diff --git a/xftp-web/README.md b/xftp-web/README.md
index 7c63e824b..b7154ee71 100644
--- a/xftp-web/README.md
+++ b/xftp-web/README.md
@@ -28,7 +28,7 @@ const servers = [
   parseXFTPServer("xftp://server2..."),
   parseXFTPServer("xftp://server3..."),
 ]
-const encrypted = encryptFileForUpload(fileBytes, "photo.jpg")
+const encrypted = await encryptFileForUpload(fileBytes, "photo.jpg")
 const {rcvDescriptions, sndDescription, uri} = await uploadFile(agent, servers, encrypted, {
   onProgress: (uploaded, total) => console.log(`${uploaded}/${total}`),
 })
diff --git a/xftp-web/src/agent.ts b/xftp-web/src/agent.ts
index 509acecef..8704065a1 100644
--- a/xftp-web/src/agent.ts
+++ b/xftp-web/src/agent.ts
@@ -4,7 +4,7 @@
 // file descriptions, and DEFLATE-compressed URI encoding.
 
 import pako from "pako"
-import {encryptFile, encodeFileHeader} from "./crypto/file.js"
+import {encryptFileAsync, encodeFileHeader} from "./crypto/file.js"
 import {generateEd25519KeyPair, encodePubKeyEd25519, encodePrivKeyEd25519, decodePrivKeyEd25519, ed25519KeyPairFromSeed} from "./crypto/keys.js"
 import {sha512Streaming} from "./crypto/digest.js"
 import {prepareChunkSizes, prepareChunkSpecs, getChunkDigest, fileSizeLen, authTagSize} from "./protocol/chunks.js"
@@ -79,7 +79,11 @@ export function decodeDescriptionURI(fragment: string): FileDescription {
 
 // -- Upload
 
-export function encryptFileForUpload(source: Uint8Array, fileName: string): EncryptedFileInfo {
+export async function encryptFileForUpload(
+  source: Uint8Array,
+  fileName: string,
+  onProgress?: (done: number, total: number) => void
+): Promise<EncryptedFileInfo> {
   const key = new Uint8Array(32)
   const nonce = new Uint8Array(24)
   crypto.getRandomValues(key)
@@ -89,7 +93,7 @@ export function encryptFileForUpload(source: Uint8Array, fileName: string): Encr
   const payloadSize = Number(fileSize) + fileSizeLen + authTagSize
   const chunkSizes = prepareChunkSizes(payloadSize)
   const encSize = BigInt(chunkSizes.reduce((a, b) => a + b, 0))
-  const encData = encryptFile(source, fileHdr, key, nonce, fileSize, encSize)
+  const encData = await encryptFileAsync(source, fileHdr, key, nonce, fileSize, encSize, onProgress)
   const digest = sha512Streaming([encData])
   console.log(`[AGENT-DBG] encrypt: encData.len=${encData.length} digest=${_dbgHex(digest, 64)} chunkSizes=[${chunkSizes.join(',')}]`)
   return {encData, digest, key, nonce, chunkSizes}
@@ -229,7 +233,7 @@ async function uploadRedirectDescription(
 ): Promise<FileDescription> {
   const yaml = encodeFileDescription(innerFd)
   const yamlBytes = new TextEncoder().encode(yaml)
-  const enc = encryptFileForUpload(yamlBytes, "")
+  const enc = await encryptFileForUpload(yamlBytes, "")
   const specs = prepareChunkSpecs(enc.chunkSizes)
 
   const chunkJobs = specs.map((spec, i) => ({
diff --git a/xftp-web/src/crypto/file.ts b/xftp-web/src/crypto/file.ts
index 0088a680b..a6468a3f7 100644
--- a/xftp-web/src/crypto/file.ts
+++ b/xftp-web/src/crypto/file.ts
@@ -69,6 +69,48 @@ export function encryptFile(
   return concatBytes(hdr, encSource, encPad, tag)
 }
 
+// Async variant: encrypts source in 64KB slices, yielding between each to avoid blocking the main thread.
+// Produces identical output to encryptFile.
+const ENCRYPT_SLICE = 65536
+
+export async function encryptFileAsync(
+  source: Uint8Array,
+  fileHdr: Uint8Array,
+  key: Uint8Array,
+  nonce: Uint8Array,
+  fileSize: bigint,
+  encSize: bigint,
+  onProgress?: (done: number, total: number) => void
+): Promise<Uint8Array> {
+  const state = sbInit(key, nonce)
+  const lenStr = encodeInt64(fileSize)
+  const padLen = Number(encSize - AUTH_TAG_SIZE - fileSize - 8n)
+  if (padLen < 0) throw new Error("encryptFile: encSize too small")
+  const totalOut = Number(encSize)
+  const out = new Uint8Array(totalOut)
+  let outOff = 0
+  // Header (small, no yield needed)
+  const hdr = sbEncryptChunk(state, concatBytes(lenStr, fileHdr))
+  out.set(hdr, outOff); outOff += hdr.length
+  // Source in 64KB slices, yielding between each
+  for (let off = 0; off < source.length; off += ENCRYPT_SLICE) {
+    const end = Math.min(off + ENCRYPT_SLICE, source.length)
+    const enc = sbEncryptChunk(state, source.subarray(off, end))
+    out.set(enc, outOff); outOff += enc.length
+    onProgress?.(end, source.length)
+    await new Promise<void>(r => setTimeout(r, 0))
+  }
+  // Padding (small, no yield needed)
+  const padding = new Uint8Array(padLen)
+  padding.fill(0x23)
+  const encPad = sbEncryptChunk(state, padding)
+  out.set(encPad, outOff); outOff += encPad.length
+  // Auth tag
+  const tag = sbAuth(state)
+  out.set(tag, outOff)
+  return out
+}
+
 // -- Decryption (FileTransfer.Crypto:decryptChunks)
 
 // Decrypt one or more XFTP chunks into a FileHeader and file content.