AXX File Documentation

Summary

An .axx file is a file encrypted by AxCrypt, a widely used free/freemium file-encryption tool. Its contents, and even the original filename, are protected with AES, so it cannot be opened without the password used to encrypt it. To open one, install AxCrypt from axcrypt.net, double-click the .axx, and enter that password. Its MIME type is application/octet-stream. There is no converter and no backdoor: without the password the data is unrecoverable by design.

Technical details

FeatureValue
Full nameAxCrypt Encrypted File
File extension.axx
MIME typeapplication/octet-stream
Format typeAES-encrypted file container (binary)
DeveloperAxCrypt AB (formerly Axantum Software AB, Sweden)
Introduced2001 (AxCrypt 1.x); AxCrypt 2.x since 2016
Open standardNo — AxCrypt-defined format (source is open)
Encryption (v1)AES-128 in CBC mode
Encryption (v2)AES-256, account/password key wrapping
Magic numberC0 B9 07 2E 4F 93 F1 46 A0 15 79 2C A1 D9 E8 21
Signature length16-byte fixed GUID at offset 0
IntegrityHMAC-SHA-512 over the file to detect tampering
FilenameOriginal name stored encrypted inside the container
CompressionOptional, before encryption
Namingreport.docx → report-docx.axx
RecoveryNone — no master key or backdoor
PlatformsWindows, macOS, Android, iOS
Related extensions.axc
Official siteaxcrypt.net
File signature (magic bytes)
C0 B9 07 2E 4F 93 F1 46 A0 15 79 2C A1 D9 E8 21

Offset 0, 16 bytes. This fixed GUID marks the AxCrypt format and is the only unencrypted, human-recognisable part of the file. It is followed by typed header sections (version, key-wrap, encryption parameters, HMAC) and then the AES-encrypted, optionally compressed payload. The original filename is stored inside the encrypted region, so the .axx name alone reveals nothing about the source file’s type.

What is an AXX file?

An .axx file is a container produced by AxCrypt, a file-encryption program first released in 2001 by the Swedish developer Axantum Software, now AxCrypt AB. When you encrypt a file, AxCrypt writes a new file with the same base name plus the .axx extension — report.docx becomes report-docx.axx — and can securely delete the original. The result is a self-contained encrypted object: a fixed header that identifies the format, a set of typed metadata sections, an integrity tag, and the AES-encrypted payload.

The single most important fact about an .axx is that the password is the only key. AxCrypt keeps no master key and has no backdoor, so a correctly encrypted file cannot be opened without the passphrase that made it. That is the encryption working as designed, not a defect. The sections below describe the actual container layout, the AES modes AxCrypt uses, how the key is derived and wrapped, and why the format is safe to hold but impossible to recover without the secret.

The 16-byte GUID and the header sections

Every AxCrypt file begins with the same 16-byte GUID, C0 B9 07 2E 4F 93 F1 46 A0 15 79 2C A1 D9 E8 21, at offset 0. This marker is the one part of the file that is not encrypted, and it is how any tool recognises the format regardless of the .axx name. Immediately after it comes a sequence of typed header sections, each a length-prefixed, type-tagged block.

offset 0   : 16-byte GUID  (C0 B9 07 2E ...)   format marker
then       : header sections, each = [length][type][data]
             - version / preamble
             - key-wrap section (wrapped data-encrypting key)
             - encryption info (IV, iterations)
             - compression flag
             - encrypted metadata (original filename, times)
             - HMAC section (integrity tag)
payload    : AES ciphertext of the (optionally compressed) file

The header sections carry everything a decryptor needs except the password: the format version, the wrapped key, the initialisation vector, whether the payload was compressed before encryption, and the encrypted metadata. Storing them as typed sections lets the format evolve, older readers skip section types they do not understand, which is how AxCrypt 2 files can be extended without breaking the parser.

AES-128 and AES-256: the two format generations

AxCrypt exists in two generations that differ in their cipher strength and key handling. AxCrypt 1.x encrypts the payload with AES-128. AxCrypt 2.x (since 2016) uses AES-256 and ties decryption to an AxCrypt account password. Both use AES as a block cipher over the file data, with an initialisation vector stored in the header so identical plaintext blocks do not produce identical ciphertext.

