Node.js src/crypto documentation

Welcome. You've found your way to the Node.js native crypto subsystem.

Do not be afraid.

While crypto may be a dark, mysterious, and forboding subject; and while this directory may be filled with many *.h and *.cc files, finding your way around is not too difficult. And I can promise you that a Gru will not jump out of the shadows and eat you (well, "promise" may be a bit too strong, a Gru may jump out of the shadows and eat you if you live in a place where such things are possible).

Finding your way around

All of the code in this directory is structured into units organized by function or crypto protocol.

The following provide generalized utility declarations that are used throughout the various other crypto files and other parts of Node.js:

• crypto_util.h / crypto_util.cc (Core crypto definitions)
• crypto_common.h / crypto_common.cc (Shared TLS utility functions)
• crypto_bio.h / crypto_bio.cc (Custom OpenSSL i/o implementation)

Of these, crypto_util.h and crypto_util.cc are the most important, as they provide the core declarations and utility functions used most extensively throughout the rest of the code.

The rest of the files are structured by their function, as detailed in the following table:

When new crypto protocols are added, they will be added into their own crypto_ *.h and *.cc files.

Helpful concepts

Node.js currently uses OpenSSL to provide it's crypto substructure. (Some custom Node.js distributions -- such as Electron -- use BoringSSL instead.)

This section aims to explain some of the utilities that have been provided to make working with the OpenSSL APIs a bit easier.

Pointer types

Most of the key OpenSSL types need to be explicitly freed when they are no longer needed. Failure to do so introduces memory leaks. To make this easier (and less error prone), the crypto_util.h defines a number of smart-pointer aliases that should be used:

using X509Pointer = DeleteFnPtr<X509, X509_free>;
using BIOPointer = DeleteFnPtr<BIO, BIO_free_all>;
using SSLCtxPointer = DeleteFnPtr<SSL_CTX, SSL_CTX_free>;
using SSLSessionPointer = DeleteFnPtr<SSL_SESSION, SSL_SESSION_free>;
using SSLPointer = DeleteFnPtr<SSL, SSL_free>;
using PKCS8Pointer = DeleteFnPtr<PKCS8_PRIV_KEY_INFO, PKCS8_PRIV_KEY_INFO_free>;
using EVPKeyPointer = DeleteFnPtr<EVP_PKEY, EVP_PKEY_free>;
using EVPKeyCtxPointer = DeleteFnPtr<EVP_PKEY_CTX, EVP_PKEY_CTX_free>;
using EVPMDCtxPointer = DeleteFnPtr<EVP_MD_CTX, EVP_MD_CTX_free>;
using RSAPointer = DeleteFnPtr<RSA, RSA_free>;
using ECPointer = DeleteFnPtr<EC_KEY, EC_KEY_free>;
using BignumPointer = DeleteFnPtr<BIGNUM, BN_clear_free>;
using NetscapeSPKIPointer = DeleteFnPtr<NETSCAPE_SPKI, NETSCAPE_SPKI_free>;
using ECGroupPointer = DeleteFnPtr<EC_GROUP, EC_GROUP_free>;
using ECPointPointer = DeleteFnPtr<EC_POINT, EC_POINT_free>;
using ECKeyPointer = DeleteFnPtr<EC_KEY, EC_KEY_free>;
using DHPointer = DeleteFnPtr<DH, DH_free>;
using ECDSASigPointer = DeleteFnPtr<ECDSA_SIG, ECDSA_SIG_free>;
using HMACCtxPointer = DeleteFnPtr<HMAC_CTX, HMAC_CTX_free>;
using CipherCtxPointer = DeleteFnPtr<EVP_CIPHER_CTX, EVP_CIPHER_CTX_free>;

Examples of these being used are pervasive through the src/crypto code.

ByteSource

The ByteSource class is a helper utility representing a read-only byte array. Instances can either wrap external ("foreign") data sources, such as an ArrayBuffer (v8::BackingStore), or allocated data.

