diff --git a/src/txUtil.ts b/src/txUtil.ts
new file mode 100644
index 0000000..d3fa511
--- /dev/null
+++ b/src/txUtil.ts
@@ -0,0 +1,191 @@
+// forked from https://sensiblecontract.org
+
+import {
+    Addr,
+    ByteString,
+    Constants,
+    SmartContractLib,
+    Utils,
+    assert,
+    hash160,
+    hash256,
+    method,
+    slice,
+    toByteString,
+} from 'scrypt-ts'
+
+type Output = {
+    satoshis: bigint
+    script: ByteString
+}
+
+export class TxUtil extends SmartContractLib {
+    @method()
+    static readOutput(tx: ByteString, outputIndex: bigint): Output {
+        // first 4 bytes version
+        // 1 byte input num, only support max 3
+        let pos: bigint = 4n
+        let ninputs: bigint = Utils.fromLEUnsigned(slice(tx, pos, pos + 1n))
+        pos = pos + 1n
+        let script: ByteString = toByteString('')
+        let satoshis: bigint = 0n
+        // max support 3 input
+        // input
+        assert(ninputs <= 3n)
+        for (let i = 0; i < 3; i++) {
+            if (i < ninputs) {
+                // output point 36 bytes
+                pos = pos + 36n
+                // 1 byte var
+                // script code + 4 bytes sequence
+                let varLen: bigint = Utils.fromLEUnsigned(
+                    slice(tx, pos, pos + 1n)
+                )
+                if (varLen < 253) {
+                    let scriptLen: bigint = varLen
+                    pos = pos + 1n + scriptLen + 4n
+                } else if (varLen == 253n) {
+                    let scriptLen: bigint = Utils.fromLEUnsigned(
+                        slice(tx, pos + 1n, pos + 3n)
+                    )
+                    pos = pos + 3n + scriptLen + 4n
+                } else if (varLen == 254n) {
+                    let scriptLen: bigint = Utils.fromLEUnsigned(
+                        slice(tx, pos + 1n, pos + 5n)
+                    )
+                    pos = pos + 5n + scriptLen + 4n
+                } else {
+                    let scriptLen: bigint = Utils.fromLEUnsigned(
+                        slice(tx, pos + 1n, pos + 9n)
+                    )
+                    pos = pos + 9n + scriptLen + 4n
+                }
+            }
+        }
+
+        let noutputs: bigint = Utils.fromLEUnsigned(slice(tx, pos, pos + 1n))
+        pos = pos + 1n
+        assert(noutputs <= 3n)
+        for (let i = 0n; i < 3n; i++) {
+            if (i < noutputs) {
+                // 8 bytes value
+                let sats: bigint = Utils.fromLEUnsigned(
+                    slice(tx, pos, pos + 8n)
+                )
+                pos = pos + 8n
+                // script code
+                let varLen: bigint = Utils.fromLEUnsigned(
+                    slice(tx, pos, pos + 1n)
+                )
+                let scriptLen: bigint = 0n
+                if (varLen < 253n) {
+                    scriptLen = varLen
+                    pos = pos + 1n + scriptLen
+                } else if (varLen == 253n) {
+                    scriptLen = Utils.fromLEUnsigned(
+                        slice(tx, pos + 1n, pos + 3n)
+                    )
+                    pos = pos + 3n + scriptLen
+                } else if (varLen == 254n) {
+                    scriptLen = Utils.fromLEUnsigned(
+                        slice(tx, pos + 1n, pos + 5n)
+                    )
+                    pos = pos + 5n + scriptLen
+                } else {
+                    scriptLen = Utils.fromLEUnsigned(
+                        slice(tx, pos + 1n, pos + 9n)
+                    )
+                    pos = pos + 9n + scriptLen
+                }
+                if (i == outputIndex) {
+                    script = slice(tx, pos - scriptLen, pos)
+                    satoshis = sats
+                }
+            }
+        }
+
+        // 4 bytes locktime
+        return { satoshis, script }
+    }
+
+    @method()
+    static getScriptCodeFromOutput(output: ByteString): ByteString {
+        return Utils.readVarint(slice(output, 8n))
+    }
+
+    @method()
+    static getVarOpLen(length: bigint): bigint {
+        let res: bigint = 0n
+        if (length <= 75n) {
+            res = 1n
+        } else if (length <= 255n) {
+            res = 2n
+        } else if (length <= 65535n) {
+            res = 3n
+        } else {
+            res = 5n
+        }
+        return res
+    }
+
+    @method()
+    static getVarOpLenOpt(length: bigint): bigint {
+        let res: bigint = 0n
+        if (length <= 75n) {
+            res = 1n
+        } else {
+            res = 2n
+        }
+        return res
+    }
+
+    @method()
+    static genBsvOutput(satoshis: bigint, address: Addr): ByteString {
+        let output = toByteString('')
+        if (satoshis > 0n) {
+            let outputScript = Utils.buildPublicKeyHashScript(address)
+            output = Utils.buildOutput(outputScript, satoshis)
+        }
+        return output
+    }
+
+    // get i-th outpoint's txid
+    @method()
+    static getPrevoutTxid(prevouts: ByteString, i: bigint): ByteString {
+        let offset: bigint = i * Constants.OutpointLen
+        return slice(prevouts, offset, offset + Constants.TxIdLen)
+    }
+
+    // get i-th outpoint's output index
+    @method()
+    static getPrevoutOutputIdx(prevouts: ByteString, i: bigint): bigint {
+        let offset = i * Constants.OutpointLen
+        return Utils.fromLEUnsigned(
+            slice(
+                prevouts,
+                offset + Constants.TxIdLen,
+                offset + Constants.OutpointLen
+            )
+        )
+    }
+
+    // verify a contract, called in another input, by its hash
+    @method()
+    static verifyContractByHash(
+        prevouts: ByteString,
+        inputIdx: bigint,
+        prevTx: ByteString,
+        contractScriptHash: Addr
+    ): boolean {
+        // validate the tx containing the called contract
+        const prevTxId = TxUtil.getPrevoutTxid(prevouts, inputIdx)
+        assert(hash256(prevTx) == prevTxId)
+
+        // validate the called contract, i.e., its locking script
+        const outputIndex = TxUtil.getPrevoutOutputIdx(prevouts, inputIdx)
+        const contractScript = TxUtil.readOutput(prevTx, outputIndex).script
+        assert(Addr(contractScript) == contractScriptHash)
+
+        return true
+    }
+}