diff --git a/clients/js/src/cipher.ts b/clients/js/src/cipher.ts
index bffd072d..00ed75b4 100644
--- a/clients/js/src/cipher.ts
+++ b/clients/js/src/cipher.ts
@@ -82,7 +82,10 @@ export abstract class Cipher {
   public abstract publicKey: Uint8Array;
   public abstract epoch?: number;
 
-  public abstract encrypt(plaintext: Uint8Array): {
+  public abstract encrypt(
+    plaintext: Uint8Array,
+    nonce?: Uint8Array,
+  ): {
     ciphertext: Uint8Array;
     nonce: Uint8Array;
   };
@@ -93,7 +96,10 @@ export abstract class Cipher {
   ): Uint8Array;
 
   /** Encrypts the plaintext and encodes it for sending. */
-  public encryptCall(calldata?: BytesLike | null): BytesLike {
+  public encryptCall(
+    calldata?: BytesLike | null,
+    nonce?: Uint8Array,
+  ): BytesLike {
     // Txs without data are just balance transfers, and all data in those is public.
     if (calldata === undefined || calldata === null || calldata.length === 0)
       return '';
@@ -104,7 +110,8 @@ export abstract class Cipher {
 
     const innerEnvelope = cborEncode({ body: getBytes(calldata) });
 
-    const { ciphertext, nonce } = this.encrypt(innerEnvelope);
+    let ciphertext: Uint8Array;
+    ({ ciphertext, nonce } = this.encrypt(innerEnvelope, nonce));
 
     const envelope: Envelope = {
       format: this.kind,
@@ -251,11 +258,14 @@ export class X25519DeoxysII extends Cipher {
     this.cipher = new deoxysii.AEAD(new Uint8Array(this.key)); // deoxysii owns the input
   }
 
-  public encrypt(plaintext: Uint8Array): {
+  public encrypt(
+    plaintext: Uint8Array,
+    nonce?: Uint8Array,
+  ): {
     ciphertext: Uint8Array;
     nonce: Uint8Array;
   } {
-    const nonce = randomBytes(deoxysii.NonceSize);
+    nonce = nonce ?? randomBytes(deoxysii.NonceSize);
     const ciphertext = this.cipher.encrypt(nonce, plaintext);
     return { nonce, ciphertext };
   }
diff --git a/clients/js/test/cipher.spec.ts b/clients/js/test/cipher.spec.ts
index 0e00287b..c1b6062c 100644
--- a/clients/js/test/cipher.spec.ts
+++ b/clients/js/test/cipher.spec.ts
@@ -2,7 +2,10 @@
 
 import nacl from 'tweetnacl';
 import { hexlify, getBytes } from 'ethers';
-import { X25519DeoxysII } from '@oasisprotocol/sapphire-paratime';
+import {
+  isCalldataEnveloped,
+  X25519DeoxysII,
+} from '@oasisprotocol/sapphire-paratime';
 