• If a pointer to external data is used to create a ByteSource, that pointer must remain valid until the ByteSource is destroyed.
• If allocated data is used, then it must have been allocated using OpenSSL's allocator. It will be freed automatically when the ByteSource is destroyed.

ArrayBufferOrViewContents

The ArrayBufferOrViewContents class is a helper utility that abstracts ArrayBuffer, TypedArray, or DataView inputs and provides access to their underlying data pointers. It is used extensively through src/crypto to make it easier to deal with inputs that allow any ArrayBuffer-backed object.

The lifetime of ArrayBufferOrViewContents should not exceed the lifetime of its input.

Key objects

Most crypto operations involve the use of keys -- cryptographic inputs that protect data. There are three general types of keys:

• Secret Keys (Symmetric)
• Public Keys (Asymmetric)
• Private Keys (Asymmetric)

Secret keys consist of a variable number of bytes. They are "symmetrical" in that the same key used to encrypt data, or generate a signature, must be used to decrypt or validate that signature. If two people are exchanging messages encrypted using a secret key, both of them must have access to the same secret key data.

Public and Private keys always come in pairs. When one is used to encrypt data or generate a signature, the other is used to decrypt or validate the signature. The Public key is intended to be shared and can be shared openly. The Private key must be kept secret and known only to the owner of the key.

The src/crypto subsystem uses several objects to represent keys. These objects are structured in a way to allow key data to be shared across multiple threads (the Node.js main thread, Worker Threads, and the libuv threadpool).

Refer to crypto_keys.h and crypto_keys.cc for all code relating to the core key objects.

KeyObjectData

KeyObjectData is an internal thread-safe structure used to wrap either an EVPKeyPointer (for Public or Private keys) or a ByteSource containing a Secret key. It is the shared backing representation used by KeyObject, CryptoKey, and native crypto jobs that operate on key material.

KeyObjectHandle

KeyObjectHandle is the internal JavaScript-visible C++ handle for a KeyObjectData. It exposes operations that internal JavaScript uses to initialize, inspect, compare, and export key material. Native code passes KeyObjectData across threads and jobs; a KeyObjectHandle is created when JavaScript needs access to those operations and is kept out of user-visible KeyObject own properties.

KeyObject

A KeyObject is the public Node.js-specific API for keys. It extends a native NativeKeyObject, which stores KeyObjectData for structured cloning. The JavaScript API surface reads its key type and a KeyObjectHandle through a hidden native-backed slot tuple, caching that tuple in a private field outside user-visible own properties. Derived metadata, such as symmetric key size and asymmetric key details, is read from the cached handle and appended lazily to the same private-field cache.

CryptoKey

A CryptoKey is the Web Crypto API key type. In the Node.js implementation, public CryptoKey instances are backed by a native NativeCryptoKey, not by a KeyObject. NativeCryptoKey stores the same KeyObjectData representation as KeyObject, plus the Web Crypto internal slots ([[extractable]], [[algorithm]], and [[usages]]).

CryptoJob

All operations that are not either Stream-based or single-use functions are built around the CryptoJob class.

A CryptoJob encapsulates a single crypto operation that can be invoked synchronously, asynchronously, or as a Web Crypto API Promise-based job.

The CryptoJob class itself is a C++ template that takes a single CryptoJobTraits struct as a parameter. The CryptoJobTraits provides the implementation detail of the job.

There are (currently) three basic CryptoJob specializations:

• CipherJob (defined in src/crypto_cipher.h) -- Used for encrypt and decrypt operations.
• KeyGenJob (defined in src/crypto_keygen.h) -- Used for secret and key pair generation operations.
• DeriveBitsJob (defined in src/crypto_util.h) -- Used for key and byte derivation operations.

Every CryptoJobTraits provides two fundamental operations:

• Configuration -- Processes input arguments when a CryptoJob instance is created.
• Implementation -- Provides the specific implementation of the operation.

The Configuration is typically provided by an AdditionalConfig() method, the signature of which is slightly different for each of the above CryptoJob specializations. Despite the signature differences, the purpose of the AdditionalConfig() function remains the same: to process input arguments and set the properties on the CryptoJob's parameters object.

