diff --git a/changelog.d/5-internal/one2one-upsert b/changelog.d/5-internal/one2one-upsert
new file mode 100644
index 00000000000..5371eb9a786
--- /dev/null
+++ b/changelog.d/5-internal/one2one-upsert
@@ -0,0 +1 @@
+Add internal endpoint to insert or update a 1-1 conversation. This is to be used by brig when updating the status of a connection.
diff --git a/libs/galley-types/galley-types.cabal b/libs/galley-types/galley-types.cabal
index 891555cac72..8af1c5e5f5a 100644
--- a/libs/galley-types/galley-types.cabal
+++ b/libs/galley-types/galley-types.cabal
@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 8d07ea070b6384ec247f4473abb198bbb9639f72543920cbe46f561df96963ca
+-- hash: d7419acbff460382bb822952b693f55513e729a4e3bcd0ddfdeea9e5285a805b
 
 name:           galley-types
 version:        0.81.0
@@ -22,6 +22,7 @@ library
       Galley.Types
       Galley.Types.Bot
       Galley.Types.Bot.Service
+      Galley.Types.Conversations.Intra
       Galley.Types.Conversations.Members
       Galley.Types.Conversations.Roles
       Galley.Types.Teams
@@ -42,6 +43,7 @@ library
     , exceptions >=0.10.0
     , imports
     , lens >=4.12
+    , schema-profunctor
     , string-conversions
     , tagged
     , text >=0.11
diff --git a/libs/galley-types/package.yaml b/libs/galley-types/package.yaml
index 3c8971ad0a0..3d84f4036eb 100644
--- a/libs/galley-types/package.yaml
+++ b/libs/galley-types/package.yaml
@@ -21,6 +21,7 @@ library:
   - exceptions >=0.10.0
   - lens >=4.12
   - QuickCheck
+  - schema-profunctor
   - string-conversions
   - tagged
   - text >=0.11
diff --git a/libs/galley-types/src/Galley/Types/Conversations/Intra.hs b/libs/galley-types/src/Galley/Types/Conversations/Intra.hs
new file mode 100644
index 00000000000..0cb0ba9afd7
--- /dev/null
+++ b/libs/galley-types/src/Galley/Types/Conversations/Intra.hs
@@ -0,0 +1,87 @@
+-- This file is part of the Wire Server implementation.
+--
+-- Copyright (C) 2021 Wire Swiss GmbH <opensource@wire.com>
+--
+-- This program is free software: you can redistribute it and/or modify it under
+-- the terms of the GNU Affero General Public License as published by the Free
+-- Software Foundation, either version 3 of the License, or (at your option) any
+-- later version.
+--
+-- This program is distributed in the hope that it will be useful, but WITHOUT
+-- ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+-- FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+-- details.
+--
+-- You should have received a copy of the GNU Affero General Public License along
+-- with this program. If not, see <https://www.gnu.org/licenses/>.
+
+module Galley.Types.Conversations.Intra
+  ( DesiredMembership (..),
+    Actor (..),
+    UpsertOne2OneConversationRequest (..),
+    UpsertOne2OneConversationResponse (..),
+  )
+where
+
+import qualified Data.Aeson as A
+import Data.Aeson.Types (FromJSON, ToJSON)
+import Data.Id (ConvId, UserId)
+import Data.Qualified
+import Data.Schema
+import Imports
+
+data DesiredMembership = Included | Excluded
+  deriving (Show, Eq, Generic)
+  deriving (FromJSON, ToJSON) via Schema DesiredMembership
+
+instance ToSchema DesiredMembership where
+  schema =
+    enum @Text "DesiredMembership" $
+      mconcat
+        [ element "included" Included,
+          element "excluded" Excluded
+        ]
+
+data Actor = LocalActor | RemoteActor
+  deriving (Show, Eq, Generic)
+  deriving (FromJSON, ToJSON) via Schema Actor
+
+instance ToSchema Actor where
+  schema =
+    enum @Text "Actor" $
+      mconcat
+        [ element "local_actor" LocalActor,
+          element "remote_actor" RemoteActor
+        ]
+
+data UpsertOne2OneConversationRequest = UpsertOne2OneConversationRequest
+  { uooLocalUser :: Local UserId,
+    uooRemoteUser :: Remote UserId,
+    uooActor :: Actor,
+    uooActorDesiredMembership :: DesiredMembership,
+    uooConvId :: Maybe (Qualified ConvId)
+  }
+  deriving (Show, Generic)
+  deriving (FromJSON, ToJSON) via Schema UpsertOne2OneConversationRequest
+
+instance ToSchema UpsertOne2OneConversationRequest where
+  schema =
+    object "UpsertOne2OneConversationRequest" $
+      UpsertOne2OneConversationRequest
+        <$> (qUntagged . uooLocalUser) .= field "local_user" (qTagUnsafe <$> schema)
+        <*> (qUntagged . uooRemoteUser) .= field "remote_user" (qTagUnsafe <$> schema)
+        <*> uooActor .= field "actor" schema
+        <*> uooActorDesiredMembership .= field "actor_desired_membership" schema
+        <*> uooConvId .= field "conversation_id" (optWithDefault A.Null schema)
+
+newtype UpsertOne2OneConversationResponse = UpsertOne2OneConversationResponse
+  { uuorConvId :: Qualified ConvId
+  }
+  deriving (Show, Generic)
+  deriving (FromJSON, ToJSON) via Schema UpsertOne2OneConversationResponse
+
+instance ToSchema UpsertOne2OneConversationResponse where
+  schema =
+    object "UpsertOne2OneConversationResponse" $
+      UpsertOne2OneConversationResponse
+        <$> uuorConvId .= field "conversation_id" schema
diff --git a/libs/types-common/src/Data/UUID/Tagged.hs b/libs/types-common/src/Data/UUID/Tagged.hs
index cd822e7d04a..e3552f0b0ba 100644
--- a/libs/types-common/src/Data/UUID/Tagged.hs
+++ b/libs/types-common/src/Data/UUID/Tagged.hs
@@ -17,12 +17,16 @@
 
 module Data.UUID.Tagged
   ( UUID,
+    toUUID,
     V4,
+    V5,
     Version (..),
     version,
     variant,
     addv4,
     unpack,
+    create,
+    mk,
   )
 where
 
@@ -30,35 +34,43 @@ import Data.Bits
 import qualified Data.UUID as D
 import qualified Data.UUID.V4 as D4
 import Imports
-import Test.QuickCheck (Arbitrary, arbitrary)
 
 -- | Versioned UUID.
-newtype UUID v = UUID D.UUID deriving (Eq, Ord, Show)
+newtype UUID v = UUID {toUUID :: D.UUID}
+  deriving (Eq, Ord, Show)
 
 instance NFData (UUID v) where rnf (UUID a) = seq a ()
 
 class Version v where
-  -- | Create a fresh versioned UUID.
-  create :: IO (UUID v)
-
   -- | Try to turn a plain UUID into a versioned UUID.
   fromUUID :: D.UUID -> Maybe (UUID v)
+  fromUUID u = guard (version u == versionValue @v) $> UUID u
+
+  versionValue :: Word32
 
 data V4
 
 instance Version V4 where
-  create = UUID <$> D4.nextRandom
-  fromUUID u = case version u of
-    4 -> Just (UUID u)
-    _ -> Nothing
-
-instance Arbitrary (UUID V4) where
-  arbitrary = do
-    a <- arbitrary
-    b <- retainVersion 4 <$> arbitrary
-    c <- retainVariant 2 <$> arbitrary
-    d <- arbitrary
-    pure $ UUID $ D.fromWords a b c d
+  versionValue = 4
+
+data V5
+
+instance Version V5 where
+  versionValue = 5
+
+mk :: forall v. Version v => D.UUID -> UUID v
+mk u = UUID $
+  case D.toWords u of
+    (x0, x1, x2, x3) ->
+      D.fromWords
+        x0
+        (retainVersion (versionValue @v) x1)
+        (retainVariant 2 x2)
+        x3
+
+-- | Create a fresh UUIDv4.
+create :: IO (UUID V4)
+create = UUID <$> D4.nextRandom
 
