How to sign and verify Ed25519 signatures in Go

1package main
2 
3import (
4	"crypto/ed25519"
5	"crypto/x509"
6	"encoding/hex"
7	"encoding/pem"
8	"fmt"
9	"log"
10	"os"
11)
12 
13func main() {
14	pemBytes, err := os.ReadFile("key.pem")
15	if err != nil {
16		log.Fatal(err)
17	}
18	block, _ := pem.Decode(pemBytes) // "PRIVATE KEY" — PKCS#8
19	if block == nil {
20		log.Fatal("key.pem: no PEM block")
21	}
22	parsed, err := x509.ParsePKCS8PrivateKey(block.Bytes)
23	if err != nil {
24		log.Fatal(err)
25	}
26	privateKey, ok := parsed.(ed25519.PrivateKey) // a value type, not a pointer
27	if !ok {
28		log.Fatalf("expected an Ed25519 key, got %T", parsed)
29	}
30 
31	message, err := os.ReadFile("message.txt")
32	if err != nil {
33		log.Fatal(err)
34	}
35	signature := ed25519.Sign(privateKey, message) // the message itself, not a digest
36 
37	if err := os.WriteFile("sig.bin", signature, 0o644); err != nil {
38		log.Fatal(err)
39	}
40	fmt.Println("signature:", hex.EncodeToString(signature))
41}
42 

1package main
2 
3import (
4	"crypto/ed25519"
5	"crypto/x509"
6	"encoding/pem"
7	"fmt"
8	"log"
9	"os"
10)
11 
12func main() {
13	pemBytes, err := os.ReadFile("pub.pem")
14	if err != nil {
15		log.Fatal(err)
16	}
17	block, _ := pem.Decode(pemBytes) // "PUBLIC KEY" — SPKI
18	if block == nil {
19		log.Fatal("pub.pem: no PEM block")
20	}
21	parsed, err := x509.ParsePKIXPublicKey(block.Bytes)
22	if err != nil {
23		log.Fatal(err)
24	}
25	publicKey, ok := parsed.(ed25519.PublicKey)
26	if !ok {
27		log.Fatalf("expected an Ed25519 key, got %T", parsed)
28	}
29 
30	message, err := os.ReadFile("message.txt")
31	if err != nil {
32		log.Fatal(err)
33	}
34	signature, err := os.ReadFile("sig.bin")
35	if err != nil {
36		log.Fatal(err)
37	}
38 
39	fmt.Println("valid:", ed25519.Verify(publicKey, message, signature))
40	fmt.Println("tampered message valid:", ed25519.Verify(publicKey, []byte("hello ed25520"), signature))
41}
42