The parameters object is specific to each CryptoJob type, and is stored with the CryptoJob. It holds all of the inputs that are used by the Implementation. The inputs held by the parameters must be threadsafe.

The AdditionalConfig() function is always called when the CryptoJob instance is being created.

The Implementation function is unique to each of the CryptoJob specializations and will either be called synchronously within the current thread or from within the libuv threadpool.

Every CryptoJob instance exposes a run() function to the JavaScript layer. When called, run() will either dispatch the job to the libuv threadpool, invoke the Implementation function synchronously, or return a Promise for Web Crypto API jobs. If invoked synchronously, run() will return a JavaScript array. The first value in the array is either an Error or undefined. If the operation was successful, the second value in the array will contain the result of the operation. Typically, the result is an ArrayBuffer, but certain CryptoJob types can alter the output.

If the CryptoJob is processed asynchronously, then the job must have an ondone property whose value is a function that is invoked when the operation is complete. This function will be called with two arguments. The first is either an Error or undefined, and the second is the result of the operation if successful.

If the CryptoJob is processed as a Web Crypto API job, then run() returns a Promise. Operation-specific failures are rejected with an OperationError, and successful jobs resolve with the Web Crypto API result shape expected by the JavaScript implementation.

For CipherJob types, the output is always an ArrayBuffer.

For KeyGenJob types, the output is either a single KeyObject, or an array containing a Public/Private key pair represented either as a KeyObjectHandle object or a Buffer. Web Crypto API key generation jobs return a CryptoKey or a CryptoKeyPair object.

For DeriveBitsJob type output is typically an ArrayBuffer but can be other values (RandomBytesJob for instance, fills an input buffer and always returns undefined).

Errors

ThrowCryptoError and the THROW_ERR_CRYPTO_* macros

The ThrowCryptoError() is a legacy utility that will throw a JavaScript exception containing details collected from OpenSSL about a failed operation. ThrowCryptoError() should only be used when necessary to report low-level OpenSSL failures.

In node_errors.h, there are a number of ERR_CRYPTO_* macro definitions that define semantically specific errors. These can be called from within the C++ code as functions, like THROW_ERR_CRYPTO_INVALID_IV(env). These methods should be used to throw JavaScript errors when necessary.

Crypto API patterns

Operation mode

All crypto functions in Node.js operate in one of these modes:

• Synchronous single-call
• Asynchronous single-call
• Web Crypto API Promise-based
• Stream-oriented

It is often possible to perform various operations across multiple modes. For instance, cipher and decipher operations can be performed in any of the three modes.

Synchronous single-call operations are always blocking. They perform their actions immediately.

// Example synchronous single-call operation
const a = new Uint8Array(10);
const b = new Uint8Array(10);
crypto.timingSafeEqual(a, b);

Asynchronous single-call operations generally perform a number of synchronous input validation steps, but then defer the actual crypto-operation work to the libuv threadpool.

// Example asynchronous single-call operation
const buf = new Uint8Array(10);
crypto.randomFill(buf, (err, buf) => {
  console.log(buf);
});

For the legacy Node.js crypto API, asynchronous single-call operations use the traditional Node.js callback pattern, as illustrated in the previous randomFill() example. In the Web Crypto API (accessible via globalThis.crypto), all asynchronous single-call operations are Promise-based.

// Example Web Crypto API asynchronous single-call operation
const { subtle } = globalThis.crypto;

subtle.generateKeys({ name: 'HMAC', length: 256 }, true, ['sign'])
  .then((key) => {
    console.log(key);
  })
  .catch((error) => {
    console.error('an error occurred');
  });

In nearly every case, asynchronous single-call operations make use of the libuv threadpool to perform crypto operations off the main event loop thread.

Stream-oriented operations use an object to maintain state over multiple individual synchronous steps. The steps themselves can be performed over time.

// Example stream-oriented operation
const hash = crypto.createHash('sha256');
let updates = 10;
setTimeout(() => {
  hash.update('hello world');
  setTimeout(() => {
    console.log(hash.digest();)
  }, 1000);
}, 1000);