certificates/acme/challenge.go

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package acme
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import (
"context"
"crypto"
"crypto/sha256"
"crypto/subtle"
"crypto/tls"
"encoding/asn1"
"encoding/base64"
"encoding/hex"
"encoding/json"
"errors"
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"fmt"
"io"
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"net"
"net/url"
"reflect"
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"strings"
"time"
"go.step.sm/crypto/jose"
)
type ChallengeType string
const (
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// HTTP01 is the http-01 ACME challenge type
HTTP01 ChallengeType = "http-01"
// DNS01 is the dns-01 ACME challenge type
DNS01 ChallengeType = "dns-01"
// TLSALPN01 is the tls-alpn-01 ACME challenge type
TLSALPN01 ChallengeType = "tls-alpn-01"
)
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// Challenge represents an ACME response Challenge type.
type Challenge struct {
ID string `json:"-"`
AccountID string `json:"-"`
AuthorizationID string `json:"-"`
Value string `json:"-"`
Type ChallengeType `json:"type"`
Status Status `json:"status"`
Token string `json:"token"`
ValidatedAt string `json:"validated,omitempty"`
URL string `json:"url"`
Error *Error `json:"error,omitempty"`
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}
// ToLog enables response logging.
func (ch *Challenge) ToLog() (interface{}, error) {
b, err := json.Marshal(ch)
if err != nil {
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return nil, WrapErrorISE(err, "error marshaling challenge for logging")
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}
return string(b), nil
}
// Validate attempts to validate the challenge. Stores changes to the Challenge
// type using the DB interface.
// satisfactorily validated, the 'status' and 'validated' attributes are
// updated.
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func (ch *Challenge) Validate(ctx context.Context, db DB, jwk *jose.JSONWebKey) error {
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// If already valid or invalid then return without performing validation.
if ch.Status != StatusPending {
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return nil
}
switch ch.Type {
case HTTP01:
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return http01Validate(ctx, ch, db, jwk)
case DNS01:
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return dns01Validate(ctx, ch, db, jwk)
case TLSALPN01:
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return tlsalpn01Validate(ctx, ch, db, jwk)
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default:
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return NewErrorISE("unexpected challenge type '%s'", ch.Type)
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}
}
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func http01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error {
u := &url.URL{Scheme: "http", Host: http01ChallengeHost(ch.Value), Path: fmt.Sprintf("/.well-known/acme-challenge/%s", ch.Token)}
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vc := MustClientFromContext(ctx)
resp, err := vc.Get(u.String())
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if err != nil {
return storeError(ctx, db, ch, false, WrapError(ErrorConnectionType, err,
"error doing http GET for url %s", u))
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}
defer resp.Body.Close()
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if resp.StatusCode >= 400 {
return storeError(ctx, db, ch, false, NewError(ErrorConnectionType,
"error doing http GET for url %s with status code %d", u, resp.StatusCode))
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}
body, err := io.ReadAll(resp.Body)
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if err != nil {
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return WrapErrorISE(err, "error reading "+
"response body for url %s", u)
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}
keyAuth := strings.TrimSpace(string(body))
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expected, err := KeyAuthorization(ch.Token, jwk)
if err != nil {
return err
}
if keyAuth != expected {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"keyAuthorization does not match; expected %s, but got %s", expected, keyAuth))
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}
// Update and store the challenge.
ch.Status = StatusValid
ch.Error = nil
ch.ValidatedAt = clock.Now().Format(time.RFC3339)
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if err = db.UpdateChallenge(ctx, ch); err != nil {
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return WrapErrorISE(err, "error updating challenge")
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}
return nil
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}
// http01ChallengeHost checks if a Challenge value is an IPv6 address
// and adds square brackets if that's the case, so that it can be used
// as a hostname. Returns the original Challenge value as the host to
// use in other cases.
func http01ChallengeHost(value string) string {
if ip := net.ParseIP(value); ip != nil && ip.To4() == nil {
value = "[" + value + "]"
}
return value
}
func tlsAlert(err error) uint8 {
var opErr *net.OpError
if errors.As(err, &opErr) {
v := reflect.ValueOf(opErr.Err)
if v.Kind() == reflect.Uint8 {
return uint8(v.Uint())
}
}
return 0
}
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func tlsalpn01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error {
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config := &tls.Config{
NextProtos: []string{"acme-tls/1"},
// https://tools.ietf.org/html/rfc8737#section-4
// ACME servers that implement "acme-tls/1" MUST only negotiate TLS 1.2
// [RFC5246] or higher when connecting to clients for validation.
