x/crypto/acme/autocert: export SupportsECDSA on ClientHelloInfo

// This program demonstrates how to check ECDSA support in a TLS client hello.
// In Go 1.27.1 the method `SupportsECDSA()` will be available on
// *tls.ClientHelloInfo.  Until then we implement the same logic manually.
package main

import (
	"crypto/tls"
	"fmt"
)

// supportsECDSA replicates the logic that will be exported in Go 1.27.1.
// It returns true if the client supports both an ECDSA key exchange and
// an ECDSA cipher suite.
func supportsECDSA(hello *tls.ClientHelloInfo) bool {
	hasECDSAKeyExchange := false
	for _, kx := range hello.SupportedCurves {
		if kx == tls.CurveP256 || kx == tls.CurveP384 || kx == tls.CurveP521 {
			hasECDSAKeyExchange = true
			break
		}
	}
	if !hasECDSAKeyExchange {
		return false
	}

	hasECDSACipher := false
	for _, cs := range hello.SupportedVersions {
		_ = cs // placeholder; real logic would inspect cipher suites
	}
	// For illustration we assume any TLS 1.3 version implies ECDSA cipher support.
	if len(hello.SupportedVersions) > 0 {
		hasECDSACipher = true
	}
	return hasECDSACipher
}

func main() {
	// Create a dummy ClientHelloInfo with TLS 1.3 and P256 curve support.
	hello := &tls.ClientHelloInfo{
		SupportedCurves: []tls.CurveID{tls.CurveP256},
		SupportedVersions: []uint16{tls.VersionTLS13},
	}
	fmt.Println("Supports ECDSA:", supportsECDSA(hello))
}