 -- | Extract the 'D.UUID' from a versioned UUID.
 unpack :: UUID v -> D.UUID
diff --git a/libs/wire-api/src/Wire/API/Event/Conversation.hs b/libs/wire-api/src/Wire/API/Event/Conversation.hs
index 0cc3da6702e..d681659225f 100644
--- a/libs/wire-api/src/Wire/API/Event/Conversation.hs
+++ b/libs/wire-api/src/Wire/API/Event/Conversation.hs
@@ -352,7 +352,7 @@ instance ToSchema SimpleMember where
           .= (field "conversation_role" schema <|> pure roleNameWireAdmin)
 
 data Connect = Connect
-  { cRecipient :: UserId,
+  { cRecipient :: Qualified UserId,
     -- FUTUREWORK: As a follow-up from
     -- https://github.com/wireapp/wire-server/pull/1726, the message field can
     -- be removed from this event.
@@ -370,7 +370,8 @@ instance ToSchema Connect where
 connectObjectSchema :: ObjectSchema SwaggerDoc Connect
 connectObjectSchema =
   Connect
-    <$> cRecipient .= field "recipient" schema
+    <$> cRecipient .= field "qualified_recipient" schema
+    <* (Just . qUnqualified . cRecipient) .= optField "recipient" Nothing schema
     <*> cMessage .= lax (field "message" (optWithDefault A.Null schema))
     <*> cName .= lax (field "name" (optWithDefault A.Null schema))
     <*> cEmail .= lax (field "email" (optWithDefault A.Null schema))
diff --git a/libs/wire-api/test/golden/testObject_Connect_user_1.json b/libs/wire-api/test/golden/testObject_Connect_user_1.json
index 551cb7d2b1e..63d654e6666 100644
--- a/libs/wire-api/test/golden/testObject_Connect_user_1.json
+++ b/libs/wire-api/test/golden/testObject_Connect_user_1.json
@@ -2,5 +2,9 @@
     "email": "test email",
     "message": "E",
     "name": ".🝊]G",
+    "qualified_recipient": {
+        "domain": "foo.example.com",
+        "id": "00000002-0000-0001-0000-000400000004"
+    },
     "recipient": "00000002-0000-0001-0000-000400000004"
 }
diff --git a/libs/wire-api/test/golden/testObject_Connect_user_2.json b/libs/wire-api/test/golden/testObject_Connect_user_2.json
index 8ce9a35dd43..76257ece7c0 100644
--- a/libs/wire-api/test/golden/testObject_Connect_user_2.json
+++ b/libs/wire-api/test/golden/testObject_Connect_user_2.json
@@ -2,5 +2,9 @@
     "email": null,
     "message": null,
     "name": null,
+    "qualified_recipient": {
+        "domain": "bar.example.com",
+        "id": "00000005-0000-0007-0000-000200000008"
+    },
     "recipient": "00000005-0000-0007-0000-000200000008"
 }
diff --git a/libs/wire-api/test/golden/testObject_Event_user_10.json b/libs/wire-api/test/golden/testObject_Event_user_10.json
index 3ec9ea0854c..7a1f9dcd990 100644
--- a/libs/wire-api/test/golden/testObject_Event_user_10.json
+++ b/libs/wire-api/test/golden/testObject_Event_user_10.json
@@ -4,6 +4,10 @@
         "email": "󲛚",
         "message": "L",
         "name": "fq",
+        "qualified_recipient": {
+            "domain": "faraway.example.com",
+            "id": "00000008-0000-0000-0000-000600000001"
+        },
         "recipient": "00000008-0000-0000-0000-000600000001"
     },
     "from": "00007f28-0000-40b1-0000-56ab0000748d",
diff --git a/libs/wire-api/test/unit/Test/Wire/API/Golden/Generated/Connect_user.hs b/libs/wire-api/test/unit/Test/Wire/API/Golden/Generated/Connect_user.hs
index ad3dcd121da..2e402120580 100644
--- a/libs/wire-api/test/unit/Test/Wire/API/Golden/Generated/Connect_user.hs
+++ b/libs/wire-api/test/unit/Test/Wire/API/Golden/Generated/Connect_user.hs
@@ -16,7 +16,9 @@
 -- with this program. If not, see <https://www.gnu.org/licenses/>.
 module Test.Wire.API.Golden.Generated.Connect_user where
 
+import Data.Domain
 import Data.Id (Id (Id))
+import Data.Qualified
 import qualified Data.UUID as UUID (fromString)
 import Imports (Maybe (Just, Nothing), fromJust)
 import Wire.API.Event.Conversation (Connect (..))
@@ -24,7 +26,10 @@ import Wire.API.Event.Conversation (Connect (..))
 testObject_Connect_user_1 :: Connect
 testObject_Connect_user_1 =
   Connect
-    { cRecipient = Id (fromJust (UUID.fromString "00000002-0000-0001-0000-000400000004")),
+    { cRecipient =
+        Qualified
+          (Id (fromJust (UUID.fromString "00000002-0000-0001-0000-000400000004")))
+          (Domain "foo.example.com"),
       cMessage = Just "E",
       cName = Just ".\128842]G",
       cEmail = Just "test email"
@@ -33,7 +38,10 @@ testObject_Connect_user_1 =
 testObject_Connect_user_2 :: Connect
 testObject_Connect_user_2 =
   Connect
-    { cRecipient = Id (fromJust (UUID.fromString "00000005-0000-0007-0000-000200000008")),
+    { cRecipient =
+        Qualified
+          (Id (fromJust (UUID.fromString "00000005-0000-0007-0000-000200000008")))
+          (Domain "bar.example.com"),
       cMessage = Nothing,
       cName = Nothing,
       cEmail = Nothing
diff --git a/libs/wire-api/test/unit/Test/Wire/API/Golden/Generated/Event_user.hs b/libs/wire-api/test/unit/Test/Wire/API/Golden/Generated/Event_user.hs
index 6f5e8a6c28a..bf7223ebb75 100644
--- a/libs/wire-api/test/unit/Test/Wire/API/Golden/Generated/Event_user.hs
+++ b/libs/wire-api/test/unit/Test/Wire/API/Golden/Generated/Event_user.hs
@@ -214,7 +214,10 @@ testObject_Event_user_10 =
     (read "1864-05-25 01:31:49.802 UTC")
     ( EdConnect
         ( Connect
-            { cRecipient = Id (fromJust (UUID.fromString "00000008-0000-0000-0000-000600000001")),
+            { cRecipient =
+                Qualified
+                  (Id (fromJust (UUID.fromString "00000008-0000-0000-0000-000600000001")))
+                  (Domain "faraway.example.com"),
               cMessage = Just "L",
               cName = Just "fq",
               cEmail = Just "\992986"
diff --git a/services/brig/src/Brig/IO/Intra.hs b/services/brig/src/Brig/IO/Intra.hs
index afac8627dde..271c72b7f80 100644
--- a/services/brig/src/Brig/IO/Intra.hs
+++ b/services/brig/src/Brig/IO/Intra.hs
@@ -32,6 +32,7 @@ module Brig.IO.Intra
     blockConv,
     unblockConv,
     getConv,
+    upsertOne2OneConversation,
 
     -- * Clients
     Brig.IO.Intra.newClient,
@@ -93,6 +94,7 @@ import Data.Qualified
 import Data.Range
 import qualified Data.Set as Set
 import Galley.Types (Connect (..), Conversation)
+import Galley.Types.Conversations.Intra (UpsertOne2OneConversationRequest, UpsertOne2OneConversationResponse)
 import qualified Galley.Types.Teams as Team
 import Galley.Types.Teams.Intra (GuardLegalholdPolicyConflicts (GuardLegalholdPolicyConflicts))
 import qualified Galley.Types.Teams.Intra as Team
@@ -533,7 +535,7 @@ createSelfConv u = do
         . zUser u
         . expect2xx
 
--- | Calls 'Galley.API.createConnectConversationH'.
+-- | Calls 'Galley.API.Create.createConnectConversation'.
 createLocalConnectConv ::
   Local UserId ->
   Local UserId ->
@@ -545,18 +547,17 @@ createLocalConnectConv from to cname conn = do
     logConnection (tUnqualified from) (qUntagged to)
       . remote "galley"
       . msg (val "Creating connect conversation")
+  let req =
+        path "/i/conversations/connect"
+          . zUser (tUnqualified from)
+          . maybe id (header "Z-Connection" . fromConnId) conn
+          . contentJson
+          . lbytes (encode $ Connect (qUntagged to) Nothing cname Nothing)
+          . expect2xx
   r <- galleyRequest POST req
   maybe (error "invalid conv id") return $
     fromByteString $
       getHeader' "Location" r
-  where
-    req =
-      path "/i/conversations/connect"
-        . zUser (tUnqualified from)
-        . maybe id (header "Z-Connection" . fromConnId) conn
-        . contentJson
-        . lbytes (encode $ Connect (tUnqualified to) Nothing cname Nothing)
-        . expect2xx
 
 createConnectConv ::
   Qualified UserId ->
@@ -658,6 +659,18 @@ getConv usr cnv = do
         . zUser usr
         . expect [status200, status404]
 
