diff --git a/SignalServiceKit/src/Messages/Attachments/OWSAttachmentsProcessor.m b/SignalServiceKit/src/Messages/Attachments/OWSAttachmentsProcessor.m
index b7962e9fc..0654b2d66 100644
--- a/SignalServiceKit/src/Messages/Attachments/OWSAttachmentsProcessor.m
+++ b/SignalServiceKit/src/Messages/Attachments/OWSAttachmentsProcessor.m
@@ -250,6 +250,7 @@ static const CGFloat kAttachmentDownloadProgressTheta = 0.001f;
     NSData *plaintext = [Cryptography decryptAttachment:cipherText
                                                 withKey:attachment.encryptionKey
                                                  digest:attachment.digest
+                                           unpaddedSize:attachment.byteCount
                                                   error:&decryptError];
 
     if (decryptError) {
diff --git a/SignalServiceKit/src/Util/Cryptography.h b/SignalServiceKit/src/Util/Cryptography.h
index 9a35081e8..f0369969d 100755
--- a/SignalServiceKit/src/Util/Cryptography.h
+++ b/SignalServiceKit/src/Util/Cryptography.h
@@ -59,6 +59,7 @@ typedef NS_ENUM(NSInteger, TSMACType) {
 + (NSData *)decryptAttachment:(NSData *)dataToDecrypt
                       withKey:(NSData *)key
                        digest:(nullable NSData *)digest
+                 unpaddedSize:(UInt32)unpaddedSize
                         error:(NSError **)error;
 
 + (NSData *)encryptAttachmentData:(NSData *)attachmentData
diff --git a/SignalServiceKit/src/Util/Cryptography.m b/SignalServiceKit/src/Util/Cryptography.m
index 0ec59232b..cf4b7586f 100755
--- a/SignalServiceKit/src/Util/Cryptography.m
+++ b/SignalServiceKit/src/Util/Cryptography.m
@@ -297,6 +297,7 @@ const NSUInteger kAES256_KeyByteLength = 32;
 + (NSData *)decryptAttachment:(NSData *)dataToDecrypt
                       withKey:(NSData *)key
                        digest:(nullable NSData *)digest
+                 unpaddedSize:(UInt32)unpaddedSize
                         error:(NSError **)error;
 {
     if (digest.length <= 0) {
@@ -328,14 +329,51 @@ const NSUInteger kAES256_KeyByteLength = 32;
     NSData *hmac = [dataToDecrypt
         subdataWithRange:NSMakeRange([dataToDecrypt length] - HMAC256_OUTPUT_LENGTH, HMAC256_OUTPUT_LENGTH)];
 
-    return [Cryptography decryptCBCMode:encryptedAttachment
-                                    key:encryptionKey
-                                     IV:iv
-                                version:nil
-                                HMACKey:hmacKey
-                               HMACType:TSHMACSHA256AttachementType
-                           matchingHMAC:hmac
-                                 digest:digest];
+    NSData *paddedPlainText = [Cryptography decryptCBCMode:encryptedAttachment
+                                                       key:encryptionKey
+                                                        IV:iv
+                                                   version:nil
+                                                   HMACKey:hmacKey
+                                                  HMACType:TSHMACSHA256AttachementType
+                                              matchingHMAC:hmac
+                                                    digest:digest];
+    if (unpaddedSize == 0) {
+        // Work around for legacy iOS client's which weren't setting padding size.
+        // Since we know those clients pre-date attachment padding we return the entire data.
+        DDLogWarn(@"%@ Decrypted attachment with unspecified size.", self.tag);
+        return paddedPlainText;
+    } else {
+        if (unpaddedSize > paddedPlainText.length) {
+            *error = OWSErrorWithCodeDescription(
+                OWSErrorCodeFailedToDecryptMessage, NSLocalizedString(@"ERROR_MESSAGE_INVALID_MESSAGE", @""));
+            return nil;
+        }
+
+        if (unpaddedSize == paddedPlainText.length) {
+            DDLogInfo(@"%@ decrypted unpadded attachment.", self.tag);
+            return [paddedPlainText copy];
+        } else {
+            unsigned long paddingSize = paddedPlainText.length - unpaddedSize;
+            DDLogInfo(@"%@ decrypted padded attachment with unpaddedSize: %u, paddingSize: %lu",
+                self.tag,
+                unpaddedSize,
+                paddingSize);
+            return [paddedPlainText subdataWithRange:NSMakeRange(0, unpaddedSize)];
+        }
+    }
+}
+
++ (unsigned long)paddedSize:(unsigned long)unpaddedSize
+{
+    // Don't enable this until clients are sufficiently rolled out.
+    BOOL shouldPad = NO;
+    if (shouldPad) {
+        // Note: This just rounds up to the nearsest power of two,
+        // but the actual padding scheme is TBD
+        return pow(2, ceil( log2( unpaddedSize )));
+    } else {
+        return unpaddedSize;
+    }
 }
 
