Java

Understanding a few ECDSA Public Key Formats

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Describes a few ECDSA public key formats. Chilkat2 Java

import com.chilkatsoft.chilkat2.BinData;
import com.chilkatsoft.chilkat2.ChilkatException;
import com.chilkatsoft.chilkat2.PublicKey;
import com.chilkatsoft.chilkat2.StringBuilder;

public class ChilkatExample {

    public static void main(String[] args) {
        // Here we have the output of the following openssl command:  openssl ec -in key.pem -pubout -text

        // Private-Key: (256 bit)
        // priv:
        //     0e:63:25:8a:73:3c:71:b6:c0:e7:a3:0f:94:b9:74:
        //     e0:be:bd:46:18:be:40:7e:66:9e:21:99:85:0e:ed:
        //     87:2d
        // pub:
        //     04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:
        //     b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:
        //     39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:
        //     2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:
        //     12:b7:b8:6a:43
        // ASN1 OID: prime256v1
        // NIST CURVE: P-256
        // writing EC key
        // -----BEGIN PUBLIC KEY-----
        // MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
        // Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
        // -----END PUBLIC KEY-----

        // The public key is shown in two different formats.
        // The first is this:

        // pub:
        //     04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:
        //     b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:
        //     39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:
        //     2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:
        //     12:b7:b8:6a:43

        // It is the ANSI X9.63 format.
        // 65-bytes are the uncompressed public key (04 || X || Y)

        // This is the same public key, but in PEM format

        // -----BEGIN PUBLIC KEY-----
        // MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
        // Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
        // -----END PUBLIC KEY-----

        // It contains ASN.1 that more explicitly identifies the key type.
        //
        // SEQUENCE (2 elem)
        //   SEQUENCE (2 elem)
        //     OBJECT IDENTIFIER 1.2.840.10045.2.1 ecPublicKey (ANSI X9.62 public key type)
        //     OBJECT IDENTIFIER 1.2.840.10045.3.1.7 prime256v1 (ANSI X9.62 named elliptic curve)
        //   BIT STRING (520 bit) 000001000101110100011010010011111101100110111101010010011001111011100...

        // PEM format can be loaded into a Chilkat public key object like this:

        StringBuilder sbPem = new StringBuilder();
        sbPem.append("-----BEGIN PUBLIC KEY-----\r\n");
        sbPem.append("MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET\r\n");
        sbPem.append("Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==\r\n");
        sbPem.append("-----END PUBLIC KEY-----\r\n");

        PublicKey pubKey1 = new PublicKey();
        try {
            pubKey1.loadFromString(sbPem.getAsString());
        } catch (ChilkatException e) {
            System.out.println(e.getLastErrorText());
            return;
        }

        // The X9.63 format can be loaded like this:
        StringBuilder sbHex = new StringBuilder();
        sbHex.append("04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:");
        sbHex.append("b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:");
        sbHex.append("39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:");
        sbHex.append("2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:");
        sbHex.append("12:b7:b8:6a:43");

        // Get rid of the ":" chars.
        int numReplaced = sbHex.replace(":", "");

        // We'll need to convert hex to base64..
        BinData bdKey = new BinData();
        bdKey.appendEncoded(sbHex.getAsString(), "hex");

        PublicKey pubKey2 = new PublicKey();
        try {
            pubKey2.loadFromString(bdKey.getEncoded("base64"));
        } catch (ChilkatException e) {
            System.out.println(e.getLastErrorText());
            return;
        }

        // Let's get the key in pubKey2 as PEM.
        // It should be idential to the PEM above.
        System.out.println(pubKey2.getPem(true));

        // Here's the output.  You can see it's the same as the PEM above..

        // -----BEGIN PUBLIC KEY-----
        // MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
        // Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
        // -----END PUBLIC KEY-----
    }
}