mirror of https://github.com/oxen-io/session-ios
Loki session reset (#14)
* Added session reset. * Hooked up session reset internals to UI. * Send empty message when we have received an end session message. * Verify incoming PreKeyWhisperMessage. * Fix indentations in SessionReset.mdpull/16/head
Mikunj Varsani
6 years ago
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GitHub
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fb1e27d633
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Subproject commit 7f42f93c7df8127331d26d0109170a0524f67f7b
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Subproject commit 870d1b5be23fd8fb5d68af6c20e36b3ed5dcde0f
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# Loki Session Reset
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## Signal
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Since Signal uses a centralised server, creating sessions is easy as the prekeys can be easily fetched.
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The process is as follows:
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1. `A` deletes all their sessions and sends `End Session` to `B`
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- `A` contacts the server and creates a new session
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2. `B` Gets this message and deletes all sessions.
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3. `B` Sends a message with a newly created session
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- `B` contacted server and established this
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4. `A` and `B` now have the same sessions so they can delete any archived ones.
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## Loki
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Loki doesn't have a centralised server and thus we need to change the process above with something similar.
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We have to introduce a session reset state `sessionState` which can take the following states:
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- `none`: No session reset is in progress
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- `initiated`: We have initiated the session reset
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- `received`: We have received a session reset from the other user
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The new process is as follows:
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1. `A` Sends `End Session` with a `PreKeyBundle` and archives its own session.
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- `sessionState = initiated`
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- The session is archived as we could get a message from `B` using the archived session, so we still want to be able to decrypt that.
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- We can show `Session reset in progress`
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2. `B` Gets this message and saves the `PreKeyBundle` and archives its own sessions.
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- `sessionState = received`
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- `B` sends an empty message, which will trigger a new session to be created.
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- `B` deletes the `PreKeyBundle` once session is created.
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- We can show `Session reset in progress`
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3. `A` and `B` both do the routine below when receiving messages.
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### Upon receiving message (Only applies to PreKey and Cipher messages)
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- Store the current active session `PS`
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- Decrypt the message
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- Decrypting a message can cause the active session to change
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- If `sessionState == none` then it means that we haven't started session reset and we can abort.
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- Get the current session `CS`
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- If `PS` is `nil` then abort as we didn't have a session before.
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- If `CS != PS` then sessions were changed.
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- If `sessionState == received` then it means that the sender used an old session to contact us. We need to wait for them to use the new one.
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- Archive `CS` and set the session to `PS`
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- If `sessionState == initiated` then it means that the sender acknowledged our session reset and sent a message with a new session
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- Delete all session except `CS`
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- `sessionState = none`
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- Send an empty message to confirm session adoption
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- We can show `Session reset done`
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- If `CS == PS` then sessions were the same.
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- If `sessionState == received` then it means that the new session we created is the one the sender used for sending message. We have successfully adopted the new session.
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- Delete all sessions except `PS`
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- `sessionState = none`
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- We can show `Session reset done`
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@ -0,0 +1,25 @@
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/// Loki: Refer to Docs/SessionReset.md for explanations
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#import "SessionCipher.h"
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NS_ASSUME_NONNULL_BEGIN
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extern NSString *const kNSNotificationName_SessionAdopted;
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extern NSString *const kNSNotificationKey_ContactPubKey;
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@interface SessionCipher (Loki)
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/**
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Decrypt the given `CipherMessage`.
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This function is a wrapper around `throws_decrypt:protocolContext:` and adds on the custom loki session handling ontop.
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Refer to SignalServiceKit/Loki/Docs/SessionReset.md for overview on how it works.
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@param whisperMessage The cipher message.
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@param protocolContext The protocol context (YapDatabaseReadWriteTransaction)
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@return The decrypted data.
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*/
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- (NSData *)throws_lokiDecrypt:(id<CipherMessage>)whisperMessage protocolContext:(nullable id)protocolContext NS_SWIFT_UNAVAILABLE("throws objc exceptions");
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@end
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NS_ASSUME_NONNULL_END
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@ -0,0 +1,181 @@
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/// Loki: Refer to Docs/SessionReset.md for explanations
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#import "SessionCipher+Loki.h"
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#import "NSNotificationCenter+OWS.h"
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#import "PreKeyWhisperMessage.h"
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#import "OWSPrimaryStorage+Loki.h"
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#import "TSContactThread.h"
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#import <YapDatabase/YapDatabase.h>
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NSString *const kNSNotificationName_SessionAdopted = @"kNSNotificationName_SessionAdopted";
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NSString *const kNSNotificationKey_ContactPubKey = @"kNSNotificationKey_ContactPubKey";
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@interface SessionCipher ()
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@property (nonatomic, readonly) NSString *recipientId;
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@property (nonatomic, readonly) int deviceId;
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@property (nonatomic, readonly) id<SessionStore> sessionStore;
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@property (nonatomic, readonly) id<PreKeyStore> prekeyStore;
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@end
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@implementation SessionCipher (Loki)
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- (NSData *)throws_lokiDecrypt:(id<CipherMessage>)whisperMessage protocolContext:(nullable id)protocolContext
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{
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// Our state before we decrypt the message
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SessionState *_Nullable state = [self getCurrentState:protocolContext];
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// While decrypting our state may change internally
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NSData *plainText = [self throws_decrypt:whisperMessage protocolContext:protocolContext];
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// Loki: Verify incoming friend request messages
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if (!state) {
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[self throws_verifyFriendRequestAcceptPreKeyForMessage:whisperMessage protocolContext:protocolContext];
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}
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// Loki: Handle any session resets
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[self handleSessionReset:whisperMessage previousState:state protocolContext:protocolContext];
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return plainText;
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}
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/// Get the current session state
