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-rw-r--r--internal/contentenc/content.go24
1 files changed, 18 insertions, 6 deletions
diff --git a/internal/contentenc/content.go b/internal/contentenc/content.go
index e841ad0..f48080f 100644
--- a/internal/contentenc/content.go
+++ b/internal/contentenc/content.go
@@ -121,6 +121,22 @@ func (be *ContentEnc) DecryptBlocks(ciphertext []byte, firstBlockNo uint64, file
return pBuf.Bytes(), err
}
+// concatAD concatenates the block number and the file ID to a byte blob
+// that can be passed to AES-GCM as associated data (AD).
+// Result is: aData = blockNo.bigEndian + fileID.
+func concatAD(blockNo uint64, fileID []byte) (aData []byte) {
+ if fileID != nil && len(fileID) != headerIDLen {
+ // fileID is nil when decrypting the master key from the config file
+ log.Panicf("wrong fileID length: %d", len(fileID))
+ }
+ const lenUint64 = 8
+ // Preallocate space to save an allocation in append()
+ aData = make([]byte, lenUint64, lenUint64+headerIDLen)
+ binary.BigEndian.PutUint64(aData, blockNo)
+ aData = append(aData, fileID...)
+ return aData
+}
+
// DecryptBlock - Verify and decrypt GCM block
//
// Corner case: A full-sized block of all-zero ciphertext bytes is translated
@@ -157,9 +173,7 @@ func (be *ContentEnc) DecryptBlock(ciphertext []byte, blockNo uint64, fileID []b
// Decrypt
plaintext := be.pBlockPool.Get()
plaintext = plaintext[:0]
- aData := make([]byte, 8)
- aData = append(aData, fileID...)
- binary.BigEndian.PutUint64(aData, blockNo)
+ aData := concatAD(blockNo, fileID)
plaintext, err := be.cryptoCore.AEADCipher.Open(plaintext, nonce, ciphertext, aData)
if err != nil {
@@ -257,9 +271,7 @@ func (be *ContentEnc) doEncryptBlock(plaintext []byte, blockNo uint64, fileID []
log.Panic("wrong nonce length")
}
// Block is authenticated with block number and file ID
- aData := make([]byte, 8)
- binary.BigEndian.PutUint64(aData, blockNo)
- aData = append(aData, fileID...)
+ aData := concatAD(blockNo, fileID)
// Get a cipherBS-sized block of memory, copy the nonce into it and truncate to
// nonce length
cBlock := be.cBlockPool.Get()