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// Copyright 2017 Google Inc.
//
// 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 gcpkms
import (
"encoding/base64"
"fmt"
"hash/crc32"
"google.golang.org/api/cloudkms/v1"
"github.com/tink-crypto/tink-go/v2/tink"
)
// gcpAEAD represents a GCP KMS service to a particular URI.
type gcpAEAD struct {
keyName string
kms cloudkms.Service
}
var _ tink.AEAD = (*gcpAEAD)(nil)
// newGCPAEAD returns a new GCP KMS service.
func newGCPAEAD(keyName string, kms *cloudkms.Service) tink.AEAD {
return &gcpAEAD{
keyName: keyName,
kms: *kms,
}
}
// Encrypt calls GCP KMS to encrypt the plaintext with associatedData and returns the resulting ciphertext.
// It returns an error if the call to KMS fails or if the response returned by KMS does not pass integrity verification
// (http://cloud.google.com/kms/docs/data-integrity-guidelines#calculating_and_verifying_checksums).
func (a *gcpAEAD) Encrypt(plaintext, associatedData []byte) ([]byte, error) {
req := &cloudkms.EncryptRequest{
Plaintext: base64.URLEncoding.EncodeToString(plaintext),
PlaintextCrc32c: computeChecksum(plaintext),
AdditionalAuthenticatedData: base64.URLEncoding.EncodeToString(associatedData),
AdditionalAuthenticatedDataCrc32c: computeChecksum(associatedData),
// Send the integrity verification fields even if their value is 0.
ForceSendFields: []string{"PlaintextCrc32c", "AdditionalAuthenticatedDataCrc32c"},
}
resp, err := a.kms.Projects.Locations.KeyRings.CryptoKeys.Encrypt(a.keyName, req).Do()
if err != nil {
return nil, err
}
if !resp.VerifiedPlaintextCrc32c {
return nil, fmt.Errorf("KMS request for %q is missing the checksum field plaintext_crc32c, and other information may be missing from the response. Please retry a limited number of times in case the error is transient", a.keyName)
}
if !resp.VerifiedAdditionalAuthenticatedDataCrc32c {
return nil, fmt.Errorf("KMS request for %q is missing the checksum field additional_authenticated_data_crc32c, and other information may be missing from the response. Please retry a limited number of times in case the error is transient", a.keyName)
}
ciphertext, err := base64.StdEncoding.DecodeString(resp.Ciphertext)
if err != nil {
return nil, err
}
if resp.CiphertextCrc32c != computeChecksum(ciphertext) {
return nil, fmt.Errorf("KMS response corrupted in transit for %q: the checksum in field ciphertext_crc32c did not match the data in field ciphertext. Please retry in case this is a transient error", a.keyName)
}
return ciphertext, nil
}
// Decrypt calls GCP KMS to decrypt the ciphertext with with associatedData and returns the resulting plaintext.
// It returns an error if the call to KMS fails or if the response returned by KMS does not pass integrity verification
// (http://cloud.google.com/kms/docs/data-integrity-guidelines#calculating_and_verifying_checksums).
func (a *gcpAEAD) Decrypt(ciphertext, associatedData []byte) ([]byte, error) {
req := &cloudkms.DecryptRequest{
Ciphertext: base64.URLEncoding.EncodeToString(ciphertext),
CiphertextCrc32c: computeChecksum(ciphertext),
AdditionalAuthenticatedData: base64.URLEncoding.EncodeToString(associatedData),
AdditionalAuthenticatedDataCrc32c: computeChecksum(associatedData),
// Send the integrity verification fields even if their value is 0.
ForceSendFields: []string{"CiphertextCrc32c", "AdditionalAuthenticatedDataCrc32c"},
}
resp, err := a.kms.Projects.Locations.KeyRings.CryptoKeys.Decrypt(a.keyName, req).Do()
if err != nil {
return nil, err
}
plaintext, err := base64.StdEncoding.DecodeString(resp.Plaintext)
if err != nil {
return nil, err
}
if resp.PlaintextCrc32c != computeChecksum(plaintext) {
return nil, fmt.Errorf("KMS response corrupted in transit for %q: the checksum in field plaintext_crc32c did not match the data in field plaintext. Please retry in case this is a transient error", a.keyName)
}
return plaintext, nil
}
// crc32cTable is used to compute checksums. It is defined as a package level variable to avoid
// re-computation on every CRC calculation.
var crc32cTable = crc32.MakeTable(crc32.Castagnoli)
// computeChecksum returns the checksum that corresponds to the input value as an int64.
func computeChecksum(value []byte) int64 {
return int64(crc32.Checksum(value, crc32cTable))
}
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