# [RustCrypto]: Constant-Time Base64[![crate][crate-image]][crate-link][![Docs][docs-image]][docs-link][![Build Status][build-image]][build-link]![Apache2/MIT licensed][license-image]![Rust Version][rustc-image][![Project Chat][chat-image]][chat-link]Pure Rust implementation of Base64 ([RFC 4648]).Implements multiple Base64 alphabets without data-dependent branches or lookuptables, thereby providing portable "best effort" constant-time operation.Supports `no_std` environments and avoids heap allocations in the core API(but also provides optional `alloc` support for convenience).[Documentation][docs-link]## AboutThis crate implements several Base64 alphabets in constant-time for sidechannelresistance, aimed at purposes like encoding/decoding the "PEM" format used tostore things like cryptographic private keys (i.e. in the [`pem-rfc7468`] crate).The paper [Util::Lookup: Exploiting key decoding in cryptographic libraries][Util::Lookup]demonstrates how the leakage from non-constant-time Base64 parsers can be usedto practically extract RSA private keys from SGX enclaves.The padded variants require (`=`) padding. Unpadded variants expresslyreject such padding.Whitespace is expressly disallowed, with the exception of the[`Decoder::new_wrapped`] and [`Encoder::new_wrapped`] modes which providefixed-width line wrapping.## Supported Base64 variants- Standard Base64: `[A-Z]`, `[a-z]`, `[0-9]`, `+`, `/`- URL-safe Base64: `[A-Z]`, `[a-z]`, `[0-9]`, `-`, `_`- bcrypt Base64: `.`, `/`, `[A-Z]`, `[a-z]`, `[0-9]`- `crypt(3)` Base64: `.`, `-`, `[0-9]`, `[A-Z]`, `[a-z]`- PBKDF2 Base64: `[A-Z]`, `[a-z]`, `[0-9]`, `.`, `/`## Minimum Supported Rust Version (MSRV) PolicyMSRV increases are not considered breaking changes and can happen in patch releases.The crate MSRV accounts for all supported targets and crate feature combinations, excludingexplicitly unstable features.## LicenseLicensed under either of: * [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0) * [MIT license](http://opensource.org/licenses/MIT)at your option.### ContributionUnless you explicitly state otherwise, any contribution intentionally submittedfor inclusion in the work by you, as defined in the Apache-2.0 license, shall bedual licensed as above, without any additional terms or conditions.[//]: # (badges)[crate-image]: https://img.shields.io/crates/v/base64ct?logo=rust[crate-link]: https://crates.io/crates/base64ct[docs-image]: https://docs.rs/base64ct/badge.svg[docs-link]: https://docs.rs/base64ct/[build-image]: https://github.com/RustCrypto/formats/actions/workflows/base64ct.yml/badge.svg[build-link]: https://github.com/RustCrypto/formats/actions/workflows/base64ct.yml[license-image]: https://img.shields.io/badge/license-Apache2.0/MIT-blue.svg[rustc-image]: https://img.shields.io/badge/rustc-1.85+-blue.svg[chat-image]: https://img.shields.io/badge/zulip-join_chat-blue.svg[chat-link]: https://rustcrypto.zulipchat.com/#narrow/stream/300570-formats[//]: # (links)[RustCrypto]: https://github.com/rustcrypto[RFC 4648]: https://tools.ietf.org/html/rfc4648[`pem-rfc7468`]: https://github.com/RustCrypto/formats/tree/master/pem-rfc7468[Util::Lookup]: https://arxiv.org/pdf/2108.04600.pdf[`Decoder::new_wrapped`]: https://docs.rs/base64ct/latest/base64ct/struct.Decoder.html#method.new_wrapped[`Encoder::new_wrapped`]: https://docs.rs/base64ct/latest/base64ct/struct.Encoder.html#method.new_wrapped