 + (NSData *)encryptAttachmentData:(NSData *)attachmentData
@@ -352,8 +390,13 @@ const NSUInteger kAES256_KeyByteLength = 32;
     [attachmentKey appendData:hmacKey];
     *outKey = [attachmentKey copy];
 
+    // Apply any padding
+    unsigned long desiredSize = [self paddedSize:attachmentData.length];
+    NSMutableData *paddedAttachmentData = [attachmentData mutableCopy];
+    paddedAttachmentData.length = desiredSize;
+
     // Encrypt
-    size_t bufferSize = [attachmentData length] + kCCBlockSizeAES128;
+    size_t bufferSize = [paddedAttachmentData length] + kCCBlockSizeAES128;
     void *buffer = malloc(bufferSize);
 
     if (buffer == NULL) {
@@ -368,8 +411,8 @@ const NSUInteger kAES256_KeyByteLength = 32;
                                           [encryptionKey bytes],
                                           [encryptionKey length],
                                           [iv bytes],
-                                          [attachmentData bytes],
-                                          [attachmentData length],
+                                          [paddedAttachmentData bytes],
+                                          [paddedAttachmentData length],
                                           buffer,
                                           bufferSize,
                                           &bytesEncrypted);
@@ -382,22 +425,22 @@ const NSUInteger kAES256_KeyByteLength = 32;
 
     NSData *cipherText = [NSData dataWithBytesNoCopy:buffer length:bytesEncrypted freeWhenDone:YES];
 
-    NSMutableData *encryptedAttachmentData = [NSMutableData data];
-    [encryptedAttachmentData appendData:iv];
-    [encryptedAttachmentData appendData:cipherText];
+    NSMutableData *encryptedPaddedData = [NSMutableData data];
+    [encryptedPaddedData appendData:iv];
+    [encryptedPaddedData appendData:cipherText];
 
     // compute hmac of: iv || encrypted data
     NSData *hmac =
-        [Cryptography truncatedSHA256HMAC:encryptedAttachmentData withHMACKey:hmacKey truncation:HMAC256_OUTPUT_LENGTH];
+        [Cryptography truncatedSHA256HMAC:encryptedPaddedData withHMACKey:hmacKey truncation:HMAC256_OUTPUT_LENGTH];
     DDLogVerbose(@"%@ computed hmac: %@", self.tag, hmac);
 
-    [encryptedAttachmentData appendData:hmac];
+    [encryptedPaddedData appendData:hmac];
 
     // compute digest of: iv || encrypted data || hmac
-    *outDigest = [self computeSHA256Digest:encryptedAttachmentData];
+    *outDigest = [self computeSHA256Digest:encryptedPaddedData];
     DDLogVerbose(@"%@ computed digest: %@", self.tag, *outDigest);
 
-    return [encryptedAttachmentData copy];
+    return [encryptedPaddedData copy];
 }
 
 + (nullable NSData *)encryptAESGCMWithData:(NSData *)plaintext key:(OWSAES256Key *)key