[PM-5693] KeyStore implementation

[dani-garcia]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-5693

📔 Objective

Migrated from bitwarden/sdk-sm#979

I've been looking into using memfd_secret to protect keys in memory on Linux.

The memfd_secret API is similar to malloc in that we get some memory range allocated for us, but this memory is only visible to the process that has access to the file descriptor, which should protect us from any memory dumping from anything other than a kernel driver.

https://man.archlinux.org/man/memfd_secret.2.en

Using this requires changing how we store keys, so this is the perfect moment to go through removing the raw keys from the API surface.

Changes

• Added a KeyRef trait and SymmetricKeyRef/AsymmetricKeyRef subtraits to represent the key types. Also a small macro to quickly implement them. KeyRef implementations are user defined types that implements Eq+Hash+Copy, and don't contain any key material
• KeyEncryptable/KeyDecryptable are now Decryptable/Encryptable, and instead of taking the key as parameter, they take a CryptoServiceContext and a KeyRef. This way keys are never exposed to the clients.
• KeyLocator trait is replaced by UsesKey. This trait only returns the KeyRef of the key required to decrypt it, instead of fetching the key itself.
• Created a KeyStore trait, with some simple CRUD methods. We have two implementations, one based on memfd_secret for Linux, and another one based on a Rust boxed slice, that also applies mlock if possible.
• Because both mlock and memfd_secret apply their protection over a specified memory area, we need a Rust data structure that is laid out linearly and also doesn't reallocate by itself. Also because memfd_secret allocates the memory for us, we can't use a std type like Vec (reason is the Vec must be allocated by the Rust allocator [ref]). This basically only leaves us with a Rust slice, on top of which we'd need to implement insert/get/delete. To allow for reuse and easier testing I've created SliceKeyStore, which implements the CRUD methods from the KeyStore trait on top of a plain slice. Then the actual mlock and memfd_secret implementations just need to implement a trait casting their data to a slice. The data is stored as a slice of Option<(KeyRef, KeyMaterial)>, and the keys are unique and sorted in the slice for easier lookups.
• Created CryptoService, which contains the KeyStores and some encrypt/decrypt utility functions. From a CryptoService you can also initialize a CryptoServiceContext
• A CryptoServiceContext contains a read only view of the keys inside the CryptoService, plus a read-write ephemeral key store, for use by Decryptable/Encryptable implementations when they need to temporarily store some keys (Cipher keys, attachment keys, send keys...).

Migrated the codebase to use these changes in a separate PR: bitwarden/sdk-sm#1117

📸 Screenshots

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• Contributor guidelines followed
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• CI builds passed
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• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation
  team

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  issue and could potentially benefit from discussion
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[dani-garcia]

Split the secure storage implementations to #125 so this is easier to review and merge.

[Hinton]

I believe we can generalize this code slightly.

Suggested change

	impl<Ids: KeyIds> Encryptable<Ids, Ids::Symmetric, EncString> for &str {
	fn encrypt(
	&self,
	ctx: &mut KeyStoreContext<Ids>,
	key: Ids::Symmetric,
	) -> Result<EncString, CryptoError> {
	self.as_bytes().encrypt(ctx, key)
	}
	}
	impl<Ids: KeyIds> Encryptable<Ids, Ids::Asymmetric, AsymmetricEncString> for &str {
	fn encrypt(
	&self,
	ctx: &mut KeyStoreContext<Ids>,
	key: Ids::Asymmetric,
	) -> Result<AsymmetricEncString, CryptoError> {
	self.as_bytes().encrypt(ctx, key)
	}
	}
	impl<Ids, K, E> Encryptable<Ids, K, E> for &str
	where
	Ids: KeyIds,
	K: KeyId,
	E: EncStringType,
	[u8]: Encryptable<Ids, K, E>,
	{
	fn encrypt(&self, ctx: &mut KeyStoreContext<Ids>, key: K) -> Result<E, CryptoError> {
	self.as_bytes().encrypt(ctx, key)
	}
	}

And define a common trait for EncString.

pub trait EncStringType {}
impl EncStringType for EncString {}
impl EncStringType for AsymmetricEncString {}

[dani-garcia]

This is something Matt also brought up when trying to do authenticated data. I think the best path forward would be to separate encryptable/decrytable into encryptable/decryptable and encodable/decodable, something like this.

impl <T: Encodable<Vec<u8>> Encryptable<EncString> for T
impl <T: Encodable<Vec<u8>> Encryptable< AsymmetricEncString> for T

impl Encodable<Vec<u8>> for &str
impl Encodable<Vec<u8>> for String

[Hinton]

Can we add some quick module docs? I.e. //!

[Hinton]

I wonder if the naming here is slightly misleading. We're not decrypting it we're adding it to the context?

[dani-garcia]

Well, we're doing both: We decrypt the key and add it to the context. Originally i was planning to make it a generic function like decrypt_key_into_store, but I couldn't get the typing to do what I wanted, which explains the long names. (And calling it decrypt_symmetric_key_into_store_using_symmetric_key is just way too long)