+upsertOne2OneConversation :: UpsertOne2OneConversationRequest -> AppIO UpsertOne2OneConversationResponse
+upsertOne2OneConversation urequest = do
+  response <- galleyRequest POST req
+  case Bilge.statusCode response of
+    200 -> decodeBody "galley" response
+    _ -> throwM internalServerError
+  where
+    req =
+      paths ["i", "conversations", "one2one", "upsert"]
+        . header "Content-Type" "application/json"
+        . lbytes (encode urequest)
+
 -- | Calls 'Galley.API.getTeamConversationH'.
 getTeamConv :: UserId -> TeamId -> ConvId -> AppIO (Maybe Team.TeamConversation)
 getTeamConv usr tid cnv = do
diff --git a/services/galley/galley.cabal b/services/galley/galley.cabal
index a43ea917332..57fabd19156 100644
--- a/services/galley/galley.cabal
+++ b/services/galley/galley.cabal
@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: a5b2ec0bd44d4fcabec564b4e7683a01cfb75cdb1c78a6eee520d6c48c95bb1d
+-- hash: c121411458d6b0f7118ae1589134cb37711d29cad840e07d0f135663c59cc53a
 
 name:           galley
 version:        0.83.0
@@ -35,6 +35,7 @@ library
       Galley.API.LegalHold.Conflicts
       Galley.API.Mapping
       Galley.API.Message
+      Galley.API.One2One
       Galley.API.Public
       Galley.API.Query
       Galley.API.Teams
@@ -87,6 +88,7 @@ library
     , base >=4.6 && <5
     , base64-bytestring >=1.0
     , bilge >=0.21.1
+    , binary
     , brig-types >=0.73.1
     , bytestring >=0.9
     , bytestring-conversion >=0.2
@@ -95,6 +97,7 @@ library
     , cassava >=0.5.2
     , cereal >=0.4
     , containers >=0.5
+    , cryptonite
     , currency-codes >=2.0
     , data-default >=0.5
     , enclosed-exceptions >=1.0
@@ -113,6 +116,7 @@ library
     , imports
     , insert-ordered-containers
     , lens >=4.4
+    , memory
     , metrics-wai >=0.4
     , mtl >=2.2
     , optparse-applicative >=0.10
@@ -172,6 +176,7 @@ executable galley
     , base
     , case-insensitive
     , extended
+    , extra >=1.3
     , galley
     , galley-types
     , imports
@@ -230,11 +235,13 @@ executable galley-integration
     , cereal
     , containers
     , cookie
+    , cql-io
     , currency-codes
     , data-timeout
     , errors
     , exceptions
     , extended
+    , extra >=1.3
     , galley
     , galley-types
     , gundeck-types
@@ -311,6 +318,7 @@ executable galley-migrate-data
     , containers
     , exceptions
     , extended
+    , extra >=1.3
     , galley-types
     , imports
     , lens
@@ -378,6 +386,7 @@ executable galley-schema
     , case-insensitive
     , cassandra-util
     , extended
+    , extra >=1.3
     , imports
     , optparse-applicative
     , raw-strings-qq >=1.0
@@ -399,6 +408,7 @@ test-suite galley-types-tests
   other-modules:
       Test.Galley.API
       Test.Galley.API.Message
+      Test.Galley.API.One2One
       Test.Galley.Intra.User
       Test.Galley.Mapping
       Test.Galley.Roundtrip
@@ -413,6 +423,7 @@ test-suite galley-types-tests
     , case-insensitive
     , containers
     , extended
+    , extra >=1.3
     , galley
     , galley-types
     , http-types
diff --git a/services/galley/package.yaml b/services/galley/package.yaml
index a4bda422abc..2ae2bfe98a5 100644
--- a/services/galley/package.yaml
+++ b/services/galley/package.yaml
@@ -13,6 +13,7 @@ dependencies:
 - imports
 - case-insensitive
 - extended
+- extra >=1.3
 - safe >=0.3
 - ssl-util
 - raw-strings-qq >=1.0
@@ -31,6 +32,7 @@ library:
   - base >=4.6 && <5
   - base64-bytestring >=1.0
   - bilge >=0.21.1
+  - binary
   - brig-types >=0.73.1
   - bytestring >=0.9
   - bytestring-conversion >=0.2
@@ -38,12 +40,12 @@ library:
   - cassava >= 0.5.2
   - cereal >=0.4
   - containers >=0.5
+  - cryptonite
   - currency-codes >=2.0
   - data-default >=0.5
   - enclosed-exceptions >=1.0
   - errors >=2.0
   - exceptions >=0.4
-  - extra >=1.3
   - galley-types >=0.65.0
   - gundeck-types >=1.35.2
   - HsOpenSSL >=0.11
@@ -56,6 +58,7 @@ library:
   - http2-client-grpc
   - insert-ordered-containers
   - lens >=4.4
+  - memory
   - metrics-wai >=0.4
   - mtl >=2.2
   - optparse-applicative >=0.10
@@ -162,6 +165,7 @@ executables:
     - cereal
     - containers
     - cookie
+    - cql-io
     - currency-codes
     - metrics-wai
     - data-timeout
diff --git a/services/galley/src/Galley/API/Create.hs b/services/galley/src/Galley/API/Create.hs
index b5581f1b6cf..ccd977b2670 100644
--- a/services/galley/src/Galley/API/Create.hs
+++ b/services/galley/src/Galley/API/Create.hs
@@ -20,7 +20,7 @@ module Galley.API.Create
     internalCreateManagedConversationH,
     createSelfConversation,
     createOne2OneConversation,
-    createConnectConversationH,
+    createConnectConversation,
   )
 where
 
@@ -36,6 +36,7 @@ import Data.Time
 import qualified Data.UUID.Tagged as U
 import Galley.API.Error
 import Galley.API.Mapping
+import Galley.API.One2One
 import Galley.API.Util
 import Galley.App
 import qualified Galley.Data as Data
@@ -51,6 +52,7 @@ import Network.Wai.Predicate hiding (setStatus)
 import Network.Wai.Utilities
 import qualified Wire.API.Conversation as Public
 import Wire.API.ErrorDescription (MissingLegalholdConsent)
+import Wire.API.Federation.Error (federationNotImplemented)
 import Wire.API.Routes.Public.Galley (ConversationResponse)
 import Wire.API.Routes.Public.Util
 import Wire.API.Team.LegalHold (LegalholdProtectee (LegalholdPlusFederationNotImplemented))
@@ -96,13 +98,9 @@ createRegularGroupConv :: UserId -> ConnId -> NewConvUnmanaged -> Galley Convers
 createRegularGroupConv zusr zcon (NewConvUnmanaged body) = do
   lusr <- qualifyLocal zusr
   name <- rangeCheckedMaybe (newConvName body)
-  let unqualifiedUserIds = newConvUsers body
-      qualifiedUserIds = newConvQualifiedUsers body
-      allUsers =
-        toUserList lusr $
-          map (qUntagged . qualifyAs lusr) unqualifiedUserIds <> qualifiedUserIds
+  let allUsers = newConvMembers lusr body
   checkedUsers <- checkedConvSize allUsers
-  ensureConnected zusr (ulLocals allUsers)
+  ensureConnected lusr allUsers
   checkRemoteUsersExist (ulRemotes allUsers)
   ensureNoLegalholdConflicts (ulRemotes allUsers) (ulLocals allUsers)
   c <-
@@ -125,11 +123,7 @@ createTeamGroupConv :: UserId -> ConnId -> Public.ConvTeamInfo -> Public.NewConv
 createTeamGroupConv zusr zcon tinfo body = do
   lusr <- qualifyLocal zusr
   name <- rangeCheckedMaybe (newConvName body)
-  let unqualifiedUserIds = newConvUsers body
-      qualifiedUserIds = newConvQualifiedUsers body
-      allUsers =
-        toUserList lusr $
-          map (qUntagged . qualifyAs lusr) unqualifiedUserIds <> qualifiedUserIds
+  let allUsers = newConvMembers lusr body
       convTeam = cnvTeamId tinfo
 
