Add NIP-44 style padding support to Nostr DM encryption

The bespoke "v2" DM envelope (XChaCha20-Poly1305 over the raw UTF-8
rumor JSON) leaks exact plaintext length to relays. Add NIP-44 v2
style payload framing: a 2-byte big-endian length prefix followed by
zero padding to NIP-44's calc_padded_len buckets (32-byte minimum,
power-of-two-derived chunks), carried in a new "v3:" envelope that
reuses the existing key derivation and AEAD.

Compatibility: deployed clients hard-reject any envelope that does not
start with "v2:", and the v2 payload is raw JSON, so padded payloads
cannot be introduced inside v2 either. This lands phase 1 of a
two-phase rollout: decrypt accepts both v2 (unpadded) and v3 (padded),
while encrypt keeps emitting v2, gated by
NostrProtocol.sendPaddedEnvelope (defaults to false with the rollout
plan documented inline). Flip the flag once v3-capable clients are
widely deployed.

Unpad validates that the claimed length's bucket exactly matches the
authenticated payload size, so a tampered or malformed length prefix
fails closed. Tests cover pad/unpad round-trips, the NIP-44 bucket
vectors, v3 round-trip, legacy v2 decrypt, default-envelope pinning,
and tampered length prefixes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bitchat/Nostr/NostrProtocol.swift

@@ -328,26 +328,46 @@ struct NostrProtocol {
         return try NostrEvent(from: rumorDict)
     }
     
-    // MARK: - Encryption (NIP-44 v2)
+    // MARK: - Encryption (NIP-44 v2/v3)
 
-    private static func encrypt(
+    /// Whether outgoing DMs use the padded "v3:" envelope.
+    ///
+    /// TWO-PHASE ROLLOUT — DO NOT FLIP YET. Deployed clients hard-reject any
+    /// ciphertext that does not start with "v2:" (`decrypt` below, as shipped,
+    /// throws `invalidCiphertext` on unknown version prefixes), and the "v2:"
+    /// payload is the raw UTF-8 rumor JSON, so a padded payload cannot be
+    /// smuggled inside "v2:" without breaking old receivers either. Enabling
+    /// this today would strand every client in the field.
+    ///
+    /// Phase 1 (this change): ship decrypt-side support for "v3:" everywhere.
+    /// Phase 2 (future release, once phase-1 clients are widely deployed):
+    /// set this to true so outgoing DMs stop leaking plaintext length to
+    /// relays.
+    static let sendPaddedEnvelope = false
+
+    /// Internal (rather than private) so tests can exercise both envelope
+    /// versions directly.
+    static func encrypt(
         plaintext: String,
         recipientPubkey: String,
-        senderKey: P256K.Schnorr.PrivateKey
+        senderKey: P256K.Schnorr.PrivateKey,
+        padded: Bool = NostrProtocol.sendPaddedEnvelope
     ) throws -> String {
 
         guard let recipientPubkeyData = Data(hexString: recipientPubkey) else {
             throw NostrError.invalidPublicKey
         }
 
-        // Encrypting message (NIP-44 v2: XChaCha20-Poly1305, versioned)
+        // Encrypting message (XChaCha20-Poly1305, versioned envelope)
 
         // Derive shared secret
         let sharedSecret = try deriveSharedSecret(
             privateKey: senderKey,
             publicKey: recipientPubkeyData
         )
-        // Derive NIP-44 v2 symmetric key (HKDF-SHA256 with label in info)
+        // Derive NIP-44 v2 symmetric key (HKDF-SHA256 with label in info).
+        // The v3 envelope deliberately reuses the same key derivation; it only
+        // changes the payload framing (length prefix + padding).
         let key = try deriveNIP44V2Key(from: sharedSecret)
 
         // 24-byte random nonce for XChaCha20-Poly1305
@@ -356,24 +376,37 @@ struct NostrProtocol {
             SecRandomCopyBytes(kSecRandomDefault, 24, ptr.baseAddress!)
         }
 
+        // v2 payload: raw UTF-8 plaintext (length leaks to relays)
+        // v3 payload: NIP-44 style [2-byte BE length][plaintext][zero padding]
         let pt = Data(plaintext.utf8)
-        let sealed = try XChaCha20Poly1305Compat.seal(plaintext: pt, key: key, nonce24: nonce24)
+        let payload = padded ? try NIP44Padding.pad(pt) : pt
+        let sealed = try XChaCha20Poly1305Compat.seal(plaintext: payload, key: key, nonce24: nonce24)
 
