1INTRO(3MONOCYPHER) LOCAL INTRO(3MONOCYPHER)
2
4 intro — introduction to Monocypher
5
7 Monocypher is a cryptographic library. It provides functions for authen‐
8 ticated encryption, hashing, password hashing and key derivation, key ex‐
9 change, and public key signatures.
10 Authenticated encryption
12 crypto_lock(3monocypher) and crypto_unlock(3monocypher) use the Chacha20
13 cipher and the Poly1305 one-time authenticator.
14 Chacha20 is a stream cipher based on a cryptographic hash function. It
16 runs efficiently on a wide variety of hardware, and unlike AES naturally
17 runs in constant time on all hardware.
18 Poly1305 is a one-time authenticator, derived from Carter & Wegman uni‐
20 versal hashing. It is very fast and very simple.
21 For specialised needs, crypto_chacha20(3monocypher) and
23 crypto_poly1305(3monocypher) are available to implement constructions in‐
24 volving them. Whenever possible, crypto_lock(3monocypher) should be pre‐
25 ferred, however.
26 Hashing
28 crypto_blake2b(3monocypher) implements the Blake2b hash. Blake2b com‐
29 bines the security of SHA-3 and the speed of MD5. It is immune to length
30 extension attacks and provides a keyed mode that makes it a safe, easy to
31 use authenticator.
32 Password hashing and key derivation
34 crypto_argon2i(3monocypher) implements the Argon2i resource intensive
35 hash algorithm, which can be used to hash passwords for storage and to
36 derive keys from passwords. Argon2 won the password hashing competition
37 in 2015. Unlike Scrypt, Argon2i is immune to timing attacks.
38 Key exchange
40 crypto_key_exchange(3monocypher) implements X25519, an elliptic curve
41 Diffie Hellman key exchange algorithm based on Curve25519. X25519 de‐
42 rives a shared secret from two private/public key pairs. It is fast,
43 simple, and relatively easy to implement securely.
44 For specialised protocols that require indistinguishability from random
46 noise, crypto_curve_to_hidden(3monocypher) gives the option to disguise
47 ephemeral (one-time use) X25519 public keys as random noise.
48 Public key signatures
50 crypto_sign(3monocypher) and crypto_check(3monocypher) implement EdDSA,
51 with Curve25519 and Blake2b. This is the same as the more famous
52 Ed25519, with SHA-512 replaced by the faster and more secure Blake2b.
53 For highly specialised needs, it is possible to use a custom hash func‐
55 tion with EdDSA; see
56 crypto_sign_init_first_pass_custom_hash(3monocypher).
57 Constant time comparison
59 crypto_verify16(3monocypher), crypto_verify32(3monocypher), and
60 crypto_verify64(3monocypher) compare buffers in constant time. They
61 should be used to compare secrets to prevent timing attacks.
62 Memory wipe
64 crypto_wipe(3monocypher) wipes a buffer. It is meant to erase secrets
65 when they are no longer needed, to reduce the chances of leaks.
66 Optional code
68 If Monocypher was compiled and installed with USE_ED25519, SHA-512 func‐
69 tions become available as well. See crypto_ed25519_sign(3monocypher),
70 crypto_ed25519_sign_init_first_pass(3monocypher),
71 crypto_sha512(3monocypher), and crypto_hmac_sha512(3monocypher).
72
74 crypto_argon2i(3monocypher), crypto_argon2i_general(3monocypher),
75 crypto_blake2b(3monocypher), crypto_blake2b_final(3monocypher),
76 crypto_blake2b_general(3monocypher),
77 crypto_blake2b_general_init(3monocypher),
78 crypto_blake2b_init(3monocypher), crypto_blake2b_update(3monocypher),
79 crypto_chacha20(3monocypher), crypto_chacha20_ctr(3monocypher),
80 crypto_check(3monocypher), crypto_check_final(3monocypher),
81 crypto_check_init(3monocypher),
82 crypto_check_init_custom_hash(3monocypher),
83 crypto_check_update(3monocypher), crypto_curve_to_hidden(3monocypher),
84 crypto_from_eddsa_private(3monocypher),
85 crypto_from_eddsa_public(3monocypher), crypto_hchacha20(3monocypher),
86 crypto_hidden_key_pair(3monocypher), crypto_hidden_to_curve(3monocypher),
87 crypto_ietf_chacha20(3monocypher), crypto_ietf_chacha20_ctr(3monocypher),
88 crypto_key_exchange(3monocypher), crypto_lock(3monocypher),
89 crypto_lock_aead(3monocypher), crypto_poly1305(3monocypher),
90 crypto_poly1305_final(3monocypher), crypto_poly1305_init(3monocypher),
91 crypto_poly1305_update(3monocypher), crypto_sign(3monocypher),
92 crypto_sign_final(3monocypher), crypto_sign_init_first_pass(3monocypher),
93 crypto_sign_init_first_pass_custom_hash(3monocypher),
94 crypto_sign_init_second_pass(3monocypher),
95 crypto_sign_public_key(3monocypher),
96 crypto_sign_public_key_custom_hash(3monocypher),
97 crypto_sign_update(3monocypher), crypto_unlock(3monocypher),
98 crypto_unlock_aead(3monocypher), crypto_verify16(3monocypher),
99 crypto_verify32(3monocypher), crypto_verify64(3monocypher),
100 crypto_wipe(3monocypher), crypto_x25519(3monocypher),
101 crypto_x25519_dirty_fast(3monocypher),
102 crypto_x25519_dirty_small(3monocypher),
103 crypto_x25519_inverse(3monocypher),
104 crypto_x25519_public_key(3monocypher), crypto_xchacha20(3monocypher),
105 crypto_xchacha20_ctr(3monocypher)
106 Optional code
108 crypto_from_ed25519_private(3monocypher),
109 crypto_from_ed25519_public(3monocypher),
110 crypto_ed25519_check(3monocypher),
111 crypto_ed25519_check_init(3monocypher),
112 crypto_ed25519_check_update(3monocypher),
113 crypto_ed25519_check_final(3monocypher),
114 crypto_ed25519_public_key(3monocypher), crypto_ed25519_sign(3monocypher),
115 crypto_ed25519_sign_init_first_pass(3monocypher),
116 crypto_ed25519_sign_init_second_pass(3monocypher),
117 crypto_ed25519_sign_final(3monocypher), crypto_hmac_sha512(3monocypher),
118 crypto_hmac_sha512_init(3monocypher),
119 crypto_hmac_sha512_update(3monocypher),
120 crypto_hmac_sha512_final(3monocypher) crypto_sha512(3monocypher),
121 crypto_sha512_init(3monocypher), crypto_sha512_update(3monocypher),
122 crypto_sha512_final(3monocypher)
123
125 Using cryptography securely is difficult. Flaws that never manifest un‐
126 der normal use might be exploited by a clever adversary. Cryptographic
127 libraries are easy to misuse. Even Monocypher allows a number of fatal
128 mistakes.