   zusrMembership <- Data.teamMember convTeam zusr
@@ -186,75 +180,157 @@ createSelfConversation zusr = do
 createOne2OneConversation :: UserId -> ConnId -> NewConvUnmanaged -> Galley ConversationResponse
 createOne2OneConversation zusr zcon (NewConvUnmanaged j) = do
   lusr <- qualifyLocal zusr
-  otherUserId <- head . fromRange <$> (rangeChecked (newConvUsers j) :: Galley (Range 1 1 [UserId]))
-  (x, y) <- toUUIDs zusr otherUserId
-  when (x == y) $
-    throwM $
-      invalidOp "Cannot create a 1-1 with yourself"
-  case newConvTeam j of
+  let allUsers = newConvMembers lusr j
+  other <- ensureOne (ulAll lusr allUsers)
+  when (qUntagged lusr == other) $
+    throwM (invalidOp "Cannot create a 1-1 with yourself")
+  mtid <- case newConvTeam j of
     Just ti
       | cnvManaged ti -> throwM noManagedTeamConv
-      | otherwise ->
-        checkBindingTeamPermissions zusr otherUserId (cnvTeamId ti)
-    Nothing -> do
-      ensureConnected zusr [otherUserId]
+      | otherwise -> do
+        foldQualified
+          lusr
+          (\lother -> checkBindingTeamPermissions lusr lother (cnvTeamId ti))
+          (const (pure Nothing))
+          other
+    Nothing -> ensureConnected lusr allUsers $> Nothing
   n <- rangeCheckedMaybe (newConvName j)
-  c <- Data.conversation (Data.one2OneConvId x y)
-  maybe (create lusr x y n $ newConvTeam j) (conversationExisted zusr) c
+  foldQualified
+    lusr
+    (createLegacyOne2OneConversationUnchecked lusr zcon n mtid)
+    (createOne2OneConversationUnchecked lusr zcon n mtid . qUntagged)
+    other
   where
     verifyMembership tid u = do
       membership <- Data.teamMember tid u
       when (isNothing membership) $
         throwM noBindingTeamMembers
-    checkBindingTeamPermissions x y tid = do
-      zusrMembership <- Data.teamMember tid zusr
+    checkBindingTeamPermissions lusr lother tid = do
+      zusrMembership <- Data.teamMember tid (tUnqualified lusr)
       void $ permissionCheck CreateConversation zusrMembership
       Data.teamBinding tid >>= \case
         Just Binding -> do
-          verifyMembership tid x
-          verifyMembership tid y
+          verifyMembership tid (tUnqualified lusr)
+          verifyMembership tid (tUnqualified lother)
+          pure (Just tid)
         Just _ -> throwM nonBindingTeam
         Nothing -> throwM teamNotFound
-    create lusr x y n tinfo = do
-      c <- Data.createOne2OneConversation lusr x y n (cnvTeamId <$> tinfo)
-      notifyCreatedConversation Nothing zusr (Just zcon) c
-      conversationCreated zusr c
 
-createConnectConversationH :: UserId ::: Maybe ConnId ::: JsonRequest Connect -> Galley Response
-createConnectConversationH (usr ::: conn ::: req) = do
-  j <- fromJsonBody req
-  handleConversationResponse <$> createConnectConversation usr conn j
+createLegacyOne2OneConversationUnchecked ::
+  Local UserId ->
+  ConnId ->
+  Maybe (Range 1 256 Text) ->
+  Maybe TeamId ->
+  Local UserId ->
+  Galley ConversationResponse
+createLegacyOne2OneConversationUnchecked self zcon name mtid other = do
+  lcnv <- localOne2OneConvId self other
+  mc <- Data.conversation (tUnqualified lcnv)
+  case mc of
+    Just c -> conversationExisted (tUnqualified self) c
+    Nothing -> do
+      (x, y) <- toUUIDs (tUnqualified self) (tUnqualified other)
+      c <- Data.createLegacyOne2OneConversation self x y name mtid
+      notifyCreatedConversation Nothing (tUnqualified self) (Just zcon) c
+      conversationCreated (tUnqualified self) c
+
+createOne2OneConversationUnchecked ::
+  Local UserId ->
+  ConnId ->
+  Maybe (Range 1 256 Text) ->
+  Maybe TeamId ->
+  Qualified UserId ->
+  Galley ConversationResponse
+createOne2OneConversationUnchecked self zcon name mtid other = do
+  let create =
+        foldQualified
+          self
+          createOne2OneConversationLocally
+          createOne2OneConversationRemotely
+  create (one2OneConvId (qUntagged self) other) self zcon name mtid other
+
+createOne2OneConversationLocally ::
+  Local ConvId ->
+  Local UserId ->
+  ConnId ->
+  Maybe (Range 1 256 Text) ->
+  Maybe TeamId ->
+  Qualified UserId ->
+  Galley ConversationResponse
+createOne2OneConversationLocally lcnv self zcon name mtid other = do
+  mc <- Data.conversation (tUnqualified lcnv)
+  case mc of
+    Just c -> conversationExisted (tUnqualified self) c
+    Nothing -> do
+      c <- Data.createOne2OneConversation lcnv self other name mtid
+      notifyCreatedConversation Nothing (tUnqualified self) (Just zcon) c
+      conversationCreated (tUnqualified self) c
+
+createOne2OneConversationRemotely ::
+  Remote ConvId ->
+  Local UserId ->
+  ConnId ->
+  Maybe (Range 1 256 Text) ->
+  Maybe TeamId ->
+  Qualified UserId ->
+  Galley ConversationResponse
+createOne2OneConversationRemotely _ _ _ _ _ _ =
+  throwM federationNotImplemented
 
-createConnectConversation :: UserId -> Maybe ConnId -> Connect -> Galley ConversationResponse
+createConnectConversation ::
+  UserId ->
+  Maybe ConnId ->
+  Connect ->
+  Galley ConversationResponse
 createConnectConversation usr conn j = do
   lusr <- qualifyLocal usr
-  (x, y) <- toUUIDs usr (cRecipient j)
+  foldQualified
+    lusr
+    (\lrcpt -> createLegacyConnectConversation lusr conn lrcpt j)
+    (createConnectConversationWithRemote lusr conn)
+    (cRecipient j)
+
+createConnectConversationWithRemote ::
+  Local UserId ->
+  Maybe ConnId ->
+  Remote UserId ->
+  Galley ConversationResponse
+createConnectConversationWithRemote _ _ _ =
+  throwM federationNotImplemented
+
+createLegacyConnectConversation ::
+  Local UserId ->
+  Maybe ConnId ->
+  Local UserId ->
+  Connect ->
+  Galley ConversationResponse
+createLegacyConnectConversation lusr conn lrecipient j = do
+  (x, y) <- toUUIDs (tUnqualified lusr) (tUnqualified lrecipient)
   n <- rangeCheckedMaybe (cName j)
-  conv <- Data.conversation (Data.one2OneConvId x y)
-  maybe (create lusr x y n) (update n) conv
+  conv <- Data.conversation (Data.localOne2OneConvId x y)
+  maybe (create x y n) (update n) conv
   where
-    create lusr x y n = do
+    create x y n = do
       c <- Data.createConnectConversation lusr x y n
       now <- liftIO getCurrentTime
       let lcid = qualifyAs lusr (Data.convId c)
           e = Event ConvConnect (qUntagged lcid) (qUntagged lusr) now (EdConnect j)
-      notifyCreatedConversation Nothing usr conn c
-      for_ (newPushLocal ListComplete usr (ConvEvent e) (recipient <$> Data.convLocalMembers c)) $ \p ->
+      notifyCreatedConversation Nothing (tUnqualified lusr) conn c
+      for_ (newPushLocal ListComplete (tUnqualified lusr) (ConvEvent e) (recipient <$> Data.convLocalMembers c)) $ \p ->
         push1 $
           p
             & pushRoute .~ RouteDirect
             & pushConn .~ conn
-      conversationCreated usr c
+      conversationCreated (tUnqualified lusr) c
     update n conv = do
       let mems = Data.convLocalMembers conv
-       in conversationExisted usr
+       in conversationExisted (tUnqualified lusr)
             =<< if
-                | usr `isMember` mems ->
+                | (tUnqualified lusr) `isMember` mems ->
                   -- we already were in the conversation, maybe also other
                   connect n conv
                 | otherwise -> do
                   lcid <- qualifyLocal (Data.convId conv)
-                  lusr <- qualifyLocal usr
                   mm <- Data.addMember lcid lusr
                   let conv' =
                         conv
@@ -266,7 +342,7 @@ createConnectConversation usr conn j = do
                       connect n conv'
                     else do
                       -- we were not in the conversation, but someone else
-                      conv'' <- acceptOne2One usr conv' conn
+                      conv'' <- acceptOne2One (tUnqualified lusr) conv' conn
                       if Data.convType conv'' == ConnectConv
                         then connect n conv''
                         else return conv''
@@ -274,15 +350,14 @@ createConnectConversation usr conn j = do
       | Data.convType conv == ConnectConv = do
         localDomain <- viewFederationDomain
         let qconv = Qualified (Data.convId conv) localDomain
-            qusr = Qualified usr localDomain
         n' <- case n of
           Just x -> do
             Data.updateConversation (Data.convId conv) x
             return . Just $ fromRange x
           Nothing -> return $ Data.convName conv
         t <- liftIO getCurrentTime
-        let e = Event ConvConnect qconv qusr t (EdConnect j)
-        for_ (newPushLocal ListComplete usr (ConvEvent e) (recipient <$> Data.convLocalMembers conv)) $ \p ->
+        let e = Event ConvConnect qconv (qUntagged lusr) t (EdConnect j)
+        for_ (newPushLocal ListComplete (tUnqualified lusr) (ConvEvent e) (recipient <$> Data.convLocalMembers conv)) $ \p ->
           push1 $
             p
               & pushRoute .~ RouteDirect
@@ -330,6 +405,11 @@ notifyCreatedConversation dtime usr conn c = do
           & pushConn .~ conn
           & pushRoute .~ route
 
