Using Ethereum compressed public key to create JWE

[Ron Kreutzer]

How do I setup an encrypter when starting with an Ethereum compressed public key string, like ‘0x032be803b18b3ce72d9760c8f736cb08cf3eb42c5ed2d907616afca15a2e3fb4c0’?

I’m attempting to encrypt a JWT into a JWE, and I think it needs to be a ECDHEntrypter.

[Yavor Vasilev]

This depends on the key type / curve and what the compression algorithm is.

[Ron Kreutzer]

For anyone else with this issue, I think I have it working now. When creating the JWK, the x parameter is the compressed public key minus the left 4 characters (0x03 in the example above).

            // Create encrypter
            var jwk = '{"kty":"OKP","crv":"X25519","x":"#local.x#","use":"enc","kid":"1"}'
            var jwk2 = createObject("java", "com.nimbusds.jose.jwk.OctetKeyPair").parse(jwk);
            var encrypter = createObject("java", "com.nimbusds.jose.crypto.X25519Encrypter").init(jwk2);
            // Create header & payload
            var jweHeader = createObject("java", "com.nimbusds.jose.JWEHeader").parse('{"alg":"ECDH-ES","enc":"XC20P","cty":"JWT"}');
            var payload2 = createObject("java", "com.nimbusds.jose.Payload").init(arguments.payload);
            // Creates the JWE object with payload
            jweObject = createObject("java", "com.nimbusds.jose.JWEObject").init(jweHeader,payload2);
            // Encrypt the JWT
            jweObject.encrypt(encrypter);
            // Serialize the JWE to compact form
            jwe = jweObject.serialize();

‌

[Yavor Vasilev]

When you write compressed, is this truly compressed (e.g. ZIP), or simply hex encoded?

[Ron Kreutzer]

An uncompressed Ethereum public key contains both the x and y axis coordinates of the point on the curve. The compressed key contains just the x axis and an indicator for whether the y axis is positive or negative. Both are hex encoded. More info at https://medium.com/asecuritysite-when-bob-met-alice/02-03-or-04-so-what-are-compressed-and-uncompressed-public-keys-6abcb57efeb6