MinVersion: tls.VersionTLS12,
ServerName: serverName(ch),
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InsecureSkipVerify: true, // we expect a self-signed challenge certificate
}
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hostPort := net.JoinHostPort(ch.Value, "443")
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vc := MustClientFromContext(ctx)
conn, err := vc.TLSDial("tcp", hostPort, config)
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if err != nil {
// With Go 1.17+ tls.Dial fails if there's no overlap between configured
// client and server protocols. When this happens the connection is
// closed with the error no_application_protocol(120) as required by
// RFC7301. See https://golang.org/doc/go1.17#ALPN
if tlsAlert(err) == 120 {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
"cannot negotiate ALPN acme-tls/1 protocol for tls-alpn-01 challenge"))
}
return storeError(ctx, db, ch, false, WrapError(ErrorConnectionType, err,
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"error doing TLS dial for %s", hostPort))
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}
defer conn.Close()
cs := conn.ConnectionState()
certs := cs.PeerCertificates
if len(certs) == 0 {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"%s challenge for %s resulted in no certificates", ch.Type, ch.Value))
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}
if cs.NegotiatedProtocol != "acme-tls/1" {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"cannot negotiate ALPN acme-tls/1 protocol for tls-alpn-01 challenge"))
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}
leafCert := certs[0]
// if no DNS names present, look for IP address and verify that exactly one exists
if len(leafCert.DNSNames) == 0 {
if len(leafCert.IPAddresses) != 1 || !leafCert.IPAddresses[0].Equal(net.ParseIP(ch.Value)) {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"incorrect certificate for tls-alpn-01 challenge: leaf certificate must contain a single IP address or DNS name, %v", ch.Value))
}
} else {
if len(leafCert.DNSNames) != 1 || !strings.EqualFold(leafCert.DNSNames[0], ch.Value) {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"incorrect certificate for tls-alpn-01 challenge: leaf certificate must contain a single IP address or DNS name, %v", ch.Value))
}
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}
idPeAcmeIdentifier := asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 31}
idPeAcmeIdentifierV1Obsolete := asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 30, 1}
foundIDPeAcmeIdentifierV1Obsolete := false
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keyAuth, err := KeyAuthorization(ch.Token, jwk)
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if err != nil {
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return err
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}
hashedKeyAuth := sha256.Sum256([]byte(keyAuth))
for _, ext := range leafCert.Extensions {
if idPeAcmeIdentifier.Equal(ext.Id) {
if !ext.Critical {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"incorrect certificate for tls-alpn-01 challenge: acmeValidationV1 extension not critical"))
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}
var extValue []byte
rest, err := asn1.Unmarshal(ext.Value, &extValue)
if err != nil || len(rest) > 0 || len(hashedKeyAuth) != len(extValue) {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"incorrect certificate for tls-alpn-01 challenge: malformed acmeValidationV1 extension value"))
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}
if subtle.ConstantTimeCompare(hashedKeyAuth[:], extValue) != 1 {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"incorrect certificate for tls-alpn-01 challenge: "+
"expected acmeValidationV1 extension value %s for this challenge but got %s",
hex.EncodeToString(hashedKeyAuth[:]), hex.EncodeToString(extValue)))
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}
ch.Status = StatusValid
ch.Error = nil
ch.ValidatedAt = clock.Now().Format(time.RFC3339)
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if err = db.UpdateChallenge(ctx, ch); err != nil {
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return WrapErrorISE(err, "tlsalpn01ValidateChallenge - error updating challenge")
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}
return nil
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}
if idPeAcmeIdentifierV1Obsolete.Equal(ext.Id) {
foundIDPeAcmeIdentifierV1Obsolete = true
}
}
if foundIDPeAcmeIdentifierV1Obsolete {
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"incorrect certificate for tls-alpn-01 challenge: obsolete id-pe-acmeIdentifier in acmeValidationV1 extension"))
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}
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType,
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"incorrect certificate for tls-alpn-01 challenge: missing acmeValidationV1 extension"))
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}
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func dns01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey) error {
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// Normalize domain for wildcard DNS names
// This is done to avoid making TXT lookups for domains like
// _acme-challenge.*.example.com
// Instead perform txt lookup for _acme-challenge.example.com
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domain := strings.TrimPrefix(ch.Value, "*.")