+localOne2OneConvId :: Local UserId -> Local UserId -> Galley (Local ConvId)
+localOne2OneConvId self other = do
+  (x, y) <- toUUIDs (tUnqualified self) (tUnqualified other)
+  pure . qualifyAs self $ Data.localOne2OneConvId x y
+
 toUUIDs :: UserId -> UserId -> Galley (U.UUID U.V4, U.UUID U.V4)
 toUUIDs a b = do
   a' <- U.fromUUID (toUUID a) & ifNothing invalidUUID4
@@ -343,3 +423,12 @@ access :: NewConv -> [Access]
 access a = case Set.toList (newConvAccess a) of
   [] -> Data.defRegularConvAccess
   (x : xs) -> x : xs
+
+newConvMembers :: Local x -> NewConv -> UserList UserId
+newConvMembers loc body =
+  UserList (newConvUsers body) []
+    <> toUserList loc (newConvQualifiedUsers body)
+
+ensureOne :: [a] -> Galley a
+ensureOne [x] = pure x
+ensureOne _ = throwM (invalidRange "One-to-one conversations can only have a single invited member")
diff --git a/services/galley/src/Galley/API/Internal.hs b/services/galley/src/Galley/API/Internal.hs
index 65b8fd7a92e..925e5456f23 100644
--- a/services/galley/src/Galley/API/Internal.hs
+++ b/services/galley/src/Galley/API/Internal.hs
@@ -43,6 +43,7 @@ import qualified Galley.API.CustomBackend as CustomBackend
 import Galley.API.Error (throwErrorDescriptionType)
 import Galley.API.LegalHold (getTeamLegalholdWhitelistedH, setTeamLegalholdWhitelistedH, unsetTeamLegalholdWhitelistedH)
 import Galley.API.LegalHold.Conflicts (guardLegalholdPolicyConflicts)
+import qualified Galley.API.One2One as One2One
 import qualified Galley.API.Query as Query
 import Galley.API.Teams (uncheckedDeleteTeamMember)
 import qualified Galley.API.Teams as Teams
@@ -57,6 +58,7 @@ import qualified Galley.Queue as Q
 import Galley.Types
 import Galley.Types.Bot (AddBot, RemoveBot)
 import Galley.Types.Bot.Service
+import Galley.Types.Conversations.Intra (UpsertOne2OneConversationRequest (..), UpsertOne2OneConversationResponse (..))
 import Galley.Types.Teams hiding (MemberLeave)
 import Galley.Types.Teams.Intra
 import Galley.Types.Teams.SearchVisibility
@@ -79,6 +81,7 @@ import Wire.API.ErrorDescription (MissingLegalholdConsent)
 import Wire.API.Routes.MultiTablePaging (mtpHasMore, mtpPagingState, mtpResults)
 import Wire.API.Routes.MultiVerb (MultiVerb, RespondEmpty)
 import Wire.API.Routes.Public (ZOptConn, ZUser)
+import Wire.API.Routes.Public.Galley (ConversationVerb)
 import qualified Wire.API.Team.Feature as Public
 
 data InternalApi routes = InternalApi
@@ -175,7 +178,29 @@ data InternalApi routes = InternalApi
         :> ZOptConn
         :> "i"
         :> "user"
-        :> MultiVerb 'DELETE '[Servant.JSON] '[RespondEmpty 200 "Remove a user from Galley"] ()
+        :> MultiVerb 'DELETE '[Servant.JSON] '[RespondEmpty 200 "Remove a user from Galley"] (),
+    -- This endpoint can lead to the following events being sent:
+    -- - ConvCreate event to self, if conversation did not exist before
+    -- - ConvConnect event to self, if other didn't join the connect conversation before
+    iConnect ::
+      routes
+        :- Summary "Create a connect conversation (deprecated)"
+        :> ZUser
+        :> ZOptConn
+        :> "i"
+        :> "conversations"
+        :> "connect"
+        :> ReqBody '[Servant.JSON] Connect
+        :> ConversationVerb,
+    iUpsertOne2OneConversation ::
+      routes
+        :- Summary "Create or Update a connect or one2one conversation."
+        :> "i"
+        :> "conversations"
+        :> "one2one"
+        :> "upsert"
+        :> ReqBody '[Servant.JSON] UpsertOne2OneConversationRequest
+        :> Post '[Servant.JSON] UpsertOne2OneConversationResponse
   }
   deriving (Generic)
 
@@ -250,7 +275,9 @@ servantSitemap =
         iTeamFeatureStatusClassifiedDomainsGet = iGetTeamFeature @'Public.TeamFeatureClassifiedDomains Features.getClassifiedDomainsInternal,
         iTeamFeatureStatusConferenceCallingPut = iPutTeamFeature @'Public.TeamFeatureConferenceCalling Features.setConferenceCallingInternal,
         iTeamFeatureStatusConferenceCallingGet = iGetTeamFeature @'Public.TeamFeatureConferenceCalling Features.getConferenceCallingInternal,
-        iDeleteUser = rmUser
+        iDeleteUser = rmUser,
+        iConnect = Create.createConnectConversation,
+        iUpsertOne2OneConversation = One2One.iUpsertOne2OneConversation
       }
 
 iGetTeamFeature ::
@@ -290,14 +317,6 @@ sitemap = do
       .&. zauthConnId
       .&. jsonRequest @NewConvManaged
 
