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// Copyright 2021 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 signature
import (
"crypto"
"crypto/rsa"
"fmt"
"hash"
"math/big"
"github.com/tink-crypto/tink-go/v2/subtle"
commonpb "github.com/tink-crypto/tink-go/v2/proto/common_go_proto"
)
const (
rsaMinModulusSizeInBits = 2048
rsaDefaultPublicExponent = 65537
)
// RSAValidModulusSizeInBits the size in bits for an RSA key.
func RSAValidModulusSizeInBits(m int) error {
if m < rsaMinModulusSizeInBits {
return fmt.Errorf("modulus size too small, must be >= %d", rsaMinModulusSizeInBits)
}
return nil
}
// RSAValidPublicExponent validates a public RSA exponent.
func RSAValidPublicExponent(e int) error {
// crypto/rsa uses the following hardcoded public exponent value.
if e != rsaDefaultPublicExponent {
return fmt.Errorf("invalid public exponent")
}
return nil
}
// HashSafeForSignature checks whether a hash function is safe to use with digital signatures
// that require collision resistance.
func HashSafeForSignature(hashAlg string) error {
switch hashAlg {
case "SHA256", "SHA384", "SHA512":
return nil
default:
return fmt.Errorf("hash function not safe for digital signatures: %q", hashAlg)
}
}
// ValidateRSAPublicKeyParams validates a public RSA key parameters.
func ValidateRSAPublicKeyParams(hashAlg commonpb.HashType, modSizeBits int, pubExponent []byte) error {
if err := HashSafeForSignature(commonpb.HashType_name[int32(hashAlg)]); err != nil {
return err
}
if err := RSAValidModulusSizeInBits(modSizeBits); err != nil {
return err
}
e := new(big.Int).SetBytes(pubExponent)
if !e.IsInt64() {
return fmt.Errorf("public exponent can't fit in a 64 bit integer")
}
return RSAValidPublicExponent(int(e.Int64()))
}
func validRSAPublicKey(publicKey *rsa.PublicKey) error {
if err := RSAValidModulusSizeInBits(publicKey.N.BitLen()); err != nil {
return err
}
return RSAValidPublicExponent(publicKey.E)
}
func hashID(hashAlg string) (crypto.Hash, error) {
switch hashAlg {
case "SHA256":
return crypto.SHA256, nil
case "SHA384":
return crypto.SHA384, nil
case "SHA512":
return crypto.SHA512, nil
default:
return 0, fmt.Errorf("invalid hash function: %q", hashAlg)
}
}
func rsaHashFunc(hashAlg string) (func() hash.Hash, crypto.Hash, error) {
if err := HashSafeForSignature(hashAlg); err != nil {
return nil, 0, err
}
hashFunc := subtle.GetHashFunc(hashAlg)
if hashFunc == nil {
return nil, 0, fmt.Errorf("invalid hash function: %q", hashAlg)
}
hashID, err := hashID(hashAlg)
if err != nil {
return nil, 0, err
}
return hashFunc, hashID, nil
}
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