How to derive public key from private key using WebCryptoApi?

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The private key is generated using Elliptic Curve. None of the methods from the SubtleCrypto interface of the Web Crypto API seem to be able to derive a public key from a private key, correct me if I'm wrong. Do I have to use a 3rd party library for that?

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1 Answers

WebCrypto is a low level API with only a relatively small feature set. To my knowledge there is no dedicated method for deriving a public key from a private key.

However, you can export the private CryptoKey as JWK (JSON Web Key), remove the private portions, and re-import the remaining portion, which thereby becomes the public CryptoKey. The following code shows this for an ECDSA key:

async function getPublic(privateKey){
    const jwkPrivate = await crypto.subtle.exportKey("jwk", privateKey);    
    delete jwkPrivate.d;
    jwkPrivate.key_ops = ["verify"];
    return crypto.subtle.importKey("jwk", jwkPrivate, {name: "ECDSA", namedCurve: "P-256"}, true, ["verify"]);
}

async function test(){

    // Generate test key pair
    const keyPair = await crypto.subtle.generateKey({name: "ECDSA", namedCurve: "P-256"}, true, ["sign", "verify"]);

    // Derive public from private key
    const publicKeyFromPrivate = await getPublic(keyPair.privateKey)

    // Compare CryptoKeys
    console.log(keyPair.publicKey);
    console.log(publicKeyFromPrivate)

    // Compare keys (in JWK format)
    const jwkPublic = await crypto.subtle.exportKey("jwk", keyPair.publicKey);
    const jwkPublicFromPrivate = await crypto.subtle.exportKey("jwk", publicKeyFromPrivate);
    console.log(jwkPublic);
    console.log(jwkPublicFromPrivate);
}

(async () => {
    await test()
})();

As you can see, the original and the reconstructed public key are identical.

However, it should be mentioned that this solutin has one drawback: the private key must be exportable.

This post shows the same approach for RSA.

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