129 Users should follow a formal introduction to cryptography. We currently
131 recommend the Crypto 101 online course: https://www.crypto101.io/
132 Random number generation
134 Use the facilities of your operating system. Avoid user space random
135 number generators. They are easy to misuse, which has lead to countless
136 vulnerabilities in the past. For instance, the random stream may be re‐
137 peated if one is not careful with multi-threading, and forward secrecy is
138 lost without proper key erasure.
139 Different system calls are available on different systems:
141 • Recent versions of Linux (glibc >= 2.25, Linux >= 3.17), provide
143 getrandom() in <linux/random.h>. Do not set any flag.
144 • BSD, Darwin/macOS, illumos as well as Solaris provide
146 arc4random_buf() in <stdlib.h> or <bsd/stdlib.h>. This is easier to
147 use than getrandom().
148 • Windows provides BCryptGenRandom().
150 The /dev/urandom special file may be used on systems that do not provide
152 an easy to use system call. Be careful though, being a file makes
153 /dev/urandom hard to use correctly and securely. Reads may be inter‐
154 rupted, and more attacks are possible on a file than on a system call.
155 Timing attacks
157 Monocypher runs in “constant time”. There is no flow from secrets to
158 timings. No secret dependent indices, no secret dependent branches.
159 Nevertheless, there are a couple important caveats.
160 Comparing secrets should be done with constant-time comparison functions,
162 such as crypto_verify16(3monocypher), crypto_verify32(3monocypher), or
163 crypto_verify64(3monocypher). Do not use standard comparison functions.
164 They tend to stop as soon as a difference is spotted. In many cases,
165 this enables attackers to recover the secrets and destroy all security.
166 The Poly1305 authenticator, X25519, and EdDSA use multiplication. Some
168 older processors do not multiply in constant time. If the target plat‐
169 form is something other than Intel or AMD x86_64, double check how it
170 handles multiplication. In particular, ARM Cortex-M CPUs may lack
171 constant-time multiplication. Some VIA Nano x86 and x86_64 CPUs may lack
172 constant-time multiplication as well.
173 Data compression
175 Encryption does not hide the length of the input plaintext. Most com‐
176 pression algorithms work by using fewer bytes to encode previously seen
177 data or common characters. If an attacker can add data to the input be‐
178 fore it is compressed and encrypted, they can observe changes to the ci‐
179 phertext length to recover secrets from the input. Researchers have
180 demonstrated an attack on HTTPS to steal session cookies when compression
181 is enabled, dubbed “CRIME”.
182 Forward secrecy
184 Long term secrets cannot be expected to stay safe indefinitely. Users
185 may reveal them by mistake, or the host computer might have a vulnerabil‐
186 ity and be compromised. To mitigate this problem, some protocols guaran‐
187 tee that past messages are not compromised even if the long term keys
188 are. This is done by generating temporary keys, then encrypting messages
189 using them.
190 In general, secrets that went through a computer should not be compro‐
192 mised when this computer is stolen or infected at a later point.
193 A first layer of defence is to explicitly wipe secrets as soon as they
195 are no longer used. Monocypher already wipes its own temporary buffers,
196 and contexts are erased with the crypto_*_final() functions. The secret
197 keys and messages however are the responsibility of the user. Use
198 crypto_wipe(3monocypher) to erase them.
199 A second layer of defence is to ensure those secrets are not swapped to
201 disk while they are used. There are several ways to do this. The most
202 secure is to disable swapping entirely. Doing so is recommended on sen‐
203 sitive machines. Another way is to encrypt the swap partition (this is
204 less safe). Finally, swap can be disabled locally – this is often the
205 only way.
206 UNIX systems can disable swap for specific buffers with mlock(), and dis‐
208 able swap for the whole process with mlockall(). Windows can disable
209 swap for specific buffers with VirtualLock().
210 Core dumps cause similar problems. Disable them. Also beware of suspend
212 to disk (deep sleep mode), which writes all RAM to disk regardless of
213 swap policy, as well as virtual machine snapshots. Erasing secrets with
214 crypto_wipe(3monocypher) is often the only way to mitigate these dangers.
215 Undefined behaviour
217 Monocypher is a C library. C is notoriously unsafe. Using Monocypher
218 incorrectly can trigger undefined behaviour. This can lead to data cor‐
219 ruption, data theft, or even arbitrary code execution.
220 Consider binding to a safe language if possible.
222BSD March 31, 2020 BSD