diff --git a/frontend/src/components/PairingCodeEntry.vue b/frontend/src/components/PairingCodeEntry.vue
index 24ba9ca..1ca1c21 100644
--- a/frontend/src/components/PairingCodeEntry.vue
+++ b/frontend/src/components/PairingCodeEntry.vue
@@ -6,29 +6,35 @@
     </div>
 
     <form @submit.prevent="submitCode" class="pairing-form">
-      <div class="input-wrapper">
-        <input
-          ref="inputRef"
-          v-model="code"
-          type="text"
-          :placeholder="placeholder"
-          :disabled="loading || completed"
-          autocomplete="off"
-          autocapitalize="none"
-          autocorrect="off"
-          spellcheck="false"
-          class="pairing-input"
-          @input="handleInput"
-          @keydown="handleKeydown"
-        />
-        <!-- Autocomplete hint overlay -->
-        <span v-if="autocompleteHint" class="autocomplete-hint">{{ autocompleteHintDisplay }}</span>
-      </div>
-
-      <!-- Looking up state -->
-      <div v-if="lookingUp" class="lookup-status">
-        <span class="lookup-spinner"></span>
-        <span>Looking up device…</span>
+      <div class="input-row">
+        <div class="input-wrapper" :class="{ 'has-error': serverError, 'is-complete': deviceInfo && !serverError && !completed, 'is-verified': completed }">
+          <!-- Colored overlay to show invalid words in red -->
+          <span v-if="hasInvalidWord" class="input-overlay" aria-hidden="true"><span v-for="(segment, i) in coloredSegments" :key="i" :class="{ 'invalid-word': segment.invalid }">{{ segment.text }}</span></span>
+          <input
+            ref="inputRef"
+            v-model="code"
+            type="text"
+            :placeholder="placeholder"
+            :disabled="completed"
+            autocomplete="off"
+            autocapitalize="none"
+            autocorrect="off"
+            spellcheck="false"
+            class="pairing-input"
+            :class="{ 'text-hidden': hasInvalidWord }"
+            @input="handleInput"
+            @keydown="handleKeydown"
+            @click="updateAutocomplete"
+            @select="updateAutocomplete"
+          />
+          <!-- Autocomplete hint overlay: shows text before cursor + hint suffix -->
+          <span v-if="autocompleteHint && !hasInvalidWord" class="autocomplete-hint" aria-hidden="true"><span class="hint-prefix">{{ autocompletePrefix }}</span><span class="hint-suffix">{{ autocompleteSuffix }}</span></span>
+        </div>
+        <!-- Processing status beside input -->
+        <div v-if="processingStatus" class="processing-status">
+          <span class="processing-icon">{{ processingStatus === 'pow' ? '🔐' : '📡' }}</span>
+          <span class="processing-spinner-small"></span>
+        </div>
       </div>
 
       <!-- Device info display (shown when 3 words match a request) -->
@@ -45,6 +51,7 @@
           </div>
         </div>
         <button
+          ref="submitBtnRef"
           type="submit"
           :disabled="loading"
           class="btn-primary"
@@ -65,12 +72,12 @@
 </template>
 
 <script setup>
-import { computed, onMounted, onUnmounted, ref } from 'vue'
+import { computed, nextTick, onMounted, onUnmounted, ref } from 'vue'
 import { startAuthentication } from '@simplewebauthn/browser'
 import aWebSocket from '@/utils/awaitable-websocket'
 import { getSettings } from '@/utils/settings'
-import { apiJson } from '@/utils/api'
-import { getUniqueMatch, isValidWord } from '@/utils/wordlist'
+import { getUniqueMatch, isValidWord, isValidPrefix } from '@/utils/wordlist'
+import { solvePoW } from '@/utils/pow'
 
 const props = defineProps({
   title: { type: String, default: 'Help Another Device Sign In' },
@@ -81,19 +88,56 @@ const props = defineProps({
 const emit = defineEmits(['completed', 'error', 'cancelled'])
 
 const inputRef = ref(null)
+const submitBtnRef = ref(null)
 const code = ref('')
 const loading = ref(false)
-const lookingUp = ref(false)
+const isProcessing = ref(false) // Track if we're processing (solving PoW or waiting for server)
+const processingStatus = ref('') // 'pow' = computing PoW, 'server' = waiting for server
 const error = ref(null)
 const completed = ref(false)
 const completedMessage = ref('')
 const deviceInfo = ref(null)
 const autocompleteHint = ref('')
-const invalidWords = ref([]) // Track which words are invalid
-let ws = null
-let lookupTimeout = null
+const hasInvalidWord = ref(false) // Track if any word is invalid (not a valid prefix)
+const serverError = ref(false) // Track if server rejected the code
+const cursorPos = ref(0) // Track cursor position for autocomplete
 
-// Get the current word being typed (the last word without a separator)
+// Unified WebSocket state
+let ws = null
+let wsConnecting = false
+let currentChallenge = null // Current PoW challenge from server
+let currentBits = null
+let powPromise = null // Promise for background PoW computation
+let powNonce = null // Solved PoW nonce ready to use
+let lookupTimeout = null
+let lastLookedUpCode = null // Track last code we looked up to avoid duplicates
+
+// Get the word at cursor position
+function getWordAtCursor(input, cursor) {
+  if (!input || cursor < 0) return { word: '', start: 0, end: 0 }
+
+  // Find word boundaries around cursor
+  let start = cursor
+  let end = cursor
+
+  // Move start back to find word beginning
+  while (start > 0 && /[a-zA-Z]/.test(input[start - 1])) {
+    start--
+  }
+
+  // Move end forward to find word end
+  while (end < input.length && /[a-zA-Z]/.test(input[end])) {
+    end++
+  }
+
+  return {
+    word: input.slice(start, end),
+    start,
+    end
+  }
+}
+
+// Get the current word being typed (the last word without a separator) - kept for validation
 function getCurrentWord(input) {
   const match = input.match(/[a-zA-Z]+$/)
   return match ? match[0] : ''
@@ -104,30 +148,94 @@ function getWords(input) {
   return input.trim().split(/[.\s]+/).filter(w => w.length > 0)
 }
 
-// Count completed words (separated by space or dot)
-function countWords(input) {
-  return getWords(input).length
-}
-
-// Validate all words and return list of invalid ones
-function validateWords(input) {
+// Count complete words (words followed by a separator)
+function countCompleteWords(input) {
+  // Count words that are followed by a separator
+  const endsWithSeparator = /[.\s]$/.test(input)
   const words = getWords(input)
-  return words.filter(w => !isValidWord(w))
+  return endsWithSeparator ? words.length : Math.max(0, words.length - 1)
 }
 
-// Check if all words are valid
+// Check validity and return info about each word segment
+function analyzeWords(input) {
+  if (!input) return { valid: true, segments: [] }
+
+  const segments = []
+  const endsWithSeparator = /[.\s]$/.test(input)
+
+  // Parse input preserving separators
+  let pos = 0
+  const regex = /([a-zA-Z]+)|([.\s]+)/g
+  let match
+
+  while ((match = regex.exec(input)) !== null) {
+    if (match[1]) {
+      // Word
+      segments.push({ text: match[1], isWord: true, start: match.index })
+    } else if (match[2]) {
+      // Separator
+      segments.push({ text: match[2], isWord: false, start: match.index })
+    }
+  }
+
+  // Determine validity of each word
+  const words = segments.filter(s => s.isWord)
+  let allValid = true
+
+  words.forEach((wordSeg, idx) => {
+    const isLastWord = idx === words.length - 1
+    const word = wordSeg.text.toLowerCase()
+
+    if (isLastWord && !endsWithSeparator) {
+      // Currently being typed - just needs to be valid prefix
+      wordSeg.invalid = !isValidPrefix(word)
+    } else {
+      // Complete word - must be a valid word
+      wordSeg.invalid = !isValidWord(word)
+    }
+
+    if (wordSeg.invalid) allValid = false
+  })
+
+  return { valid: allValid, segments }
+}
+
+// Computed property for colored segments
+const coloredSegments = computed(() => {
+  const { segments } = analyzeWords(code.value)
+  return segments.map(s => ({
+    text: s.text,
+    invalid: s.invalid || false
+  }))
+})
+
+// Check if all completed words are valid and current word is a valid prefix
+function checkWordsValidity(input) {
+  return analyzeWords(input).valid
+}
+
+// Check if all words are valid complete words
 function allWordsValid(input) {
   const words = getWords(input)
   return words.length > 0 && words.every(w => isValidWord(w))
 }
 
