diff --git a/Signal/src/Jobs/MessageFetcherJob.swift b/Signal/src/Jobs/MessageFetcherJob.swift
index 432f6fe90..39457188b 100644
--- a/Signal/src/Jobs/MessageFetcherJob.swift
+++ b/Signal/src/Jobs/MessageFetcherJob.swift
@@ -201,10 +201,10 @@ public class MessageFetcherJob: NSObject {
 
     private func acknowledgeDelivery(envelope: SSKProtoEnvelope) {
         let request: TSRequest
-        if let source = envelope.source {
-            request = OWSRequestFactory.acknowledgeMessageDeliveryRequest(withSource: source, timestamp: envelope.timestamp)
-        } else if let serverGuid = envelope.serverGuid, envelope.hasServerTimestamp {
+        if let serverGuid = envelope.serverGuid, envelope.hasServerTimestamp {
             request = OWSRequestFactory.acknowledgeMessageDeliveryRequest(withServerGuid: serverGuid, serverTimestamp: envelope.serverTimestamp)
+        } else if let source = envelope.source {
+            request = OWSRequestFactory.acknowledgeMessageDeliveryRequest(withSource: source, timestamp: envelope.timestamp)
         } else {
             owsFailDebug("Cannot ACK message which has neither source, nor server GUID and timestamp.")
             return
diff --git a/SignalServiceKit/src/Messages/OWSMessageDecrypter.m b/SignalServiceKit/src/Messages/OWSMessageDecrypter.m
index 85b16a61a..772f47d31 100644
--- a/SignalServiceKit/src/Messages/OWSMessageDecrypter.m
+++ b/SignalServiceKit/src/Messages/OWSMessageDecrypter.m
@@ -4,6 +4,7 @@
 
 #import "OWSMessageDecrypter.h"
 #import "NSData+messagePadding.h"
+#import "NSString+SSK.h"
 #import "NotificationsProtocol.h"
 #import "OWSAnalytics.h"
 #import "OWSBlockingManager.h"
@@ -151,6 +152,12 @@ NSError *EnsureDecryptError(NSError *_Nullable error, NSString *fallbackErrorDes
 
     DecryptSuccessBlock successBlock = ^(
         OWSMessageDecryptResult *result, YapDatabaseReadWriteTransaction *transaction) {
+        // Ensure all blocked messages are discarded.
+        if ([self isEnvelopeSenderBlocked:envelope]) {
+            OWSLogInfo(@"ignoring blocked envelope: %@", envelope.source);
+            return failureBlock();
+        }
+
         // Having received a valid (decryptable) message from this user,
         // make note of the fact that they have a valid Signal account.
         [SignalRecipient markRecipientAsRegistered:result.source deviceId:result.sourceDevice transaction:transaction];
@@ -161,13 +168,20 @@ NSError *EnsureDecryptError(NSError *_Nullable error, NSString *fallbackErrorDes
     @try {
         OWSLogInfo(@"decrypting envelope: %@", [self descriptionForEnvelope:envelope]);
 
-        // We block UD messages later, after they are decrypted.
         if (envelope.type != SSKProtoEnvelopeTypeUnidentifiedSender) {
-            OWSAssertDebug(envelope.source.length > 0);
+            if (!envelope.hasSource || envelope.source.length < 1 || !envelope.source.isValidE164) {
+                OWSFailDebug(@"incoming envelope has invalid source");
+                return failureBlock();
+            }
+            if (!envelope.hasSourceDevice || envelope.sourceDevice < 1) {
+                OWSFailDebug(@"incoming envelope has invalid source device");
+                return failureBlock();
+            }
+
+            // We block UD messages later, after they are decrypted.
             if ([self isEnvelopeSenderBlocked:envelope]) {
                 OWSLogInfo(@"ignoring blocked envelope: %@", envelope.source);
-                failureBlock();
-                return;
+                return failureBlock();
             }
         }
 
