1CRYPTO_SIGN(3MONOCYPHER) LOCAL CRYPTO_SIGN(3MONOCYPHER)
2
4 crypto_sign, crypto_check, crypto_sign_public_key — public key signatures
5
7 #include <monocypher.h>
8 void
10 crypto_sign_public_key(uint8_t public_key[32],
11 const uint8_t secret_key[32]);
12 void
14 crypto_sign(uint8_t signature[64], const uint8_t secret_key[32],
15 const uint8_t public_key[32], const uint8_t *message,
16 size_t message_size);
17 int
19 crypto_check(const uint8_t signature[64], const uint8_t public_key[32],
20 const uint8_t *message, size_t message_size);
21
23 crypto_sign() and crypto_check() provide EdDSA public key signatures and
24 verification.
25 The arguments are:
27 signature
29 The signature.
30 secret_key
32 A 32-byte random number, known only to you. See
33 intro(3monocypher) about random number generation (use your oper‐
34 ating system's random number generator). Do not use the same
35 private key for both signatures and key exchanges. The public
36 keys are different, and revealing both may leak information.
37 public_key
39 The public key, generated from secret_key with
40 crypto_sign_public_key().
41 message
43 Message to sign.
44 message_size
46 Length of message, in bytes.
47 signature and message may overlap.
49 crypto_sign_public_key() computes the public key of the specified secret
51 key.
52 crypto_sign() signs a message with secret_key. The public key is op‐
54 tional, and will be recomputed if not provided. This recomputation dou‐
55 bles the execution time.
56 crypto_check() checks that a given signature is genuine. Meaning, only
58 someone who had the private key could have signed the message. It does
59 not run in constant time. It does not have to in most threat models, be‐
60 cause nothing is secret: everyone knows the public key, and the signature
61 and message are rarely secret. If the message needs to be secret, use
62 crypto_key_exchange(3monocypher) and crypto_lock_aead(3monocypher) in‐
63 stead.
64 An incremental interface is available; see
66 crypto_sign_init_first_pass(3monocypher).
67
69 crypto_sign_public_key() and crypto_sign() return nothing.
70 crypto_check() returns 0 for legitimate messages and -1 for forgeries.
72
74 The following examples assume the existence of arc4random_buf(), which
75 fills the given buffer with cryptographically secure random bytes. If
76 arc4random_buf() does not exist on your system, see intro(3monocypher)
77 for advice about how to generate cryptographically secure random bytes.
78 Generate a public key from a random secret key:
80 uint8_t sk[32]; /* Random secret key */
82 uint8_t pk[32]; /* Matching public key */
83 arc4random_buf(sk, 32);
84 crypto_sign_public_key(pk, sk);
85 /* Wipe the secret key if it is no longer needed */
86 crypto_wipe(sk, 32);
87 Sign a message:
89 uint8_t sk [32]; /* Secret key from above */
91 const uint8_t pk [32]; /* Matching public key */
92 const uint8_t message [11] = "Lorem ipsu"; /* Message to sign */
93 uint8_t signature[64];
94 crypto_sign(signature, sk, pk, message, 10);
95 /* Wipe the secret key if it is no longer needed */
96 crypto_wipe(sk, 32);
97 Check the above:
99 const uint8_t pk [32]; /* Their public key */
101 const uint8_t message [11] = "Lorem ipsu"; /* Signed message */
102 const uint8_t signature[64]; /* Signature to check */
103 if (crypto_check(signature, pk, message, 10)) {
104 /* Message is corrupted, abort processing */
105 } else {
106 /* Message is genuine */
107 }
108
110 crypto_blake2b(3monocypher), crypto_key_exchange(3monocypher),
111 crypto_lock(3monocypher), intro(3monocypher)
112
114 These functions implement PureEdDSA with Curve25519 and Blake2b, as de‐
115 scribed in RFC 8032. This is the same as Ed25519, with Blake2b instead
116 of SHA-512.
117
119 The crypto_sign(), crypto_check(), and crypto_sign_public_key() functions
120 appeared in Monocypher 0.2.
121 Starting with Monocypher 2.0.5, modified signatures abusing the inherent
123 signature malleability property of EdDSA now cause a non-zero return
124 value of crypto_check(); in prior versions, such signatures would be ac‐
125 cepted.
126 A critical security vulnerability that caused all-zero signatures to be
128 accepted was introduced in Monocypher 0.3; it was fixed in Monocypher
129 1.1.1 and 2.0.4.
130
132 Signature malleability
133 Signature malleability is the ability of an attacker to produce a valid
134 signature with knowledge of only an existing signature and the public
135 key. That is, given a message, a signature and a public key, an attacker
136 could generate a new signature for the same message that is valid under
137 the same public key. Monocypher prevents signature malleability by only
138 accepting signatures in canonical form.
139 On the other hand, EdDSA signatures are not unique like cryptographic
141 hashes. The signing procedure is deterministic by specification and
142 crypto_sign() follows this specification. However, someone with the pri‐
143 vate key can generate arbitrarily many valid, canonical, different signa‐
144 tures of the same message. Because of this, never assume that signatures
145 are unique.
146 Fault injection and power analysis
148 Fault injection (also known as glitching) and power analysis may be used
149 to manipulate the resulting signature and recover the secret key in some
150 cases. This requires hardware access. If attackers are expected to have
151 such access and have the relevant equipment, you may try use the incre‐
152 mental interface provided by crypto_sign_init_first_pass(3monocypher) to
153 mitigate the side channel attacks. Note that there may still be other
154 power-related side channels (such as if the CPU leaks information when an
155 operation overflows a register) that must be considered.
156BSD September 26, 2020 BSD