-// Compute autocomplete hint for display
-const autocompleteHintDisplay = computed(() => {
+// Compute autocomplete prefix (text up to and including current word at cursor)
+const autocompletePrefix = computed(() => {
   if (!autocompleteHint.value || !code.value) return ''
-  const currentWord = getCurrentWord(code.value)
-  if (!currentWord) return ''
-  // Return the typed text + remaining hint characters
-  return code.value + autocompleteHint.value.slice(currentWord.length)
+  const { word, end } = getWordAtCursor(code.value, cursorPos.value)
+  if (!word) return ''
+  // Return text up to end of current word (this part is transparent)
+  return code.value.slice(0, end)
+})
+
+// Compute autocomplete suffix (the completion part only)
+const autocompleteSuffix = computed(() => {
+  if (!autocompleteHint.value || !code.value) return ''
+  const { word } = getWordAtCursor(code.value, cursorPos.value)
+  if (!word) return ''
+  // Return only the remaining characters of the hint
+  return autocompleteHint.value.slice(word.length)
 })
 
 // Check if we have exactly 3 valid words
@@ -141,38 +249,122 @@ function normalizeCode(input) {
   return input.trim().toLowerCase().split(/[.\s]+/).filter(w => w).join('.')
 }
 
-// Update autocomplete hint based on current input
-function updateAutocomplete() {
-  const currentWord = getCurrentWord(code.value)
-  const wordCount = countWords(code.value)
+// Start solving PoW in background
+function startPowSolving() {
+  if (!currentChallenge || powPromise) return
 
-  // Don't autocomplete if we already have 3 words or no current word
-  if (wordCount >= 3 || !currentWord || currentWord.length < 1) {
+  const challenge = Uint8Array.from(atob(currentChallenge), c => c.charCodeAt(0))
+  powPromise = solvePoW(challenge, currentBits).then(nonce => {
+    powNonce = nonce
+    powPromise = null
+  })
+}
+
+// Get the solved PoW nonce, waiting if necessary
+async function getPowNonce() {
+  if (powNonce) {
+    const nonce = powNonce
+    powNonce = null
+    return nonce
+  }
+  if (powPromise) {
+    await powPromise
+    const nonce = powNonce
+    powNonce = null
+    return nonce
+  }
+  // Need to solve now
+  if (!currentChallenge) {
+    throw new Error('No PoW challenge available')
+  }
+  const challenge = Uint8Array.from(atob(currentChallenge), c => c.charCodeAt(0))
+  return await solvePoW(challenge, currentBits)
+}
+
+// Update challenge from server response and start solving
+function updateChallenge(pow) {
+  if (pow?.challenge) {
+    currentChallenge = pow.challenge
+    currentBits = pow.bits
+    powNonce = null
+    powPromise = null
+    // Start solving in background immediately
+    startPowSolving()
+  }
+}
+
+// Connect to the unified WebSocket endpoint
+async function ensureConnection() {
+  if (ws || wsConnecting) return
+
+  wsConnecting = true
+  try {
+    const settings = await getSettings()
+    const authHost = settings?.auth_host
+
+    // Single unified endpoint - no code in URL!
+    const wsPath = '/auth/ws/remote-auth/pair'
+    const wsUrl = authHost && location.host !== authHost ? `//${authHost}${wsPath}` : wsPath
+
+    ws = await aWebSocket(wsUrl)
+
+    // First message is PoW challenge
+    const msg = await ws.receive_json()
+    updateChallenge(msg.pow)
+  } catch (err) {
+    console.error('WebSocket connection error:', err)
+    ws = null
+  } finally {
+    wsConnecting = false
+  }
+}
+
+// Update autocomplete hint based on current input and cursor position
+function updateAutocomplete() {
+  // Update cursor position
+  cursorPos.value = inputRef.value?.selectionStart ?? code.value.length
+
+  const { word, end } = getWordAtCursor(code.value, cursorPos.value)
+  const completeWordCount = countCompleteWords(code.value)
+
+  // Don't autocomplete if:
+  // - we already have 3 complete words
+  // - no current word at cursor
+  // - cursor is not at the end of the word (user is editing middle of word)
+  if (completeWordCount >= 3 || !word || word.length < 1 || cursorPos.value !== end) {
     autocompleteHint.value = ''
     return
   }
 
   // Find unique match for current prefix
-  const match = getUniqueMatch(currentWord.toLowerCase())
-  if (match && match !== currentWord.toLowerCase()) {
+  const match = getUniqueMatch(word.toLowerCase())
+  if (match && match !== word.toLowerCase()) {
     autocompleteHint.value = match
   } else {
     autocompleteHint.value = ''
   }
 }
 
-// Apply autocomplete - complete the current word
+// Apply autocomplete - complete the current word at cursor
 function applyAutocomplete() {
   if (!autocompleteHint.value) return false
 
-  const currentWord = getCurrentWord(code.value)
-  if (!currentWord) return false
+  const { word, start, end } = getWordAtCursor(code.value, cursorPos.value)
+  if (!word) return false
+
+  // Replace the current word with the full word + space
+  const before = code.value.slice(0, start)
+  const after = code.value.slice(end)
+  code.value = before + autocompleteHint.value + ' ' + after
+
+  // Move cursor to after the inserted word + space
+  const newPos = start + autocompleteHint.value.length + 1
+  nextTick(() => {
+    inputRef.value?.setSelectionRange(newPos, newPos)
+    cursorPos.value = newPos
+  })
 
-  // Replace the current partial word with the full word + space
-  const beforeCurrent = code.value.slice(0, code.value.length - currentWord.length)
-  code.value = beforeCurrent + autocompleteHint.value + ' '
   autocompleteHint.value = ''
-
   return true
 }
 
@@ -189,16 +381,22 @@ function handleInput() {
   // Reset device info when code changes
   deviceInfo.value = null
   error.value = null
-  invalidWords.value = []
+  serverError.value = false
+
+  // Check if any word is invalid (not a valid prefix or complete word)
+  hasInvalidWord.value = !checkWordsValidity(code.value)
 
   const words = getWords(code.value)
 
+  // Connect to WebSocket on first meaningful input (start solving PoW early)
+  if (words.length >= 1 && !ws && !wsConnecting) {
+    ensureConnection()
+  }
+
   // Check for invalid words when we have 3 words
   if (words.length === 3) {
-    const invalid = validateWords(code.value)
-    if (invalid.length > 0) {
-      invalidWords.value = invalid
-      error.value = `Unknown word${invalid.length > 1 ? 's' : ''}: ${invalid.join(', ')}`
+    if (!allWordsValid(code.value)) {
+      // Don't show error message - the red input is enough feedback
       return
     }
 
@@ -210,32 +408,118 @@ function handleInput() {
 }
 
 async function lookupDeviceInfo() {
-  if (!hasThreeValidWords.value || loading.value) return
+  // Don't start a new lookup if we're already processing or loading
+  if (isProcessing.value || loading.value) return
 