The compatibility rule catches people out: AxCrypt 2 can open old 1.x files, but AxCrypt 1 cannot open files made by AxCrypt 2, because the version-2 format and its AES-256 key handling did not exist when version 1 was written. The classic “I can’t open my old .axx” problem is almost always someone trying to open a version-2 file with an old version-1 build, or vice versa. Using the current AxCrypt app resolves the direction that is supported.

Key derivation and the wrapped data key

AxCrypt does not encrypt your file directly with your password. It uses a two-level key scheme. A random data-encrypting key is generated per file and used with AES to encrypt the payload. Your password is passed through a key-derivation step (a salted, many-iteration hash designed to be slow, so brute-force guessing is expensive) to produce a key-encrypting key. That key-encrypting key then wraps (encrypts) the data key, and only the wrapped form is stored in the header’s key-wrap section.

This indirection has two consequences worth understanding. First, changing a file’s password only re-wraps the data key; it does not re-encrypt the whole payload, so a password change is fast. Second, the iteration count in the key-derivation step is deliberately high to slow down offline password guessing, which is why a weak passphrase, not the AES cipher, is the practical weak point of any encrypted file. The AES-256 itself has no known feasible break; a short or common password does.

The HMAC integrity tag and the hidden filename

AxCrypt files are authenticated, not just encrypted. The container includes an HMAC section, a keyed hash (HMAC-SHA-512 in version 2) computed over the file’s contents. On decryption AxCrypt recomputes the HMAC and compares it, so any corruption or tampering — a flipped bit, an appended byte, an attempt to swap in different ciphertext — is detected and the decryption is refused rather than silently producing garbage. This is what makes the format tamper-evident.

The original filename is itself stored inside the encrypted region, in the encrypted metadata section, not in the .axx name. That is why report-docx.axx tells an observer only that the source ended in .docx; a fully encrypted metadata block hides the real name and type until you decrypt. After a successful decrypt, AxCrypt reads that stored name and restores the file to its real name and extension.

Why a forgotten password means permanent loss, and other risks

The .axx container is a legitimate, well-built encryption format with no executable code, so holding one is safe. The genuine risks are practical, and all of them are consequences of strong cryptography rather than flaws in it.

First, a forgotten password is unrecoverable. Because AxCrypt keeps no master key and the data key is wrapped by a key derived only from your password, there is mathematically no way to open the file without it. Resetting your AxCrypt account does not decrypt files that were encrypted under the old password; those files stay locked. This is the security guarantee, not a bug, and it is why any “AXX password recovery” service is at best a brute-force gamble against your passphrase strength.

Second, searches for “open axx” and “axx viewer” surface fake “universal file viewer” downloads that cannot decrypt anything and often bundle adware or unwanted programs. Only the real AxCrypt app from axcrypt.net can open an .axx, and only with the correct password; nothing else can. Third, like any encrypted attachment, an .axx can wrap malicious content, so the encryption hiding a file’s type also hides malware from scanners until it is decrypted — decrypt only files from senders you trust.

Frequently asked questions

Can I open an AXX file without the password?

No. The payload is AES-encrypted and the data key is wrapped by a key derived from your password. AxCrypt holds no master key and there is no backdoor, so without the correct password the contents are unrecoverable. Any third-party “AXX viewer” that claims otherwise cannot decrypt the file.

Why won’t my old .axx open in the new AxCrypt?

AxCrypt 2 opens AxCrypt 1.x files, but AxCrypt 1 cannot open files made by AxCrypt 2, which use a different AES-256 format. Use the current AxCrypt app and the original encryption password; the supported upgrade direction is old-to-new.

The .axx name doesn’t say what the original file was — why?

AxCrypt stores the original filename encrypted inside the container, in the metadata section, rather than in the .axx name. You cannot tell the source type from the outside; after decrypting, AxCrypt restores the real name and extension.

References