diff --git a/packages/tlock/src/commitment.test.ts b/packages/tlock/src/commitment.test.ts
index fc11cb3..da42255 100644
--- a/packages/tlock/src/commitment.test.ts
+++ b/packages/tlock/src/commitment.test.ts
@@ -8,6 +8,7 @@ import {
   encodeBidPreimage,
   fromHex,
   i128ToBeBytes,
+  isValidHex,
   toHex,
 } from "./commitment.js";
 
@@ -64,11 +65,49 @@ test("fromHex decodes lowercase, uppercase, and prefixed values", () => {
 test("fromHex accepts empty input as an empty byte array", () => {
   assert.deepEqual([...fromHex("")], []);
   assert.deepEqual([...fromHex("0x")], []);
+  assert.deepEqual([...fromHex("0X")], []);
 });
 
-test("fromHex rejects odd-length and non-hex input", () => {
+test("fromHex rejects odd-length, non-hex, and non-string input", () => {
   assert.throws(() => fromHex("abc"), /odd hex length/);
+  assert.throws(() => fromHex("0xabc"), /odd hex length/);
+  assert.throws(() => fromHex("0Xabc"), /odd hex length/);
   assert.throws(() => fromHex("zz"), /invalid hex characters/);
   assert.throws(() => fromHex("12 3"), /invalid hex characters/);
+  assert.throws(() => fromHex("12g4"), /invalid hex characters/);
   assert.throws(() => fromHex("0x12gg"), /invalid hex characters/);
+  assert.throws(() => fromHex(123 as any), /hex must be a string/);
+  assert.throws(() => fromHex(null as any), /hex must be a string/);
 });
+
+test("isValidHex accepts valid even-length hex strings", () => {
+  assert.equal(isValidHex("abcdef"), true);
+  assert.equal(isValidHex("ABCDEF"), true);
+  assert.equal(isValidHex("0xAbCdEf"), true);
+  assert.equal(isValidHex("0XABCDEF"), true);
+  assert.equal(isValidHex(""), true);
+  assert.equal(isValidHex("0x"), true);
+  assert.equal(isValidHex("0X"), true);
+  assert.equal(isValidHex("00"), true);
+  assert.equal(isValidHex("0x1234567890abcdefABCDEF"), true);
+});
+
+test("isValidHex rejects odd-length, non-hex, and non-string inputs", () => {
+  assert.equal(isValidHex("abc"), false);
+  assert.equal(isValidHex("0xabc"), false);
+  assert.equal(isValidHex("0Xabc"), false);
+  assert.equal(isValidHex("zz"), false);
+  assert.equal(isValidHex("12 3"), false);
+  assert.equal(isValidHex("12g4"), false);
+  assert.equal(isValidHex("0x12gg"), false);
+  assert.equal(isValidHex(123 as any), false);
+  assert.equal(isValidHex(null as any), false);
+  assert.equal(isValidHex(undefined as any), false);
+  assert.equal(isValidHex({} as any), false);
+});
+
+test("toHex produces clean lowercase hex strings", () => {
+  assert.equal(toHex(new Uint8Array([0x00, 0x0f, 0xab, 0xcd, 0xef])), "000fabcdef");
+  assert.equal(toHex(new Uint8Array([])), "");
+});
+
diff --git a/packages/tlock/src/commitment.ts b/packages/tlock/src/commitment.ts
index 9994867..cab1009 100644
--- a/packages/tlock/src/commitment.ts
+++ b/packages/tlock/src/commitment.ts
@@ -77,10 +77,24 @@ export function toHex(bytes: Uint8Array): string {
     .join("");
 }
 