-  lookingUp.value = true
+  // Check if current input has 3 valid words
+  if (!hasThreeValidWords.value) return
+
+  const normalizedCode = normalizeCode(code.value)
+
+  // Skip if we already successfully looked up this exact code
+  if (normalizedCode === lastLookedUpCode && deviceInfo.value) {
+    return
+  }
+
+  isProcessing.value = true
+  processingStatus.value = 'pow'
   error.value = null
+  serverError.value = false
 
   try {
-    const normalizedCode = normalizeCode(code.value)
-    const info = await apiJson(`/auth/api/remote-auth-info?code=${encodeURIComponent(normalizedCode)}`)
-    deviceInfo.value = info
-  } catch (err) {
-    // 404 means no matching request found
-    if (err.status === 404) {
-      error.value = 'No device found with this code. Check the code and try again.'
-    } else {
-      console.error('Lookup error:', err)
-      error.value = err.message || 'Failed to look up device'
+    // Ensure we have a connection
+    await ensureConnection()
+
+    if (!ws) {
+      throw new Error('Failed to connect')
     }
+
+    // Get PoW solution (may wait for background computation)
+    const nonce = await getPowNonce()
+    const powB64 = btoa(String.fromCharCode(...nonce))
+
+    // Take the field value NOW (after PoW is solved) - user may have changed it
+    const currentCode = normalizeCode(code.value)
+
+    // Check if we still have 3 valid words
+    if (!hasThreeValidWords.value) {
+      // User changed input while we were solving PoW - abort this lookup
+      return
+    }
+
+    // Now waiting for server
+    processingStatus.value = 'server'
+
+    // Send current code + PoW
+    ws.send_json({
+      code: currentCode,
+      pow: powB64
+    })
+
+    // Receive response
+    const res = await ws.receive_json()
+
+    // Update challenge for next request
+    updateChallenge(res.pow)
+
+    if (res.status === 'error') {
+      serverError.value = true
+      deviceInfo.value = null
+      lastLookedUpCode = null
+      // Don't select text - let user continue editing
+      // User needs to change the code before we retry
+    } else if (res.status === 'found' && res.host) {
+      // Success! Set field to normalized value
+      code.value = currentCode.replace(/\./g, ' ')
+      deviceInfo.value = {
+        host: res.host,
+        user_agent_pretty: res.user_agent_pretty
+      }
+      lastLookedUpCode = currentCode
+      // Focus the submit button after device info loads
+      nextTick(() => {
+        submitBtnRef.value?.focus()
+      })
+    } else {
+      serverError.value = true
+      deviceInfo.value = null
+      lastLookedUpCode = null
+    }
+  } catch (err) {
+    console.error('Lookup error:', err)
+    serverError.value = true
     deviceInfo.value = null
+    lastLookedUpCode = null
+    // Close broken connection
+    if (ws) {
+      ws.close()
+      ws = null
+    }
   } finally {
-    lookingUp.value = false
+    isProcessing.value = false
+    processingStatus.value = ''
   }
 }
 
 function handleKeydown(event) {
-  // Tab or Space triggers autocomplete if we have a hint
-  if ((event.key === 'Tab' || event.key === ' ') && autocompleteHint.value) {
+  // Tab triggers autocomplete if we have a hint
+  if (event.key === 'Tab') {
+    if (autocompleteHint.value) {
+      const applied = applyAutocomplete()
+      if (applied) {
+        event.preventDefault()
+        return
+      }
+    }
+    // Prevent Tab from moving focus if input has content (keep user in the input)
+    if (code.value.trim()) {
+      event.preventDefault()
+    }
+    return
+  }
+
+  // Space triggers autocomplete if we have a hint
+  if (event.key === ' ' && autocompleteHint.value) {
     const applied = applyAutocomplete()
     if (applied) {
       event.preventDefault()
@@ -250,25 +534,38 @@ async function submitCode() {
   error.value = null
 
   try {
-    const settings = await getSettings()
-    const authHost = settings?.auth_host
+    // Ensure we still have the connection
+    if (!ws) {
+      await ensureConnection()
+    }
 
-    const normalizedCode = normalizeCode(code.value)
+    if (!ws) {
+      throw new Error('Failed to connect')
+    }
 
-    const wsPath = `/auth/ws/remote-auth/pair/${encodeURIComponent(normalizedCode)}`
-    const wsUrl = authHost && location.host !== authHost ? `//${authHost}${wsPath}` : wsPath
+    // Get PoW solution for authenticate request
+    const nonce = await getPowNonce()
+    const powB64 = btoa(String.fromCharCode(...nonce))
 
-    ws = await aWebSocket(wsUrl)
+    // Send authenticate request with PoW
+    ws.send_json({
+      authenticate: true,
+      pow: powB64
+    })
 
     // Receive authentication options
     const res = await ws.receive_json()
 
-    if (res.status) {
-      throw new Error(res.detail || `Connection failed: ${res.status}`)
+    if (res.status === 'error') {
+      throw new Error(res.error || 'Authentication failed')
+    }
+
+    if (!res.optionsJSON) {
+      throw new Error(res.detail || 'Failed to get authentication options')
     }
 
     // Perform WebAuthn authentication
-    const authResponse = await startAuthentication(res.optionsJSON || res)
+    const authResponse = await startAuthentication(res.optionsJSON)
     ws.send_json(authResponse)
 
     // Wait for confirmation
@@ -279,7 +576,7 @@ async function submitCode() {
       completedMessage.value = result.message || 'The other device is now logged in.'
       emit('completed')
     } else {
-      throw new Error(result.detail || 'Authentication failed')
+      throw new Error(result.detail || result.error || 'Authentication failed')
     }
   } catch (err) {
     console.error('Pairing error:', err)
@@ -290,6 +587,7 @@ async function submitCode() {
     emit('error', message)
   } finally {
     loading.value = false
+    // Close the WebSocket after authentication attempt
     if (ws) {
       ws.close()
       ws = null
@@ -300,12 +598,24 @@ async function submitCode() {
 function reset() {
   code.value = ''
   error.value = null
+  serverError.value = false
   completed.value = false
   completedMessage.value = ''
   deviceInfo.value = null
-  lookingUp.value = false
+  isProcessing.value = false
+  processingStatus.value = ''
   autocompleteHint.value = ''
-  invalidWords.value = []
+  hasInvalidWord.value = false
+  lastLookedUpCode = null
+  // Close WebSocket on reset
+  if (ws) {
+    ws.close()
+    ws = null
+  }
+  currentChallenge = null
+  currentBits = null
+  powPromise = null
+  powNonce = null
 }
 
 onMounted(() => {
@@ -353,9 +663,56 @@ defineExpose({ reset })
   gap: 0.5rem;
 }
 
+.input-row {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+}
+
 .input-wrapper {
   position: relative;
   display: flex;
+  max-width: 21ch;
+}
+
+.input-wrapper.has-error .pairing-input {
+  border-color: var(--color-error, #ef4444);
+}
+
+.input-wrapper.has-error .pairing-input:focus {
+  border-color: var(--color-error, #ef4444);
+  box-shadow: 0 0 0 2px var(--color-error-alpha, rgba(239, 68, 68, 0.2));
+}
+
+.input-wrapper.is-complete .pairing-input {
+  border-color: var(--color-success, #10b981);
+}
+
+.input-wrapper.is-complete .pairing-input:focus {
+  border-color: var(--color-success, #10b981);
+  box-shadow: 0 0 0 2px var(--color-success-alpha, rgba(16, 185, 129, 0.2));
+}
+
+.input-wrapper.is-verified .pairing-input {
+  border-color: var(--color-success, #10b981);
+  background: var(--color-success-bg, rgba(16, 185, 129, 0.1));
+}
+
+.input-overlay {
+  position: absolute;
+  left: 0;
+  top: 0;
+  padding: 0.625rem 0.75rem;
+  font-size: 1rem;
+  font-family: inherit;
+  color: var(--color-text);
+  pointer-events: none;
+  white-space: pre;
+  border: 1px solid transparent;
+}
+
+.input-overlay .invalid-word {
+  color: var(--color-error, #ef4444);
 }
 
 .pairing-input {
@@ -367,6 +724,11 @@ defineExpose({ reset })
   border-radius: var(--radius-sm, 4px);
   background: var(--color-surface);
   color: var(--color-text);
+  caret-color: var(--color-text);
+}
+
+.pairing-input.text-hidden {
+  color: transparent;
 }
 
