Elliptic Curve Keys

This project defines a wrapper for Elliptic Curve (EC) private and public keys.

• Curves;
• Basic Construction
  • Randomly generated keys
  • Instantiating from encoded keys
  • Instantiating with objects
• ECKey properties
• ECKey functions
  • asPublicECKey()
  • computeSecret(otherKey)
  • createECDH()
  • createSign(hash)
  • createVerify(hash)
  • toBuffer(format)
  • toString(format)
  • toJSON()
• MIT License

Curves

This wrapper supports onl the three main curves listed below:

OpenSSL Curve Name	RFC-7518 (6.2.1.1)	ASN.1 OID
prime256v1	P-256	1.2.840.10045.3.1.7
secp384k1	P-256K non standard	1.3.132.0.10
secp384r1	P-384	1.3.132.0.34
secp521r1	P-521	1.3.132.0.35

Both the OpenSSL names and RFC-7518 (JWA/JWK) names can be used as parameters to the methods in the ECKey class.

Please be aware that NodeJS (and OpenSSL) support a large number of curves (see openssl ecparam -list_curves for a full list), but for brevity this implementation restricts to the three mentioned above.

PLEASE NOTE: The P-256K curve name (crv parameter) used when serializing a key using the secp384k1 curve is not standard, and NOT interoperable with other systems.

See the IANA registry for all known (and interoperable) curve names.

The P-256K name used might change at ANY time.

Basic construction

To use, start importing the main ECKey class:

1const ECKey = require('ec-key');

Randomly generated keys

To create a random ECKey instance simply call the createECKey static method, optionally specifying a curve name (defaults to prime256v1):

1// Create a new (random) ECKey instance using the secp521r1 curve
2var randomKey = ECKey.createECKey('P-521');

Instantiating from encoded keys

To import an existing private or public key, simply invoke the constructor with a String or a Buffer and the format in which the key is encoded:

1// Create a new ECKey instance from a base-64 spki string
2var key = new ECKey('MFkwEw ... 3w06qg', 'spki');

For Buffers and base64-encoded Strings the constructor supports both the pkcs8 (or rfc5208) and spki (or rfc5280) formats.

Additionally, the pem format is supported for unencoded Strings and Buffers:

1// Load up a PEM file and wrap it into a
2var pem = fs.readFileSync('./key.pem');
3var key = new ECKey(pem, 'pem');

Instantiating with objects

Instances of the ECKey class can also be created from very simple object.

For example JWKs can be used directly, and whereas in the example below the crv, x and y values will be considered, kty and kid will be ignored.

1/// Simply create from a JWK object
2var key = new ECKey({
3  "kty":"EC",
4  "crv":"P-256",
5  "x":"f83OJ3D2xF1Bg8vub9tLe1gHMzV76e8Tus9uPHvRVEU",
6  "y":"x_FEzRu9m36HLN_tue659LNpXW6pCyStikYjKIWI5a0",
7  "kid":"Public key used in JWS spec Appendix A.3 example"
8})

The following values are recognized:

• curve or crv: the curve type for this key, identified either by its OpenSSL or its RFC-7518 (JWA/JWK) name.

With regards to coordinates:

• d: the private d coordinate for the elliptic curve point, either as a Buffer, or a base64-encoded String of the coordinate's big endian representation
• x: the public x coordinate for the elliptic curve point, either as a Buffer, or a base64-encoded String of the coordinate's big endian representation
• y: the public y coordinate for the elliptic curve point, either as a Buffer, or a base64-encoded String of the coordinate's big endian representation

And also:

• publicKey: the uncompressed and prefixed (0x04) concatenation of the x and y public coordinates' big endian representation, as described in SEC-1 ECC section 2.3.3
• privateKey: the private d coordinate for the elliptic curve point, either as a Buffer, or a base64-encoded String of the coordinate's big endian representation

ECKey properties

The following enumerable properties are available for instances of ECKey:

• curve: the EC key curve name in OpenSSL format (e.g. prime256v1)
• isPrivateECKey: a boolean indicating whther this instance represents a private or public EC key.
• x: the public x coordinate's big endian representation for the elliptic curve point as a Buffer
• y: the public y coordinate's big endian representation for the elliptic curve point as a Buffer
• d: the private d coordinate's big endian representation for the elliptic curve point as a Buffer

Additionally the following properties are available, but not enumerable:

• jsonCurve: the EC key curve name in RFC-7518 format (e.g. P-256)
• publicCodePoint: the uncompressed and prefixed (0x04) concatenation of the x and y public coordinates' big endian representation, as described in SEC-1 ECC section 2.3.3.

