Expand description
This is cretrit
(pronounced “creTRIT”), a Rust library for performing comparisonrevealing cryptographic operations (encryption, comparison) on arbitrary data.
It implements the twovalue and threevalue encrypted comparison algorithms described in the paper OrderRevealing Encryption: New Constructions, Applications, and Lower Bounds by Kevin Lewi and David J. Wu, of Stanford University.
What this library allows you to do is to take plaintext values, encrypt them, and then perform comparison operations on the encrypted forms that produce results identical to the equivalent comparison operations on the plaintexts. This is typically useful for performing ordering (where the comparison is “is this value lessthan, equalto, or greaterthan that value?”) and equality (“is this value equalto or notequalto that value?”). However the library is designed to accommodate other comparison operators, in case the need arises. To provide implementation examples, and satisfy common use cases, generic ordering and equality ciphertext types are also provided.
If the ideas in this library intrigue you, but the idea of fiddling around with these lowlevel primitives sounds a bit tedious, you may wish to check out the rest of the Enquo Project. The Enquo Project exists to provide encrypted, queryable datastores for everyone, which uses comparisonrevealing cryptography extensively.
Security Status
This library has NOT been audited by any competent third party for implementation flaws. If you would like to undertake, or sponsor, such an audit, please get in touch.
If you believe you have found a security flaw in this library, an email to security@enquo.org
would be appreciated.
Usage
To use the existing ordering and equality types, you just have to select a cipher suite, and then use
the module in that cipher suite that corresponds to the operation you wish to perform.
At present, only one cipher suite is available, named aes128v1
, and there are ore
(orderrevealing encryption) and ere
(equalityrevealing encryption) modules.
From there, you instantiate a Cipher
whose generic parameters represent the number of blocks (N
) and the “width” of each block (the number of values representable by each block, W
), giving it a key to use for encryption.
For example:
// Let's do some orderrevealing encryption!
use cretrit::aes128v1::ore;
// Always use cryptographicallysecure random keys!
let key = ChaCha20Rng::from_entropy().gen::<[u8; 32]>();
// This cipher has four blocks, the value of each is in the range
// 0255. Hence, this cipher can represent the ordering of values
// between 0 and 256^41 (aka 2**321), which corresponds to a 32bit
// unsigned integer.
let cipher = ore::Cipher::<4, 256>::new(&key)?;
This cipher is how you encrypt plaintexts.
Internally, plaintexts are an array of the value of each block, and you can use that representation if you like.
For encrypting unsigned integers, there are implementations of the TryFrom
trait that allow you to pass the integers in directly, like this:
let forty_two: u32 = 42;
let ore_forty_two = cipher.full_encrypt(&forty_two.try_into()?)?;
let over_nine_thousand: u32 = 9001;
let ore_over_nine_thousand = cipher.full_encrypt(&over_nine_thousand.try_into()?)?;
If the value you’re trying to turn into a plaintext can’t be represented by the plaintext type you’re using, a cretrit::Error::RangeError
will be returned.
Ciphertexts for the orderrevealing and equalityrevealing encryption schemes implement Ord
, Eq
, and the Partial*
variants as appropriate.
Thus, you can just compare the outputs of the encrypt
function like they were any other value:
assert!(ore_forty_two != ore_over_nine_thousand);
assert!(ore_forty_two < ore_over_nine_thousand);
You can also serialise and deserialise ciphertexts to/from u8
vectors, which allows you to store them in files, databases, etc.
A simple example of roundtripping a ciphertext:
// Pull in the necessary trait
use cretrit::SerializableCipherText;
let v = ore_forty_two.to_vec()?;
// When deserialising a ciphertext, you need to specify the cipher parameters
// so that the types line up.
let new_forty_two = ore::CipherText::<4, 256>::from_slice(&v)?;
// Once it's deserialised, it's back to its original form and ready to
// go!
assert!(new_forty_two == ore_forty_two);
assert!(new_forty_two != ore_over_nine_thousand);
assert!(new_forty_two < ore_over_nine_thousand);
Terminology
To help make sense of everything, here’s some of the terms that we use in the codebase and documentation.

ComparisonRevealing Encryption: a generic encryption scheme which produces ciphertexts which can be compared against one another to determine a defined relationship between the plaintexts from which the ciphertexts were produced. Ideally, the ciphertexts do not reveal any other information about the two plaintexts or their relationship to each other.

OrderRevealing Encryption: a form of comparisonrevealing encryption, which produces ciphertexts which reveal the relative ordering of ciphertexts, without giving any indication of the actual value of the underlying plaintexts. This is done by revealing whether a ciphertext is lessthan, equalto, or greaterthan any other ciphertext, which is all that is necessary to order any collection of ciphertexts.

EqualityRevealing Encryption: a form or comparisonrevealing encryption, which produces ciphertexts which reveal whether the plaintext value behind two ciphertexts are equal, or not. While orderrevealing encryption can also be used to reveal equality, this form is useful when there is no welldefined ordering of a set of values, or you specifically do not which to reveal that ordering. The ciphertexts produced by equalityrevealing encryption are also smaller than those produced by orderrevealing encryption.

Cipher Suite: a collection of cryptographic primitives which, in combination, are needed to perform the complete set of comparisonrevealing encryption operations. Multiple cipher suites may be defined to upgrade security, or provide increased performance.

Cipher: a combination of a cipher suite and comparison operator which, together, provide the ability to encrypt a plaintext into a particular comparisonrevealing form.

Plaintext Block: to keep ciphertext sizes under control, the LewiWu scheme breaks a single large plaintext into smaller blocks. Each block can represent values of a certain range, and a single plaintext has a certain number of blocks. These parameters control the size and leakage of the corresponding ciphertext.
Contributing
For general guidelines for contributions, see CONTRIBUTING.md.
Licence
Unless otherwise stated, everything in this repo is covered by the following licence statement (the MIT licence):
Copyright (C) 2022 Matt Palmer <matt@enquo.org>
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
Modules
 ComparisonRevealing Encryption using AES128 as the PseudoRandom Function and Hash Function.
Structs
 Something capable of turning
PlainText
s into comparableCipherText
s by means of encryption.  A ComparisonRevealing Encrypted value.
 A plaintext suitable for encrypting using a comparisonrevealing scheme.
Enums
 Error type for all Cretrit operations
Traits
 Provide the ability to serialise/deserialise a ciphertext