Efficient and customizable data-encoding functions like base64, base32, and hex

This crate provides little-endian ASCII base-conversion encodings for bases of size 2, 4, 8, 16, 32, and 64. It supports:

• padding for streaming
• canonical encodings (e.g. trailing bits are checked)
• in-place encoding and decoding functions
• partial decoding functions (e.g. for error recovery)
• character translation (e.g. for case-insensitivity)
• most and least significant bit-order
• ignoring characters when decoding (e.g. for skipping newlines)
• wrapping the output when encoding
• no-std environments with default-features = false, features = ["alloc"]
• no-alloc environments with default-features = false

You may use the binary or the website to play around.

Examples

This crate provides predefined encodings as constants. These constants are of type Encoding. This type provides encoding and decoding functions with in-place or allocating variants. Here is an example using the allocating encoding function of BASE64:

use data_encoding::BASE64;
assert_eq!(BASE64.encode(b"Hello world"), "SGVsbG8gd29ybGQ=");

Here is an example using the in-place decoding function of BASE32:

use data_encoding::BASE32;
let input = b"JBSWY3DPEB3W64TMMQ======";
let mut output = vec![0; BASE32.decode_len(input.len()).unwrap()];
let len = BASE32.decode_mut(input, &mut output).unwrap();
assert_eq!(&output[0 .. len], b"Hello world");

You are not limited to the predefined encodings. You may define your own encodings (with the same correctness and performance properties as the predefined ones) using the Specification type:

use data_encoding::Specification;
let hex = {
    let mut spec = Specification::new();
    spec.symbols.push_str("0123456789abcdef");
    spec.encoding().unwrap()
};
assert_eq!(hex.encode(b"hello"), "68656c6c6f");

You may use the macro library to define a compile-time custom encoding:

use data_encoding::Encoding;
use data_encoding_macro::new_encoding;
const HEX: Encoding = new_encoding!{
    symbols: "0123456789abcdef",
    translate_from: "ABCDEF",
    translate_to: "abcdef",
};
const BASE64: Encoding = new_encoding!{
    symbols: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",
    padding: '=',
};

Properties

The HEXUPPER, BASE32, BASE32HEX, BASE64, and BASE64URL predefined encodings conform to RFC4648.

In general, the encoding and decoding functions satisfy the following properties:

• They are deterministic: their output only depends on their input
• They have no side-effects: they do not modify any hidden mutable state
• They are correct: encoding followed by decoding gives the initial data
• They are canonical (unless is_canonical returns false): decoding followed by encoding gives the initial data

This last property is usually not satisfied by base64 implementations. This is a matter of choice and this crate has made the choice to let the user choose. Support for canonical encoding as described by the RFC is provided. But it is also possible to disable checking trailing bits, to add characters translation, to decode concatenated padded inputs, and to ignore some characters. Note that non-canonical encodings may be an attack vector as described in Base64 Malleability in Practice.

Since the RFC specifies the encoding function on all inputs and the decoding function on all possible encoded outputs, the differences between implementations come from the decoding function which may be more or less permissive. In this crate, the decoding function of canonical encodings rejects all inputs that are not a possible output of the encoding function. Here are some concrete examples of decoding differences between this crate, the base64 crate, and the base64 GNU program:

We can summarize these discrepancies as follows:

This crate permits to disable checking trailing bits. It permits to ignore some characters. It permits to translate characters. It permits to use unpadded encodings. However, for padded encodings, support for concatenated inputs cannot be disabled. This is simply because it doesn't make sense to use padding if it is not to support concatenated inputs.