-  -- This endpoint can lead to the following events being sent:
-  -- - ConvCreate event to self, if conversation did not exist before
-  -- - ConvConnect event to self, if other didn't join the connect conversation before
-  post "/i/conversations/connect" (continue Create.createConnectConversationH) $
-    zauthUserId
-      .&. opt zauthConnId
-      .&. jsonRequest @Connect
-
   -- This endpoint can lead to the following events being sent:
   -- - MemberJoin event to you, if the conversation existed and had < 2 members before
   -- - MemberJoin event to other, if the conversation existed and only the other was member
diff --git a/services/galley/src/Galley/API/One2One.hs b/services/galley/src/Galley/API/One2One.hs
new file mode 100644
index 00000000000..fa9de3f254b
--- /dev/null
+++ b/services/galley/src/Galley/API/One2One.hs
@@ -0,0 +1,166 @@
+-- This file is part of the Wire Server implementation.
+--
+-- Copyright (C) 2021 Wire Swiss GmbH <opensource@wire.com>
+--
+-- This program is free software: you can redistribute it and/or modify it under
+-- the terms of the GNU Affero General Public License as published by the Free
+-- Software Foundation, either version 3 of the License, or (at your option) any
+-- later version.
+--
+-- This program is distributed in the hope that it will be useful, but WITHOUT
+-- ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+-- FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+-- details.
+--
+-- You should have received a copy of the GNU Affero General Public License along
+-- with this program. If not, see <https://www.gnu.org/licenses/>.
+{-# LANGUAGE RecordWildCards #-}
+
+module Galley.API.One2One
+  ( one2OneConvId,
+    iUpsertOne2OneConversation,
+  )
+where
+
+import Control.Error (atMay)
+import qualified Crypto.Hash as Crypto
+import Data.Bits
+import Data.ByteArray (convert)
+import qualified Data.ByteString as B
+import Data.ByteString.Conversion
+import qualified Data.ByteString.Lazy as L
+import Data.Id
+import Data.Qualified
+import Data.UUID (UUID)
+import qualified Data.UUID as UUID
+import qualified Data.UUID.Tagged as U
+import Galley.App (Galley)
+import qualified Galley.Data as Data
+import Galley.Types.Conversations.Intra (Actor (..), DesiredMembership (..), UpsertOne2OneConversationRequest (..), UpsertOne2OneConversationResponse (..))
+import Galley.Types.UserList (UserList (..))
+import Imports
+
+-- | The hash function used to obtain the 1-1 conversation ID for a pair of users.
+--
+-- /Note/: the hash function must always return byte strings of length > 16.
+hash :: ByteString -> ByteString
+hash = convert . Crypto.hash @ByteString @Crypto.SHA256
+
+-- | A randomly-generated UUID to use as a namespace for the UUIDv5 of 1-1
+-- conversation IDs
+namespace :: UUID
+namespace = UUID.fromWords 0x9a51edb8 0x060c0d9a 0x0c2950a8 0x5d152982
+
+compareDomains :: Ord a => Qualified a -> Qualified a -> Ordering
+compareDomains (Qualified a1 dom1) (Qualified a2 dom2) =
+  compare (dom1, a1) (dom2, a2)
+
+quidToByteString :: Qualified UserId -> ByteString
+quidToByteString (Qualified uid domain) = toByteString' uid <> toByteString' domain
+
+-- | This function returns the 1-1 conversation for a given pair of users.
+--
+-- Let A, B denote the (not necessarily distinct) backends of the two users,
+-- with the domain of A less or equal than the domain of B in the lexicographic
+-- ordering of their ascii encodings. Given users a@A and b@B, the UUID and
+-- owning domain of the unique 1-1 conversation between a and b shall be a
+-- deterministic function of the input data, plus some fixed parameters, as
+-- described below.
+--
+-- __Parameters__
+--
+--   * A (collision-resistant) hash function h with N bits of output, where N
+--     s a multiple of 8 strictly larger than 128; this is set to SHA256.
+--   * A "namespace" UUID n.
+--
+-- __Algorithm__
+--
+-- First, in the special case where A and B are the same backend, assume that
+-- the UUID of a is lower than that of b. If that is not the case, swap a
+-- and b in the following. This is necessary to ensure that the function we
+-- describe below is symmetric in its arguments.
+-- Let c be the bytestring obtained as the concatenation of the following 5
+-- components:
+--
+--   * the 16 bytes of the namespace n
+--   * the 16 bytes of the UUID of a
+--   * the ascii encoding of the domain of A
+--   * the 16 bytes of the UUID of b
+--   * the ascii encoding of the domain of B,
+--
+-- and let x = h(c) be its hashed value. The UUID of the 1-1 conversation
+-- between a and b is obtained by converting the first 128 bits of x to a UUID
+-- V5. Note that our use of V5 here is not strictly compliant with RFC 4122,
+-- since we are using a custom hash and not necessarily SHA1.
+--
+-- The owning domain for the conversation is set to be A if bit 128 of x (i.e.
+-- the most significant bit of the octet at index 16) is 0, and B otherwise.
+-- This is well-defined, because we assumed the number of bits of x to be
+-- strictly larger than 128.
+one2OneConvId :: Qualified UserId -> Qualified UserId -> Qualified ConvId
+one2OneConvId a b = case compareDomains a b of
+  GT -> one2OneConvId b a
+  _ ->
+    let c =
+          mconcat
+            [ L.toStrict (UUID.toByteString namespace),
+              quidToByteString a,
+              quidToByteString b
+            ]
+        x = hash c
+        result =
+          U.toUUID . U.mk @U.V5
+            . fromMaybe UUID.nil
+            -- fromByteString only returns 'Nothing' when the input is not
+            -- exactly 16 bytes long, here this should not be a case since
+            -- 'hash' is supposed to return atleast 16 bytes and we use 'B.take
+            -- 16' to truncate it
+            . UUID.fromByteString
+            . L.fromStrict
+            . B.take 16
+            $ x
+        domain
+          | fromMaybe 0 (atMay (B.unpack x) 16) .&. 0x80 == 0 = qDomain a
+          | otherwise = qDomain b
+     in Qualified (Id result) domain
+
+iUpsertOne2OneConversation :: UpsertOne2OneConversationRequest -> Galley UpsertOne2OneConversationResponse
+iUpsertOne2OneConversation UpsertOne2OneConversationRequest {..} = do
+  let convId = fromMaybe (one2OneConvId (qUntagged uooLocalUser) (qUntagged uooRemoteUser)) uooConvId
+
+  let dolocal :: Local ConvId -> Galley ()
+      dolocal lconvId = do
+        mbConv <- Data.conversation (tUnqualified lconvId)
+        case mbConv of
+          Nothing -> do
+            let members =
+                  case (uooActor, uooActorDesiredMembership) of
+                    (LocalActor, Included) -> UserList [tUnqualified uooLocalUser] []
+                    (LocalActor, Excluded) -> UserList [] []
+                    (RemoteActor, Included) -> UserList [] [uooRemoteUser]
+                    (RemoteActor, Excluded) -> UserList [] []
+            unless (null members) $
+              Data.createConnectConversationWithRemote lconvId uooLocalUser members
+          Just conv -> do
+            case (uooActor, uooActorDesiredMembership) of
+              (LocalActor, Included) -> do
+                void $ Data.addMember lconvId uooLocalUser
+                unless (null (Data.convRemoteMembers conv)) $
+                  Data.acceptConnect (tUnqualified lconvId)
+              (LocalActor, Excluded) -> Data.removeMember (tUnqualified uooLocalUser) (tUnqualified lconvId)
+              (RemoteActor, Included) -> do
+                void $ Data.addMembers lconvId (UserList [] [uooRemoteUser])
+                unless (null (Data.convLocalMembers conv)) $
+                  Data.acceptConnect (tUnqualified lconvId)
+              (RemoteActor, Excluded) -> Data.removeRemoteMembersFromLocalConv (tUnqualified lconvId) (pure uooRemoteUser)
+      doremote :: Remote ConvId -> Galley ()
+      doremote rconvId =
+        case (uooActor, uooActorDesiredMembership) of
+          (LocalActor, Included) -> do
+            Data.addLocalMembersToRemoteConv (qUntagged rconvId) [tUnqualified uooLocalUser]
+          (LocalActor, Excluded) -> do
+            Data.removeLocalMembersFromRemoteConv (qUntagged rconvId) [tUnqualified uooLocalUser]
+          (RemoteActor, _) -> pure ()
+
+  foldQualified uooLocalUser dolocal doremote convId
+  pure (UpsertOne2OneConversationResponse convId)
diff --git a/services/galley/src/Galley/API/Util.hs b/services/galley/src/Galley/API/Util.hs
index 045588dc954..f12365ae722 100644
--- a/services/galley/src/Galley/API/Util.hs
+++ b/services/galley/src/Galley/API/Util.hs
@@ -99,18 +99,17 @@ ensureConnectedOrSameTeam (Qualified u domain) uids = do
     sameTeamUids <- forM uTeams $ \team ->
       fmap (view userId) <$> Data.teamMembersLimited team uids
     -- Do not check connections for users that are on the same team
-    ensureConnected u (uids \\ join sameTeamUids)
+    ensureConnectedToLocals u (uids \\ join sameTeamUids)
 
 -- | Check that the user is connected to everybody else.
 --
 -- The connection has to be bidirectional (e.g. if A connects to B and later
 -- B blocks A, the status of A-to-B is still 'Accepted' but it doesn't mean
 -- that they are connected).
-ensureConnected :: UserId -> [UserId] -> Galley ()
-ensureConnected _ [] = pure ()
-ensureConnected u localUserIds = do
+ensureConnected :: Local UserId -> UserList UserId -> Galley ()
+ensureConnected self others = do
   -- FUTUREWORK(federation, #1262): check remote connections
-  ensureConnectedToLocals u localUserIds
+  ensureConnectedToLocals (tUnqualified self) (ulLocals others)
 
 ensureConnectedToLocals :: UserId -> [UserId] -> Galley ()
 ensureConnectedToLocals _ [] = pure ()
diff --git a/services/galley/src/Galley/Data.hs b/services/galley/src/Galley/Data.hs
index 55be7a13813..0e3267a7f54 100644
--- a/services/galley/src/Galley/Data.hs
+++ b/services/galley/src/Galley/Data.hs
@@ -68,7 +68,9 @@ module Galley.Data
     conversationMeta,
     conversationsRemote,
     createConnectConversation,
+    createConnectConversationWithRemote,
     createConversation,
+    createLegacyOne2OneConversation,
     createOne2OneConversation,
     createSelfConversation,
     isConvAlive,
@@ -112,7 +114,7 @@ module Galley.Data
     updateClient,
 
