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hmacsha.go
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hmacsha.go
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// Package hmac is the default implementation for generating and validating challenges. It uses HMAC-SHA256 to
// generate and validate challenges.
package hmac
import (
"encoding/base64"
"fmt"
"strings"
"sync"
"github.com/gtank/cryptopasta"
"github.com/ory/fosite"
"github.com/pkg/errors"
)
// HMACStrategy is responsible for generating and validating challenges.
type HMACStrategy struct {
AuthCodeEntropy int
GlobalSecret []byte
sync.Mutex
}
const (
// key should be at least 256 bit long, making it
minimumEntropy = 32
// the secrets (client and global) should each have at least 16 characters making it harder to guess them
minimumSecretLength = 32
)
var b64 = base64.URLEncoding.WithPadding(base64.NoPadding)
// Generate generates a token and a matching signature or returns an error.
// This method implements rfc6819 Section 5.1.4.2.2: Use High Entropy for Secrets.
func (c *HMACStrategy) Generate() (string, string, error) {
c.Lock()
defer c.Unlock()
if len(c.GlobalSecret) < minimumSecretLength {
return "", "", errors.Errorf("Secret for signing HMAC-SHA256 is expected to be 32 byte long, got %d byte", len(c.GlobalSecret))
}
var signingKey [32]byte
copy(signingKey[:], c.GlobalSecret)
if c.AuthCodeEntropy < minimumEntropy {
c.AuthCodeEntropy = minimumEntropy
}
// When creating secrets not intended for usage by human users (e.g.,
// client secrets or token handles), the authorization server should
// include a reasonable level of entropy in order to mitigate the risk
// of guessing attacks. The token value should be >=128 bits long and
// constructed from a cryptographically strong random or pseudo-random
// number sequence (see [RFC4086] for best current practice) generated
// by the authorization server.
tokenKey, err := RandomBytes(c.AuthCodeEntropy)
if err != nil {
return "", "", errors.WithStack(err)
}
signature := cryptopasta.GenerateHMAC(tokenKey, &signingKey)
encodedSignature := b64.EncodeToString(signature)
encodedToken := fmt.Sprintf("%s.%s", b64.EncodeToString(tokenKey), encodedSignature)
return encodedToken, encodedSignature, nil
}
// Validate validates a token and returns its signature or an error if the token is not valid.
func (c *HMACStrategy) Validate(token string) error {
if len(c.GlobalSecret) < minimumSecretLength {
return errors.Errorf("Secret for signing HMAC-SHA256 is expected to be 32 byte long, got %d byte", len(c.GlobalSecret))
}
var signingKey [32]byte
copy(signingKey[:], c.GlobalSecret)
split := strings.Split(token, ".")
if len(split) != 2 {
return errors.WithStack(fosite.ErrInvalidTokenFormat)
}
tokenKey := split[0]
tokenSignature := split[1]
if tokenKey == "" || tokenSignature == "" {
return errors.WithStack(fosite.ErrInvalidTokenFormat)
}
decodedTokenSignature, err := b64.DecodeString(tokenSignature)
if err != nil {
return errors.WithStack(err)
}
decodedTokenKey, err := b64.DecodeString(tokenKey)
if err != nil {
return errors.WithStack(err)
}
if !cryptopasta.CheckHMAC(decodedTokenKey, decodedTokenSignature, &signingKey) {
// Hash is invalid
return errors.WithStack(fosite.ErrTokenSignatureMismatch)
}
return nil
}
func (c *HMACStrategy) Signature(token string) string {
split := strings.Split(token, ".")
if len(split) != 2 {
return ""
}
return split[1]
}