How to sign and verify Ed25519 signatures in Kotlin

1import java.io.File
2import java.security.KeyFactory
3import java.security.Signature
4import java.security.spec.PKCS8EncodedKeySpec
5import java.util.Base64
6import java.util.HexFormat
7 
8fun main() {
9    val pem = File("key.pem").readText()
10    val pkcs8 = Base64.getMimeDecoder().decode(pem.replace(Regex("-----[^-]+-----"), ""))
11    val privateKey = KeyFactory.getInstance("Ed25519").generatePrivate(PKCS8EncodedKeySpec(pkcs8))
12 
13    val signature = Signature.getInstance("Ed25519").run {
14        initSign(privateKey)
15        update(File("message.txt").readBytes()) // the message itself: Ed25519 hashes internally
16        sign()
17    }
18 
19    File("sig.bin").writeBytes(signature)
20    println("signature: ${HexFormat.of().formatHex(signature)}")
21}

1import java.io.File
2import java.security.KeyFactory
3import java.security.PublicKey
4import java.security.Signature
5import java.security.spec.X509EncodedKeySpec
6import java.util.Base64
7 
8fun isValid(key: PublicKey, message: ByteArray, signature: ByteArray): Boolean =
9    Signature.getInstance("Ed25519").run {
10        initVerify(key)
11        update(message)
12        verify(signature) // false for a wrong signature, no exception
13    }
14 
15fun main() {
16    val pem = File("pub.pem").readText()
17    val spki = Base64.getMimeDecoder().decode(pem.replace(Regex("-----[^-]+-----"), ""))
18    val publicKey = KeyFactory.getInstance("Ed25519").generatePublic(X509EncodedKeySpec(spki))
19 
20    val message = File("message.txt").readBytes()
21    val signature = File("sig.bin").readBytes()
22    println("valid: ${isValid(publicKey, message, signature)}")
23    println("tampered message valid: ${isValid(publicKey, "hello ed25520".toByteArray(), signature)}")
24}