Use constant size generic parameter for random bytes generation

All uses of `get_random()` were in the form of:

  `&get_random(vec![0u8; SIZE])`

with `SIZE` being a constant.

Building a `Vec` is unnecessary for two reasons. First, it uses a
very short-lived dynamic memory allocation. Second, a `Vec` is a
resizable object, which is useless in those context when random
data have a fixed size and will only be read.

`get_random_bytes()` takes a constant as a generic parameter and
returns an array with the requested number of random bytes.

Stack safety analysis: the random bytes will be allocated on the
caller stack for a very short time (until the encoding function has
been called on the data). In some cases, the random bytes take
less room than the `Vec` did (a `Vec` is 24 bytes on a 64 bit
computer). The maximum used size is 180 bytes, which makes it
for 0.008% of the default stack size for a Rust thread (2MiB),
so this is a non-issue.

Also, most of the uses of those random bytes are to encode them
using an `Encoding`. The function `crypto::encode_random_bytes()`
generates random bytes and encode them with the provided
`Encoding`, leading to code deduplication.

`generate_id()` has also been converted to use a constant generic
parameter as well since the length of the requested String is always
a constant.

1 files changed, 14 insertions, 7 deletions

diff --git a/src/crypto.rs b/src/crypto.rs
index be9680cb..daf5124d 100644
--- a/src/crypto.rs
+++ b/src/crypto.rs
@@ -3,7 +3,7 @@
 //
 use std::num::NonZeroU32;
 
-use data_encoding::HEXLOWER;
+use data_encoding::{Encoding, HEXLOWER};
 use ring::{digest, hmac, pbkdf2};
 
 static DIGEST_ALG: pbkdf2::Algorithm = pbkdf2::PBKDF2_HMAC_SHA256;
@@ -38,17 +38,24 @@ pub fn hmac_sign(key: &str, data: &str) -> String {
 //
 
 pub fn get_random_64() -> Vec<u8> {
-    get_random(vec![0u8; 64])
+    get_random_bytes::<64>().to_vec()
 }
 
-pub fn get_random(mut array: Vec<u8>) -> Vec<u8> {
+/// Return an array holding `N` random bytes.
+pub fn get_random_bytes<const N: usize>() -> [u8; N] {
     use ring::rand::{SecureRandom, SystemRandom};
 
+    let mut array = [0; N];
     SystemRandom::new().fill(&mut array).expect("Error generating random values");
 
     array
 }
 
+/// Encode random bytes using the provided function.
+pub fn encode_random_bytes<const N: usize>(e: Encoding) -> String {
+    e.encode(&get_random_bytes::<N>())
+}
+
 /// Generates a random string over a specified alphabet.
 pub fn get_random_string(alphabet: &[u8], num_chars: usize) -> String {
     // Ref: https://rust-lang-nursery.github.io/rust-cookbook/algorithms/randomness.html
@@ -77,18 +84,18 @@ pub fn get_random_string_alphanum(num_chars: usize) -> String {
     get_random_string(ALPHABET, num_chars)
 }
 
-pub fn generate_id(num_bytes: usize) -> String {
-    HEXLOWER.encode(&get_random(vec![0; num_bytes]))
+pub fn generate_id<const N: usize>() -> String {
+    encode_random_bytes::<N>(HEXLOWER)
 }
 
 pub fn generate_send_id() -> String {
     // Send IDs are globally scoped, so make them longer to avoid collisions.
-    generate_id(32) // 256 bits
+    generate_id::<32>() // 256 bits
 }
 
 pub fn generate_attachment_id() -> String {
     // Attachment IDs are scoped to a cipher, so they can be smaller.
-    generate_id(10) // 80 bits
+    generate_id::<10>() // 80 bits
 }
 
 /// Generates a numeric token for email-based verifications.