cmd: add tlsinfo.

cmd/tlsinfo/README.txt

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+tlsinfo: show TLS version, cipher, and peer certificates
+---------------------------------------------------------
+
+Description
+  tlsinfo connects to a TLS server and prints the negotiated TLS version and
+  cipher suite, followed by details for each certificate in the server’s
+  presented chain (as provided by the server).
+
+Usage
+  tlsinfo <hostname:port>
+
+Output
+  The program prints the negotiated protocol and cipher, then one section per
+  certificate in the order received from the server. Example fields:
+    TLS Version: TLS 1.3
+    Cipher Suite: TLS_AES_128_GCM_SHA256
+    Certificate 1
+        Subject: CN=example.com, O=Example Corp, C=US
+        Issuer: CN=Example Root CA, O=Example Corp, C=US
+        DNS Names: [example.com www.example.com]
+        Not Before: 2025-01-01 00:00:00 +0000 UTC
+        Not After:  2026-01-01 23:59:59 +0000 UTC
+
+Examples
+  # Inspect a public HTTPS endpoint
+  tlsinfo example.com:443
+
+Notes
+  - Verification is intentionally disabled (InsecureSkipVerify=true). The tool
+    does not validate the server certificate or hostname; it is for inspection
+    only.
+  - The SNI/ServerName is inferred from <hostname> when applicable.
+  - You must specify a port (e.g., 443 for HTTPS).
+  - The entire certificate chain is printed exactly as presented by the server.

cmd/tlsinfo/main.go

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+package main
+
+import (
+	"crypto/tls"
+	"crypto/x509"
+	"fmt"
+	"os"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Printf("Usage: %s ‹hostname:port>\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	hostPort := os.Args[1]
+	conn, err := tls.Dial("tcp", hostPort, &tls.Config{
+		InsecureSkipVerify: true,
+	})
+
+	if err != nil {
+		fmt.Printf("Failed to connect to the TLS server: %v\n", err)
+		os.Exit(1)
+	}
+	defer conn.Close()
+	state := conn.ConnectionState()
+	printConnectionDetails(state)
+}
+
+func printConnectionDetails(state tls.ConnectionState) {
+	version := tlsVersion(state.Version)
+	cipherSuite := tls.CipherSuiteName(state.CipherSuite)
+	fmt.Printf("TLS Version: %s\n", version)
+	fmt.Printf("Cipher Suite: %s\n", cipherSuite)
+	printPeerCertificates(state.PeerCertificates)
+}
+
+func tlsVersion(version uint16) string {
+	switch version {
+
+	case tls.VersionTLS13:
+		return "TLS 1.3"
+	case tls.VersionTLS12:
+
+		return "TLS 1.2"
+	case tls.VersionTLS11:
+		return "TLS 1.1"
+	case tls.VersionTLS10:
+		return "TLS 1.0"
+	default:
+		return "Unknown"
+	}
+}
+
+func printPeerCertificates(certificates []*x509.Certificate) {
+	for i, cert := range certificates {
+		fmt.Printf("Certificate %d\n", i+1)
+		fmt.Printf("\tSubject: %s\n", cert.Subject)
+		fmt.Printf("\tIssuer: %s\n", cert.Issuer)
+		fmt.Printf("\tDNS Names:	%v\n", cert.DNSNames)
+		fmt.Printf("\tNot Before: %s\n:", cert.NotBefore)
+		fmt.Printf("\tNot After: %s\n", cert.NotAfter)
+	}
+}