ECKey functions

asPublicECKey()

Return this instance if this key is a public key, or create a new ECKey instance not including the private components of the key.

computeSecret(otherKey)

A simple shortcut for createECDH().computeSecret(otherKey) as explained below.

createECDH()

Create a standard Node ECDH object instance whose computeSecret(...) function accepts also ECKey (as in, this class) instances.

createSign(hash)

Create a standard Node Sign object whose sign(...) function is automatically populated with this instance.

• hash: the hashing function to use for generating the signature, normally one of SHA256, SHA384 or SHA512.

1// Create a signature of the message "the quick brown fox" with a random key
2var message = "the quick brown fox";
3var key = ECKey.createECKey('P-384');
4var signature = key.createSign('SHA384')
5                   .update(message)
6                   .sign('base64');

createVerify(hash)

Create a standard Node Verify object whose verify(...) function is automatically populated with this instance.

• hash: the hashing function to use for generating the signature, normally one of SHA256, SHA384 or SHA512.

1// Verify the signature calcuated above
2key.createVerify('SHA384')
3   .update(message)
4   .verify(signature, 'base64');

toBuffer(format)

Encode this EC key, optionally using the specified format (defaults to pem).

Formats supported are as follows:

• pem: return a Buffer containing the ascii represtation of the OpenSSL PEM format
  • equivalent to new Buffer(key.toString('pem'), 'ascii') and provided for convenience only
• rfc5951: (private keys only) returns the encoding of this key as specified by RFC-5951
• pkcs8 or rfc5208: (private keys only) returns the PKCS8 encoding of this key as specified by RFC-5208
• spki or rfc5280: (public keys only) returns the SPKI encoding of this key as specified by RFC-5280

toString(format)

Encode this EC key, optionally using the specified format (defaults to pem).

Formats supported are as follows:

• pem: return the key in OpenSSL's PEM format
• rfc5951: (private keys only) returns the encoding of this key as specified by RFC-5951, wrapped with a header and footer as outlined in section 4
• pkcs8 or rfc5208: (private keys only) returns the PKCS8 encoding of this key as specified by RFC-5208 encoded in base64
• spki or rfc5280: (public keys only) returns the SPKI encoding of this key as specified by RFC-5280 encoded in base64

toJSON()

Formats this ECKey as a JSON Web Key as specified by RFC-7517.

Please note that his function will also be called by the JSON.stringify(...) function.

1// Convert a PEM to a JWK in one easy step
2
3var pem = `-----BEGIN PRIVATE KEY-----
4MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgzr+Twxehecu0VYoC
5XUBL1Z4h3H28gPnJ5MP0AcOixAOhRANCAAS6pMWMMndZxOPSC9ui6sUUbmeK6dIi
6k3ZwTmm0SE7G+tYon5C57aVek5qH4y4OipbSLfbsIQuOkt0G8Vu1KZ3u
7-----END PRIVATE KEY-----`;
8
9var key = new ECKey(pem, 'pem');
10
11var jwk = JSON.stringify(key, null, 2);
12
13console.log(jwk);
14
15// This will result in the following output:
16// {
17//   "kty": "EC",
18//   "crv": "P-256",
19//   "x": "uqTFjDJ3WcTj0gvbourFFG5niunSIpN2cE5ptEhOxvo",
20//   "y": "1iifkLntpV6TmofjLg6KltIt9uwhC46S3QbxW7Upne4",
21//   "d": "zr-Twxehecu0VYoCXUBL1Z4h3H28gPnJ5MP0AcOixAM"
22// }