 .pairing-input:focus {
@@ -392,23 +754,34 @@ defineExpose({ reset })
   padding: 0.625rem 0.75rem;
   font-size: 1rem;
   font-family: inherit;
-  color: var(--color-text-muted);
-  opacity: 0.5;
   pointer-events: none;
   white-space: pre;
 }
 
-.lookup-status {
+.autocomplete-hint .hint-prefix {
+  color: transparent;
+}
+
+.autocomplete-hint .hint-suffix {
+  color: var(--color-text-muted);
+  opacity: 0.6;
+}
+
+.processing-status {
   display: flex;
   align-items: center;
-  gap: 0.5rem;
+  gap: 0.25rem;
   font-size: 0.875rem;
   color: var(--color-text-muted);
 }
 
-.lookup-spinner {
-  width: 14px;
-  height: 14px;
+.processing-icon {
+  font-size: 0.875rem;
+}
+
+.processing-spinner-small {
+  width: 12px;
+  height: 12px;
   border: 2px solid var(--color-border);
   border-top-color: var(--color-primary);
   border-radius: 50%;
diff --git a/frontend/src/components/RemoteAuthComplete.vue b/frontend/src/components/RemoteAuthComplete.vue
index 4b21bab..cb168a8 100644
--- a/frontend/src/components/RemoteAuthComplete.vue
+++ b/frontend/src/components/RemoteAuthComplete.vue
@@ -61,6 +61,7 @@ import { computed, onMounted, onUnmounted, reactive, ref } from 'vue'
 import { startAuthentication } from '@simplewebauthn/browser'
 import aWebSocket from '@/utils/awaitable-websocket'
 import { getSettings, uiBasePath } from '@/utils/settings'
+import { solvePoW } from '@/utils/pow'
 
 const props = defineProps({
   token: { type: String, required: true }
@@ -118,6 +119,18 @@ async function authenticate() {
 
     ws = await aWebSocket(wsUrl)
 
+    // First message is PoW challenge
+    const powChallenge = await ws.receive_json()
+    if (powChallenge.pow) {
+      const challenge = Uint8Array.from(atob(powChallenge.pow.challenge), c => c.charCodeAt(0))
+      const powStart = performance.now()
+      const nonce = await solvePoW(challenge, powChallenge.pow.bits)
+      const powTime = performance.now() - powStart
+      const nonceVal = nonce.reduce((acc, b, i) => acc + BigInt(b) * (1n << BigInt(i * 8)), 0n)
+      console.log(`PoW solved: ${Number(nonceVal)} iterations in ${(powTime / 1000).toFixed(3)}s (${(powTime / Number(nonceVal)).toFixed(3)}ms/iter)`)
+      ws.send_json({ pow: btoa(String.fromCharCode(...nonce)) })
+    }
+
     // Receive authentication options
     const res = await ws.receive_json()
 
diff --git a/frontend/src/components/RemoteAuthLinkModal.vue b/frontend/src/components/RemoteAuthLinkModal.vue
index 70811a6..8168ed6 100644
--- a/frontend/src/components/RemoteAuthLinkModal.vue
+++ b/frontend/src/components/RemoteAuthLinkModal.vue
@@ -108,6 +108,7 @@ import QRCode from 'qrcode/lib/browser'
 import aWebSocket from '@/utils/awaitable-websocket'
 import { getSettings } from '@/utils/settings'
 import { formatDate } from '@/utils/helpers'
+import { solvePoW } from '@/utils/pow'
 
 const props = defineProps({
   active: { type: Boolean, default: false },
@@ -152,6 +153,18 @@ async function startRemoteAuth() {
 
     ws = await aWebSocket(wsUrl)
 
+    // First message is PoW challenge
+    const powChallenge = await ws.receive_json()
+    if (powChallenge.pow) {
+      const challenge = Uint8Array.from(atob(powChallenge.pow.challenge), c => c.charCodeAt(0))
+      const powStart = performance.now()
+      const nonce = await solvePoW(challenge, powChallenge.pow.bits)
+      const powTime = performance.now() - powStart
+      const nonceVal = nonce.reduce((acc, b, i) => acc + BigInt(b) * (1n << BigInt(i * 8)), 0n)
+      console.log(`PoW solved: ${Number(nonceVal)} iterations in ${(powTime / 1000).toFixed(3)}s (${(powTime / Number(nonceVal)).toFixed(3)}ms/iter)`)
+      ws.send_json({ pow: btoa(String.fromCharCode(...nonce)) })
+    }
+
     // Receive the remote auth token, pairing code, and URL
     const res = await ws.receive_json()
 