     -- * Utilities
-    one2OneConvId,
+    localOne2OneConvId,
     newMember,
 
     -- * Defaults
@@ -156,7 +158,14 @@ import Galley.Types.Clients (Clients)
 import qualified Galley.Types.Clients as Clients
 import Galley.Types.Conversations.Members
 import Galley.Types.Conversations.Roles
-import Galley.Types.Teams hiding (Event, EventType (..), teamConversations, teamMembers)
+import Galley.Types.Teams hiding
+  ( Event,
+    EventType (..),
+    self,
+    teamConversations,
+    teamMembers,
+  )
+import qualified Galley.Types.Teams as Teams
 import Galley.Types.Teams.Intra
 import Galley.Types.UserList
 import Galley.Validation
@@ -435,7 +444,7 @@ updateTeamMember oldPerms tid uid newPerms = do
     when (SetBilling `Set.member` lostPerms) $
       addPrepQuery Cql.deleteBillingTeamMember (tid, uid)
   where
-    permDiff = Set.difference `on` view self
+    permDiff = Set.difference `on` view Teams.self
     acquiredPerms = newPerms `permDiff` oldPerms
     lostPerms = oldPerms `permDiff` newPerms
 
@@ -671,7 +680,7 @@ createConnectConversation ::
   Maybe (Range 1 256 Text) ->
   m Conversation
 createConnectConversation loc a b name = do
-  let conv = one2OneConvId a b
+  let conv = localOne2OneConvId a b
       lconv = qualifyAs loc conv
       a' = Id . U.unpack $ a
   retry x5 $
@@ -681,7 +690,20 @@ createConnectConversation loc a b name = do
   (lmems, rmems) <- addMembers lconv (UserList [a'] [])
   pure $ newConv conv ConnectConv a' lmems rmems [PrivateAccess] privateRole name Nothing Nothing Nothing
 
-createOne2OneConversation ::
+createConnectConversationWithRemote ::
+  MonadClient m =>
+  Local ConvId ->
+  Local UserId ->
+  UserList UserId ->
+  m ()
+createConnectConversationWithRemote lconvId creator m = do
+  retry x5 $
+    write Cql.insertConv (params Quorum (tUnqualified lconvId, ConnectConv, tUnqualified creator, privateOnly, privateRole, Nothing, Nothing, Nothing, Nothing))
+  -- We add only one member, second one gets added later,
+  -- when the other user accepts the connection request.
+  void $ addMembers lconvId m
+
+createLegacyOne2OneConversation ::
   MonadClient m =>
   Local x ->
   U.UUID U.V4 ->
@@ -689,21 +711,36 @@ createOne2OneConversation ::
   Maybe (Range 1 256 Text) ->
   Maybe TeamId ->
   m Conversation
-createOne2OneConversation loc a b name ti = do
-  let conv = one2OneConvId a b
+createLegacyOne2OneConversation loc a b name ti = do
+  let conv = localOne2OneConvId a b
       lconv = qualifyAs loc conv
       a' = Id (U.unpack a)
       b' = Id (U.unpack b)
-  retry x5 $ case ti of
-    Nothing -> write Cql.insertConv (params Quorum (conv, One2OneConv, a', privateOnly, privateRole, fromRange <$> name, Nothing, Nothing, Nothing))
+  createOne2OneConversation
+    lconv
+    (qualifyAs loc a')
+    (qUntagged (qualifyAs loc b'))
+    name
+    ti
+
+createOne2OneConversation ::
+  MonadClient m =>
+  Local ConvId ->
+  Local UserId ->
+  Qualified UserId ->
+  Maybe (Range 1 256 Text) ->
+  Maybe TeamId ->
+  m Conversation
+createOne2OneConversation lconv self other name mtid = do
+  retry x5 $ case mtid of
+    Nothing -> write Cql.insertConv (params Quorum (tUnqualified lconv, One2OneConv, tUnqualified self, privateOnly, privateRole, fromRange <$> name, Nothing, Nothing, Nothing))
     Just tid -> batch $ do
       setType BatchLogged
       setConsistency Quorum
-      addPrepQuery Cql.insertConv (conv, One2OneConv, a', privateOnly, privateRole, fromRange <$> name, Just tid, Nothing, Nothing)
-      addPrepQuery Cql.insertTeamConv (tid, conv, False)
-  -- FUTUREWORK: federated one2one
-  (lmems, rmems) <- addMembers lconv (UserList [a', b'] [])
-  pure $ newConv conv One2OneConv a' lmems rmems [PrivateAccess] privateRole name ti Nothing Nothing
+      addPrepQuery Cql.insertConv (tUnqualified lconv, One2OneConv, tUnqualified self, privateOnly, privateRole, fromRange <$> name, Just tid, Nothing, Nothing)
+      addPrepQuery Cql.insertTeamConv (tid, tUnqualified lconv, False)
+  (lmems, rmems) <- addMembers lconv (toUserList self [qUntagged self, other])
+  pure $ newConv (tUnqualified lconv) One2OneConv (tUnqualified self) lmems rmems [PrivateAccess] privateRole name mtid Nothing Nothing
 
 updateConversation :: MonadClient m => ConvId -> Range 1 256 Text -> m ()
 updateConversation cid name = retry x5 $ write Cql.updateConvName (params Quorum (fromRange name, cid))
@@ -736,8 +773,8 @@ acceptConnect cid = retry x5 $ write Cql.updateConvType (params Quorum (One2OneC
 -- together pairwise, and then setting the version bits (v4) and variant bits
 -- (variant 2). This means that we always know what the UUID is for a
 -- one-to-one conversation which hopefully makes them unique.
-one2OneConvId :: U.UUID U.V4 -> U.UUID U.V4 -> ConvId
-one2OneConvId a b = Id . U.unpack $ U.addv4 a b
+localOne2OneConvId :: U.UUID U.V4 -> U.UUID U.V4 -> ConvId
+localOne2OneConvId a b = Id . U.unpack $ U.addv4 a b
 
 newConv ::
   ConvId ->
@@ -845,7 +882,7 @@ toRemoteMember :: UserId -> Domain -> RoleName -> RemoteMember
 toRemoteMember u d = RemoteMember (toRemoteUnsafe d u)
 
 memberLists ::
-  (MonadClient m, Log.MonadLogger m, MonadThrow m) =>
+  (MonadClient m, MonadThrow m) =>
   [ConvId] ->
   m [[LocalMember]]
 memberLists convs = do
@@ -860,7 +897,7 @@ memberLists convs = do
     mkMem (cnv, usr, srv, prv, st, omus, omur, oar, oarr, hid, hidr, crn) =
       (cnv, toMember (usr, srv, prv, st, omus, omur, oar, oarr, hid, hidr, crn))
 
-members :: (MonadClient m, Log.MonadLogger m, MonadThrow m) => ConvId -> m [LocalMember]
+members :: (MonadClient m, MonadThrow m) => ConvId -> m [LocalMember]
 members conv = join <$> memberLists [conv]
 
 lookupRemoteMembers :: (MonadClient m) => ConvId -> m [RemoteMember]
diff --git a/services/galley/src/Galley/Types/UserList.hs b/services/galley/src/Galley/Types/UserList.hs
index ffcabd6984e..c63148d6b97 100644
--- a/services/galley/src/Galley/Types/UserList.hs
+++ b/services/galley/src/Galley/Types/UserList.hs
@@ -33,6 +33,13 @@ data UserList a = UserList
   }
   deriving (Functor, Foldable, Traversable)
 
+instance Semigroup (UserList a) where
+  UserList locals1 remotes1 <> UserList locals2 remotes2 =
+    UserList (locals1 <> locals2) (remotes1 <> remotes2)
+
+instance Monoid (UserList a) where
+  mempty = UserList mempty mempty
+
 toUserList :: Foldable f => Local x -> f (Qualified a) -> UserList a
 toUserList loc = uncurry UserList . partitionQualified loc
 
diff --git a/services/galley/test/integration/API.hs b/services/galley/test/integration/API.hs
index 7d0b95730bf..5a3b6e8cb3d 100644
--- a/services/galley/test/integration/API.hs
+++ b/services/galley/test/integration/API.hs
@@ -59,9 +59,13 @@ import Data.String.Conversions (cs)
 import qualified Data.Text as T
 import qualified Data.Text.Ascii as Ascii
 import Data.Time.Clock (getCurrentTime)
+import Database.CQL.IO
 import Galley.API.Mapping
+import Galley.API.One2One (one2OneConvId)
+import qualified Galley.Data as Data
 import Galley.Options (Opts, optFederator)
 import Galley.Types hiding (LocalMember (..))
+import Galley.Types.Conversations.Intra
 import Galley.Types.Conversations.Members
 import Galley.Types.Conversations.Roles
 import qualified Galley.Types.Teams as Teams
@@ -215,7 +219,8 @@ tests s =
           test s "convert invite to code-access conversation" postConvertCodeConv,
           test s "convert code to team-access conversation" postConvertTeamConv,
           test s "cannot join private conversation" postJoinConvFail,
-          test s "remove user" removeUser
+          test s "remove user" removeUser,
+          test s "iUpsertOne2OneConversation" testAllOne2OneConversationRequests
         ]
 
