How to verify an ECDSA signature in Go

1package main
2 
3import (
4	"crypto/ecdsa"
5	"crypto/sha256"
6	"crypto/x509"
7	"encoding/pem"
8	"fmt"
9	"os"
10)
11 
12func main() {
13	pemBytes, _ := os.ReadFile("pub.pem")
14	block, _ := pem.Decode(pemBytes) // "PUBLIC KEY" — SPKI
15	pub, err := x509.ParsePKIXPublicKey(block.Bytes)
16	if err != nil {
17		panic(err)
18	}
19	ecPub := pub.(*ecdsa.PublicKey)
20 
21	message, _ := os.ReadFile("message.txt")
22	signature, _ := os.ReadFile("sig.der") // ASN.1 DER
23 
24	digest := sha256.Sum256(message) // Go verifies a digest, not the message
25	if ecdsa.VerifyASN1(ecPub, digest[:], signature) {
26		fmt.Println("valid")
27	} else {
28		fmt.Println("INVALID")
29	}
30}

1r := new(big.Int).SetBytes(rawSig[:32]) // needs "math/big"
2s := new(big.Int).SetBytes(rawSig[32:])
3ok := ecdsa.Verify(ecPub, digest[:], r, s)

How it works

1. pem.Decode extracts the DER block from BEGIN PUBLIC KEY; x509.ParsePKIXPublicKey parses the SPKI structure and returns any — type-assert to *ecdsa.PublicKey.
2. Unlike Node.js and Python, Go does not hash for you: compute sha256.Sum256(message) and pass the digest slice.
3. ecdsa.VerifyASN1 consumes ASN.1 DER (OpenSSL's output). For raw 64-byte signatures, load each half into a big.Int and call ecdsa.Verify.

Gotchas

• Passing the message where the digest belongs is the classic Go ECDSA bug — verification simply returns false. The digest argument should be exactly the hash size (32 bytes for SHA-256).
• If your PEM says BEGIN CERTIFICATE, parse it with x509.ParseCertificate and use cert.PublicKey — ParsePKIXPublicKey does not accept certificates.
• VerifyASN1 uses a strict DER parser: trailing bytes or non-minimal integers are rejected. A signature that passes elsewhere but fails in Go may be sloppily encoded — inspect it in the converter.

Related recipes

• How to sign a message with ECDSA in Go
• How to convert an ECDSA signature between DER and raw in Go
• How to verify an ES256 JWT in Go