+const HEX_RE = /^[0-9a-fA-F]*$/;
+
+/**
+ * Returns true if `hex` (optionally `0x` or `0X` prefixed) is a valid, even-length hexadecimal string.
+ */
+export function isValidHex(hex: string): boolean {
+  if (typeof hex !== "string") return false;
+  const clean = hex.startsWith("0x") || hex.startsWith("0X") ? hex.slice(2) : hex;
+  return clean.length % 2 === 0 && HEX_RE.test(clean);
+}
+
 export function fromHex(hex: string): Uint8Array {
-  const clean = /^0x/i.test(hex) ? hex.slice(2) : hex;
+  if (typeof hex !== "string") {
+    throw new Error("hex must be a string");
+  }
+  const clean = hex.startsWith("0x") || hex.startsWith("0X") ? hex.slice(2) : hex;
   if (clean.length % 2 !== 0) throw new Error("odd hex length");
-  if (!/^[0-9a-fA-F]*$/.test(clean)) {
+  if (!HEX_RE.test(clean)) {
     throw new Error("invalid hex characters");
   }
   const out = new Uint8Array(clean.length / 2);
@@ -89,3 +103,4 @@ export function fromHex(hex: string): Uint8Array {
   }
   return out;
 }
+
diff --git a/packages/tlock/src/index.ts b/packages/tlock/src/index.ts
index 0cdd65f..36d1a49 100644
--- a/packages/tlock/src/index.ts
+++ b/packages/tlock/src/index.ts
@@ -6,6 +6,7 @@ export {
   beBytesToI128,
   toHex,
   fromHex,
+  isValidHex,
   VALUE_BYTES,
   NONCE_BYTES,
   PREIMAGE_BYTES,
diff --git a/packages/tlock/src/seal.test.ts b/packages/tlock/src/seal.test.ts
index 66543ca..7a081e0 100644
--- a/packages/tlock/src/seal.test.ts
+++ b/packages/tlock/src/seal.test.ts
@@ -115,3 +115,54 @@ test(
     await assert.rejects(openBid(sealed.ciphertext, client));
   },
 );
+
+test("sealBid rejects non-positive or non-integer round numbers", async () => {
+  const client = quicknet();
+  const nonce = generateNonce();
+  const value = 100n;
+
+  await assert.rejects(
+    () => sealBid({ value, nonce, round: 0, client }),
+    (err: any) => err instanceof RangeError && /round must be a positive integer/.test(err.message),
+  );
+  await assert.rejects(
+    () => sealBid({ value, nonce, round: -5, client }),
+    (err: any) => err instanceof RangeError && /round must be a positive integer/.test(err.message),
+  );
+  await assert.rejects(
+    () => sealBid({ value, nonce, round: 1.5, client }),
+    (err: any) => err instanceof RangeError && /round must be a positive integer/.test(err.message),
+  );
+  await assert.rejects(
+    () => sealBid({ value, nonce, round: NaN, client }),
+    (err: any) => err instanceof RangeError && /round must be a positive integer/.test(err.message),
+  );
+});
+
+test("sealBid rejects invalid nonce lengths", async () => {
+  const client = quicknet();
+  const value = 100n;
+  const round = 1000;
+
+  await assert.rejects(
+    () => sealBid({ value, nonce: new Uint8Array(16), round, client }),
+    /nonce must be 32 bytes/,
+  );
+  await assert.rejects(
+    () => sealBid({ value, nonce: new Uint8Array(31), round, client }),
+    /nonce must be 32 bytes/,
+  );
+  await assert.rejects(
+    () => sealBid({ value, nonce: new Uint8Array(33), round, client }),
+    /nonce must be 32 bytes/,
+  );
+});
+
+test("openBid rejects empty ciphertext", async () => {
+  const client = quicknet();
+  await assert.rejects(
+    () => openBid(new Uint8Array(0), client),
+    /ciphertext is empty/,
+  );
+});
+
diff --git a/packages/tlock/src/seal.ts b/packages/tlock/src/seal.ts
index 0cfdf83..c235983 100644
--- a/packages/tlock/src/seal.ts
+++ b/packages/tlock/src/seal.ts
@@ -42,6 +42,13 @@ export function generateNonce(): Uint8Array {
 export async function sealBid(params: SealBidParams): Promise<SealedBid> {
   const { value, nonce, round, client, identity, auditorPublicKey } = params;
 
+  if (!Number.isInteger(round) || round < 1) {
+    throw new RangeError(`round must be a positive integer, got ${round}`);
+  }
+  if (!nonce || nonce.length !== NONCE_BYTES) {
+    throw new Error(`nonce must be ${NONCE_BYTES} bytes, got ${nonce?.length}`);
+  }
+
   const preimage = encodeBidPreimage(value, nonce);
   const h = commitment(value, nonce);
   const armored = await timelockEncrypt(round, TlockBuffer.from(preimage), client);
@@ -68,6 +75,9 @@ export async function openBid(
   ciphertext: Uint8Array,
   client: DrandClient,
 ): Promise<OpenedBid> {
+  if (!ciphertext || ciphertext.length === 0) {
+    throw new Error("ciphertext is empty");
+  }
   const armored = utf8Decode.decode(ciphertext);
   const plaintext = await timelockDecrypt(armored, client);
   return decodeBidPreimage(Uint8Array.from(plaintext));