-        // v2: base64url(nonce24 || ciphertext || tag)
+        // version prefix + base64url(nonce24 || ciphertext || tag)
         var combined = Data()
         combined.append(nonce24)
         combined.append(sealed.ciphertext)
         combined.append(sealed.tag)
-        return "v2:" + Base64URLCoding.encode(combined)
+        return (padded ? "v3:" : "v2:") + Base64URLCoding.encode(combined)
     }
 
-    private static func decrypt(
+    /// Internal (rather than private) so tests can exercise both envelope
+    /// versions directly.
+    static func decrypt(
         ciphertext: String,
         senderPubkey: String,
         recipientKey: P256K.Schnorr.PrivateKey
     ) throws -> String {
-        // Expect NIP-44 v2 format
+        // Accept both the legacy unpadded "v2:" envelope and the padded "v3:"
-        guard ciphertext.hasPrefix("v2:") else { throw NostrError.invalidCiphertext }
+        // envelope (see `sendPaddedEnvelope` for the rollout plan).
+        let isPadded: Bool
+        if ciphertext.hasPrefix("v2:") {
+            isPadded = false
+        } else if ciphertext.hasPrefix("v3:") {
+            isPadded = true
+        } else {
+            throw NostrError.invalidCiphertext
+        }
         let encoded = String(ciphertext.dropFirst(3))
         guard let data = Base64URLCoding.decode(encoded),
               data.count > (24 + 16),
@@ -399,18 +432,23 @@ struct NostrProtocol {
         }
 
         // If 32 bytes (x-only) try both parities, otherwise single try
+        let payload: Data
         if senderPubkeyData.count == 32 {
             let even = Data([0x02]) + senderPubkeyData
             if let pt = try? attemptDecrypt(using: even) {
-                return String(data: pt, encoding: .utf8) ?? ""
+                payload = pt
-            }
-            let odd = Data([0x03]) + senderPubkeyData
-            let pt = try attemptDecrypt(using: odd)
-            return String(data: pt, encoding: .utf8) ?? ""
             } else {
-            let pt = try attemptDecrypt(using: senderPubkeyData)
+                let odd = Data([0x03]) + senderPubkeyData
-            return String(data: pt, encoding: .utf8) ?? ""
+                payload = try attemptDecrypt(using: odd)
             }
+        } else {
+            payload = try attemptDecrypt(using: senderPubkeyData)
+        }
+
+        // The AEAD tag has already authenticated the payload; unpadding
+        // failures here mean a malformed sender, not a wrong key.
+        let plaintextData = isPadded ? try NIP44Padding.unpad(payload) : payload
+        return String(data: plaintextData, encoding: .utf8) ?? ""
     }
     
     private static func deriveSharedSecret(
@@ -641,6 +679,60 @@ enum NostrError: Error {
     case encryptionFailed
 }
 
+// MARK: - NIP-44 style padding (v3 envelope payload framing)
+
+/// Payload framing for the padded "v3:" envelope, modeled on NIP-44 v2:
+/// `[2-byte big-endian plaintext length][plaintext][zero padding]`, where the
+/// total is padded to `paddedLength(for:)` — power-of-two-derived buckets with
+/// a 32-byte minimum — so ciphertext length no longer reveals exact plaintext
+/// length to relays.
+enum NIP44Padding {
+    static let minPaddedLength = 32
+    static let maxPlaintextLength = 65535
+
+    /// NIP-44's calc_padded_len: pad to 32 bytes minimum, then to a chunk
+    /// granularity of max(32, nextPowerOfTwo/8).
+    static func paddedLength(for unpaddedLength: Int) -> Int {
+        guard unpaddedLength > minPaddedLength else { return minPaddedLength }
+        // Smallest power of two strictly greater than (unpaddedLength - 1).
+        let nextPower = 1 << (Int.bitWidth - (unpaddedLength - 1).leadingZeroBitCount)
+        let chunk = nextPower <= 256 ? 32 : nextPower / 8
+        return chunk * ((unpaddedLength - 1) / chunk + 1)
+    }
+
+    /// Prefix plaintext with its 2-byte big-endian length and zero-pad to the
+    /// bucketed length. Rejects empty plaintexts and plaintexts that do not
+    /// fit the 16-bit length prefix.
+    static func pad(_ plaintext: Data) throws -> Data {
+        let length = plaintext.count
+        guard length >= 1, length <= maxPlaintextLength else {
+            throw NostrError.encryptionFailed
+        }
+        let padded = paddedLength(for: length)
+        var result = Data(capacity: 2 + padded)
+        result.append(UInt8(length >> 8))
+        result.append(UInt8(length & 0xFF))
+        result.append(plaintext)
+        result.append(Data(count: padded - length))
+        return result
+    }
+
+    /// Read the 2-byte length prefix, validate the total padded size matches
+    /// it exactly, and return the plaintext. Throws on any inconsistency so a
+    /// malformed (already-authenticated) payload can never over- or
+    /// under-read.
+    static func unpad(_ padded: Data) throws -> Data {
+        guard padded.count >= 2 else { throw NostrError.invalidCiphertext }
+        let start = padded.startIndex
+        let length = Int(padded[start]) << 8 | Int(padded[start + 1])
+        guard length >= 1,
+              padded.count == 2 + paddedLength(for: length) else {
+            throw NostrError.invalidCiphertext
+        }
+        return padded.subdata(in: (start + 2)..<(start + 2 + length))
+    }
+}
+
 // MARK: - NIP-44 v2 helpers (XChaCha20-Poly1305)
 
