AES-256 encryption/decryption security tool for files | ZeroTools

The requirements for file protection in modern browser environments are extremely strict, and the core of this is AES-256-GCM, a symmetric key encryption method using Galois/Counter Mode, implemented in the crypto.subtle interface of the Web Crypto API.

AES-256-GCM goes beyond simply ensuring confidentiality and serves as a representative authenticated cipher that verifies data integrity while encrypting it.

This tool executes this advanced encryption algorithm directly on the client side to completely conceal files without going through the network layer.

The plaintext data input by the user is encrypted through block-by-block Galois field multiplication in the browser's isolated memory space, leaving no trace on the server side.

This completely local processing model eliminates the risk of external eavesdropping and is the last line of defense against leakage of sensitive information.

Additionally, the parallel processing power provided by GCM mode is highly compatible with modern multi-core processors, minimizing delays in the encryption process even for large files.

It structurally eliminates the padding oracle vulnerabilities that were an issue with conventional CBC mode, making it the most reliable encryption model at present for encrypting static data called files.

In order to generate the highly randomized 256-bit encryption key required by AES-256-GCM from a human-memorable character string called a password, this organization employs Password-Based Key Derivation Function 2 called PBKDF2.

Specifically, we specify SHA-256 as the internal hash function and intentionally apply a huge number of iterations, or iterations, 100,000 times.

This computational burden acts as a strong delay defense against brute force and dictionary attacks. Even if an attacker attempts to search for the key using dedicated hardware, each guess would require 100,000 hash calculations, making it mathematically impossible to crack the password within a realistic amount of time.

Furthermore, by adding a unique salt obtained from a cryptographically secure pseudo-random number generator during the key derivation process, the final key generated will be completely different every time even if the password is the same, completely nullifying pre-computation attacks using rainbow tables.

Passwords entered by users are only temporarily held in memory, and a strict memory management design is in place to ensure that they are destroyed as soon as hash calculation and key derivation are complete.

The essential elements to ensure the security of AES-256-GCM are the 96-bit initialization vector or IV and the 128-bit authentication tag.

The initialization vector acts as a nonce that is uniquely generated for each encryption execution, and transforms the output ciphertext into something unpredictable even if the combination of plaintext and encryption key is the same.

If the initialization vector were fixed, the cryptographic foundation of GCM would collapse instantly, and high-quality random numbers would be assigned under strict entropy control.

On the other hand, a 128-bit authentication tag is automatically added to the end of encrypted data, and is responsible for instantly detecting the fact that even a single bit of a file has been altered by a third party.

During the decryption process, the Web Crypto API first verifies the mathematical integrity of this authentication tag and only allows plaintext output if the calculated tag and the tag attached to the file are an exact match.

This strict authentication mechanism rejects any attempts by malicious attackers to manipulate the contents by cutting and pasting the ciphertext, ensuring the integrity of the files.

In order to process large files ranging from hundreds of megabytes to gigabytes without failure on the browser, advanced memory management and asynchronous processing are required.

This tool makes full use of modern browsers' File API and ArrayBuffer to expand the target file in memory as a continuous buffer stream rather than in chunks.

At this time, the encryption functions of the Web Crypto API are executed asynchronously based on promises, allowing intensive cryptographic operations to be performed in the background without interfering with the main thread's UI drawing.

Raw data read into memory is immediately converted to ciphertext, and the plaintext buffer before encryption is immediately released for garbage collection, completely preventing unnecessary memory occupation and memory leaks.

This entire lifecycle is completed within the user's local device, and there is no unintentional writing to external storage or temporary areas, creating an extremely robust architecture that physically and logically blocks the generation of residual data that could become a breeding ground for shadow IT.

Upon successful completion of the encryption process, the system combines the generated ciphertext body, the 96-bit initialization vector, and the salt used for key derivation into a single binary structure, constructing a huge Blob object.

This combined data stream is treated as a file with a special dot enc extension and is downloaded directly to the user's local file system.

This download process is performed via a virtual object URL generated within the browser, so it does not consume any network bandwidth and does not require any communication with an external server.

At the time of decryption, by reading this dot enc file again, the salt and initialization vector stored at the beginning are accurately parsed, and combined with the password entered by the user, the legitimate decryption key is reconstructed.

This optimized and structured binary format is highly portable and compatible, allowing consistent file exchange in any environment, regardless of OS or platform differences.

Highly encrypted files are at their best when shared with third parties through inherently insecure channels such as cloud storage or email.

In practices such as backing up confidential engineering drawings, financial data, or databases containing personal information, transmitting data in plain text carries an extremely high risk.

By using this tool to convert the target file into AES-256-GCM robust ciphertext, even if the data is intercepted during the transfer route or the cloud storage server is compromised, the data contents will not be leaked.

It also provides a strong barrier against loss or theft of physical media when storing archives on a local external hard drive or USB memory.

By simply using a separately shared password via secure out-of-band communication and the decryption function of this tool, the receiving party can securely access the original confidential information using a standard web browser without having to install any special software on the device, providing great practical convenience and reducing operating costs.