### 22 releases

Uses new Rust 2021

0.7.1 | May 6, 2022 |
---|---|

0.7.0 | Jan 14, 2022 |

0.7.0-pre.0 | Nov 12, 2021 |

0.6.1 | Apr 21, 2021 |

0.1.5 | Jul 29, 2020 |

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# secp256kFUN!

A mid-level rust secp256k1 elliptic curve cryptography library that's optimized for fun! Here, fun means:

**type safety**: Error cases you would typically have to deal with when using other APIs are ruled out at compile time using rust's type system.**abstraction**: The library exposes two simple abstractions*Points*and*Scalars*so you can do clean textbook implementations of crypto.**unoptimizable**: The most straightforward way of expressing a certain operation on the group is also the most efficient way.

Fun does not mean (yet -- please help!):

**well reviewed or tested**: This code is fresh and experimental and not rigorously tested.**side-channel resistant**: There has been no empirical investigation into whether this library or the underlying parity/libsecp256k1 is resistant against timing attacks etc.**performant**: The library is in general not as performant as libsecp256k1.

The goal is for this library to let researchers experiment with ideas, have them work on Bitcoin *and* to enjoy it!
*High-level* libraries like rust-secp256k1 make it difficult to implement exotic cryptographic schemes correctly and efficiently.
*Low-level* libraries like parity/libsecp256k1 make it possible but the resulting code is often error prone and difficult to read.

## Use

`[``dependencies``]`
`secp256kfun ``=` `"`0.7`"`

### Should use?

This library is ready for production as long what you are trying to produce is **fun and amusement!**.
If you want to engineer something solid that a lot of people's money will depend on, this library is a very very risky choice.
Here are some alternatives:

- rust-secp256k1 - the rust bindings to the libsecp256k1 that Bitcoin itself uses
- k256 - the arithmetic of this library is (by default) based on this library.
- ristretto - in the case you don't need to use secp256k1

## Documentation

# Features

Here's the distinguishing features of this library.

## The Zero Element

Both secp256k1 points and scalars have a notional *zero* element.
Unfortunately, in things surrounding Bitcoin, the zero scalar and zero point are illegal values in most cases.

solves these difficulties using `secp256kfun`*marker types*.
Points and Scalars are marked with

or `Zero`

at compile time (by default, `NonZero`

).
So if you declare your function with a `NonZero`

type, passing a `NonZero`

type will be a compile time error as shown below:`Zero`

`use` `secp256kfun``::``{``marker``::``*``,` Scalar`,` Point`,`G`,`g`}``;`
`//` a randomly selected Scalar will never be zero (statistically unreachable)
`let` x `=` `Scalar``::`random`(``&``mut` `rand``::`thread_rng`(``)``)``;`
`dbg!``(``&`x`)``;` `//` Scalar<.., NonZero>
`//` Multiplying a NonZero scalar by G (which is also NonZero) results in a NonZero point
`let` X `=` `g!``(`x `*` G`)``;`
`dbg!``(``&`X`)` `//` Point<..,NonZero>
`let` Y `=` `g!``(``-`x `*` G`)`
`//` An addition **can** lead to a zero so the result is marked Zero
`let` sum `=` `g!``(`X `+` Y`)``;`
`dbg!``(``&`sum`)``;` `//` Point<.., Zero>
`//` Now let's say I naively decide to use this value as my public key...
`let` public_key `=` sum`.``normalize``(``)``;`
`//` BOOM! This is a compile time Error! 🎉
`send_pubkey_to_bob``(``&`public_key`)``;`
`fn` `send_pubkey_to_bob``(``public_key``:` `&`Point`)`` ``{`
`unimplemented!``(``)`
`}`

This gives us:

`error[E0308]:`` mismatched types`
`--``>` src/lib.rs:77:20
`|`
`17` `|` `send_pubkey_to_bob``(``&``public_key`)`;`
`|` `^^^^^^^^^^^`` expected struct `````secp256kfun::marker::NonZero````, found struct ````secp256kfun::marker::Zero````