diff --git a/frontend/src/utils/pow-test.js b/frontend/src/utils/pow-test.js
new file mode 100644
index 0000000..ea67adf
--- /dev/null
+++ b/frontend/src/utils/pow-test.js
@@ -0,0 +1,46 @@
+import { solvePoW, verifyPoW, nonceToNumber } from './pow.js'
+
+const TRIALS = 20
+const REQUIRED_BITS = 18
+
+async function test() {
+  console.log(`Running ${TRIALS} trials with ${REQUIRED_BITS} required zero bits...\n`)
+
+  const times = []
+  const iterations = []
+
+  for (let trial = 1; trial <= TRIALS; trial++) {
+    const challenge = crypto.getRandomValues(new Uint8Array(8))
+
+    const start = performance.now()
+    let lastIterations = 0
+    const nonce = await solvePoW(challenge, REQUIRED_BITS, {
+      onProgress: (n) => { lastIterations = n }
+    })
+    const elapsed = performance.now() - start
+
+    const valid = await verifyPoW(challenge, nonce, REQUIRED_BITS)
+    const nonceVal = nonceToNumber(nonce)
+
+    times.push(elapsed)
+    iterations.push(Number(nonceVal))
+
+    console.log(`Trial ${trial.toString().padStart(2)}: ${(elapsed / 1000).toFixed(3)}s, nonce=${nonceVal}, valid=${valid}`)
+  }
+
+  const avgTime = times.reduce((a, b) => a + b, 0) / times.length
+  const avgIter = iterations.reduce((a, b) => a + b, 0) / iterations.length
+  const minTime = Math.min(...times)
+  const maxTime = Math.max(...times)
+
+  console.log('\n--- Summary ---')
+  console.log(`Trials: ${TRIALS}`)
+  console.log(`Required bits: ${REQUIRED_BITS}`)
+  console.log(`Avg time: ${(avgTime / 1000).toFixed(3)}s`)
+  console.log(`Min time: ${(minTime / 1000).toFixed(3)}s`)
+  console.log(`Max time: ${(maxTime / 1000).toFixed(3)}s`)
+  console.log(`Avg iterations: ${Math.round(avgIter)}`)
+  console.log(`Expected iterations: ~${Math.pow(2, REQUIRED_BITS)}`)
+}
+
+test()
diff --git a/frontend/src/utils/pow.js b/frontend/src/utils/pow.js
new file mode 100644
index 0000000..5da4122
--- /dev/null
+++ b/frontend/src/utils/pow.js
@@ -0,0 +1,142 @@
+/**
+ * Proof of Work utility using SHA-512
+ *
+ * The PoW challenge requires finding a nonce such that
+ * SHA-512(challenge || nonce) has a specified number of leading zero bits.
+ */
+
+/**
+ * Solve a Proof of Work challenge
+ *
+ * @param {Uint8Array|ArrayBuffer} challenge - 8-byte server-provided challenge
+ * @param {number} requiredZeroBits - Number of leading zero bits required (1-32)
+ * @param {object} [options] - Optional parameters
+ * @param {AbortSignal} [options.signal] - AbortSignal to cancel the operation
+ * @param {function} [options.onProgress] - Callback for progress updates (iterations count)
+ * @returns {Promise<Uint8Array>} The successful 8-byte nonce (little-endian)
+ * @throws {Error} If challenge is invalid or operation is aborted
+ */
+export async function solvePoW(challenge, requiredZeroBits, options = {}) {
+  const { signal, onProgress } = options
+  const startTime = performance.now()
+
+  // Validate inputs
+  const challengeBytes = challenge instanceof ArrayBuffer
+    ? new Uint8Array(challenge)
+    : challenge
+
+  if (!(challengeBytes instanceof Uint8Array) || challengeBytes.length !== 8) {
+    throw new Error('Challenge must be exactly 8 bytes')
+  }
+
+  if (requiredZeroBits < 1 || requiredZeroBits > 32) {
+    throw new Error('Required zero bits must be between 1 and 32')
+  }
+
+  // Prepare the buffer: challenge (8 bytes) || nonce (8 bytes)
+  const data = new Uint8Array(16)
+  data.set(challengeBytes, 0)
+  // Nonce area data[8..15] starts at zero
+
+  // Precalculate mask and byte count for the required zero bits
+  const fullZeroBytes = requiredZeroBits >>> 3
+  const remainingBits = requiredZeroBits & 7
+  const partialMask = remainingBits ? (0xFF << (8 - remainingBits)) & 0xFF : 0
+
+  while (true) {
+    // Increment 64-bit little-endian nonce
+    for (let i = 8; i < 16; i++) if (++data[i] !== 256) break
+    // Calculate SHA-512 hash and verify leading zero bits
+    const hash = new Uint8Array(await crypto.subtle.digest('SHA-512', data))
+    if (hash[0]) continue  // Quick check - most hashes fail here
+    // Yield to UI periodically and check abort signal
+    if (!(hash[1] & 0x0F)) {
+      await new Promise(r => setTimeout(r, 0))
+      if (signal?.aborted) throw new DOMException('PoW operation aborted', 'AbortError')
+    }
+    // Check all full zero bytes
+    let valid = true
+    for (let i = 1; i < fullZeroBytes; i++) {
+      if (hash[i]) { valid = false; break }
+    }
+    if (!valid) continue
+    if (partialMask && (hash[fullZeroBytes] & partialMask)) continue
+    break
+  }
+  const iterations = Number(nonceToNumber(data.slice(8, 16)))
+  const elapsed = (performance.now() - startTime) / 1000
+  const iterPerSec = Math.round(iterations / elapsed)
+  console.log(`PoW solved in ${elapsed.toFixed(1)}s (${(iterPerSec / 1e3).toFixed(0)} k-it/s)`)
+  return data.slice(8, 16)
+}
+
+
+/**
+ * Verify a PoW solution
+ *
+ * @param {Uint8Array|ArrayBuffer} challenge - 8-byte server-provided challenge
+ * @param {Uint8Array|ArrayBuffer} nonce - 8-byte client-provided nonce
+ * @param {number} requiredZeroBits - Number of leading zero bits required
+ * @returns {Promise<boolean>} True if the solution is valid
+ */
+export async function verifyPoW(challenge, nonce, requiredZeroBits) {
+  const challengeBytes = challenge instanceof ArrayBuffer
+    ? new Uint8Array(challenge)
+    : challenge
+
+  const nonceBytes = nonce instanceof ArrayBuffer
+    ? new Uint8Array(nonce)
+    : nonce
+
+  if (challengeBytes.length !== 8 || nonceBytes.length !== 8) {
+    return false
+  }
+
+  const data = new Uint8Array(16)
+  data.set(challengeBytes, 0)
+  data.set(nonceBytes, 8)
+
+  const hashBuffer = await crypto.subtle.digest('SHA-512', data)
+  const hash = new Uint8Array(hashBuffer)
+
+  // Check leading zero bits using same logic as solvePoW
+  const fullZeroBytes = requiredZeroBits >>> 3
+  const remainingBits = requiredZeroBits & 7
+  const partialMask = remainingBits ? (0xFF << (8 - remainingBits)) & 0xFF : 0
+
+  for (let i = 0; i < fullZeroBytes; i++) {
+    if (hash[i] !== 0) return false
+  }
+  if (partialMask && (hash[fullZeroBytes] & partialMask) !== 0) {
+    return false
+  }
+  return true
+}
+
+/**
+ * Convert a nonce Uint8Array to a BigInt (little-endian)
+ * @param {Uint8Array} nonce - 8-byte little-endian nonce
+ * @returns {bigint} The nonce as a BigInt
+ */
+export function nonceToNumber(nonce) {
+  let value = 0n
+  for (let i = 7; i >= 0; i--) {
+    value = (value << 8n) | BigInt(nonce[i])
+  }
+  return value
+}
+
+/**
+ * Convert a BigInt to a nonce Uint8Array (little-endian)
+ * @param {bigint} value - The value to convert
+ * @returns {Uint8Array} 8-byte little-endian representation
+ */
+export function numberToNonce(value) {
+  const nonce = new Uint8Array(8)
+  let v = BigInt(value)
+  for (let i = 0; i < 8; i++) {
+    nonce[i] = Number(v & 0xFFn)
+    v >>= 8n
+  }
+  return nonce
+}
diff --git a/frontend/src/utils/wordlist.js b/frontend/src/utils/wordlist.js
new file mode 100644
index 0000000..2a8592f
--- /dev/null
+++ b/frontend/src/utils/wordlist.js
@@ -0,0 +1,61 @@
+// Auto-generated from paskia/util/wordlist.py - do not edit manually
+// A custom list of 1024 common 3-6 letter words, with unique 3-prefixes and no prefix words
+
+export const words = ["able", "about", "absent", "abuse", "access", "acid", "across", "act", "adapt", "add", "adjust", "admit", "adult", "advice", "affair", "afraid", "again", "age", "agree", "ahead", "aim", "air", "aisle", "alarm", "album", "alert", "alien", "all", "almost", "alone", "alpha", "also", "alter", "always", "amazed", "among", "amused", "anchor", "angle", "animal", "ankle", "annual", "answer", "any", "apart", "appear", "april", "arch", "are", "argue", "army", "around", "array", "art", "ascent", "ash", "ask", "aspect", "assume", "asthma", "atom", "attack", "audit", "august", "aunt", "author", "avoid", "away", "awful", "axis", "baby", "back", "bad", "bag", "ball", "bamboo", "bank", "bar", "base", "battle", "beach", "become", "beef", "before", "begin", "behind", "below", "bench", "best", "better", "beyond", "bid", "bike", "bind", "bio", "birth", "bitter", "black", "bleak", "blind", "blood", "blue", "board", "body", "boil", "bomb", "bone", "book", "border", "boss", "bottom", "bounce", "bowl", "box", "boy", "brain", "bread", "bring", "brown", "brush", "bubble", "buck", "budget", "build", "bulk", "bundle", "burden", "bus", "but", "buyer", "buzz", "cable", "cache", "cage", "cake", "call", "came", "can", "car", "case", "catch", "cause", "cave", "celery", "cement", "census", "cereal", "change", "check", "child", "choice", "chunk", "cigar", "circle", "city", "civil", "class", "clean", "client", "close", "club", "coast", "code", "coffee", "coil", "cold", "come", "cool", "copy", "core", "cost", "cotton", "couch", "cover", "coyote", "craft", "cream", "crime", "cross", "cruel", "cry", "cube", "cue", "cult", "cup", "curve", "custom", "cute", "cycle", "dad", "damage", "danger", "daring", "dash", "dawn", "day", "deal", "debate", "decide", "deer", "define", "degree", "deity", "delay", "demand", "denial", "depth", "derive", "design", "detail", "device", "dial", "dice", "die", "differ", "dim", "dinner", "direct", "dish", "divert", "dizzy", "doctor", "dog", "dollar", "domain", "donate", "door", "dose", "double", "dove", "draft", "dream