Crate b64ct

Pure Rust implementation of "B64" encoding, a minified subset of the standard Base64 encoding (RFC 4648, section 4) used by the PHC string format.

B64 Encoding

Supports the Base64 character subset: [A-Z], [a-z], [0-9], +, / but disallows padding (=) as well as extra whitespace.

Full spec is given in the the PHC string format specification:

https://github.com/P-H-C/phc-string-format/blob/master/phc-sf-spec.md#b64

The B64 encoding is the standard Base64 encoding (RFC 4648, section 4) except that the padding = signs are omitted, and extra characters (whitespace) are not allowed:

• Input is split into successive groups of bytes. Each group, except possibly the last one, contains exactly three bytes.

• For a group of bytes b0, b1 and b2, compute the following value:

  x = (b0 << 16) + (b1 << 8) + b2

  Then split x into four 6-bit values y0, y1, y2 and y3 such that:

  x = (y0 << 18) + (y1 << 12) + (y2 << 6) + y3

• Each 6-bit value is encoded into a character in the [A-Za-z0-9+/] alphabet, in that order:

  • A..Z = 0 to 25
  • a..z = 26 to 51
  • 0..9 = 52 to 61
  • + = 62
  • / = 63

• If the last group does not contain exactly three bytes, then:

  1. The group is completed with one or two bytes of value 0x00, then processed as above.
  2. The resulting sequence of characters is truncated to its first two characters (if the group initially contained a single byte) or to its first three characters (if the group initially contained two bytes).

A B64-encoded value thus yields a string whose length, taken modulo 4, can be equal to 0, 2 or 3, but not to 1. Take note that a sequence of characters of the right length may still be an invalid encoding if it defines some non-zero trailing bits in the last incomplete group; producers MUST set the trailing bits to 0, while consumers MAY ignore them, or MAY reject such invalid encodings.

Implementation

Implemented without data-dependent branches or lookup tables, thereby providing portable "best effort" constant-time operation.

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

https://github.com/Sc00bz/ConstTimeEncoding/blob/master/base64.cpp

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

Structs

InvalidEncodingError	Invalid encoding of provided "B64" string.
InvalidLengthError	Insufficient output buffer length.

Enums

Error

Generic error, union of InvalidLengthError and InvalidEncodingError.

Functions

decode	"B64" decode the given source byte slice into the provided destination buffer.
decode_in_place	Decode B64-encoded string in-place.
decode_vec	alloc Decode a "B64"-encoded string into a byte vector.
decoded_len	Get the length of the output from decoding the provided "B64"-encoded input.
encode	Encode the input byte slice as "B64", writing the result into the provided destination slice, and returning an ASCII-encoded string value.
encode_string	alloc Encode the input byte slice as a "B64"-encoded String.
encoded_len	Get the "B64"-encoded length of the given byte slice.