1package jwt
2
3import (
4	"crypto"
5	"crypto/hmac"
6	"errors"
7)
8
9// Implements the HMAC-SHA family of signing methods 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 the signature of HSXXX tokens.  Returns nil if the signature is valid.
49func (m *SigningMethodHMAC) Verify(signingString, signature string, key interface{}) error {
50	// Verify the key is the right type
51	keyBytes, ok := key.([]byte)
52	if !ok {
53		return ErrInvalidKeyType
54	}
55
56	// Decode signature, for comparison
57	sig, err := DecodeSegment(signature)
58	if err != nil {
59		return err
60	}
61
62	// Can we use the specified hashing method?
63	if !m.Hash.Available() {
64		return ErrHashUnavailable
65	}
66
67	// This signing method is symmetric, so we validate the signature
68	// by reproducing the signature from the signing string and key, then
69	// comparing that against the provided signature.
70	hasher := hmac.New(m.Hash.New, keyBytes)
71	hasher.Write([]byte(signingString))
72	if !hmac.Equal(sig, hasher.Sum(nil)) {
73		return ErrSignatureInvalid
74	}
75
76	// No validation errors.  Signature is good.
77	return nil
78}
79
80// Implements the Sign method from SigningMethod for this signing method.
81// Key must be []byte
82func (m *SigningMethodHMAC) Sign(signingString string, key interface{}) (string, error) {
83	if keyBytes, ok := key.([]byte); ok {
84		if !m.Hash.Available() {
85			return "", ErrHashUnavailable
86		}
87
88		hasher := hmac.New(m.Hash.New, keyBytes)
89		hasher.Write([]byte(signingString))
90
91		return EncodeSegment(hasher.Sum(nil)), nil
92	}
93
94	return "", ErrInvalidKeyType
95}
96