 describe('X25519DeoxysII', () => {
   it('key derivation', () => {
@@ -30,7 +33,9 @@ describe('X25519DeoxysII', () => {
     const cipher = X25519DeoxysII.ephemeral(nacl.box.keyPair().publicKey);
     for (let i = 1; i < 512; i += 30) {
       const expected = nacl.randomBytes(i);
-      const decoded = cipher.decryptCall(cipher.encryptCall(expected));
+      const encrypted = cipher.encryptCall(expected);
+      expect(isCalldataEnveloped(encrypted)).toStrictEqual(true);
+      const decoded = cipher.decryptCall(encrypted);
       expect(hexlify(decoded)).toEqual(hexlify(expected));
     }
   });
diff --git a/contracts/contracts/CalldataEncryption.sol b/contracts/contracts/CalldataEncryption.sol
new file mode 100644
index 00000000..b1e7358c
--- /dev/null
+++ b/contracts/contracts/CalldataEncryption.sol
@@ -0,0 +1,102 @@
+// SPDX-License-Identifier: Apache-2.0
+
+pragma solidity ^0.8.0;
+
+import {Subcall} from "./Subcall.sol";
+import {Sapphire} from "./Sapphire.sol";
+import "./CBOR.sol" as CBOR;
+
+function _deriveKey(
+    bytes32 in_peerPublicKey,
+    Sapphire.Curve25519SecretKey in_x25519_secret
+) view returns (bytes32) {
+    return
+        Sapphire.deriveSymmetricKey(
+            Sapphire.Curve25519PublicKey.wrap(in_peerPublicKey),
+            in_x25519_secret
+        );
+}
+
+function _encryptInner(
+    bytes memory in_data,
+    Sapphire.Curve25519SecretKey in_x25519_secret,
+    bytes15 nonce,
+    bytes32 peerPublicKey
+) view returns (bytes memory out_encrypted) {
+    bytes memory plaintextEnvelope = abi.encodePacked(
+        hex"a1", // map(1)
+        hex"64", //     text(4) "body"
+        "body",
+        CBOR.encodeBytes(in_data)
+    );
+
+    out_encrypted = Sapphire.encrypt(
+        _deriveKey(peerPublicKey, in_x25519_secret),
+        nonce,
+        plaintextEnvelope,
+        ""
+    );
+}
+
+function encryptCallData(bytes memory in_data)
+    view
+    returns (bytes memory out_encrypted)
+{
+    if (in_data.length == 0) {
+        return "";
+    }
+
+    Sapphire.Curve25519PublicKey myPublic;
+    Sapphire.Curve25519SecretKey mySecret;
+
+    (myPublic, mySecret) = Sapphire.generateCurve25519KeyPair("");
+
+    bytes15 nonce = bytes15(Sapphire.randomBytes(15, ""));
+
+    Subcall.CallDataPublicKey memory cdpk;
+    uint256 epoch;
+
+    (epoch, cdpk) = Subcall.coreCallDataPublicKey();
+
+    return encryptCallData(in_data, myPublic, mySecret, nonce, epoch, cdpk.key);
+}
+
+function encryptCallData(
+    bytes memory in_data,
+    Sapphire.Curve25519PublicKey myPublic,
+    Sapphire.Curve25519SecretKey mySecret,
+    bytes15 nonce,
+    uint256 epoch,
+    bytes32 peerPublicKey
+) view returns (bytes memory out_encrypted) {
+    if (in_data.length == 0) {
+        return "";
+    }
+
+    bytes memory inner = _encryptInner(in_data, mySecret, nonce, peerPublicKey);
+
+    return
+        abi.encodePacked(
+            hex"a2", //  map(2)
+            hex"64", //      text(4) "body"
+            "body",
+            hex"a4", //          map(4)
+            hex"62", //              text(2) "pk"
+            "pk",
+            hex"5820", //                 bytes(32)
+            myPublic,
+            hex"64", //              text(4) "data"
+            "data",
+            CBOR.encodeBytes(inner), //     bytes(n) inner
+            hex"65", //              text(5) "epoch"
+            "epoch",
+            CBOR.encodeUint(epoch), //      unsigned(epoch)
+            hex"65", //              text(5) "nonce"
+            "nonce",
+            hex"4f", //                  bytes(15) nonce
+            nonce,
+            hex"66", //      text(6) "format"
+            "format",
+            hex"01" //      unsigned(1)
+        );
+}
diff --git a/contracts/contracts/tests/TestCalldataEncryption.sol b/contracts/contracts/tests/TestCalldataEncryption.sol
new file mode 100644
index 00000000..3f40bccb
--- /dev/null
+++ b/contracts/contracts/tests/TestCalldataEncryption.sol
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: Apache-2.0
+
+pragma solidity ^0.8.0;
+
+import { Sapphire } from "../Sapphire.sol";
+import { encryptCallData } from "../CalldataEncryption.sol";
+import { EIP155Signer } from "../EIP155Signer.sol";
+
+contract TestCalldataEncryption {
+    function testEncryptCallData(
+        bytes memory in_data,
+        Sapphire.Curve25519PublicKey myPublic,
+        Sapphire.Curve25519SecretKey mySecret,
+        bytes15 nonce,
+        uint256 epoch,
