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Notes for keyring developers

Introduction

This document is aimed at developers that want to improve the existing keyring backends, or want to implement new backends. You don't need it for using keyring.

Backend implementation details

macOS Keychain

On macOS, the keyrings are store in files. We handle the user's keyrings, these are in ~/Library/Keychains. They are files with extension .keychain (before Sierra) or .keychain-db (starting from Sierra).

Whenever a keyring is specified, it can be a symbolic name (e.g. login), or an absolute filename (e.g. /Users/gaborcsardi/Library/Keychains/login.keychain). If a symbolic name is specified, we look for both .keychain and .keychain-db files:

  • If the .keychain file exists, we use that.
  • Otherwise, if the .keychain-db file exists, we use that.
  • Otherwise, if the system is Sierra or later, we use .keychain-db.
  • Otherwise we use the .keychain file.

Windows Credential Store

We use the old API to the Windows Credential Store. See e.g. https://msdn.microsoft.com/en-us/library/windows/desktop/aa374804%28v%3Dvs.85%29.aspx and also the CREDENTIAL, CredWrite, CredDelete, CredEnumerate there. The reason for this is that MinGW does not provide a wrapper to the new API, introduced in Windows 8.x. This also means that we don't have access to the secrets stored by recent Windows web browsers (IE and Edge).

Multiple keyrings

The old Windows Credential Store does not support multiple keyrings, and it does not support locking and unlocking the (single) keyring, either.

For every (non-default) keyring, we create a credential, with target name keyring::. This credential contains metadata about the keyring. It currently has the following (DCF, Debian Control File) format:

Version: 1.0.0
Verify: NgF+vkkNsOoSnXVXt249u6xknskhDasMIhE8Uuzpl/w=
Salt: some random salt

The Verify tag is used to check if the keyring password that was specified to unlock the keyring, was correct.

The Salt tag is used to salt the SHA256 hash, to make it more secure. It is generated randomly when the keyring is created.

When a keyring is unlocked, the user specifies the pass phrase of the keyring. We create the SHA256 hash of this pass phrase, and this will be the AES key to encrypt/decrypt the items in the keyring. When unlocking a keyring, we use the Verify field, to see if the supplied password indeed hashes to the correct AES key. If it can decrypt the Verify string, then it is correct.

We also store the AES key in the keyring, in a session credential with target name keyring::unlocked. A session credential's life time is the life time of a single login session. The AES key is stored in a base64 encoded form, e.g.:

JvL7srqc0X1vVnqbSayFnIkJZoe2xMOWoDh+aBR9DJc=

The credentials of the keyring itself have target names as keyring:service:username, where the username may be empty. If keyring is empty, then the credential is considered to be on the default keyring, and it is not encrypted. Credentials on other keyrings are encrypted using the AES key of the keyring. The random initialization vector of the encryption is stored as the first 16 bytes of the keyring item.

When we set a key, we need to:

  1. Check if the key is on the default keyring.
  2. If 1. is TRUE, then just set the key, using target name :service:username (username might be empty, servicename not), and finish.
  3. Check if the keyring exists.
  4. If 3. is FALSE, then error and finish.
  5. Check that the keyring is unlocked.
  6. If 5. is FALSE, then prompt the user and unlock the keyring.
  7. Encrypt the key with the AES key, and store the encrypted key using target name keyring:service:username (again, username might be empty, service name not).

When we get a key, we need to:

  1. Check if the key is on the default keyring.
  2. If 1. is TRUE, then we just get the key, using target name :service:username.
  3. Check if the keyring is locked.
  4. If 3. is TRUE, then prompt the user and unlock the keyring.
  5. Get the AES key from the unlocked keyring.
  6. Get the key and use the AES key to decrypt it.

To unlock a keyring we need to:

  1. Get the keyring password and SHA256 hash it.
  2. Try to decrypt the Verify field of the keyring metadata, to see if the password was correct.
  3. If the password was not correct, then error out.
  4. If the password was correct, then store the AES key under target name keyring::unlocked.

The C functions for this backend do not know about multiple keyrings at all, we just use them to get/set/delete/list "regular" credentials in the credential store.

Secret Service API

The Secret Service API works on DBUS, and multiple daemons support it. On GNOME based systems, typically gnome-keyring, and on KDE based systems typically KWallet is used.

Linux systems lacking a GUI typically do not run a secret service daemon, and keyring cannot use the OS credential store on these systems. The default backend is the environment variable based one (backend_env) on these systems.

Prompts

For the Secret Service backend, it is currently not possible to specify a password for some functions. Instead, the password is read in interactively on this backend, by the system. This currently happens when a new keyring is created, and when a keyring is unlocked.

Unloading the package

libsecret uses libglib, and libglib does not allow unloading the package. More precisely, if you unload the keyring package on Linux, then some libglib threads will stay around. If you then reload the package, R will crash.

This is a known limitation of libglib, and there is currently no way around it. For keyring a workaround would be to use libdbus to communicate with the Secret Service daemon directly. libdbus is a standalone DBUS library, it does not use libglib. There is an issue for this in our issue tracker, but no plans currently about when it would happen: https://github.com/r-lib/keyring/issues/15

This limitation is especially annoying for development with devtools, because devtools::load_all(), devtools::test(), etc. try to unload and reload the package, which results in crashes. A workaround is to run the tests from the command line:

R -e 'devtools::test()'

Environment variable backend

When an item has a username on this backend, the service name and the username is separated with a colon (:) character in the created environment variable:

service:username

Note that shells typically cannot read or write these environment variables. Programming languages, i.e. Sys.getenv() and Sys.setenv() in R, or the similar functions in Python, C, etc. can read and write them just fine.

Limits on secret sizes

Various backends (and platforms) have various limits on the size of the secrets they can store. In general, the safest to assume that the secret can be maximum about 400 bytes long. If you want to store something longer, encrypt it with AES, and store it in a regular file, and store the AES key in the keyring. See the openssl package for AES encryption and decryption.

macOS Keychain

The macOS keychain can store very big secrets. In our experiments it had no problems with a secret of 1GB, although that is already a little slow to encrypt and decrypt. The real limit is probably (much) higher, and it could be the limit on the keychain file's size.

Windows Credential Store

According to the documentation in the Windows Credential Store the maximum secret size is 512 bytes: https://msdn.microsoft.com/en-us/library/windows/desktop/aa374788%28v%3Dvs.85%29.aspx

In practice, on Windows 8.1 we managed to store secrets of up to 2560 bytes. Other Windows versions might have other limits, though. Note that if you use the non-default keyring, then the actual secret stored by keyring is longer, because it is encrypted manually, and also BASE64 encoded. Hence the advised 400 bytes limit.

Linux Secret Service

According to our tests with the Secret Service, it can store secrets of at least 25MB. We used an Ubuntu 16.04 system and the gnome-keyring daemon for this. Note, however, that these large secrets only work if you first create a small keyring item, and then update its contents. Creating a large keyring item straight away gives an error.

Testing the package

Testing the package on CIs or local machines is tricky. Here are some notes.

Test keyrings and keyring items

When running tests (and examples), the package only creates keyrings with a name prefix Rkeyringtest, and on other keyrings it only creates services with the name prefix R-keyring-test-service-. This makes it easier to remove the leftover keyrings and keyring items manually from the system keyring.
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