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// Copyright 2017 The LUCI Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package tokensigning
import (
"context"
"encoding/base64"
"github.com/golang/protobuf/proto"
"go.chromium.org/luci/common/retry/transient"
"go.chromium.org/luci/server/auth/signing"
)
// Signer knows how to sign protos and serialize/encode signed result.
type Signer struct {
// Signer is the actual signer: it knows how to sign blobs.
Signer signing.Signer
// SigningContext is prepended to the token blob before it is signed.
//
// It exists to avoid cross-protocol attacks, when same key is used to sign
// different kinds of tokens. An attacker may get a token of kind A, and use
// it in place of a token of kind B. This may produce unexpected (possibly
// bad) results, especially for proto-serialized tokens (that all use small
// integers for message tags).
//
// By using different SigningContext strings per token kind we ensure tokens
// are recognized as correctly signed only when they are used in an
// appropriate context.
//
// SigningContext should be some arbitrary constant string that designates the
// usage of the token. We actually prepend SigningContext + '\x00' to the
// token blob.
//
// If SigningContext is "", this mechanism is completely skipped.
SigningContext string
// Encoding is base64 encoding to use (or RawURLEncoding if nil).
Encoding *base64.Encoding
// Wrap takes a blob with token body, the signature and signing key details
// and returns a proto to serialize/encode and return as the final signed
// token.
Wrap func(unwrapped *Unwrapped) proto.Message
}
// SignToken serializes the body, signs it and returns serialized envelope.
//
// Produces base64 URL-safe token or an error (possibly transient).
func (s *Signer) SignToken(c context.Context, body proto.Message) (string, error) {
info, err := s.Signer.ServiceInfo(c)
if err != nil {
return "", transient.Tag.Apply(err)
}
blob, err := proto.Marshal(body)
if err != nil {
return "", err
}
withCtx := prependSigningContext(blob, s.SigningContext)
keyID, sig, err := s.Signer.SignBytes(c, withCtx)
if err != nil {
return "", transient.Tag.Apply(err)
}
tok, err := proto.Marshal(s.Wrap(&Unwrapped{
Body: blob,
RsaSHA256Sig: sig,
SignerID: info.ServiceAccountName,
KeyID: keyID,
}))
if err != nil {
return "", err
}
enc := s.Encoding
if enc == nil {
enc = base64.RawURLEncoding
}
return enc.EncodeToString(tok), nil
}