@@ -421,20 +435,13 @@ NSError *EnsureDecryptError(NSError *_Nullable error, NSString *fallbackErrorDes
             }
 
             NSString *source = decryptResult.senderRecipientId;
-            if (source.length < 1) {
+            if (source.length < 1 || !source.isValidE164) {
                 NSString *errorDescription = @"Invalid UD sender.";
                 OWSFailDebug(@"%@", errorDescription);
                 NSError *error = OWSErrorWithCodeDescription(OWSErrorCodeFailedToDecryptUDMessage, errorDescription);
                 return failureBlock(error);
             }
 
-            if ([self.blockingManager.blockedPhoneNumbers containsObject:source]) {
-                OWSLogInfo(@"ignoring blocked UD envelope: %@", envelope.source);
-                NSError *error = OWSErrorWithCodeDescription(
-                    OWSErrorCodeFailedToDecryptUDMessage, @"ignoring blocked UD envelope");
-                return failureBlock(error);
-            }
-
             long sourceDeviceId = decryptResult.senderDeviceId;
             if (sourceDeviceId < 1 || sourceDeviceId > UINT32_MAX) {
                 NSString *errorDescription = @"Invalid UD sender device id.";
@@ -478,7 +485,7 @@ NSError *EnsureDecryptError(NSError *_Nullable error, NSString *fallbackErrorDes
     OWSAssert(trustRootData);
     OWSAssert(trustRootData.length == ECCKeyLength + 1);
     NSError *error;
-    ECPublicKey *_Nullable trustRoot = [[ECPublicKey alloc] initWithKeyData:[trustRootData removeKeyType] error:&error];
+    ECPublicKey *_Nullable trustRoot = [[ECPublicKey alloc] initWithSerializedKeyData:trustRootData error:&error];
     if (error || !trustRoot) {
         // This exits.
         OWSFail(@"Invalid UD trust root.");
diff --git a/SignalServiceKit/src/Messages/OWSMessageManager.m b/SignalServiceKit/src/Messages/OWSMessageManager.m
index 74d597059..91091efd4 100644
--- a/SignalServiceKit/src/Messages/OWSMessageManager.m
+++ b/SignalServiceKit/src/Messages/OWSMessageManager.m
@@ -208,12 +208,15 @@ NS_ASSUME_NONNULL_BEGIN
 
     OWSLogInfo(@"handling decrypted envelope: %@", [self descriptionForEnvelope:envelope]);
 
-    if (!envelope.source.isValidE164) {
+    if (!envelope.hasSource || envelope.source.length < 1 || !envelope.source.isValidE164) {
         OWSFailDebug(@"incoming envelope has invalid source");
         return;
     }
+    if (!envelope.hasSourceDevice || envelope.sourceDevice < 1) {
+        OWSFailDebug(@"incoming envelope has invalid source device");
+        return;
+    }
 
-    OWSAssertDebug(envelope.source.length > 0);
     OWSAssertDebug(![self isEnvelopeSenderBlocked:envelope]);
 
     switch (envelope.type) {
diff --git a/SignalServiceKit/src/Messages/OWSMessageSend.swift b/SignalServiceKit/src/Messages/OWSMessageSend.swift
index a913cfa4c..a8e4c6bb4 100644
--- a/SignalServiceKit/src/Messages/OWSMessageSend.swift
+++ b/SignalServiceKit/src/Messages/OWSMessageSend.swift
@@ -5,6 +5,10 @@
 import Foundation
 import SignalMetadataKit
 
+// Corresponds to a single effort to send a message to a given recipient,
+// which may span multiple attempts.  Note that group messages may be sent
+// to multiple recipients and therefore require multiple instances of
+// OWSMessageSend.
 @objc
 public class OWSMessageSend: NSObject {
     @objc
@@ -17,7 +21,6 @@ public class OWSMessageSend: NSObject {
     @objc
     public let recipient: SignalRecipient
 
-    // TODO: Should this be per-recipient or per-message?
     private static let kMaxRetriesPerRecipient: Int = 3
 
     @objc