What Is a BKS File? How to Open Bouncy Castle Keystores

2 Which Kind of BKS File Do You Have? Read the first eight bytes, then check the folder

BKS files can be tiny or big, so read only the start. The short script below does that and prints one line. Save it as check_bks.py and replace keystore.bks with your BKS file name.

import struct, sys
data = open(sys.argv[1], "rb").read()
head = data[:8]
if head[:4] == b"\xfe\xed\xfe\xed":
    print("JKS: a Java KeyStore, not a BKS file")
elif head[:2] == b"\x30\x82":
    print("DER data: PKCS12 or BCFKS, not a BKS file")
elif head[:3] == b"\0\0\0" and head[3] in (1, 2) and head[4:] == b"\0\0\0\x14":
    count = struct.unpack(">i", data[28:32])[0]
    print("Version", head[3], "| iterations", count, "| size", len(data), "bytes")
    if head[3] == 1:
        print("Version 1: an older BKS file or a UBER file. The start does not show which.")
elif head[:2] in (b"\xff\xfe", b"\xfe\xff") or head[:3] == b"\xef\xbb\xbf":
    print("Text with a byte order mark: maybe an NTBackup selection list")
else:
    print("First bytes", head.hex(" "), "- not a keystore")

python check_bks.py keystore.bks
Version 2 | iterations 1992 | size 2919 bytes

We ran it on all 18 test files, and it named every kind correctly. It reads only the start, so a cut or changed BKS file still reads as version 2. The table shows the starts it knows.

Three more clues besides the bytes

3 What Is Inside a BKS File? A header, typed entries and a checksum

Think of a binder with labeled pockets. Labels are aliases, and pockets store certificates or locked keys. Anyone can read the labels. Only the password opens the locks.

The 32-byte header

The salt and the count feed the password check. Every BKS file gets a new salt, so two files with the same password still look different.

The entries

After the header comes one entry after another. Every entry starts with a type byte. We saw three types in our 18 files.

Certificates are stored as DER bytes, not as PEM text, so a search for BEGIN CERTIFICATE finds nothing. The alias and the text X.509 do show in a plain text viewer. The private key bytes are encrypted.

BKS files with only type 01 entries work as truststores, like our 127-certificate bundle. Files with type 04 entries are keystores in the full sense, because they store keys.

00 00 00 02 00 00 00 14                           version 2, salt length 20
67 24 ed c9 92 55 5c 3f 85 f3 1c 59 36 1b a1 fa   salt, first 16 of 20 bytes
87 a6 15 89                                       salt, last 4 bytes
00 00 06 d4                                       1,748 iterations
00                                                end marker, no entries
a6 76 12 c5 b0 a9 d2 ce f0 50 d4 1b 2e db a4 19   checksum, first 16 of 20 bytes
4b 15 c4 47                                       checksum, last 4 bytes

The 20-byte checksum

The last 20 bytes are the checksum, and the password creates it. Version 1 BKS files used a weak one. CERT says version 1 used a 16-bit key for it, which allows only 65,536 keys. Bouncy Castle 1.47 moved the default to version 2 with a 160-bit key.

UBER files look different inside

Bouncy Castle also offers the UBER store type, and the type named BouncyCastle made the same output in our test. Both start with 00 00 00 01. They encrypt everything after the header, so no alias shows in a text viewer. Our three UBER-type files failed when we read them as BKS.

9 Is a BKS File an NTBackup Backup? Not a backup, a list of what to back up

NTBackup also worked from a command line. Microsoft's ntbackup page shows the call with an at sign before the file name. It calls these BKS files backup selection files and says you must create them in the graphical version of Backup.

Other sites agree. FileInfo calls it an NTBackup settings file with plain text lists of folders and files. The file-extensions.org entry calls it a scheduled backup script from Windows NT and 2000 and marks the entry obsolete. It adds that text editors can view it.

The last three rows are user reports from Covington Innovations, mysysadmintips and Cadzow. We did not repeat any of them, because we could not use NTBackup.

The encoding trap

The reports disagree about how to save the BKS file. The sources call it plain text, and they also say Unicode. Both can be true, because UTF-16 is text. We saved the same two lines of folder paths five ways and checked them.

Covington reports that the first kind worked and that a mark made NTBackup reject the file. The file command cannot name that kind. Our five files did not come from NTBackup, and we could not test which one it accepts. If lists fail, check the encoding first.

Open an NTBackup BKS file

1. 1 Read the first bytes. Text means a list and not a keystore.
2. 2 Copy the BKS file, then open the copy in Notepad.
3. 3 Read the lines. Lines are folder or file paths, and Cadzow shows that they can also name server items.
4. 4 Covington's fix was to save as Unicode in Notepad and delete the first two bytes with a hex editor. We did not test it.

Find the backup the list made

The list only names what to save. The backup it creates is a BKF file, and the BKF guide for NTBackup archives explains how to open one. These three pages describe viewers and Microsoft's command.

SysTools says its viewer has Quick, Deep and Range Based scan modes and previews files for free. Freeviewer says the same three modes exist and that its demo only previews BKF and ZIP files. Both say saving needs the paid version. We tested neither.

12 What Else Does the BKS Ending Mean? A backup list, a spreadsheet and a few more

If your BKS file does not start with 00 00 00 02 or 00 00 00 01, it may be none of the keystore kinds. Three sites list other meanings, and the table shows what they say.

The file-extensions.org list names five BKS records: Works backups, two IBM BookManager records, Windows scheduled backup scripts and NTBackup settings. The FILExt page names the Bouncy Castle keystore, the NTBackup script, BookManager, Capego and the Works backup.

We found no public sample of the Works, BookManager or Capego kinds, so we say nothing about their bytes. All three sites are directories, and none of them explains how it counted.

13 What Is the BKS File Format Made Of? The specification facts we could check

About the file signature

The first four bytes separate version 2 from the rest, and the next four were 00 00 00 14 in all 15 of our BKS and UBER files. FILExt lists the same two starts as the most common. We did not find a media type for BKS files that we could confirm.

What we left out

We did not read the Bouncy Castle source code. The layout above comes from our own 18 files, and keytool and pyjks agreed with it. We made no version 1 BKS file, because Bouncy Castle 1.77 does not offer that type.

We did not use NTBackup, KeyStore Explorer, Portecle, SysTools BKF Viewer or Freeviewer BKF Viewer. All 18 keystores and all five selection lists are ours. None came from a phone, an app or NTBackup. So we say nothing about those files beyond what the cited pages state.

We also found no Android page that lists BKS, so the only source for Android is the CERT note. We did not test the Works, BookManager or Capego kinds.

Sources checked

For the format, the Wikipedia entry on Bouncy Castle, the CERT note on version 1 files and the CVE-2018-5382 record. For the NTBackup list, Microsoft page on the ntbackup command and the Wikipedia entry on NTBackup.

For the endings, FileInfo's BKS entry. For user reports, the posts by Covington, mysysadmintips and Cadzow linked above. Every test count came from our own tests in October 2026.