diff --git a/internal/contentenc/offsets.go b/internal/contentenc/offsets.go
index 97d7116..5ad0da4 100644
--- a/internal/contentenc/offsets.go
+++ b/internal/contentenc/offsets.go
@@ -54,6 +54,11 @@ func (be *ContentEnc) CipherSizeToPlainSize(cipherSize uint64) uint64 {
 
 	overhead := be.BlockOverhead()*blockCount + HeaderLen
 
+	if overhead > cipherSize {
+		tlog.Warn.Printf("cipherSize %d < overhead %d: corrupt file\n", cipherSize, overhead)
+		return 0
+	}
+
 	return cipherSize - overhead
 }