", "drive", "drop", "drum", "dry", "duck", "dumb", "dune", "during", "dust", "dutch", "dwarf", "eager", "early", "east", "echo", "eco", "edge", "edit", "effort", "egg", "eight", "either", "elbow", "elder", "elite", "else", "embark", "emerge", "emily", "employ", "enable", "end", "enemy", "engine", "enjoy", "enlist", "enough", "enrich", "ensure", "entire", "envy", "equal", "era", "erode", "error", "erupt", "escape", "essay", "estate", "ethics", "evil", "evoke", "exact", "excess", "exist", "exotic", "expect", "extent", "eye", "fabric", "face", "fade", "faith", "fall", "family", "fan", "far", "father", "fault", "feel", "female", "fence", "fetch", "fever", "few", "fiber", "field", "figure", "file", "find", "first", "fish", "fit", "fix", "flat", "flesh", "flight", "float", "fluid", "fly", "foam", "focus", "fog", "foil", "follow", "food", "force", "fossil", "found", "fox", "frame", "fresh", "friend", "frog", "fruit", "fuel", "fun", "fury", "future", "gadget", "gain", "galaxy", "game", "gap", "garden", "gas", "gate", "gauge", "gaze", "genius", "ghost", "giant", "gift", "giggle", "ginger", "girl", "give", "glass", "glide", "globe", "glue", "goal", "god", "gold", "good", "gospel", "govern", "gown", "grant", "great", "grid", "group", "grunt", "guard", "guess", "guide", "gulf", "gun", "gym", "habit", "hair", "half", "hammer", "hand", "happy", "hard", "hat", "have", "hawk", "hay", "hazard", "head", "hedge", "height", "help", "hen", "hero", "hidden", "high", "hill", "hint", "hip", "hire", "hobby", "hockey", "hold", "home", "honey", "hood", "hope", "horse", "host", "hotel", "hour", "hover", "how", "hub", "huge", "human", "hungry", "hurt", "hybrid", "ice", "icon", "idea", "idle", "ignore", "ill", "image", "immune", "impact", "income", "index", "infant", "inhale", "inject", "inmate", "inner", "input", "inside", "into", "invest", "iron", "island", "issue", "italy", "item", "ivory", "jacket", "jaguar", "james", "jar", "jazz", "jeans", "jelly", "jewel", "job", "joe", "joke", "joy", "judge", "juice", "july", "jump", "june", "just", "kansas", "kate", "keep", "kernel", "key", "kick", "kid", "kind", "kiss", "kit", "kiwi", "knee", "knife", "know", "labor", "lady", "lag", "lake", "lamp", "laptop", "large", "later", "laugh", "lava", "law", "layer", "lazy", "leader", "left", "legal", "lemon", "length", "lesson", "letter", "level", "liar", "libya", "lid", "life", "light", "like", "limit", "line", "lion", "liquid", "list", "little", "live", "lizard", "load", "local", "logic", "long", "loop", "lost", "loud", "love", "low", "loyal", "lucky", "lumber", "lunch", "lust", "luxury", "lyrics", "mad", "magic", "main", "major", "make", "male", "mammal", "man", "map", "market", "mass", "matter", "maze", "mccoy", "meadow", "media", "meet", "melt", "member", "men", "mercy", "mesh", "method", "middle", "milk", "mimic", "mind", "mirror", "miss", "mix", "mobile", "model", "mom", "monkey", "moon", "more", "mother", "mouse", "move", "much", "muffin", "mule", "must", "mutual", "myself", "myth", "naive", "name", "napkin", "narrow", "nasty", "nation", "near", "neck", "need", "nephew", "nerve", "nest", "net", "never", "news", "next", "nice", "night", "noble", "noise", "noodle", "normal", "nose", "note", "novel", "now", "number", "nurse", "nut", "oak", "obey", "object", "oblige", "obtain", "occur", "ocean", "odor", "off", "often", "oil", "okay", "old", "olive", "omit", "once", "one", "onion", "online", "open", "opium", "oppose", "option", "orange", "orbit", "order", "organ", "orient", "orphan", "other", "outer", "oval", "oven", "own", "oxygen", "oyster", "ozone", "pact", "paddle", "page", "pair", "palace", "panel", "paper", "parade", "past", "path", "pause", "pave", "paw", "pay", "peace", "pen", "people", "pepper", "permit", "pet", "philip", "phone", "phrase", "piano", "pick", "piece", "pig", "pilot", "pink", "pipe", "pistol", "pitch", "pizza", "place", "please", "pluck", "poem", "point", "polar", "pond", "pool", "post", "pot", "pound", "powder", "praise", "prefer", "price", "profit", "public", "pull", "punch", "pupil", "purity", "push", "put", "puzzle", "qatar", "quasi", "queen", "quite", "quoted", "rabbit", "race", "radio", "rail", "rally", "ramp", "range", "rapid", "rare", "rather", "raven", "raw", "razor", "real", "rebel", "recall", "red", "reform", "region", "reject", "relief", "remain", "rent", "reopen", "report", "result", "return", "review", "reward", "rhythm", "rib", "rich", "ride", "rifle", "right", "ring", "riot", "ripple", "risk", "ritual", "river", "road", "robot", "rocket", "room", "rose", "rotate", "round", "row", "royal", "rubber", "rude", "rug", "rule", "run", "rural", "sad", "safe", "sage", "sail", "salad", "same", "santa", "sauce", "save", "say", "scale", "scene", "school", "scope", "screen", "scuba", "sea", "second", "seed", "self", "semi", "sense", "series", "settle", "seven", "shadow", "she", "ship", "shock", "shrimp", "shy", "sick", "side", "siege", "sign", "silver", "simple", "since", "siren", "sister", "six", "size", "skate", "sketch", "ski", "skull", "slab", "sleep", "slight", "slogan", "slush", "small", "smile", "smooth", "snake", "sniff", "snow", "soap", "soccer", "soda", "soft", "solid", "son", "soon", "sort", "south", "space", "speak", "sphere", "spirit", "split", "spoil", "spring", "spy", "square", "state", "step", "still", "story", "strong", "stuff", "style", "submit", "such", "sudden", "suffer", "sugar", "suit", "summer", "sun", "supply", "sure", "swamp", "sweet", "switch", "sword", "symbol", "syntax", "syria", "system", "table", "tackle", "tag", "tail", "talk", "tank", "tape", "target", "task", "tattoo", "taxi", "team", "tell", "ten", "term", "test", "text", "that", "theme", "this", "three", "thumb", "tibet", "ticket", "tide", "tight", "tilt", "time", "tiny", "tip", "tired", "tissue", "title", "toast", "today", "toe", "toilet", "token", "tomato", "tone", "tool", "top", "torch", "toss", "total", "toward", "toy", "trade", "tree", "trial", "trophy", "true", "try", "tube", "tumble", "tunnel", "turn", "twenty", "twice", "two", "type", "ugly", "unable", "uncle", "under", "unfair", "unique", "unlock", "until", "unveil", "update", "uphold", "upon", "upper", "upset", "urban", "urge", "usage", "use", "usual", "vacuum", "vague", "valid", "van", "vapor", "vast", "vault", "vein", "velvet", "vendor", "very", "vessel", "viable", "video", "view", "villa", "violin", "virus", "visit", "vital", "vivid", "vocal", "voice", "volume", "vote", "voyage", "wage", "wait", "wall", "want", "war", "wash", "water", "wave", "way", "wealth", "web", "weird", "were", "west", "wet", "what", "when", "whip", "wide", "wife", "will", "window", "wire", "wish", "wolf", "woman", "wonder", "wood", "work", "wrap", "wreck", "write", "wrong", "xander", "xbox", "xerox", "xray", "yang", "yard", "year", "yellow", "yes", "yin", "york", "you", "zane", "zara", "zebra", "zen", "zero", "zippo", "zone", "zoo", "zorro", "zulu"];
+
+// Pre-computed prefix map for autocomplete (maps 1-3 char prefixes to matching words)
+const prefixMap = new Map();
+
+// Build prefix map on module load
+for (const word of words) {
+  for (let i = 1; i <= Math.min(word.length, 6); i++) {
+    const prefix = word.slice(0, i);
+    if (!prefixMap.has(prefix)) {
+      prefixMap.set(prefix, []);
+    }
+    prefixMap.get(prefix).push(word);
+  }
+}
+
+/**
+ * Find words matching the given prefix.
+ * @param {string} prefix - The prefix to match (case-insensitive)
+ * @returns {string[]} Array of matching words
+ */
+export function findMatches(prefix) {
+  if (!prefix) return [];
+  const normalized = prefix.toLowerCase();
+  return prefixMap.get(normalized) || [];
+}
+
+/**
+ * Get unique autocomplete suggestion if prefix matches exactly one word.
+ * @param {string} prefix - The prefix to match (case-insensitive)
+ * @returns {string|null} The matching word if unique, null otherwise
+ */
+export function getUniqueMatch(prefix) {
+  const matches = findMatches(prefix);
+  return matches.length === 1 ? matches[0] : null;
+}
+
+/**
+ * Check if a word is valid (exists in the wordlist).
+ * @param {string} word - The word to check (case-insensitive)
+ * @returns {boolean} True if the word is valid
+ */
+export function isValidWord(word) {
+  if (!word) return false;
+  const normalized = word.toLowerCase();
+  return words.includes(normalized);
+}
+
+/**
+ * Check if a string is a valid prefix (could be the start of a valid word).
+ * @param {string} prefix - The prefix to check (case-insensitive)
+ * @returns {boolean} True if the prefix matches at least one word
+ */
+export function isValidPrefix(prefix) {
+  if (!prefix) return true;  // Empty is always valid
+  const normalized = prefix.toLowerCase();
+  return prefixMap.has(normalized);
+}
diff --git a/paskia/fastapi/ws.py b/paskia/fastapi/ws.py
index 60f4314..8a1d758 100644
--- a/paskia/fastapi/ws.py
+++ b/paskia/fastapi/ws.py
@@ -1,4 +1,5 @@
 import asyncio
+import base64
 import logging
 from functools import wraps
 from uuid import UUID
@@ -11,7 +12,7 @@ from paskia.authsession import create_session, get_reset, get_session
 from paskia.fastapi import authz
 from paskia.fastapi.session import AUTH_COOKIE, infodict
 from paskia.globals import db, passkey
-from paskia.util import hostutil, passphrase
+from paskia.util import hostutil, passphrase, pow
 from paskia.util.tokens import create_token, session_key
 
