Hello all,
This is a proposal to introduce the ChaCha20 and ChaCha20-Poly1305
cipher implementations into JDK. At a high level, the plan is to
include both ChaCha20-Poly1305 and the base ChaCha20 stream cipher into
JDK as part of the SunJCE provider initially, and then add TLS cipher
suites as a follow-on feature.
Both algorithms will be CipherSpi implementations and will generally
conform to the details of that API. I will discuss below some of the
details such as which flavors of init are supported, etc.
* Instantiation
o For ChaCha20 and ChaCha20-Poly1305, the simple name will
suffice: either "ChaCha20" for the basic stream cipher or
"ChaCha20-Poly1305" for AEAD mode will work. You may however
use the 3-element transform "ChaCha20/None/NoPadding" and
"ChaCha20-Poly1305/None/NoPadding". Any other type of
transformation string will cause NoSuchAlgorithmException to be
thrown.
* Initialization
o All three engineInit methods in the CipherSpi API will be
supported. Keys provided through the various Cipher init
methods should have the algorithm String "ChaCha20" applied to
it (case-insensitive).
o For init/engineInit methods that take an AlgorithmParameterSpec,
ChaCha20 and ChaCha20-Poly1305 use different APS classes.
+ ChaCha20 will have a new ChaCha20ParameterSpec which takes a
nonce (byte[]) and a counter (int). This class will have
getter methods to return those values if desired (getNonce()
and getBlockCounter(), respectively).
+ ChaCha20-Poly1305 will use IvParameterSpec to provide the
nonce. The primary reason this is being used instead of
ChaCha20ParameterSpec is in order to make backporting to
earlier JDK releases possible. Also there's no need to set
a counter value, so it would end up being an ignored parameter.
+ For init calls where no AlgorithmParameterSpec or
AlgorithmParameter has been provided, a random nonce will be
set at initialization time. the counter value will be set
to 1. The random nonce can be retrieved using the getIV()
Cipher method or by using the getParameters() call and
parsing the output from AlgorithmParameters.getEncoded().
* Use
o ChaCha20 encrypt and decrypt operations would work as any stream
cipher would - as many bytes of ciphertext are returned from an
encrypt function as plaintext bytes submitted (and vice versa
for decrypt).
o ChaCha20-Poly1305 operates in a similar fashion to other AEAD
ciphers. For encryption operations, as many bytes are returned
as input submitted with the exception of the doFinal calls,
which would return any remaining ciphertext plus an extra 16
bytes for the tag. For decryption, individual update calls
return no plaintext. The plaintext is returned only after the
last bytes of ciphertext are provided, the authentication tag is
provided, and the doFinal call is made. Once the authentication
tag has been verified then the plaintext will be returned.
o The getOutputSize call will return the following
+ ChaCha20: Same value as the submitted input size
+ ChaCha20-Poly1305: For encrypt, the returned size will be
the input size + 16 bytes for the tag. For decryption, the
returned size will be input length - 16 bytes, or zero
(whichever is larger).
o Wrap and Unwrap: I have not been able to find a standardized
wrap/unwrap format for ChaCha20 similar to RFC 3394 for AES.
Right now the wrap() and unwrap() methods just take the encoding
of the key to be wrapped and encrypts or decrypts them
respectively. If anyone is aware of a wrapping format for
ChaCha20 please let me know. My searches have so far come up empty.
o Counter rollover protection will be enforced. For ChaCha20 and
ChaCha20-Poly1305, the cipher will cease to process input once
the 32-bit counter space has been exhausted.
o Nonce reuse protection: For both ChaCha20 and ChaCha20-Poly1305:
we will not allow reuse of the same nonce between two
consecutive init() operations.
* KeyGenerator
o There will be a new KeyGenerator algorithm called "ChaCha20"
which will create a 32-byte key suitable for use in either
ChaCha20 or ChaCha20-Poly1305 cipher instances. If you use
forms of the KeyGenerator.init() that take a variable key length
and you do something other than 32 bytes then you'll have
InvalidParameterException thrown at you.
o If you use a form of the init that takes an
AlgorithmParameterSpec it will throw
InvalidAlgorithmParameterSpecException. This is similar in
behavior to other KeyGenerators like the HmacSHA-2 family,
ARCFOUR, RC2, and AES.
* Other TBD/in-progress items
o ChaCha20Parameters: This will be added to
com.sun.crypto.provider and will be able to provide an encoding
for parameters used in ChaCha20 and ChaCha20-Poly1305 ciphers.
+ For ChaCha20-Poly1305, the default encoded form of the
AlgorithmParameters will be the AEADChaCha20Poly1305Nonce
from RFC 8103 section 3 (basically the nonce as an ASN.1
OCTET STRING of 12 bytes).
+ For ChaCha20 I have not been able to find a standardized
encoding for ChaCha20 parameters. For lack of an official
format I currently have it encoding the parameters as a
SEQUENCE of an OCTET STRING (the nonce) and an INTEGER (the
counter starting value).
# Question: If a getParameters call on a cipher is called
after the cipher has been in use for some time, should
such an encoding provide the counter's current value, or
the starting value at the time the cipher was initialized?
* Backporting
o We would like to backport this, but because we need the new
ChaCha20ParameterSpec class to set the initial counter value
ChaCha20 will not get backported.
o ChaCha20-Poly1305 however can be backported, and the use of
IvParameterSpec with ChaCha20-Poly1305 will allow this to
happen. Being able to backport ChaCha20-Poly1305 also allows
the TLS cipher suites to be backported when those get added (see
below).
o Questions concerning how far back this will be backported and in
what timeframes are still TBD.
* Things that will not be part of this proposal...
o TLS Cipher suites: Yes, we will do this, but this will be done
as follow-on work. This proposal covers just the JCA portion.
I've already got TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256,
TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256, and
TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256 cipher suites working,
so worry not! It is our plan to have these in JSSE.
Thanks to everyone who has provided feedback so far and let's set a
closure date on the discussion for two weeks from now. I think we
should be able to hammer out any questions/concerns within that timeframe.
--Jamil
