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package dkim
import (
"crypto"
"crypto/rsa"
"crypto/x509"
"encoding/base64"
"errors"
"fmt"
"net"
"strings"
"golang.org/x/crypto/ed25519"
)
type verifier interface {
Public() crypto.PublicKey
Verify(hash crypto.Hash, hashed []byte, sig []byte) error
}
type rsaVerifier struct {
*rsa.PublicKey
}
func (v rsaVerifier) Public() crypto.PublicKey {
return v.PublicKey
}
func (v rsaVerifier) Verify(hash crypto.Hash, hashed, sig []byte) error {
return rsa.VerifyPKCS1v15(v.PublicKey, hash, hashed, sig)
}
type ed25519Verifier struct {
ed25519.PublicKey
}
func (v ed25519Verifier) Public() crypto.PublicKey {
return v.PublicKey
}
func (v ed25519Verifier) Verify(hash crypto.Hash, hashed, sig []byte) error {
if !ed25519.Verify(v.PublicKey, hashed, sig) {
return errors.New("dkim: invalid Ed25519 signature")
}
return nil
}
type queryResult struct {
Verifier verifier
KeyAlgo string
HashAlgos []string
Notes string
Services []string
Flags []string
}
// QueryMethod is a DKIM query method.
type QueryMethod string
const (
// DNS TXT resource record (RR) lookup algorithm
QueryMethodDNSTXT QueryMethod = "dns/txt"
)
type txtLookupFunc func(domain string) ([]string, error)
type queryFunc func(domain, selector string, txtLookup txtLookupFunc) (*queryResult, error)
var queryMethods = map[QueryMethod]queryFunc{
QueryMethodDNSTXT: queryDNSTXT,
}
func queryDNSTXT(domain, selector string, txtLookup txtLookupFunc) (*queryResult, error) {
var txts []string
var err error
if txtLookup != nil {
txts, err = txtLookup(selector + "._domainkey." + domain)
} else {
txts, err = net.LookupTXT(selector + "._domainkey." + domain)
}
if netErr, ok := err.(net.Error); ok && netErr.Temporary() {
return nil, tempFailError("key unavailable: " + err.Error())
} else if err != nil {
return nil, permFailError("no key for signature: " + err.Error())
}
// Long keys are split in multiple parts
txt := strings.Join(txts, "")
return parsePublicKey(txt)
}
func parsePublicKey(s string) (*queryResult, error) {
params, err := parseHeaderParams(s)
if err != nil {
return nil, permFailError("key syntax error: " + err.Error())
}
res := new(queryResult)
if v, ok := params["v"]; ok && v != "DKIM1" {
return nil, permFailError("incompatible public key version")
}
p, ok := params["p"]
if !ok {
return nil, permFailError("key syntax error: missing public key data")
}
if p == "" {
return nil, permFailError("key revoked")
}
p = strings.ReplaceAll(p, " ", "")
b, err := base64.StdEncoding.DecodeString(p)
if err != nil {
return nil, permFailError("key syntax error: " + err.Error())
}
switch params["k"] {
case "rsa", "":
pub, err := x509.ParsePKIXPublicKey(b)
if err != nil {
// RFC 6376 is inconsistent about whether RSA public keys should
// be formatted as RSAPublicKey or SubjectPublicKeyInfo.
// Erratum 3017 (https://www.rfc-editor.org/errata/eid3017) proposes
// allowing both.
pub, err = x509.ParsePKCS1PublicKey(b)
if err != nil {
return nil, permFailError("key syntax error: " + err.Error())
}
}
rsaPub, ok := pub.(*rsa.PublicKey)
if !ok {
return nil, permFailError("key syntax error: not an RSA public key")
}
// RFC 8301 section 3.2: verifiers MUST NOT consider signatures using
// RSA keys of less than 1024 bits as valid signatures.
if rsaPub.Size()*8 < 1024 {
return nil, permFailError(fmt.Sprintf("key is too short: want 1024 bits, has %v bits", rsaPub.Size()*8))
}
res.Verifier = rsaVerifier{rsaPub}
res.KeyAlgo = "rsa"
case "ed25519":
if len(b) != ed25519.PublicKeySize {
return nil, permFailError(fmt.Sprintf("invalid Ed25519 public key size: %v bytes", len(b)))
}
ed25519Pub := ed25519.PublicKey(b)
res.Verifier = ed25519Verifier{ed25519Pub}
res.KeyAlgo = "ed25519"
default:
return nil, permFailError("unsupported key algorithm")
}
if hashesStr, ok := params["h"]; ok {
res.HashAlgos = parseTagList(hashesStr)
}
if notes, ok := params["n"]; ok {
res.Notes = notes
}
if servicesStr, ok := params["s"]; ok {
services := parseTagList(servicesStr)
hasWildcard := false
for _, s := range services {
if s == "*" {
hasWildcard = true
break
}
}
if !hasWildcard {
res.Services = services
}
}
if flagsStr, ok := params["t"]; ok {
res.Flags = parseTagList(flagsStr)
}
return res, nil
}
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