CODEWITHSUNDEEP
.netjvmrsa
Ian Newson
Ian Newson

Reputation: 7949

Trying to read an RSA public key generated in .NET and JVM and it's failing

, Hi, here is my .NET code:

    public static string ExportPublicKeyFile(this RSA rsa)
    {
        var privateKeyBytes = rsa.ExportRSAPublicKey();
        var builder = new StringBuilder("-----BEGIN PUBLIC KEY-----");

        var base64PrivateKeyString = Convert.ToBase64String(privateKeyBytes);
        var offset = 0;
        const int LINE_LENGTH = 64;

        for (var i = 0; i < base64PrivateKeyString.Length; ++i)
        {
            if (i % (LINE_LENGTH - 1) == 0)
                builder.Append("\n");
            builder.Append(base64PrivateKeyString[i]);
        }
        if (builder[builder.Length-1] != '\n')
            builder.Append("\n");

        builder.AppendLine("-----END PUBLIC KEY-----");
        return builder.ToString();
    }

(There's some extra fluff around the outside to save to a file)

Here's my Kotlin:

class App {
    companion object {
        @JvmStatic
        fun main(args: Array<String>) {
            //App().run()
            //val parts = "/ss/dsa".split('/')
            //println(parts)

            val key = PublicKeyReader.get("C:\\Path\\To\\public.key")

        }

        object PublicKeyReader {
            @kotlin.Throws(Exception::class)
            operator fun get(filename: String): PublicKey {
                val keyBytes: ByteArray = Files.readAllBytes(Paths.get(filename))
                val spec = X509EncodedKeySpec(keyBytes)
                val kf: java.security.KeyFactory = java.security.KeyFactory.getInstance("RSA")
                return kf.generatePublic(spec)
            }
        }

    }
}

The Kotlin isn't running an Android, it's an IntelliJ application currently.

I get the following exception when running:

Caused by: java.security.InvalidKeyException: invalid key format

I have tried removing the BEGIN\END lines manually, and it didn't help.

Upvotes: 1

Views: 270

Answers (1)

Topaco
Topaco

Reputation: 49221

In the C# code, ExportRSAPublicKey() exports the key in PKCS#1 format, DER encoded. Then the key is PEM encoded (Base64 encoding with line breaks plus header plus footer).
Presumably, with regard to header and footer, the key should be exported in X.509/SPKI format. However, for this purpose the ExportSubjectPublicKeyInfo() method must be used.

Since the Kotlin code expects the key DER encoded, on the Kotlin side a DER encoding has to be performed (removal of header, footer and line breaks, Base64 decoding).
The Kotlin code furthermore expects the key in X.509/SPKI format, which is consistent with the C# code if ExportSubjectPublicKeyInfo() is applied there.

Upvotes: 3

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