyset_id: crate::nuts::nut02::Id::from_str("00deadbeef123456").unwrap(),
            secret,
            c: crate::nuts::nut01::PublicKey::from_hex(
                "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
            )
            .unwrap(),
            witness: Some(Witness::HTLCWitness(htlc_witness)),
            dleq: None,
            p2pk_e: None,
        };

        assert!(proof.verify_htlc().is_ok());
    }

    #[test]
    fn test_htlc_empty_refund_tag_is_rejected() {
        let preimage_bytes = [42u8; 32];
        let hash = Sha256Hash::hash(&preimage_bytes);
        let hash_str = hash.to_string();

        let tags = vec![
            vec!["locktime".to_string(), "1".to_string()],
            vec!["refund".to_string()],
        ];

        let nut10_secret = Nut10Secret::new(Kind::HTLC, SecretData::new(hash_str, Some(tags)));
        let secret: SecretString = nut10_secret.try_into().unwrap();

        let htlc_witness = HTLCWitness {
            preimage: hex::encode([0xffu8; 32]),
            signatures: None,
        };

        let proof = Proof {
            amount: crate::Amount::from(1),
            keyset_id: crate::nuts::nut02::Id::from_str("00deadbeef123456").unwrap(),
            secret,
            c: crate::nuts::nut01::PublicKey::from_hex(
                "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2",
            )
            .unwrap(),
            witness: Some(Witness::HTLCWitness(htlc_witness)),
            dleq: None,
            p2pk_e: None,
        };

        assert!(matches!(
            proof.verify_htlc(),
            Err(Error::SpendConditionsNotMet)
        ));
    }
}