Pure Rust implementation of [B64], a subset of the standard Base64 encoding ([RFC 4648]) used by the [PHC string format].
Implemented without data-dependent branches or look up tables, thereby providing "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).
The following description of [B64] is quoted from the [PHC string format] spec:
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) + b2Then split
xinto four 6-bit valuesy0,y1,y2andy3such that:x = (y0 << 18) + (y1 << 12) + (y2 << 6) + y3Each 6-bit value is encoded into a character in the
[A-Za-z0-9+/]alphabet, in that order:
A..Z= 0 to 25a..z= 26 to 510..9= 52 to 61+= 62/= 63If the last group does not contain exactly three bytes, then:
- The group is completed with one or two bytes of value 0x00, then processed as above.
- 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.
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.