 private extension NostrProtocol {

bitchatTests/NostrProtocolTests.swift

@@ -289,6 +289,159 @@ struct NostrProtocolTests {
         #expect(object["limit"] as? Int == 42)
     }
 
+    // MARK: - Padding (v3 envelope)
+
+    @Test func paddedLengthMatchesNIP44Buckets() {
+        // Vectors from the NIP-44 reference test suite (calc_padded_len).
+        let vectors: [(Int, Int)] = [
+            (1, 32), (16, 32), (32, 32), (33, 64), (37, 64), (45, 64), (49, 64),
+            (64, 64), (65, 96), (100, 128), (111, 128), (200, 224), (250, 256),
+            (320, 320), (383, 384), (384, 384), (400, 448), (500, 512),
+            (512, 512), (515, 640), (700, 768), (800, 896), (900, 1024),
+            (1020, 1024), (65535, 65536)
+        ]
+        for (unpadded, expected) in vectors {
+            #expect(
+                NIP44Padding.paddedLength(for: unpadded) == expected,
+                "paddedLength(for: \(unpadded)) should be \(expected)"
+            )
+        }
+    }
+
+    @Test func padUnpadRoundTrip() throws {
+        for length in [1, 2, 31, 32, 33, 100, 320, 1020, 4096, 65535] {
+            let plaintext = Data((0..<length).map { _ in UInt8.random(in: .min ... .max) })
+            let padded = try NIP44Padding.pad(plaintext)
+            #expect(padded.count == 2 + NIP44Padding.paddedLength(for: length))
+            let unpadded = try NIP44Padding.unpad(padded)
+            #expect(unpadded == plaintext)
+        }
+    }
+
+    @Test func padHidesExactLengthWithinBucket() throws {
+        // Two plaintexts of different length in the same bucket must produce
+        // identically sized padded payloads (and thus ciphertexts).
+        let short = try NIP44Padding.pad(Data(repeating: 0x41, count: 65))
+        let long = try NIP44Padding.pad(Data(repeating: 0x42, count: 96))
+        #expect(short.count == long.count)
+    }
+
+    @Test func padRejectsOutOfRangePlaintexts() {
+        #expect(throws: (any Error).self) { try NIP44Padding.pad(Data()) }
+        #expect(throws: (any Error).self) { try NIP44Padding.pad(Data(count: 65536)) }
+    }
+
+    @Test func unpadRejectsTamperedLengthPrefix() throws {
+        var padded = try NIP44Padding.pad(Data(repeating: 0x41, count: 40))
+
+        // Claimed length larger than the actual payload
+        var tooLong = padded
+        tooLong[tooLong.startIndex] = 0xFF
+        tooLong[tooLong.startIndex + 1] = 0xFF
+        #expect(throws: NostrError.invalidCiphertext) { try NIP44Padding.unpad(tooLong) }
+
+        // Claimed length of zero
+        var zero = padded
+        zero[zero.startIndex] = 0x00
+        zero[zero.startIndex + 1] = 0x00
+        #expect(throws: NostrError.invalidCiphertext) { try NIP44Padding.unpad(zero) }
+
+        // Claimed length whose bucket does not match the payload size
+        // (payload is bucket 64; a claimed length of 20 expects bucket 32)
+        var wrongBucket = padded
+        wrongBucket[wrongBucket.startIndex] = 0x00
+        wrongBucket[wrongBucket.startIndex + 1] = 0x14
+        #expect(throws: NostrError.invalidCiphertext) { try NIP44Padding.unpad(wrongBucket) }
+
+        // Truncated payloads
+        #expect(throws: NostrError.invalidCiphertext) { try NIP44Padding.unpad(Data()) }
+        #expect(throws: NostrError.invalidCiphertext) { try NIP44Padding.unpad(Data([0x00])) }
+        padded.removeLast()
+        #expect(throws: NostrError.invalidCiphertext) { try NIP44Padding.unpad(padded) }