To fix this, the library forces you to manually mark the value as

and then deal with the case that it is `NonZero`

.`Zero`

`match` sum`.``normalize``(``)``.``mark``::``<`NonZero`>``(``)` `{`
`Some``(`public_key`)` `=>` `send_pubkey_to_bob``(``&`public_key`)``,` `//` it was actually NonZero
`None` `=>` `..` `//` deal with the case it is Zero
`}`

Or you can declare that you are confident that it can never be

## Variable time or Constant time?

If a cryptogrpahic function's execution time should be independent of its secret inputs. Otherwise, information about those inputs may leak to anyone that can measure its execution time.

In secp256kfun we try and solve this problem by allowing you to mark different inputs as

or `Public`

.
Depending on the marking the rust compiler may choose different low level operations.
Choosing faster but variable time operations for `Secret`

inputs and slower safer constant time ones for things marked as `Public`

.
In other words, the caller can decide which input are`Secret`

For example, below we have a

function which is called by the committing party with a secret value and by the verifying party when the secret value is finally revealed.
Note that we only have to write the function once and the caller decides by marking whether the function should run in constant time or variable time.`pedersen_commitment`

`use` `secp256kfun``::``{``marker``::``*``,` Point`,` Scalar`,` g`}``;`
`///` commit to a secret value x with publicly known A and B.
`fn` `pedersen_commit``(`
`A``:` `&``Point``<`impl PointType`>`, `//` Accept any kind of Point
`B``:` `&``Point``<`impl PointType`>`,
`r``:` `&``Scalar``<`impl Secrecy`>`, `//` Accept a Secret or Public Scalar
`x``:` `&``Scalar``<`impl Secrecy, Zero`>`, `//` Allow commitment to Zero
`)`` ``->` Point `{`
`//` Make the commitment
`g!``(`r `*` A `+` x `*` B`)`
`.``normalize``(``)`
`//` If the result is zero we could easily compute the discrete
`//` logarithm of B with respect to A. Since this is meant to be unknown
`//` this is computionally unreachable.
`.``expect_nonzero``(``"`computationally unreachable`"``)`
`}`
`//` public setup
`let` A `=` `secp256kfun``::`G`;` `//` use the standard basepoint for one of the points
`let` B `=` `Point``::`random`(``&``mut` `rand``::`thread_rng`(``)``)``;`
`//` Alice commits to her secret value x with randomness r
`let` r `=` `Scalar``::`random`(``&``mut` `rand``::`thread_rng`(``)``)``;`
`let` x `=` `Scalar``::`from`(``42``)``;`
`let` commitment `=` `pedersen_commit``(`A`,` `&`B`,` `&`r`,` `&`x`)``;`
`//` Imagine Later on, Bob receives the public opening (r,x) for commitment. He
`//` doesn't care about leaking these values via execution time so he marks them
`//` as public.
`let` r `=` r`.``mark``::``<`Public`>``(``)``;`
`let` x `=` x`.``mark``::``<`Public`>``(``)``;`
`//` Now he'll compute the commitment in faster variable time and check it
`//` against the original
`assert_eq!``(`commitment`,` `pedersen_commit``(`A`,` `&`B`,` `&`r`,` `&`x`)``)``;`

As of

marking things correctly does very little since we changed the arithmetic backend to k256 (it's always going to be constant time).
However this situation may improve in future versions.`v0 .7.0`

## Features

- Built-in type-safe "x-only" point compression and decompression.
- Arithmetic expression macros

and`g!`

(used above) to clearly express group operations.`s!` - Nonce derivation API to help avoid messing this up.
- Feature flags:

serialization/deserialization for binary and hex for human-readable formats (enable with`serde`

feature hex requires`serde`

feature as well).`alloc`

support`no_std`

adds`libsecp_compat`

implementations to and from rust-secp256k1 types.`From`

implementations of core types with the`proptest`

feature`proptest`

#### Dependencies

~0.5–1.8MB

~30K SLoC