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jwt.go
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jwt.go
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/*
Copyright 2014 The Kubernetes 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 serviceaccount
import (
"crypto"
"crypto/ecdsa"
"crypto/elliptic"
"crypto/rsa"
"crypto/x509"
"encoding/base64"
"fmt"
jose "gopkg.in/square/go-jose.v2"
"gopkg.in/square/go-jose.v2/jwt"
)
type TokenGenerator interface {
// GenerateToken generates a token which will identify the given
// ServiceAccount. privateClaims is an interface that will be
// serialized into the JWT payload JSON encoding at the root level of
// the payload object. Public claims take precedent over private
// claims i.e. if both claims and privateClaims have an "exp" field,
// the value in claims will be used.
GenerateToken(claims *jwt.Claims, privateClaims interface{}) (string, error)
}
// JWTTokenGenerator returns a TokenGenerator that generates signed JWT tokens, using the given privateKey.
// privateKey is a PEM-encoded byte array of a private RSA key.
func JWTTokenGenerator(iss string, privateKey interface{}) (TokenGenerator, error) {
var signer jose.Signer
var err error
switch pk := privateKey.(type) {
case *rsa.PrivateKey:
signer, err = signerFromRSAPrivateKey(pk)
if err != nil {
return nil, fmt.Errorf("could not generate signer for RSA keypair: %w", err)
}
case *ecdsa.PrivateKey:
signer, err = signerFromECDSAPrivateKey(pk)
if err != nil {
return nil, fmt.Errorf("could not generate signer for ECDSA keypair: %w", err)
}
case jose.OpaqueSigner:
signer, err = signerFromOpaqueSigner(pk)
if err != nil {
return nil, fmt.Errorf("could not generate signer for OpaqueSigner: %w", err)
}
default:
return nil, fmt.Errorf("unknown private key type %T, must be *rsa.PrivateKey, *ecdsa.PrivateKey, or jose.OpaqueSigner", privateKey)
}
return &jwtTokenGenerator{
iss: iss,
signer: signer,
}, nil
}
// keyIDFromPublicKey derives a key ID non-reversibly from a public key.
//
// The Key ID is field on a given on JWTs and JWKs that help relying parties
// pick the correct key for verification when the identity party advertises
// multiple keys.
//
// Making the derivation non-reversible makes it impossible for someone to
// accidentally obtain the real key from the key ID and use it for token
// validation.
func keyIDFromPublicKey(publicKey interface{}) (string, error) {
publicKeyDERBytes, err := x509.MarshalPKIXPublicKey(publicKey)
if err != nil {
return "", fmt.Errorf("failed to serialize public key to DER format: %w", err)
}
hasher := crypto.SHA256.New()
_, _ = hasher.Write(publicKeyDERBytes)
publicKeyDERHash := hasher.Sum(nil)
keyID := base64.RawURLEncoding.EncodeToString(publicKeyDERHash)
return keyID, nil
}
func signerFromRSAPrivateKey(keyPair *rsa.PrivateKey) (jose.Signer, error) {
keyID, err := keyIDFromPublicKey(&keyPair.PublicKey)
if err != nil {
return nil, fmt.Errorf("failed to derive keyID: %w", err)
}
// IMPORTANT: If this function is updated to support additional key sizes,
// algorithmForPublicKey in serviceaccount/openidmetadata.go must also be
// updated to support the same key sizes. Today we only support RS256.
// Wrap the RSA keypair in a JOSE JWK with the designated key ID.
privateJWK := &jose.JSONWebKey{
Algorithm: string(jose.RS256),
Key: keyPair,
KeyID: keyID,
Use: "sig",
}
signer, err := jose.NewSigner(
jose.SigningKey{
Algorithm: jose.RS256,
Key: privateJWK,
},
nil,
)
if err != nil {
return nil, fmt.Errorf("failed to create signer: %w", err)
}
return signer, nil
}
func signerFromECDSAPrivateKey(keyPair *ecdsa.PrivateKey) (jose.Signer, error) {
var alg jose.SignatureAlgorithm
switch keyPair.Curve {
case elliptic.P256():
alg = jose.ES256
case elliptic.P384():
alg = jose.ES384
case elliptic.P521():
alg = jose.ES512
default:
return nil, fmt.Errorf("unknown private key curve, must be 256, 384, or 521")
}
keyID, err := keyIDFromPublicKey(&keyPair.PublicKey)
if err != nil {
return nil, fmt.Errorf("failed to derive keyID: %w", err)
}
// Wrap the ECDSA keypair in a JOSE JWK with the designated key ID.
privateJWK := &jose.JSONWebKey{
Algorithm: string(alg),
Key: keyPair,
KeyID: keyID,
Use: "sig",
}
signer, err := jose.NewSigner(
jose.SigningKey{
Algorithm: alg,
Key: privateJWK,
},
nil,
)
if err != nil {
return nil, fmt.Errorf("failed to create signer: %w", err)
}
return signer, nil
}
func signerFromOpaqueSigner(opaqueSigner jose.OpaqueSigner) (jose.Signer, error) {
alg := jose.SignatureAlgorithm(opaqueSigner.Public().Algorithm)
signer, err := jose.NewSigner(
jose.SigningKey{
Algorithm: alg,
Key: &jose.JSONWebKey{
Algorithm: string(alg),
Key: opaqueSigner,
KeyID: opaqueSigner.Public().KeyID,
Use: "sig",
},
},
nil,
)
if err != nil {
return nil, fmt.Errorf("failed to create signer: %w", err)
}
return signer, nil
}
type jwtTokenGenerator struct {
iss string
signer jose.Signer
}
func (j *jwtTokenGenerator) GenerateToken(claims *jwt.Claims, privateClaims interface{}) (string, error) {
// claims are applied in reverse precedence
return jwt.Signed(j.signer).
Claims(privateClaims).
Claims(claims).
Claims(&jwt.Claims{
Issuer: j.iss,
}).
CompactSerialize()
}