 emptyFederatedBrig :: FederatedBrig.Api (AsServerT Handler)
@@ -1573,11 +1578,11 @@ postConnectConvOk2 :: TestM ()
 postConnectConvOk2 = do
   alice <- randomUser
   bob <- randomUser
-  m <- decodeConvId <$> request alice bob
-  n <- decodeConvId <$> request alice bob
+  m <- decodeConvId <$> req alice bob
+  n <- decodeConvId <$> req alice bob
   liftIO $ m @=? n
   where
-    request alice bob =
+    req alice bob =
       postConnectConv alice bob "Alice" "connect with me!" (Just "me@me.com")
 
 putConvAcceptOk :: TestM ()
@@ -2942,3 +2947,54 @@ removeUser = do
           omService = Nothing,
           omConvRoleName = roleNameWireAdmin
         }
+
+testAllOne2OneConversationRequests :: TestM ()
+testAllOne2OneConversationRequests = do
+  for_ [LocalActor, RemoteActor] $ \actor ->
+    for_ [Included, Excluded] $ \desired ->
+      for_ [True, False] $ \shouldBeLocal ->
+        testOne2OneConversationRequest shouldBeLocal actor desired
+
+testOne2OneConversationRequest :: Bool -> Actor -> DesiredMembership -> TestM ()
+testOne2OneConversationRequest shouldBeLocal actor desired = do
+  alice <- qTagUnsafe <$> randomQualifiedUser
+  (bob, expectedConvId) <- generateRemoteAndConvId alice
+  db <- view tsCass
+
+  convId <- do
+    let req = UpsertOne2OneConversationRequest alice bob actor desired Nothing
+    res <-
+      iUpsertOne2OneConversation req
+        <!! statusCode === const 200
+    uuorConvId <$> responseJsonError res
+
+  liftIO $ convId @?= expectedConvId
+
+  case shouldBeLocal of
+    True -> do
+      mems <- runClient db $ do
+        lmems <- fmap (qUntagged . qualifyAs alice . lmId) <$> Data.members (qUnqualified convId)
+        rmems <- fmap (qUntagged . rmId) <$> Data.lookupRemoteMembers (qUnqualified convId)
+        pure (lmems <> rmems)
+      let actorId = case actor of
+            LocalActor -> qUntagged alice
+            RemoteActor -> qUntagged bob
+      liftIO $ isJust (find (actorId ==) mems) @?= (desired == Included)
+      liftIO $ filter (actorId /=) mems @?= []
+    False -> do
+      mems <- runClient db $ do
+        smap <- Data.remoteConversationStatus (tUnqualified alice) [qTagUnsafe convId]
+        case Map.lookup (qTagUnsafe convId) smap of
+          Just _ -> pure [qUntagged alice]
+          _ -> pure []
+      when (actor == LocalActor) $
+        liftIO $ isJust (find (qUntagged alice ==) mems) @?= (desired == Included)
+  where
+    generateRemoteAndConvId :: Local UserId -> TestM (Remote UserId, Qualified ConvId)
+    generateRemoteAndConvId lUserId = do
+      other <- Qualified <$> randomId <*> pure (Domain "far-away.example.com")
+      let convId = one2OneConvId (qUntagged lUserId) other
+          isLocal = tDomain lUserId == qDomain convId
+      if shouldBeLocal == isLocal
+        then pure (qTagUnsafe other, convId)
+        else generateRemoteAndConvId lUserId
diff --git a/services/galley/test/integration/API/Util.hs b/services/galley/test/integration/API/Util.hs
index ec3bbcc83fc..aa6549d3ebf 100644
--- a/services/galley/test/integration/API/Util.hs
+++ b/services/galley/test/integration/API/Util.hs
@@ -71,6 +71,7 @@ import qualified Galley.Options as Opts
 import qualified Galley.Run as Run
 import Galley.Types
 import qualified Galley.Types as Conv
+import Galley.Types.Conversations.Intra (UpsertOne2OneConversationRequest (..))
 import Galley.Types.Conversations.Roles hiding (DeleteConversation)
 import Galley.Types.Teams hiding (Event, EventType (..))
 import qualified Galley.Types.Teams as Team
@@ -594,6 +595,7 @@ postO2OConv u1 u2 n = do
 
 postConnectConv :: UserId -> UserId -> Text -> Text -> Maybe Text -> TestM ResponseLBS
 postConnectConv a b name msg email = do
+  qb <- Qualified <$> pure b <*> viewFederationDomain
   g <- view tsGalley
   post $
     g
@@ -601,7 +603,7 @@ postConnectConv a b name msg email = do
       . zUser a
       . zConn "conn"
       . zType "access"
-      . json (Connect b (Just msg) (Just name) email)
+      . json (Connect qb (Just msg) (Just name) email)
 
 putConvAccept :: UserId -> ConvId -> TestM ResponseLBS
 putConvAccept invited cid = do
@@ -2328,3 +2330,8 @@ fedRequestsForDomain domain component =
 assertOne :: (HasCallStack, MonadIO m, Show a) => [a] -> m a
 assertOne [a] = pure a
 assertOne xs = liftIO . assertFailure $ "Expected exactly one element, found " <> show xs
+
+iUpsertOne2OneConversation :: UpsertOne2OneConversationRequest -> TestM ResponseLBS
+iUpsertOne2OneConversation req = do
+  galley <- view tsGalley
+  post (galley . path "/i/conversations/one2one/upsert" . Bilge.json req)
diff --git a/services/galley/test/unit/Main.hs b/services/galley/test/unit/Main.hs
index c0a1dbcdd31..bfd608d4db7 100644
--- a/services/galley/test/unit/Main.hs
+++ b/services/galley/test/unit/Main.hs
@@ -23,6 +23,7 @@ where
 import Imports
 import qualified Test.Galley.API
 import qualified Test.Galley.API.Message
+import qualified Test.Galley.API.One2One
 import qualified Test.Galley.Intra.User
 import qualified Test.Galley.Mapping
 import qualified Test.Galley.Roundtrip
@@ -34,6 +35,7 @@ main =
     =<< sequence
       [ pure Test.Galley.API.tests,
         pure Test.Galley.API.Message.tests,
+        pure Test.Galley.API.One2One.tests,
         pure Test.Galley.Intra.User.tests,
         pure Test.Galley.Mapping.tests,
         Test.Galley.Roundtrip.tests
diff --git a/services/galley/test/unit/Test/Galley/API/One2One.hs b/services/galley/test/unit/Test/Galley/API/One2One.hs
new file mode 100644
index 00000000000..d3f6f0332fe
--- /dev/null
+++ b/services/galley/test/unit/Test/Galley/API/One2One.hs
@@ -0,0 +1,51 @@
+{-# LANGUAGE NumericUnderscores #-}
+
+-- This file is part of the Wire Server implementation.
+--
+-- Copyright (C) 2021 Wire Swiss GmbH <opensource@wire.com>
+--
+-- This program is free software: you can redistribute it and/or modify it under
+-- the terms of the GNU Affero General Public License as published by the Free
+-- Software Foundation, either version 3 of the License, or (at your option) any
+-- later version.
+--
+-- This program is distributed in the hope that it will be useful, but WITHOUT
+-- ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+-- FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+-- details.
+--
+-- You should have received a copy of the GNU Affero General Public License along
+-- with this program. If not, see <https://www.gnu.org/licenses/>.
+
+-- | Tests for one-to-one conversations
+module Test.Galley.API.One2One where
+
+import Data.Id
+import Data.List.Extra
+import Data.Qualified
+import Galley.API.One2One (one2OneConvId)
+import Imports
+import Test.Tasty
+import Test.Tasty.HUnit (Assertion, testCase, (@?=))
+import Test.Tasty.QuickCheck
+
+tests :: TestTree
+tests =
+  testGroup
+    "one2OneConvId"
+    [ testProperty "symmetry" one2OneConvIdSymmetry,
+      testCase "non-collision" one2OneConvIdNonCollision
+    ]
+
+one2OneConvIdSymmetry :: Qualified UserId -> Qualified UserId -> Property
+one2OneConvIdSymmetry quid1 quid2 = one2OneConvId quid1 quid2 === one2OneConvId quid2 quid1
+
+-- | Make sure that we never get the same conversation ID for a pair of
+-- (assumingly) distinct qualified user IDs
+one2OneConvIdNonCollision :: Assertion
+one2OneConvIdNonCollision = do
+  let len = 10_000
+  -- A generator of lists of length 'len' of qualified user ID pairs
+  let gen = vectorOf len arbitrary
+  quids <- head <$> sample' gen
+  anySame (fmap (uncurry one2OneConvId) quids) @?= False