diff --git a/packages/SwingSet/src/netstring.js b/packages/SwingSet/src/netstring.js
new file mode 100644
index 000000000000..204af9b69a78
--- /dev/null
+++ b/packages/SwingSet/src/netstring.js
@@ -0,0 +1,81 @@
+// adapted from 'netstring-stream', https://github.com/tlivings/netstring-stream/
+import { assert } from '@agoric/assert';
+
+const { Transform } = require('stream');
+
+const COLON = 58;
+const COMMA = 44;
+
+// input is a Buffer, output is a netstring-wrapped Buffer
+export function encode(data) {
+  const prefix = Buffer.from(`${data.length}:`);
+  const suffix = Buffer.from(',');
+  return Buffer.concat([prefix, data, suffix]);
+}
+
+// input is a sequence of strings, output is a byte pipe
+export function createWriter() {
+  function transform(chunk, encoding, callback) {
+    assert.equal(encoding, 'buffer');
+    callback(encode(chunk));
+  }
+  return new Transform({ transform });
+}
+
+// Input is a Buffer containing zero or more netstrings and maybe some
+// leftover bytes. Output is zero or more decoded Buffers, one per netstring,
+// plus a Buffer of leftover bytes.
+//
+export function decode(data) {
+  // TODO: it would be more efficient to accumulate pending data in an array,
+  // rather than doing a concat each time
+  let start = 0;
+  const payloads = [];
+
+  for (;;) {
+    const colon = data.indexOf(COLON, start);
+    if (colon === -1) {
+      break; // still waiting for `${LENGTH}:`
+    }
+    const sizeString = data.toString('utf-8', start, colon);
+    const size = parseInt(sizeString, 10);
+    if (!(size > -1)) {
+      // reject NaN, all negative numbers
+      throw Error(`unparseable size '${sizeString}', should be integer`);
+    }
+    if (data.length < colon + 1 + size + 1) {
+      break; // still waiting for `${DATA}.`
+    }
+    if (data[colon + 1 + size] !== COMMA) {
+      throw Error(`malformed netstring: not terminated by comma`);
+    }
+    payloads.push(data.subarray(colon + 1, colon + 1 + size));
+    start = colon + 1 + size + 1;
+  }
+
+  const leftover = data.subarray(start);
+  return { leftover, payloads };
+}
+
+// input is a byte pipe, output is a sequence of Buffers
+export function createReader() {
+  let buffered = Buffer.from('');
+
+  function transform(chunk, encoding, callback) {
+    assert.equal(encoding, 'buffer');
+    buffered = Buffer.concat([buffered, chunk]);
+    let res = null;
+    try {
+      const { leftover, payloads } = decode(buffered);
+      buffered = leftover;
+      for (let i = 0; i < payloads.length; i += 1) {
+        this.push(payloads[i]);
+      }
+    } catch (err) {
+      res = err;
+    }
+    callback(res);
+  }
+
+  return new Transform({ transform });
+}
diff --git a/packages/SwingSet/test/test-netstring.js b/packages/SwingSet/test/test-netstring.js
new file mode 100644
index 000000000000..c9d2e7b1dbab
--- /dev/null
+++ b/packages/SwingSet/test/test-netstring.js
@@ -0,0 +1,60 @@
+import '@agoric/install-ses'; // adds 'harden' to global
+import test from 'ava';
+import { encode, decode } from '../src/netstring';
+
+test('encode', t => {
+  function eq(input, expected) {
+    const encoded = encode(Buffer.from(input));
+    const expBuf = Buffer.from(expected);
+    if (encoded.compare(expBuf) !== 0) {
+      console.log(`got : ${encoded}`);
+      console.log(`want: ${expBuf}`);
+    }
+    t.deepEqual(encoded, expBuf);
+  }
+
+  eq('', '0:,');
+  eq('a', '1:a,');
+  eq('abc', '3:abc,');
+  const umlaut = 'ümlaut';
+  t.is(umlaut.length, 6);
+  const umlautBuffer = Buffer.from(umlaut, 'utf-8');
+  t.is(umlautBuffer.length, 7);
+  const expectedBuffer = Buffer.from(`7:${umlaut},`, 'utf-8');
+  eq(umlautBuffer, expectedBuffer);
+});
+
+test('decode', t => {
+  function eq(input, expPayloads, expLeftover) {
+    const encPayloads = expPayloads.map(Buffer.from);
+    const encLeftover = Buffer.from(expLeftover);
+
+    const { payloads, leftover } = decode(Buffer.from(input));
+    t.deepEqual(payloads, encPayloads);
+    t.deepEqual(leftover, encLeftover);
+  }
+
+  eq('', [], '');
+  eq('0', [], '0');
+  eq('0:', [], '0:');
+  eq('0:,', [''], '');
+  eq('0:,1', [''], '1');
+  eq('0:,1:', [''], '1:');
+  eq('0:,1:a', [''], '1:a');
+  eq('0:,1:a,', ['', 'a'], '');
+
+  const umlaut = 'ümlaut';
+  t.is(umlaut.length, 6);
+  const umlautBuffer = Buffer.from(umlaut, 'utf-8');
+  t.is(umlautBuffer.length, 7);
+  const expectedBuffer = Buffer.from(`7:${umlaut},`, 'utf-8');
+  eq(expectedBuffer, [umlaut], '');
+
+  function bad(input, message) {
+    t.throws(() => decode(Buffer.from(input)), { message });
+  }
+
+  // bad('a', 'non-numeric length prefix');
+  bad('a:', `unparseable size 'a', should be integer`);
+  bad('1:ab', 'malformed netstring: not terminated by comma');
+});