attempt to process candidate objects in scan mode

diff --git a/deltatar/crypto.py b/deltatar/crypto.py
index c0896bb..268bbbf 100755 (executable)
--- a/deltatar/crypto.py
+++ b/deltatar/crypto.py
@@ -542,6 +542,64 @@ def locate_hdr_candidates (fd):
     return cands
 
 
+HDR_CAND_GOOD       = 0 # header marks begin of valid object
+HDR_CAND_FISHY      = 1 # inconclusive (tag mismatch, obj overlap etc.)
+HDR_CAND_JUNK       = 2 # not a header / object unreadable
+
+
+def inspect_hdr (fd, off):
+    """
+    Attempt to parse a header in *fd* at position *off*.
+
+    Returns a verdict about the quality of that header plus the parsed header
+    when readable.
+    """
+
+    _ = os.lseek (fd, off, os.SEEK_SET)
+
+    if os.lseek (fd, 0, os.SEEK_CUR) != off:
+        if PDTCRYPT_VERBOSE is True:
+            noise ("PDT: %d → dismissed (lseek() past EOF)" % off)
+        return HDR_CAND_JUNK, None
+
+    raw = os.read (fd, PDTCRYPT_HDR_SIZE)
+    if len (raw) != PDTCRYPT_HDR_SIZE:
+        if PDTCRYPT_VERBOSE is True:
+            noise ("PDT: %d → dismissed (EOF inside header)" % off)
+        return HDR_CAND_JUNK, None
+
+    try:
+        hdr = hdr_read (raw)
+    except InvalidHeader as exn:
+        if PDTCRYPT_VERBOSE is True:
+            noise ("PDT: %d → dismissed (invalid: [%s])" % (off, str (exn)))
+        return HDR_CAND_JUNK, None
+
+    obj0 = off + PDTCRYPT_HDR_SIZE
+    objX = obj0 + hdr ["ctsize"]
+
+    eof  = os.lseek (fd, 0, os.SEEK_END)
+    if eof < objX:
+        if PDTCRYPT_VERBOSE is True:
+            noise ("PDT: %d → EOF inside object (%d≤%d≤%d); adjusting size to "
+                   "%d" % (off, obj0, eof, objX, (eof - obj0)))
+        # try reading up to the end
+        hdr ["ctsize"] = eof - obj0
+        return HDR_CAND_FISHY, hdr
+
+    return HDR_CAND_GOOD, hdr
+
+
+def try_decrypt (fd, off, hdr, fname=None):
+    """
+    Attempt to decrypt the object in the (seekable) descriptor *fd* starting at
+    *off* using the metadata in *hdr*. An output file can be specified with
+    *fname*; if it is *None*, the decrypted payload will be discarded.
+    """
+
+    raise NotImplementedError
+
+
 ###############################################################################
 ## passthrough / null encryption
 ###############################################################################
@@ -1669,9 +1727,38 @@ def mode_scan (pw, fname, nacl=None):
         if PDTCRYPT_VERBOSE is True:
             noise ("PDT: scan complete: found %d candidates:" % len (cands))
             noise_output_candidates (cands)
-    finally:
+    except:
         os.close (fd)
+        raise
 
+    junk, todo = [], []
+    try:
+        for cand in cands:
+            vdt, hdr = inspect_hdr (fd, cand)
+            if vdt == HDR_CAND_JUNK:
+                junk.append (cand)
+            else:
+                off0 = cand + PDTCRYPT_HDR_SIZE
+                if PDTCRYPT_VERBOSE is True:
+                    noise ("PDT: read payload @%d: %s" % (off0, hdr_fmt (hdr)))
+                vdtt = try_decrypt (fd, off0, hdr)
+
+                if vdt == HDR_CAND_GOOD and vdtt == HDR_CAND_GOOD:
+                    noise ("PDT: %d → ✓ valid object %d–%d"
+                           % (cand, off0, off0 + hdr ["ctsize"]))
+                elif vdt == HDR_CAND_FISHY and vdtt == HDR_CAND_GOOD:
+                    noise ("PDT: %d → × object %d–%d, corrupt header"
+                           % (cand, off0, off0 + hdr ["ctsize"]))
+                elif vdt == HDR_CAND_GOOD and vdtt == HDR_CAND_FISHY:
+                    noise ("PDT: %d → × object %d–%d, problematic payload"
+                           % (cand, off0, off0 + hdr ["ctsize"]))
+                elif vdt == HDR_CAND_FISHY and vdtt == HDR_CAND_FISHY:
+                    noise ("PDT: %d → × object %d–%d, corrupt header, problematic "
+                           "ciphertext" % (cand, off0, off0 + hdr ["ctsize"]))
+                else:
+                    raise Unreachable
+    finally:
+        os.close (fd)
 
 def usage (err=False):
     out = print