 
@@ -44,6 +45,39 @@ def websocket_error_handler(func):
 app = FastAPI()
 
 
+async def _require_pow(ws: WebSocket) -> None:
+    """Send a PoW challenge and verify the client's solution.
+
+    Sends: {"pow": {"challenge": "<base64>", "bits": 14}}
+    Expects: {"pow": "<base64-nonce>"}
+
+    Raises:
+        ValueError: If the PoW solution is invalid
+    """
+    challenge = pow.generate_challenge()
+    bits = pow.DEFAULT_POW_BITS
+
+    await ws.send_json({
+        "pow": {
+            "challenge": base64.b64encode(challenge).decode(),
+            "bits": bits,
+        }
+    })
+
+    response = await ws.receive_json()
+    nonce_b64 = response.get("pow")
+    if not nonce_b64:
+        raise ValueError("PoW solution required")
+
+    try:
+        nonce = base64.b64decode(nonce_b64)
+    except Exception:
+        raise ValueError("Invalid PoW nonce encoding")
+
+    if not pow.verify_pow(challenge, nonce, bits):
+        raise ValueError("Invalid PoW solution")
+
+
 def _validate_origin(ws: WebSocket) -> str:
     """Extract and validate origin from WebSocket request headers.
 
@@ -212,10 +246,11 @@ async def websocket_remote_auth_request(ws: WebSocket):
 
     Flow:
     1. Client connects
-    2. Server creates a remote auth token and sends it with URL/expiry/pairing_code
-    3. Server waits for another device to authenticate via /remote-auth/complete
-    4. When auth completes, server sends session_token to this client
-    5. Client can then use the session token to set a cookie
+    2. Server sends PoW challenge, client solves and responds
+    3. Server creates a remote auth token and sends it with URL/expiry/pairing_code
+    4. Server waits for another device to authenticate via /remote-auth/complete
+    5. When auth completes, server sends session_token to this client
+    6. Client can then use the session token to set a cookie
     """
     origin = _validate_origin(ws)
     host = origin.split("://", 1)[1]
@@ -223,6 +258,9 @@ async def websocket_remote_auth_request(ws: WebSocket):
     if remoteauth.instance is None:
         raise ValueError("Remote authentication is not available")
 
+    # Require PoW before creating the request
+    await _require_pow(ws)
+
     metadata = infodict(ws, "remote-auth-request")
 
     # Create the remote auth request
@@ -332,18 +370,22 @@ async def websocket_remote_auth_complete(ws: WebSocket, token: str):
 
     Flow:
     1. Client opens the remote auth link and connects here
-    2. Server verifies the token is valid
-    3. Server sends WebAuthn options
-    4. Client authenticates with passkey
-    5. Server creates session for the REQUESTING device's host
-    6. Server notifies the requesting device via the callback
-    7. Server sends confirmation to this client
+    2. Server sends PoW challenge, client solves and responds
+    3. Server verifies the token is valid
+    4. Server sends WebAuthn options
+    5. Client authenticates with passkey
+    6. Server creates session for the REQUESTING device's host
+    7. Server notifies the requesting device via the callback
+    8. Server sends confirmation to this client
     """
     origin = _validate_origin(ws)
 
     if remoteauth.instance is None:
         raise ValueError("Remote authentication is not available")
 
+    # Require PoW before revealing if token is valid
+    await _require_pow(ws)
+
     # Validate the remote auth token
     request = await remoteauth.instance.get_request(token)
     if request is None:
@@ -406,85 +448,168 @@ async def websocket_remote_auth_complete(ws: WebSocket, token: str):
     )
 
 
-@app.websocket("/remote-auth/pair/{code}")
+@app.websocket("/remote-auth/pair")
 @websocket_error_handler
-async def websocket_remote_auth_pair(ws: WebSocket, code: str):
+async def websocket_remote_auth_pair(ws: WebSocket):
     """Complete a remote authentication request using a pairing code.
 
     This endpoint is called from the user's profile on the authenticating device.
     The user enters the pairing code displayed on the requesting device.
 
-    Flow:
-    1. User on Device B (with passkey) enters pairing code from Device A
-    2. Server looks up the remote auth request by pairing code
-    3. Server sends WebAuthn options
-    4. User authenticates with passkey
-    5. Server creates session for Device A's host with Device A's metadata
-    6. Server notifies Device A via the callback
-    7. Server sends confirmation to Device B
+    Redesigned protocol (no keywords in URL):
+    1. Server sends PoW challenge immediately on connect
+    2. Client sends {code: "word.word.word", pow: "<base64-nonce>"}
+    3. Server validates PoW and code:
+       - If invalid code/PoW: {status: "error", error: "...", pow: {challenge, bits}}
+       - If valid: {status: "found", host: "...", user_agent_pretty: "...", pow: {challenge, bits}}
+    4. Client can then send {authenticate: true, pow: "<nonce>"} to start WebAuthn
+    5. Server sends {optionsJSON: ...}
+    6. Client sends WebAuthn response
+    7. Server sends {status: "success", message: "..."}
     """
+    from paskia.util import useragent
+
     origin = _validate_origin(ws)
 
     if remoteauth.instance is None:
         raise ValueError("Remote authentication is not available")
 
-    # Look up the remote auth request by pairing code
-    request = await remoteauth.instance.get_request_by_pairing_code(code)
-    if request is None:
-        raise ValueError("Invalid or expired pairing code")
+    # Generate initial PoW challenge
+    challenge = pow.generate_challenge()
+    bits = pow.DEFAULT_POW_BITS
 
