Reworking bssl_crypto: don't use zero keys in examples.

Someone _probably_ wouldn't copy–paste an example to the point of using
a zero key but, just in case, since we have `rand_array` now, we might
as well use it so that the examples are safe.

Change-Id: I289cb8f27894e8f0429fbbed37a1db3ea4295ffd
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/65567
Reviewed-by: Bob Beck <bbe@google.com>

rust/bssl-crypto/src/aead.rs

@@ -31,11 +31,11 @@
 //! ```
 //! use bssl_crypto::aead::{Aead, Aes256Gcm};
 //!
-//! let key = [0u8; 32];
+//! let key = bssl_crypto::rand_array();
 //! let aead = Aes256Gcm::new(&key);
 //!
 //! let mut message_counter: u64 = 0;
-//! let mut nonce = [0u8; 12];
+//! let mut nonce = bssl_crypto::rand_array();
 //! nonce[4..].copy_from_slice(message_counter.to_be_bytes().as_slice());
 //! message_counter += 1;
 //! let plaintext = b"message";

rust/bssl-crypto/src/aes.rs

@@ -29,7 +29,7 @@
 //! ```
 //! use bssl_crypto::aes;
 //!
-//! let key_bytes = [0u8; 32];
+//! let key_bytes = bssl_crypto::rand_array();
 //! let enc_key = aes::EncryptKey::new_256(&key_bytes);
 //! let block = [0u8; aes::BLOCK_SIZE];
 //! let mut transformed_block = enc_key.encrypt(&block);

rust/bssl-crypto/src/hmac.rs

@@ -30,7 +30,7 @@
 //! ```
 //! use bssl_crypto::hmac::HmacSha256;
 //!
-//! let key = [0u8; 32];
+//! let key = bssl_crypto::rand_array();
 //! let mut ctx = HmacSha256::new(&key);
 //! ctx.update(b"hel");
 //! ctx.update(b"lo");
@@ -46,7 +46,7 @@
 //! ```
 //! use bssl_crypto::hmac::HmacSha256;
 //!
-//! let key = [0u8; 32];
+//! let key = bssl_crypto::rand_array();
 //! let mut keyed_ctx = HmacSha256::new(&key);
 //! let mut ctx1 = keyed_ctx.clone();
 //! ctx1.update(b"foo");