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@dgrijalva @patbaker82 @martinlindhe @evanphx
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package jwt
import (
"crypto"
"crypto/ecdsa"
"crypto/rand"
"errors"
"math/big"
)
var (
// Sadly this is missing from crypto/ecdsa compared to crypto/rsa
ErrECDSAVerification = errors.New("crypto/ecdsa: verification error")
)
// Implements the ECDSA family of signing methods signing methods
// Expects *ecdsa.PrivateKey for signing and *ecdsa.PublicKey for verification
type SigningMethodECDSA struct {
Name string
Hash crypto.Hash
KeySize int
CurveBits int
}
// Specific instances for EC256 and company
var (
SigningMethodES256 *SigningMethodECDSA
SigningMethodES384 *SigningMethodECDSA
SigningMethodES512 *SigningMethodECDSA
)
func init() {
// ES256
SigningMethodES256 = &SigningMethodECDSA{"ES256", crypto.SHA256, 32, 256}
RegisterSigningMethod(SigningMethodES256.Alg(), func() SigningMethod {
return SigningMethodES256
})
// ES384
SigningMethodES384 = &SigningMethodECDSA{"ES384", crypto.SHA384, 48, 384}
RegisterSigningMethod(SigningMethodES384.Alg(), func() SigningMethod {
return SigningMethodES384
})
// ES512
SigningMethodES512 = &SigningMethodECDSA{"ES512", crypto.SHA512, 66, 521}
RegisterSigningMethod(SigningMethodES512.Alg(), func() SigningMethod {
return SigningMethodES512
})
}
func (m *SigningMethodECDSA) Alg() string {
return m.Name
}
// Implements the Verify method from SigningMethod
// For this verify method, key must be an ecdsa.PublicKey struct
func (m *SigningMethodECDSA) Verify(signingString, signature string, key interface{}) error {
var err error
// Decode the signature
var sig []byte
if sig, err = DecodeSegment(signature); err != nil {
return err
}
// Get the key
var ecdsaKey *ecdsa.PublicKey
switch k := key.(type) {
case *ecdsa.PublicKey:
ecdsaKey = k
default:
return ErrInvalidKeyType
}
if len(sig) != 2*m.KeySize {
return ErrECDSAVerification
}
r := big.NewInt(0).SetBytes(sig[:m.KeySize])
s := big.NewInt(0).SetBytes(sig[m.KeySize:])
// Create hasher
if !m.Hash.Available() {
return ErrHashUnavailable
}
hasher := m.Hash.New()
hasher.Write([]byte(signingString))
// Verify the signature
if verifystatus := ecdsa.Verify(ecdsaKey, hasher.Sum(nil), r, s); verifystatus == true {
return nil
} else {
return ErrECDSAVerification
}
}
// Implements the Sign method from SigningMethod
// For this signing method, key must be an ecdsa.PrivateKey struct
func (m *SigningMethodECDSA) Sign(signingString string, key interface{}) (string, error) {
// Get the key
var ecdsaKey *ecdsa.PrivateKey
switch k := key.(type) {
case *ecdsa.PrivateKey:
ecdsaKey = k
default:
return "", ErrInvalidKeyType
}
// Create the hasher
if !m.Hash.Available() {
return "", ErrHashUnavailable
}
hasher := m.Hash.New()
hasher.Write([]byte(signingString))
// Sign the string and return r, s
if r, s, err := ecdsa.Sign(rand.Reader, ecdsaKey, hasher.Sum(nil)); err == nil {
curveBits := ecdsaKey.Curve.Params().BitSize
if m.CurveBits != curveBits {
return "", ErrInvalidKey
}
keyBytes := curveBits / 8
if curveBits%8 > 0 {
keyBytes += 1
}
// We serialize the outpus (r and s) into big-endian byte arrays and pad
// them with zeros on the left to make sure the sizes work out. Both arrays
// must be keyBytes long, and the output must be 2*keyBytes long.
rBytes := r.Bytes()
rBytesPadded := make([]byte, keyBytes)
copy(rBytesPadded[keyBytes-len(rBytes):], rBytes)
sBytes := s.Bytes()
sBytesPadded := make([]byte, keyBytes)
copy(sBytesPadded[keyBytes-len(sBytes):], sBytes)
out := append(rBytesPadded, sBytesPadded...)
return EncodeSegment(out), nil
} else {
return "", err
}
}
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