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Understanding a few ECDSA Public Key Formats

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Describes a few ECDSA public key formats. Zig

const std = @import("std");
const chilkat = @import("chilkat");

pub fn main(init: std.process.Init) !void {
    const alloc = init.arena.allocator();

    // 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:

    const sb_pem = try chilkat.StringBuilder.init();
    defer sb_pem.deinit();
    sb_pem.append("-----BEGIN PUBLIC KEY-----\r\n") catch {};
    sb_pem.append("MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET\r\n") catch {};
    sb_pem.append("Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==\r\n") catch {};
    sb_pem.append("-----END PUBLIC KEY-----\r\n") catch {};

    const pub_key1 = try chilkat.PublicKey.init();
    defer pub_key1.deinit();
    pub_key1.loadFromString(try sb_pem.getAsString(alloc)) catch {
        std.debug.print("{s}\n", .{try pub_key1.getLastErrorText(alloc)});
        return;
    };

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

    // Get rid of the ":" chars.
    _ = sb_hex.replace(":", "");

    // We'll need to convert hex to base64..
    const bd_key = try chilkat.BinData.init();
    defer bd_key.deinit();
    bd_key.appendEncoded(try sb_hex.getAsString(alloc), "hex") catch {};

    const pub_key2 = try chilkat.PublicKey.init();
    defer pub_key2.deinit();
    pub_key2.loadFromString(try bd_key.getEncoded(alloc, "base64")) catch {
        std.debug.print("{s}\n", .{try pub_key2.getLastErrorText(alloc)});
        return;
    };

    // Let's get the key in pubKey2 as PEM.
    // It should be idential to the PEM above.
    std.debug.print("{s}\n", .{try pub_key2.getPem(alloc, 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-----
}