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// Copyright 2024 Google LLC
//
// 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 ecdsa
import (
"crypto/ecdsa"
"crypto/rand"
"fmt"
"hash"
"math/big"
"github.com/tink-crypto/tink-go/v2/insecuresecretdataaccess"
"github.com/tink-crypto/tink-go/v2/internal/internalapi"
internalecdsa "github.com/tink-crypto/tink-go/v2/internal/signature/ecdsa"
"github.com/tink-crypto/tink-go/v2/key"
"github.com/tink-crypto/tink-go/v2/tink"
)
// signer is an implementation of the [tink.Signer] interface for ECDSA
// (RFC6979).
type signer struct {
key *ecdsa.PrivateKey
prefix []byte
parameters *Parameters
hashFunc func() hash.Hash
}
var _ tink.Signer = (*signer)(nil)
// NewSigner creates a new instance of [Signer].
//
// It assumes that the private key k is valid.
//
// This is an internal API.
func NewSigner(k *PrivateKey, _ internalapi.Token) (tink.Signer, error) {
params := k.publicKey.parameters
hashFunc, err := hashFunctionFromEnum(params.HashType())
if err != nil {
return nil, err
}
curve, err := curveFromTinkECDSACurveType(params.CurveType())
if err != nil {
return nil, err
}
publicPoint := k.publicKey.PublicPoint()
// The point is guaranteed to be encoded as per SEC 1 v2.0, Section 2.3.3
// https://www.secg.org/sec1-v2.pdf#page=17.08.
xy := publicPoint[1:]
ecdsaPrivateKey := new(ecdsa.PrivateKey)
ecdsaPrivateKey.PublicKey.Curve = curve
ecdsaPrivateKey.PublicKey.X = new(big.Int).SetBytes(xy[:len(xy)/2])
ecdsaPrivateKey.PublicKey.Y = new(big.Int).SetBytes(xy[len(xy)/2:])
ecdsaPrivateKey.D = new(big.Int).SetBytes(k.PrivateKeyValue().Data(insecuresecretdataaccess.Token{}))
return &signer{
key: ecdsaPrivateKey,
prefix: k.OutputPrefix(),
hashFunc: hashFunc,
parameters: params,
}, nil
}
// Sign computes a signature for the given data.
//
// The returned signature is of the form: prefix || signature, where prefix is
// the key's output prefix which can be empty, and signature is the signature
// in the encoding specified by the key's parameters.
func (e *signer) Sign(data []byte) ([]byte, error) {
h := e.hashFunc()
h.Write(data)
if e.parameters.Variant() == VariantLegacy {
h.Write([]byte{0})
}
hashed := h.Sum(nil)
switch encoding := e.parameters.SignatureEncoding(); encoding {
case IEEEP1363:
r, s, err := ecdsa.Sign(rand.Reader, e.key, hashed)
if err != nil {
return nil, err
}
sig := internalecdsa.Signature{R: r, S: s}
signatureBytes, err := internalecdsa.IEEEP1363Encode(&sig, e.key.PublicKey.Curve.Params().Name)
if err != nil {
return nil, fmt.Errorf("ecdsa_signer: signing failed: %s", err)
}
return append(e.prefix, signatureBytes...), nil
case DER:
signatureBytes, err := ecdsa.SignASN1(rand.Reader, e.key, hashed)
if err != nil {
return nil, fmt.Errorf("ecdsa_signer: signing failed: %s", err)
}
return append(e.prefix, signatureBytes...), nil
default:
return nil, fmt.Errorf("ecdsa_signer: unsupported encoding: %s", encoding)
}
}
func signerConstructor(key key.Key) (any, error) {
that, ok := key.(*PrivateKey)
if !ok {
return nil, fmt.Errorf("key is not a *ecdsa.PrivateKey")
}
return NewSigner(that, internalapi.Token{})
}
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