+
+        // Works on Data slices with non-zero startIndex
+        let sliced = try (Data([0xAB]) + NIP44Padding.pad(Data(repeating: 0x41, count: 40))).dropFirst()
+        #expect(try NIP44Padding.unpad(sliced) == Data(repeating: 0x41, count: 40))
+    }
+
+    @Test func paddedEnvelopeRoundTrip_v3() throws {
+        let sender = try NostrIdentity.generate()
+        let recipient = try NostrIdentity.generate()
+        let plaintext = "padded envelope test"
+
+        let ciphertext = try NostrProtocol.encrypt(
+            plaintext: plaintext,
+            recipientPubkey: recipient.publicKeyHex,
+            senderKey: sender.schnorrSigningKey(),
+            padded: true
+        )
+        #expect(ciphertext.hasPrefix("v3:"))
+
+        let decrypted = try NostrProtocol.decrypt(
+            ciphertext: ciphertext,
+            senderPubkey: sender.publicKeyHex,
+            recipientKey: recipient.schnorrSigningKey()
+        )
+        #expect(decrypted == plaintext)
+    }
+
+    @Test func legacyUnpaddedEnvelopeStillDecrypts_v2() throws {
+        let sender = try NostrIdentity.generate()
+        let recipient = try NostrIdentity.generate()
+        let plaintext = "legacy v2 envelope"
+
+        // What deployed clients send today.
+        let ciphertext = try NostrProtocol.encrypt(
+            plaintext: plaintext,
+            recipientPubkey: recipient.publicKeyHex,
+            senderKey: sender.schnorrSigningKey(),
+            padded: false
+        )
+        #expect(ciphertext.hasPrefix("v2:"))
+
+        let decrypted = try NostrProtocol.decrypt(
+            ciphertext: ciphertext,
+            senderPubkey: sender.publicKeyHex,
+            recipientKey: recipient.schnorrSigningKey()
+        )
+        #expect(decrypted == plaintext)
+    }
+
+    @Test func outgoingMessagesStillUseV2UntilRolloutFlagFlips() throws {
+        // Deployed clients reject anything that is not "v2:", so the padded
+        // envelope must stay off by default until decrypt-side support is
+        // widely shipped (see NostrProtocol.sendPaddedEnvelope).
+        #expect(NostrProtocol.sendPaddedEnvelope == false)
+
+        let sender = try NostrIdentity.generate()
+        let recipient = try NostrIdentity.generate()
+        let giftWrap = try NostrProtocol.createPrivateMessage(
+            content: "default envelope",
+            recipientPubkey: recipient.publicKeyHex,
+            senderIdentity: sender
+        )
+        #expect(giftWrap.content.hasPrefix("v2:"))
+    }
+
+    @Test func decryptRejectsUnknownEnvelopeVersion() throws {
+        let sender = try NostrIdentity.generate()
+        let recipient = try NostrIdentity.generate()
+        let ciphertext = try NostrProtocol.encrypt(
+            plaintext: "test",
+            recipientPubkey: recipient.publicKeyHex,
+            senderKey: sender.schnorrSigningKey(),
+            padded: false
+        )
+        let mutated = "v9:" + ciphertext.dropFirst(3)
+        #expect(throws: NostrError.invalidCiphertext) {
+            _ = try NostrProtocol.decrypt(
+                ciphertext: mutated,
+                senderPubkey: sender.publicKeyHex,
+                recipientKey: recipient.schnorrSigningKey()
+            )
+        }
+    }
+
     // MARK: - Helpers
     private static func base64URLDecode(_ s: String) -> Data? {
         var str = s.replacingOccurrences(of: "-", with: "+").replacingOccurrences(of: "_", with: "/")