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- (SessionState *_Nullable)getCurrentState:(nullable id)protocolContext {
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SessionRecord *record = [self.sessionStore loadSession:self.recipientId deviceId:self.deviceId protocolContext:protocolContext];
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SessionState *state = record.sessionState;
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// Check if session is initialized
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if (!state.hasSenderChain) {
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return nil;
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}
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return state;
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}
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/// Handle any loki session reset stuff
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- (void)handleSessionReset:(id<CipherMessage>)whisperMessage
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previousState:(SessionState *_Nullable)previousState
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protocolContext:(nullable id)protocolContext
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{
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// Don't bother doing anything if we didn't have a session before
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if (!previousState) {
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return;
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}
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OWSAssertDebug([protocolContext isKindOfClass:[YapDatabaseReadWriteTransaction class]]);
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YapDatabaseReadWriteTransaction *transaction = protocolContext;
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// Get the thread
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TSContactThread *thread = [TSContactThread getThreadWithContactId:self.recipientId transaction:transaction];
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if (!thread) {
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return;
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}
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// Bail early if no session reset is in progress
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if (thread.sessionResetState == TSContactThreadSessionResetStateNone) {
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return;
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}
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BOOL sessionResetReceived = thread.sessionResetState == TSContactThreadSessionResetStateRequestReceived;
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SessionState *_Nullable currentState = [self getCurrentState:protocolContext];
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// Check if our previous state and our current state differ
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if (!currentState || ![currentState.aliceBaseKey isEqualToData:previousState.aliceBaseKey]) {
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if (sessionResetReceived) {
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// The other user used an old session to contact us.
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// Wait for them to use a new one
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[self restoreSession:previousState protocolContext:protocolContext];
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} else {
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// Our session reset went through successfully
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// We had initiated a session reset and got a different session back from the user
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[self deleteAllSessionsExcept:currentState protocolContext:protocolContext];
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[self notifySessionAdopted];
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}
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} else if (sessionResetReceived) {
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// Our session reset went through successfully
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// We got a message with the same session from the other user
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[self deleteAllSessionsExcept:previousState protocolContext:protocolContext];
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[self notifySessionAdopted];
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}
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}
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/// Send a notification about a new session being adopted
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- (void)notifySessionAdopted
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{
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[[NSNotificationCenter defaultCenter]
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postNotificationNameAsync:kNSNotificationName_SessionAdopted
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object:nil
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userInfo:@{
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kNSNotificationKey_ContactPubKey : self.recipientId,
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}];
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}
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/// Delete all other sessions except the given one
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- (void)deleteAllSessionsExcept:(SessionState *)state protocolContext:(nullable id)protocolContext
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{
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SessionRecord *record = [self.sessionStore loadSession:self.recipientId deviceId:self.deviceId protocolContext:protocolContext];
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[record removePreviousSessionStates];
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[record setState:state];
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[self.sessionStore storeSession:self.recipientId
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deviceId:self.deviceId
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session:record
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protocolContext:protocolContext];
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}
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/// Set the given session as the active one while archiving the old one
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- (void)restoreSession:(SessionState *)state protocolContext:(nullable id)protocolContext
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{
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SessionRecord *record = [self.sessionStore loadSession:self.recipientId deviceId:self.deviceId protocolContext:protocolContext];
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// Remove the state from previous session states
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[record.previousSessionStates enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(SessionState *obj, NSUInteger idx, BOOL *stop) {
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if ([state.aliceBaseKey isEqualToData:obj.aliceBaseKey]) {
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[record.previousSessionStates removeObjectAtIndex:idx];
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*stop = true;
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}
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}];
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// Promote it so the previous state gets archived
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[record promoteState:state];
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[self.sessionStore storeSession:self.recipientId
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deviceId:self.deviceId
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session:record
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protocolContext:protocolContext];
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}
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/// Check that we have matching prekeys in the case of a `PreKeyWhisperMessage`
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/// This is so that we don't trigger a false friend request accept on unknown contacts
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- (void)throws_verifyFriendRequestAcceptPreKeyForMessage:(id<CipherMessage>)whisperMessage protocolContext:(nullable id)protocolContext {
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OWSAssertDebug([protocolContext isKindOfClass:[YapDatabaseReadTransaction class]]);
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YapDatabaseReadTransaction *transaction = protocolContext;
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/// We only want to look at `PreKeyWhisperMessage`
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if (![whisperMessage isKindOfClass:[PreKeyWhisperMessage class]]) {
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return;
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}
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/// We need the primary storage to access contact prekeys
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if (![self.prekeyStore isKindOfClass:[OWSPrimaryStorage class]]) {
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return;
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}
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PreKeyWhisperMessage *preKeyMessage = whisperMessage;
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OWSPrimaryStorage *primaryStorage = self.prekeyStore;
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PreKeyRecord *_Nullable storedPreKey = [primaryStorage getPreKeyForContact:self.recipientId transaction:transaction];
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if(!storedPreKey) {
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OWSRaiseException(@"LokiInvalidPreKey", @"Received a friend request from a pubkey for which no prekey bundle was created");
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}
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if (storedPreKey.Id != preKeyMessage.prekeyID) {
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OWSRaiseException(@"LokiPreKeyIdsDontMatch", @"Received a preKeyWhisperMessage (friend request accept) from an unknown source");
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}
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}
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@end
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