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Showing content from http://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto/encrypt below:

SubtleCrypto: encrypt() method - Web APIs

Baseline Widely available

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Note: This feature is available in Web Workers.

The encrypt() method of the SubtleCrypto interface encrypts data.

It takes as its arguments a key to encrypt with, some algorithm-specific parameters, and the data to encrypt (also known as "plaintext"). It returns a Promise which will be fulfilled with the encrypted data (also known as "ciphertext").

encrypt(algorithm, key, data)

algorithm

An object specifying the algorithm to be used and any extra parameters if required:

• To use RSA-OAEP, pass an RsaOaepParams object.
• To use AES-CTR, pass an AesCtrParams object.
• To use AES-CBC, pass an AesCbcParams object.
• To use AES-GCM, pass an AesGcmParams object.

key

A CryptoKey containing the key to be used for encryption.

data

An ArrayBuffer, a TypedArray, or a DataView containing the data to be encrypted (also known as the plaintext).

A Promise that fulfills with an ArrayBuffer containing the "ciphertext".

The promise is rejected when the following exceptions are encountered:

InvalidAccessError DOMException

Raised when the requested operation is not valid for the provided key (e.g., invalid encryption algorithm, or invalid key for the specified encryption algorithm).

OperationError DOMException

Raised when the operation failed for an operation-specific reason (e.g., algorithm parameters of invalid sizes, or AES-GCM plaintext longer than 2³⁹−256 bytes).

The Web Crypto API provides four algorithms that support the encrypt() and decrypt() operations.

One of these algorithms — RSA-OAEP — is a public-key cryptosystem.

The other three encryption algorithms here are all symmetric algorithms, and they're all based on the same underlying cipher, AES (Advanced Encryption Standard). The difference between them is the mode. The Web Crypto API supports three different AES modes:

• CTR (Counter Mode)
• CBC (Cipher Block Chaining)
• GCM (Galois/Counter Mode)

It's strongly recommended to use authenticated encryption, which includes checks that the ciphertext has not been modified by an attacker. Authentication helps protect against chosen-ciphertext attacks, in which an attacker can ask the system to decrypt arbitrary messages, and use the result to deduce information about the secret key. While it's possible to add authentication to CTR and CBC modes, they do not provide it by default and when implementing it manually one can easily make minor, but serious mistakes. GCM does provide built-in authentication, and for this reason it's often recommended over the other two AES modes.

The RSA-OAEP public-key encryption system is specified in RFC 3447.

This represents AES in Counter Mode, as specified in NIST SP800-38A.

AES is a block cipher, meaning that it splits the message into blocks and encrypts it a block at a time. In CTR mode, every time a block of the message is encrypted, an extra block of data is mixed in. This extra block is called the "counter block".

A given counter block value must never be used more than once with the same key:

• Given a message n blocks long, a different counter block must be used for every block.
• If the same key is used to encrypt more than one message, a different counter block must be used for all blocks across all messages.

Typically this is achieved by splitting the initial counter block value into two concatenated parts:

• A nonce (that is, a number that may only be used once). The nonce part of the block stays the same for every block in the message. Each time a new message is to be encrypted, a new nonce is chosen. Nonces don't have to be secret, but they must not be reused with the same key.
• A counter. This part of the block gets incremented each time a block is encrypted.

Essentially: the nonce should ensure that counter blocks are not reused from one message to the next, while the counter should ensure that counter blocks are not reused within a single message.

Note: See Appendix B of the NIST SP800-38A standard for more information.

This represents AES in Cipher Block Chaining Mode, as specified in NIST SP800-38A.

This represents AES in Galois/Counter Mode, as specified in NIST SP800-38D.

One major difference between this mode and the others is that GCM is an "authenticated" mode, which means that it includes checks that the ciphertext has not been modified by an attacker.

Note: You can try the working examples out on GitHub.

This code fetches the contents of a text box, encodes it for encryption, and encrypts it with using RSA-OAEP. See the complete code on GitHub.

function getMessageEncoding() {
  const messageBox = document.querySelector(".rsa-oaep #message");
  let message = messageBox.value;
  let enc = new TextEncoder();
  return enc.encode(message);
}

function encryptMessage(publicKey) {
  let encoded = getMessageEncoding();
  return window.crypto.subtle.encrypt(
    {
      name: "RSA-OAEP",
    },
    publicKey,
    encoded,
  );
}

This code fetches the contents of a text box, encodes it for encryption, and encrypts it using AES in CTR mode. See the complete code on GitHub.

function getMessageEncoding() {
  const messageBox = document.querySelector(".aes-ctr #message");
  let message = messageBox.value;
  let enc = new TextEncoder();
  return enc.encode(message);
}

function encryptMessage(key) {
  let encoded = getMessageEncoding();
  // counter will be needed for decryption
  counter = window.crypto.getRandomValues(new Uint8Array(16));
  return window.crypto.subtle.encrypt(
    {
      name: "AES-CTR",
      counter,
      length: 64,
    },
    key,
    encoded,
  );
}

let iv = window.crypto.getRandomValues(new Uint8Array(16));
let key = window.crypto.getRandomValues(new Uint8Array(16));
let data = new Uint8Array(12345);
// crypto functions are wrapped in promises so we have to use await and make sure the function that
// contains this code is an async function
// encrypt function wants a cryptokey object
const key_encoded = await window.crypto.subtle.importKey(
  "raw",
  key.buffer,
  "AES-CTR",
  false,
  ["encrypt", "decrypt"],
);
const encrypted_content = await window.crypto.subtle.encrypt(
  {
    name: "AES-CTR",
    counter: iv,
    length: 128,
  },
  key_encoded,
  data,
);

// Uint8Array
console.log(encrypted_content);

This code fetches the contents of a text box, encodes it for encryption, and encrypts it using AES in CBC mode. See the complete code on GitHub.

function getMessageEncoding() {
  const messageBox = document.querySelector(".aes-cbc #message");
  let message = messageBox.value;
  let enc = new TextEncoder();
  return enc.encode(message);
}

function encryptMessage(key) {
  let encoded = getMessageEncoding();
  // iv will be needed for decryption
  iv = window.crypto.getRandomValues(new Uint8Array(16));
  return window.crypto.subtle.encrypt({ name: "AES-CBC", iv }, key, encoded);
}

This code fetches the contents of a text box, encodes it for encryption, and encrypts it using AES in GCM mode. See the complete code on GitHub.

function getMessageEncoding() {
  const messageBox = document.querySelector(".aes-gcm #message");
  const message = messageBox.value;
  const enc = new TextEncoder();
  return enc.encode(message);
}

function encryptMessage(key) {
  const encoded = getMessageEncoding();
  // iv will be needed for decryption
  const iv = window.crypto.getRandomValues(new Uint8Array(12));
  return window.crypto.subtle.encrypt({ name: "AES-GCM", iv }, key, encoded);
}

• SubtleCrypto.decrypt().
• RFC 3447 specifies RSAOAEP.
• NIST SP800-38A specifies CTR mode.
• NIST SP800-38A specifies CBC mode.
• NIST SP800-38D specifies GCM mode.

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