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eab.go
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eab.go
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package api
import (
"context"
"encoding/json"
"errors"
"go.step.sm/crypto/jose"
"github.com/smallstep/certificates/acme"
)
// ExternalAccountBinding represents the ACME externalAccountBinding JWS
type ExternalAccountBinding struct {
Protected string `json:"protected"`
Payload string `json:"payload"`
Sig string `json:"signature"`
}
// validateExternalAccountBinding validates the externalAccountBinding property in a call to new-account.
func validateExternalAccountBinding(ctx context.Context, nar *NewAccountRequest) (*acme.ExternalAccountKey, error) {
acmeProv, err := acmeProvisionerFromContext(ctx)
if err != nil {
return nil, acme.WrapErrorISE(err, "could not load ACME provisioner from context")
}
if !acmeProv.RequireEAB {
//nolint:nilnil // legacy
return nil, nil
}
if nar.ExternalAccountBinding == nil {
return nil, acme.NewError(acme.ErrorExternalAccountRequiredType, "no external account binding provided")
}
eabJSONBytes, err := json.Marshal(nar.ExternalAccountBinding)
if err != nil {
return nil, acme.WrapErrorISE(err, "error marshaling externalAccountBinding into bytes")
}
eabJWS, err := jose.ParseJWS(string(eabJSONBytes))
if err != nil {
return nil, acme.WrapErrorISE(err, "error parsing externalAccountBinding jws")
}
// TODO(hs): implement strategy pattern to allow for different ways of verification (i.e. webhook call) based on configuration?
keyID, acmeErr := validateEABJWS(ctx, eabJWS)
if acmeErr != nil {
return nil, acmeErr
}
db := acme.MustDatabaseFromContext(ctx)
externalAccountKey, err := db.GetExternalAccountKey(ctx, acmeProv.ID, keyID)
if err != nil {
var ae *acme.Error
if errors.As(err, &ae) {
return nil, acme.WrapError(acme.ErrorUnauthorizedType, err, "the field 'kid' references an unknown key")
}
return nil, acme.WrapErrorISE(err, "error retrieving external account key")
}
if externalAccountKey == nil {
return nil, acme.NewError(acme.ErrorUnauthorizedType, "the field 'kid' references an unknown key")
}
if len(externalAccountKey.HmacKey) == 0 {
return nil, acme.NewError(acme.ErrorServerInternalType, "external account binding key with id '%s' does not have secret bytes", keyID)
}
if externalAccountKey.AlreadyBound() {
return nil, acme.NewError(acme.ErrorUnauthorizedType, "external account binding key with id '%s' was already bound to account '%s' on %s", keyID, externalAccountKey.AccountID, externalAccountKey.BoundAt)
}
payload, err := eabJWS.Verify(externalAccountKey.HmacKey)
if err != nil {
return nil, acme.WrapErrorISE(err, "error verifying externalAccountBinding signature")
}
jwk, err := jwkFromContext(ctx)
if err != nil {
return nil, err
}
var payloadJWK *jose.JSONWebKey
if err = json.Unmarshal(payload, &payloadJWK); err != nil {
return nil, acme.WrapError(acme.ErrorMalformedType, err, "error unmarshaling payload into jwk")
}
if !keysAreEqual(jwk, payloadJWK) {
return nil, acme.NewError(acme.ErrorUnauthorizedType, "keys in jws and eab payload do not match")
}
return externalAccountKey, nil
}
// keysAreEqual performs an equality check on two JWKs by comparing
// the (base64 encoding) of the Key IDs.
func keysAreEqual(x, y *jose.JSONWebKey) bool {
if x == nil || y == nil {
return false
}
digestX, errX := acme.KeyToID(x)
digestY, errY := acme.KeyToID(y)
if errX != nil || errY != nil {
return false
}
return digestX == digestY
}
// validateEABJWS verifies the contents of the External Account Binding JWS.
// The protected header of the JWS MUST meet the following criteria:
//
// - The "alg" field MUST indicate a MAC-based algorithm
// - The "kid" field MUST contain the key identifier provided by the CA
// - The "nonce" field MUST NOT be present
// - The "url" field MUST be set to the same value as the outer JWS
func validateEABJWS(ctx context.Context, jws *jose.JSONWebSignature) (string, *acme.Error) {
if jws == nil {
return "", acme.NewErrorISE("no JWS provided")
}
if len(jws.Signatures) != 1 {
return "", acme.NewError(acme.ErrorMalformedType, "JWS must have one signature")
}
header := jws.Signatures[0].Protected
algorithm := header.Algorithm
keyID := header.KeyID
nonce := header.Nonce
if !(algorithm == jose.HS256 || algorithm == jose.HS384 || algorithm == jose.HS512) {
return "", acme.NewError(acme.ErrorMalformedType, "'alg' field set to invalid algorithm '%s'", algorithm)
}
if keyID == "" {
return "", acme.NewError(acme.ErrorMalformedType, "'kid' field is required")
}
if nonce != "" {
return "", acme.NewError(acme.ErrorMalformedType, "'nonce' must not be present")
}
jwsURL, ok := header.ExtraHeaders["url"]
if !ok {
return "", acme.NewError(acme.ErrorMalformedType, "'url' field is required")
}
outerJWS, err := jwsFromContext(ctx)
if err != nil {
return "", acme.WrapErrorISE(err, "could not retrieve outer JWS from context")
}
if len(outerJWS.Signatures) != 1 {
return "", acme.NewError(acme.ErrorMalformedType, "outer JWS must have one signature")
}
outerJWSURL, ok := outerJWS.Signatures[0].Protected.ExtraHeaders["url"]
if !ok {
return "", acme.NewError(acme.ErrorMalformedType, "'url' field must be set in outer JWS")
}
if jwsURL != outerJWSURL {
return "", acme.NewError(acme.ErrorMalformedType, "'url' field is not the same value as the outer JWS")
}
return keyID, nil
}