-    if request.completed:
-        raise ValueError("This remote authentication has already been completed")
-
-    # The session will be created for the requesting device's host
-    target_host = request.host
-
-    # Generate authentication options (no credential restriction for remote auth)
-    options, challenge = passkey.instance.auth_generate_options(credential_ids=None)
-    await ws.send_json({"optionsJSON": options})
-
-    # Wait for client authentication response
-    credential = passkey.instance.auth_parse(await ws.receive_json())
-
-    # Fetch and verify credential
-    try:
-        stored_cred = await db.instance.get_credential_by_id(credential.raw_id)
-    except ValueError:
-        raise ValueError(
-            f"This passkey is no longer registered with {passkey.instance.rp_name}"
-        )
-
-    # Verify the credential
-    passkey.instance.auth_verify(credential, challenge, stored_cred, origin)
-
-    # Update credential last_used
-    await db.instance.login(stored_cred.user_uuid, stored_cred)
-
-    # Create a session for the REQUESTING device (with their IP/user-agent)
-    assert stored_cred.uuid is not None
-    session_token = await create_session(
-        user_uuid=stored_cred.user_uuid,
-        credential_uuid=stored_cred.uuid,
-        host=target_host,
-        ip=request.ip,
-        user_agent=request.user_agent,
-    )
-
-    # Complete the remote auth request (notifies the waiting device)
-    completed = await remoteauth.instance.complete_request(
-        token=request.key,
-        session_token=session_token,
-        user_uuid=stored_cred.user_uuid,
-        credential_uuid=stored_cred.uuid,
-    )
-
-    if not completed:
-        raise ValueError("Failed to complete remote authentication")
-
-    # Send confirmation to the authenticating device
-    await ws.send_json(
-        {
-            "status": "success",
-            "message": "Authentication successful. The other device is now logged in.",
+    await ws.send_json({
+        "pow": {
+            "challenge": base64.b64encode(challenge).decode(),
+            "bits": bits,
         }
-    )
+    })
+
+    request = None
+    webauthn_challenge = None
+
+    while True:
+        msg = await ws.receive_json()
+
+        # Validate PoW
+        nonce_b64 = msg.get("pow")
+        if not nonce_b64:
+            raise ValueError("PoW solution required")
+
+        try:
+            nonce = base64.b64decode(nonce_b64)
+        except Exception:
+            raise ValueError("Invalid PoW nonce encoding")
+
+        if not pow.verify_pow(challenge, nonce, bits):
+            # Invalid PoW - send new challenge
+            challenge = pow.generate_challenge()
+            await ws.send_json({
+                "status": "error",
+                "error": "Invalid proof of work",
+                "pow": {
+                    "challenge": base64.b64encode(challenge).decode(),
+                    "bits": bits,
+                }
+            })
+            continue
+
+        # Handle authenticate request (after successful lookup)
+        if msg.get("authenticate") and request is not None:
+            # Generate authentication options
+            options, webauthn_challenge = passkey.instance.auth_generate_options(
+                credential_ids=None
+            )
+            await ws.send_json({"optionsJSON": options})
+
+            # Wait for WebAuthn response
+            credential = passkey.instance.auth_parse(await ws.receive_json())
+
+            # Fetch and verify credential
+            try:
+                stored_cred = await db.instance.get_credential_by_id(credential.raw_id)
+            except ValueError:
+                raise ValueError(
+                    f"This passkey is no longer registered with {passkey.instance.rp_name}"
+                )
+
+            # Verify the credential
+            passkey.instance.auth_verify(
+                credential, webauthn_challenge, stored_cred, origin
+            )
+
+            # Update credential last_used
+            await db.instance.login(stored_cred.user_uuid, stored_cred)
+
+            # Create a session for the REQUESTING device
+            assert stored_cred.uuid is not None
+            session_token = await create_session(
+                user_uuid=stored_cred.user_uuid,
+                credential_uuid=stored_cred.uuid,
+                host=request.host,
+                ip=request.ip,
+                user_agent=request.user_agent,
+            )
+
+            # Complete the remote auth request (notifies the waiting device)
+            completed = await remoteauth.instance.complete_request(
+                token=request.key,
+                session_token=session_token,
+                user_uuid=stored_cred.user_uuid,
+                credential_uuid=stored_cred.uuid,
+            )
+
+            if not completed:
+                raise ValueError("Failed to complete remote authentication")
+
+            await ws.send_json({
+                "status": "success",
+                "message": "Authentication successful. The other device is now logged in.",
+            })
+            break
+
+        # Handle code lookup request
+        code = msg.get("code")
+        if not code:
+            raise ValueError("Pairing code required")
+
+        # Look up the remote auth request by pairing code
+        request = await remoteauth.instance.get_request_by_pairing_code(code)
+
+        # Generate new challenge for next request
+        challenge = pow.generate_challenge()
+
+        if request is None:
+            await ws.send_json({
+                "status": "error",
+                "error": "not_found",
+                "pow": {
+                    "challenge": base64.b64encode(challenge).decode(),
+                    "bits": bits,
+                }
+            })
+            request = None  # Reset for next attempt
+            continue
+
+        if request.completed:
+            await ws.send_json({
+                "status": "error",
+                "error": "already_completed",
+                "pow": {
+                    "challenge": base64.b64encode(challenge).decode(),
+                    "bits": bits,
+                }
+            })
+            request = None
+            continue
+
+        # Valid code found - send device info
+        await ws.send_json({
+            "status": "found",
+            "host": request.host,
+            "user_agent_pretty": useragent.compact_user_agent(request.user_agent),
+            "pow": {
+                "challenge": base64.b64encode(challenge).decode(),
+                "bits": bits,
+            }
+        })
diff --git a/paskia/util/pow.py b/paskia/util/pow.py
new file mode 100644
index 0000000..a6eda52
--- /dev/null
+++ b/paskia/util/pow.py
@@ -0,0 +1,62 @@
+"""
+Proof of Work utility using SHA-512.
+
+The PoW challenge requires finding a nonce such that
+SHA-512(challenge || nonce) has a specified number of leading zero bits.
+
+Both challenge and nonce are 8 bytes (uint64), concatenated into 16 bytes for hashing.
+The nonce is little-endian.
+"""
+
+import hashlib
+import os
+
+# Default difficulty: 16 bits means ~65k iterations on average
+DEFAULT_POW_BITS = 16
+
+
+def generate_challenge() -> bytes:
+    """Generate a random 8-byte challenge."""
+    return os.urandom(8)
+
+
+def verify_pow(challenge: bytes, nonce: bytes, required_bits: int = DEFAULT_POW_BITS) -> bool:
+    """Verify a Proof of Work solution.
+
+    Args:
+        challenge: 8-byte server-provided challenge
+        nonce: 8-byte client-provided nonce (little-endian)
+        required_bits: Number of leading zero bits required (1-32)
+
+    Returns:
+        True if the solution is valid
+    """
+    if len(challenge) != 8 or len(nonce) != 8:
+        return False
+
+    if required_bits < 1 or required_bits > 32:
+        return False
+
+    # Concatenate challenge and nonce
+    data = challenge + nonce
+
+    # Calculate SHA-512 hash
+    hash_bytes = hashlib.sha512(data).digest()
+
+    # Check leading zero bits
+    full_zero_bytes = required_bits >> 3  # required_bits // 8
+    remaining_bits = required_bits & 7     # required_bits % 8
+
+    # Check full zero bytes
+    for i in range(full_zero_bytes):
+        if hash_bytes[i] != 0:
+            return False
+
+    # Check partial byte if needed
+    if remaining_bits:
+        # Mask for remaining bits: e.g., 3 bits -> 0b11100000 = 0xE0
+        partial_mask = (0xFF << (8 - remaining_bits)) & 0xFF
+        if hash_bytes[full_zero_bytes] & partial_mask:
+            return False
+
+    return True