Crate base64ct

source ·
Expand description

§RustCrypto: Constant-Time Base64

crate Docs Build Status Apache2/MIT licensed Rust Version Project Chat

Pure Rust implementation of Base64 (RFC 4648).

Implements multiple Base64 alphabets without data-dependent branches or lookup tables, 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).



This crate implements several Base64 alphabets in constant-time for sidechannel resistance, aimed at purposes like encoding/decoding the “PEM” format used to store things like cryptographic private keys (i.e. in the pem-rfc7468 crate).

The paper Util::Lookup: Exploiting key decoding in cryptographic libraries demonstrates how the leakage from non-constant-time Base64 parsers can be used to practically extract RSA private keys from SGX enclaves.

The padded variants require (=) padding. Unpadded variants expressly reject such padding.

Whitespace is expressly disallowed, with the exception of the Decoder::new_wrapped and Encoder::new_wrapped modes which provide fixed-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]

§Minimum Supported Rust Version

This crate requires Rust 1.60 at a minimum.

We may change the MSRV in the future, but it will be accompanied by a minor version bump.


Licensed under either of:

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


§Allocating (enable alloc crate feature)

use base64ct::{Base64, Encoding};

let bytes = b"example bytestring!";
let encoded = Base64::encode_string(bytes);
assert_eq!(encoded, "ZXhhbXBsZSBieXRlc3RyaW5nIQ==");

let decoded = Base64::decode_vec(&encoded).unwrap();
assert_eq!(decoded, bytes);

§Heapless no_std usage

use base64ct::{Base64, Encoding};

const BUF_SIZE: usize = 128;

let bytes = b"example bytestring!";
assert!(Base64::encoded_len(bytes) <= BUF_SIZE);

let mut enc_buf = [0u8; BUF_SIZE];
let encoded = Base64::encode(bytes, &mut enc_buf).unwrap();
assert_eq!(encoded, "ZXhhbXBsZSBieXRlc3RyaW5nIQ==");

let mut dec_buf = [0u8; BUF_SIZE];
let decoded = Base64::decode(encoded, &mut dec_buf).unwrap();
assert_eq!(decoded, bytes);


Implemented using integer arithmetic alone without any lookup tables or data-dependent branches, thereby providing portable “best effort” constant-time operation.

Not constant-time with respect to message length (only data).

Adapted from the following constant-time C++ implementation of Base64:

Copyright (c) 2014 Steve “Sc00bz” Thomas (steve at tobtu dot com). Derived code is dual licensed MIT + Apache 2 (with permission from Sc00bz).