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vc := MustClientFromContext(ctx)
txtRecords, err := vc.LookupTxt("_acme-challenge." + domain)
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if err != nil {
return storeError(ctx, db, ch, false, WrapError(ErrorDNSType, err,
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"error looking up TXT records for domain %s", domain))
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}
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expectedKeyAuth, err := KeyAuthorization(ch.Token, jwk)
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if err != nil {
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return err
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}
h := sha256.Sum256([]byte(expectedKeyAuth))
expected := base64.RawURLEncoding.EncodeToString(h[:])
var found bool
for _, r := range txtRecords {
if r == expected {
found = true
break
}
}
if !found {
return storeError(ctx, db, ch, false, NewError(ErrorRejectedIdentifierType,
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"keyAuthorization does not match; expected %s, but got %s", expectedKeyAuth, txtRecords))
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}
// Update and store the challenge.
ch.Status = StatusValid
ch.Error = nil
ch.ValidatedAt = clock.Now().Format(time.RFC3339)
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if err = db.UpdateChallenge(ctx, ch); err != nil {
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return WrapErrorISE(err, "error updating challenge")
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}
return nil
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}
// serverName determines the SNI HostName to set based on an acme.Challenge
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// for TLS-ALPN-01 challenges RFC8738 states that, if HostName is an IP, it
// should be the ARPA address https://datatracker.ietf.org/doc/html/rfc8738#section-6.
// It also references TLS Extensions [RFC6066].
func serverName(ch *Challenge) string {
var serverName string
ip := net.ParseIP(ch.Value)
if ip != nil {
serverName = reverseAddr(ip)
} else {
serverName = ch.Value
}
return serverName
}
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// reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
// address addr suitable for rDNS (PTR) record lookup or an error if it fails
// to parse the IP address.
// Implementation taken and adapted from https://golang.org/src/net/dnsclient.go?s=780:834#L20
func reverseAddr(ip net.IP) (arpa string) {
if ip.To4() != nil {
return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa."
}
// Must be IPv6
buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
// Add it, in reverse, to the buffer
for i := len(ip) - 1; i >= 0; i-- {
v := ip[i]
buf = append(buf, hexit[v&0xF],
'.',
hexit[v>>4],
'.')
}
// Append "ip6.arpa." and return (buf already has the final .)
buf = append(buf, "ip6.arpa."...)
return string(buf)
}
// Convert unsigned integer to decimal string.
// Implementation taken from https://golang.org/src/net/parse.go
func uitoa(val uint) string {
if val == 0 { // avoid string allocation
return "0"
}
var buf [20]byte // big enough for 64bit value base 10
i := len(buf) - 1
for val >= 10 {
q := val / 10
buf[i] = byte('0' + val - q*10)
i--
val = q
}
// val < 10
buf[i] = byte('0' + val)
return string(buf[i:])
}
const hexit = "0123456789abcdef"
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// KeyAuthorization creates the ACME key authorization value from a token
// and a jwk.
func KeyAuthorization(token string, jwk *jose.JSONWebKey) (string, error) {
thumbprint, err := jwk.Thumbprint(crypto.SHA256)
if err != nil {
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return "", WrapErrorISE(err, "error generating JWK thumbprint")
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}
encPrint := base64.RawURLEncoding.EncodeToString(thumbprint)
return fmt.Sprintf("%s.%s", token, encPrint), nil
}
// storeError the given error to an ACME error and saves using the DB interface.
func storeError(ctx context.Context, db DB, ch *Challenge, markInvalid bool, err *Error) error {
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ch.Error = err
if markInvalid {
ch.Status = StatusInvalid
}
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if err := db.UpdateChallenge(ctx, ch); err != nil {
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return WrapErrorISE(err, "failure saving error to acme challenge")
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}
return nil
}