base64ct 1.6.0

Pure Rust implementation of Base64 (RFC 4648) which avoids any usages of data-dependent branches/LUTs and thereby provides portable "best effort" constant-time operation and embedded-friendly no_std support

RustCrypto: Constant-Time Base64

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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.