1package jwt
2
3import (
4 "crypto"
5 "crypto/hmac"
6 "errors"
7)
8
9// SigningMethodHMAC implements the HMAC-SHA family of signing methods.
10// Expects key type of []byte for both signing and validation
11type SigningMethodHMAC struct {
12 Name string
13 Hash crypto.Hash
14}
15
16// Specific instances for HS256 and company
17var (
18 SigningMethodHS256 *SigningMethodHMAC
19 SigningMethodHS384 *SigningMethodHMAC
20 SigningMethodHS512 *SigningMethodHMAC
21 ErrSignatureInvalid = errors.New("signature is invalid")
22)
23
24func init() {
25 // HS256
26 SigningMethodHS256 = &SigningMethodHMAC{"HS256", crypto.SHA256}
27 RegisterSigningMethod(SigningMethodHS256.Alg(), func() SigningMethod {
28 return SigningMethodHS256
29 })
30
31 // HS384
32 SigningMethodHS384 = &SigningMethodHMAC{"HS384", crypto.SHA384}
33 RegisterSigningMethod(SigningMethodHS384.Alg(), func() SigningMethod {
34 return SigningMethodHS384
35 })
36
37 // HS512
38 SigningMethodHS512 = &SigningMethodHMAC{"HS512", crypto.SHA512}
39 RegisterSigningMethod(SigningMethodHS512.Alg(), func() SigningMethod {
40 return SigningMethodHS512
41 })
42}
43
44func (m *SigningMethodHMAC) Alg() string {
45 return m.Name
46}
47
48// Verify implements token verification for the SigningMethod. Returns nil if
49// the signature is valid. Key must be []byte.
50//
51// Note it is not advised to provide a []byte which was converted from a 'human
52// readable' string using a subset of ASCII characters. To maximize entropy, you
53// should ideally be providing a []byte key which was produced from a
54// cryptographically random source, e.g. crypto/rand. Additional information
55// about this, and why we intentionally are not supporting string as a key can
56// be found on our usage guide
57// https://golang-jwt.github.io/jwt/usage/signing_methods/#signing-methods-and-key-types.
58func (m *SigningMethodHMAC) Verify(signingString string, sig []byte, key interface{}) error {
59 // Verify the key is the right type
60 keyBytes, ok := key.([]byte)
61 if !ok {
62 return newError("HMAC verify expects []byte", ErrInvalidKeyType)
63 }
64
65 // Can we use the specified hashing method?
66 if !m.Hash.Available() {
67 return ErrHashUnavailable
68 }
69
70 // This signing method is symmetric, so we validate the signature
71 // by reproducing the signature from the signing string and key, then
72 // comparing that against the provided signature.
73 hasher := hmac.New(m.Hash.New, keyBytes)
74 hasher.Write([]byte(signingString))
75 if !hmac.Equal(sig, hasher.Sum(nil)) {
76 return ErrSignatureInvalid
77 }
78
79 // No validation errors. Signature is good.
80 return nil
81}
82
83// Sign implements token signing for the SigningMethod. Key must be []byte.
84//
85// Note it is not advised to provide a []byte which was converted from a 'human
86// readable' string using a subset of ASCII characters. To maximize entropy, you
87// should ideally be providing a []byte key which was produced from a
88// cryptographically random source, e.g. crypto/rand. Additional information
89// about this, and why we intentionally are not supporting string as a key can
90// be found on our usage guide https://golang-jwt.github.io/jwt/usage/signing_methods/.
91func (m *SigningMethodHMAC) Sign(signingString string, key interface{}) ([]byte, error) {
92 if keyBytes, ok := key.([]byte); ok {
93 if !m.Hash.Available() {
94 return nil, ErrHashUnavailable
95 }
96
97 hasher := hmac.New(m.Hash.New, keyBytes)
98 hasher.Write([]byte(signingString))
99
100 return hasher.Sum(nil), nil
101 }
102
103 return nil, newError("HMAC sign expects []byte", ErrInvalidKeyType)
104}