hkdf for short links

rfcs/2026-03-20-smp-agent-web/2026-03-20-smp-agent-web-spike.md

@@ -218,7 +218,9 @@ Each step produces working, tested code. Steps 1-11 work without block encryptio
 
 **Done**. Function: `connShortLinkStrP` in `agent/protocol.ts`. Uses `base64urlDecode` from xftp-web `description.ts`.
 
-**Future**: add long link parsing (`ConnectionRequestUri`) and an either-parser that handles both short and long links.
+**Future**:
+- Add long link parsing (`ConnectionRequestUri`) and an either-parser that handles both short and long links.
+- Add `restoreShortLink`: preset servers are shortened to host-only (`SMPServerOnlyHost` - no port, no keyHash). After parsing, `restoreShortLink` looks up the full server by hostname from a preset servers list. Without this, connections to preset servers will fail. See `Agent/Protocol.hs:1692`.
 
 ### Step 7: HKDF Key Derivation
 

smp-web/src/crypto/shortLink.ts

@@ -0,0 +1,21 @@
+// Short link key derivation.
+// Mirrors: Simplex.Messaging.Crypto.ShortLink
+
+import {hkdf} from "@noble/hashes/hkdf"
+import {sha512} from "@noble/hashes/sha512"
+
+// contactShortLinkKdf (Agent/Protocol.hs:47-50)
+// hkdf("", linkKey, "SimpleXContactLink", 56) -> (linkId[24], sbKey[32])
+export function contactShortLinkKdf(linkKey: Uint8Array): {linkId: Uint8Array; sbKey: Uint8Array} {
+  const derived = hkdf(sha512, linkKey, new Uint8Array(0), "SimpleXContactLink", 56)
+  return {
+    linkId: derived.slice(0, 24),
+    sbKey: derived.slice(24, 56),
+  }
+}
+
+// invShortLinkKdf (Agent/Protocol.hs:52-53)
+// hkdf("", linkKey, "SimpleXInvLink", 32) -> sbKey[32]
+export function invShortLinkKdf(linkKey: Uint8Array): Uint8Array {
+  return hkdf(sha512, linkKey, new Uint8Array(0), "SimpleXInvLink", 32)
+}

tests/SMPWebTests.hs

@@ -18,9 +18,10 @@ import Data.List.NonEmpty (NonEmpty (..))
 import Data.Word (Word16)
 import qualified Simplex.Messaging.Agent.Protocol as AP
 import qualified Simplex.Messaging.Crypto as C
+import Simplex.Messaging.Crypto.ShortLink (contactShortLinkKdf, invShortLinkKdf)
 import Simplex.Messaging.Encoding
 import Simplex.Messaging.Encoding.String (strEncode)
-import Simplex.Messaging.Protocol (SMPServer, pattern SMPServer)
+import Simplex.Messaging.Protocol (EntityId (..), SMPServer, pattern SMPServer)
 import Simplex.Messaging.Server.Env.STM (AStoreType (..))
 import Simplex.Messaging.Server.MsgStore.Types (SMSType (..), SQSType (..))
 import Simplex.Messaging.Server.Web (attachStaticAndWS)
@@ -52,6 +53,9 @@ impWS = "import { connectSMP, sendBlock, receiveBlock } from './dist/transport/w
 impAgentProto :: String
 impAgentProto = "import { connShortLinkStrP } from './dist/agent/protocol.js';"
 
+impCryptoShortLink :: String
+impCryptoShortLink = "import { contactShortLinkKdf, invShortLinkKdf } from './dist/crypto/shortLink.js';"
+
 jsStr :: B.ByteString -> String
 jsStr bs = "'" <> BC.unpack bs <> "'"
 
@@ -154,6 +158,24 @@ smpWebTests = describe "SMP Web Client" $ do
           <> "})")
         tsEncoded `shouldBe` hsEncoded
 
+  describe "crypto/shortLink" $ do
+    describe "contactShortLinkKdf" $ do
+      it "TypeScript produces same linkId and sbKey as Haskell" $ do
+        let linkKey = AP.LinkKey $ B.pack [1..32]
+            (EntityId hsLinkId, C.SbKey hsKey) = contactShortLinkKdf linkKey
+        tsResult <- callNode $ impCryptoShortLink
+          <> "const r = contactShortLinkKdf(" <> jsUint8 (B.pack [1..32]) <> ");"
+          <> jsOut ("new Uint8Array([...r.linkId, ...r.sbKey])")
+        tsResult `shouldBe` (hsLinkId <> hsKey)
+
+    describe "invShortLinkKdf" $ do
+      it "TypeScript produces same sbKey as Haskell" $ do
+        let linkKey = AP.LinkKey $ B.pack [50..81]
+            C.SbKey hsKey = invShortLinkKdf linkKey
+        tsResult <- callNode $ impCryptoShortLink
+          <> jsOut ("invShortLinkKdf(" <> jsUint8 (B.pack [50..81]) <> ")")
+        tsResult `shouldBe` hsKey
+
   describe "agent/protocol" $ do
     describe "ConnShortLink" $ do
       it "parses simplex: contact link" $ do