+        bytes32 peerPublicKey
+    ) external view returns (bytes memory) {
+        return encryptCallData(in_data, myPublic, mySecret, nonce, epoch, peerPublicKey);
+    }
+
+    function makeExampleCall(
+        bytes calldata in_data,
+        uint64 nonce,
+        uint256 gasPrice,
+        uint64 gasLimit,
+        address myAddr,
+        bytes32 myKey
+    )
+        external view
+        returns (bytes memory)
+    {
+        EIP155Signer.EthTx memory theTx = EIP155Signer.EthTx({
+            nonce: nonce,
+            gasPrice: gasPrice,
+            gasLimit: gasLimit,
+            value: 0,
+            to: address(this),
+            chainId: block.chainid,
+            data: encryptCallData(abi.encodeCall(this.example, in_data))
+        });
+
+        return EIP155Signer.sign(myAddr, myKey, theTx);
+    }
+
+    event ExampleEvent(bytes);
+
+    function example(bytes calldata in_calldata)
+        external
+    {
+        emit ExampleEvent(in_calldata);
+    }
+}
diff --git a/contracts/test/calldata.spec.ts b/contracts/test/calldata.spec.ts
new file mode 100644
index 00000000..88a09344
--- /dev/null
+++ b/contracts/test/calldata.spec.ts
@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: Apache-2.0
+
+import { expect } from 'chai';
+import { ethers } from 'hardhat';
+import { TestCalldataEncryption } from '../typechain-types/contracts/tests';
+import {
+  boxKeyPairFromSecretKey,
+  crypto_box_SECRETKEYBYTES,
+  isCalldataEnveloped,
+  X25519DeoxysII,
+} from '@oasisprotocol/sapphire-paratime';
+import { hexlify, parseUnits, randomBytes } from 'ethers';
+import { randomInt } from 'crypto';
+
+describe('CalldataEncryption', () => {
+  let contract: TestCalldataEncryption;
+
+  before(async () => {
+    const factory = await ethers.getContractFactory('TestCalldataEncryption');
+    contract = await factory.deploy();
+    await contract.waitForDeployment();
+  });
+
+  // Ensures that the JS library provides the same results as Solidity
+  it('testEncryptCallData', async () => {
+    for (let i = 1; i < 1024; i += 1 + i / 5) {
+      const peerKeypair = boxKeyPairFromSecretKey(
+        randomBytes(crypto_box_SECRETKEYBYTES),
+      );
+      const myKeypair = boxKeyPairFromSecretKey(
+        randomBytes(crypto_box_SECRETKEYBYTES),
+      );
+      const calldata = randomBytes(i);
+      const epoch = randomInt(1 << 32);
+      const nonce = randomBytes(15);
+      const cipher = X25519DeoxysII.fromSecretKey(
+        myKeypair.secretKey,
+        peerKeypair.publicKey,
+        epoch,
+      );
+      const encryptedCall = cipher.encryptCall(calldata, nonce);
+      const result = await contract.testEncryptCallData(
+        calldata,
+        myKeypair.publicKey,
+        myKeypair.secretKey,
+        nonce,
+        epoch,
+        peerKeypair.publicKey,
+      );
+      expect(result).eq(hexlify(encryptedCall));
+    }
+  });
+
+  it('roundtrip encryption', async () => {
+    // Tests must be submitted from an account which has a balance
+    // But can't get access to the signer private key from here
+    // So, assume the 0xf39F address is being used to run tests
+    const myAddr = '0xf39Fd6e51aad88F6F4ce6aB8827279cffFb92266';
+    const myKey =
+      '0xac0974bec39a17e36ba4a6b4d238ff944bacb478cbed5efcae784d7bf4f2ff80';
+    const [signerAddr] = await ethers.getSigners();
+    expect(signerAddr).eq(myAddr);
+
+    for (let i = 1; i < 1024; i += 250) {
+      // Have the contract sign an encrypted transaction for us
+      const bytes = randomBytes(i);
+      const nonce = await ethers.provider.getTransactionCount(myAddr);
+      const gasPrice = parseUnits('100', 'gwei');
+      const gasLimit = 200000;
+      const tx = await contract.makeExampleCall(
+        bytes,
+        nonce,
+        gasPrice,
+        gasLimit,
+        myAddr,
+        myKey,
+      );
+
+      // Then broadcast transaction and make sure the result is given back to us
+      // Making sure the tx was encrypted, and data is passed correctly
+      const response = await ethers.provider.broadcastTransaction(tx);
+      expect(isCalldataEnveloped(response.data)).eq(true);
+      const receipt = await response.wait();
+      expect(receipt?.status).eq(1);
+      const parsed = contract.interface.parseLog({
+        topics: receipt!.logs[0].topics as string[],
+        data: receipt!.logs[0].data,
+      });
+      expect(parsed!.args[0]).eq(hexlify(bytes));
+    }
+  });
+});