Decrypt File Online — Restore Authenticated MTCF01 Files

To decrypt a file online with this tool, choose an .mtcenc package created by the companion file encryptor, enter its original passphrase, and decrypt locally. The workspace verifies the MTCF01 header and AES-GCM authentication tag before enabling the recovered-file download. It does not support encrypted ZIP, Office, PDF, or unknown third-party formats, and a wrong password cannot be bypassed.

Decrypt File Online At a Glance

What It Means to Decrypt a File Safely

Decryption reverses an encryption operation only when a compatible key and format are available. The file decryptor expects a specific package produced by the paired local encryption workspace, not an arbitrary password-protected document. A successful operation reconstructs the original file bytes and makes them available for download. Crucially, the interface does not consider recognizable-looking content to be proof of success. It waits for the AES-GCM authentication check to succeed, because a corrupted file can otherwise produce misleading data that appears plausible when casually opened.

Knowing the original password is necessary but not sufficient when the file was encrypted by different software. Programs use different headers, key-derivation methods, ciphers, and container structures, and the same word can lead to entirely different keys in different formats. This tool deliberately identifies its supported format before attempting key derivation. An encrypted Office document, password-protected PDF, or ZIP archive should be handled by software that implements that format. Repeatedly trying such files here will not repair them or provide reliable results.

Recognizing the Encrypted Package Header

An MTCF01 package begins with a six-byte marker, followed by a four-byte header length and a UTF-8 JSON header. The header records a format version, cipher and key derivation identifiers, a salt, a nonce, the number of PBKDF2 iterations, and basic file metadata. The parser first checks reasonable size bounds and these expected values so that malformed input does not proceed directly to expensive cryptographic work. In particular, the ciphertext length must match the declared original byte count plus the GCM authentication tag length.

These checks provide early error messages for unsupported or damaged files, but they do not authenticate the header by themselves. Someone can edit plaintext metadata in the package, so the cipher includes those exact header bytes as authenticated additional data. A modified filename, iteration count, or other header field should invalidate the final decryption, even if the JSON remains syntactically well formed. That separation between format parsing and cryptographic authentication is important: a file can be structurally recognizable while still being untrusted until verification completes.

How the Passphrase Unlocks the Content

The passphrase is not stored inside the encrypted file. Instead, the package stores a random salt and PBKDF2 iteration count used to derive the AES key from the same secret originally supplied at encryption time. When the decryptor receives the passphrase, it repeats that derivation using the package settings. A different password produces a different key, and authentication fails with an explicit message. Passwords cannot be recovered from the salt or the iteration count; those are public parameters intended to make compatible key derivation possible.

Passphrase entry requires at least twelve characters and does not accept arbitrarily long input. These boundaries reduce accidental misuse without guaranteeing that every accepted secret is hard to guess. Protect the passphrase from shoulder-surfing, shared clipboard histories, autofill in untrusted browsers, and inclusion in public support tickets. If the original secret has been lost, this tool has no administrator override, password-reset link, or master recovery key. Anyone promising effortless recovery of strongly encrypted content should be treated with caution.

Tamper Detection before a File Can Be Downloaded

AES-GCM uses an authentication tag that depends on the key, nonce, ciphertext, and any associated authenticated data. During decryption, the browser checks the tag before returning plaintext. If the content was changed in transit, partially truncated, or paired with an incorrect key, the operation fails. This is a stronger property than a conversion that merely produces bytes and leaves the user to guess whether they are correct. The interface does not enable download of recovered content after a failed authentication step, and it clears previous result state when a new attempt begins.

Authentication cannot prove that the sender is a trusted person. An attacker who knows a password can create a different valid package with that password, and a legitimate encrypted file can contain malicious or false material. Treat recovered documents with the same caution as any unknown download, including checking the source and scanning for malware where appropriate. The integrity check establishes consistency of the protected package, not a certificate of authorship, professional review, or safety of the underlying file format.

Worked Example: Restoring an Encrypted Report

Suppose a colleague sends an authenticated package named report.pdf.mtcenc through an approved file-sharing channel and provides the passphrase separately. Download the entire package, select it in this workspace, and enter the exact secret. The parser checks the envelope and the browser derives a key from the saved salt and iteration count. If AES-GCM authentication succeeds, the tool offers a file named report.pdf for download and reports the verified original byte count. The suffix is restored from authenticated metadata rather than guessed from the encrypted download name.

As a deliberate negative test on a disposable sample, replace one byte of the encrypted package or enter a different password. The expected behavior is a failure with no decrypted download. A network copy that stops before completion is similarly rejected because its data length no longer matches the expected envelope. For an important handoff, compare a digest supplied through an authenticated communication channel with a fresh file checksum generator result on the recovered file. This is an optional verification of exact bytes, not an alternative to keeping the password secret.

Avoiding Format Confusion and False Success

A file with a .mtcenc extension is not automatically valid. Extensions can be renamed without changing the underlying bytes, so the decryptor inspects a marker and structured metadata rather than trusting the visible filename alone. A common password-protected ZIP still begins with ZIP-specific bytes and remains unsupported even if renamed to .mtcenc. The same applies to encrypted PDF files and operating-system key containers. Recognizing that mismatch early avoids wasting time and prevents a false success message when a file was never opened correctly.

When a supported envelope has malformed JSON, an unexpected algorithm identifier, unsafe derivation settings, or inconsistent lengths, the tool reports an error. It does not attempt insecure fallbacks such as unauthenticated AES modes, truncated tags, or ignoring unrecognized header fields. Those shortcuts could return content without a reliable integrity guarantee. If compatibility with another encryption program is required, first determine the exact specification and use a vetted implementation rather than patching an unrelated file extension onto a package.

Local Processing, Memory Limits, and Browser Requirements

The file stays in browser memory while this operation runs, and the recovered bytes are assembled into a downloadable object only after authentication. The maximum original payload is 25 MiB, with a small permitted overhead for the package. This bounded design limits resource use but may still be slower on low-memory devices or browsers with aggressive tab suspension. Files above the limit need a streaming-capable application or organization-approved workflow. The tool is not a general-purpose archive manager and cannot decrypt folders as a single operation unless the user first creates an appropriate archive with separate software.

Modern Web Crypto requires a secure execution context, normally HTTPS or local development on localhost. If a browser does not expose the cryptographic functions, the workspace reports that limitation rather than pretending to decrypt. Local operation does not neutralize browser extensions, malicious downloads, or compromised devices. Use a trusted browser, save recovered data to an appropriate location, and check any organizational policies before handling sensitive records. The download confirmation only means the browser was asked to save the file, not that every filesystem or backup operation succeeded.

Handling Recovered Files after Authentication

An encrypted package may be suitable for transmission or cold storage, but the newly decrypted copy is plain data again. Once it is downloaded, device backups, indexing services, thumbnail previews, and document editors may create additional copies. Choose the destination folder deliberately, avoid shared machines for private material, and use the organization's approved access controls. If another person will receive the recovered document, the encryption of the original package does not automatically secure all subsequent transfers.

The paired file encryptor generates the compatible packages with a fresh random salt and nonce for each new operation. Keep a tested encrypted backup until you no longer need it, and retain a secure record of the passphrase away from the package. Where auditability matters, record which approved person supplied the file and which channel was used rather than relying solely on the package's original filename. Strong encryption can protect confidentiality in transit or storage, but safe recovery and responsible handling finish the job.

Frequently Asked Questions

Conclusion

A valid package and the exact original passphrase are both required for dependable decryption. Wait for authentication, verify the recovered file before discarding backups, and handle its plain contents with the same care that justified encryption in the first place.