struct hex [src]

Alias for std.crypto.codecs.base64_hex_ct.hex
(best-effort) constant time hexadecimal encoding and decoding.
Members

decode (Function)
decoderWithIgnore (Function)
DecoderWithIgnore (struct)
encode (Function)
Source

  pub const hex = struct {
    /// Encodes a binary buffer into a hexadecimal string.
    /// The output buffer must be twice the size of the input buffer.
    pub fn encode(encoded: []u8, bin: []const u8, comptime case: std.fmt.Case) error{SizeMismatch}!void {
        if (encoded.len / 2 != bin.len) {
            return error.SizeMismatch;
        }
        for (bin, 0..) |v, i| {
            const b: u16 = v >> 4;
            const c: u16 = v & 0xf;
            const off = if (case == .upper) 32 else 0;
            const x =
                ((87 - off + c + (((c -% 10) >> 8) & ~@as(u16, 38 - off))) & 0xff) << 8 |
                ((87 - off + b + (((b -% 10) >> 8) & ~@as(u16, 38 - off))) & 0xff);
            encoded[i * 2] = @truncate(x);
            encoded[i * 2 + 1] = @truncate(x >> 8);
        }
    }

    /// Decodes a hexadecimal string into a binary buffer.
    /// The output buffer must be half the size of the input buffer.
    pub fn decode(bin: []u8, encoded: []const u8) error{ SizeMismatch, InvalidCharacter, InvalidPadding }!void {
        if (encoded.len % 2 != 0) {
            return error.InvalidPadding;
        }
        if (bin.len < encoded.len / 2) {
            return error.SizeMismatch;
        }
        _ = decodeAny(bin, encoded, null) catch |err| {
            switch (err) {
                error.InvalidCharacter => return error.InvalidCharacter,
                error.InvalidPadding => return error.InvalidPadding,
                else => unreachable,
            }
        };
    }

    /// A decoder that ignores certain characters.
    /// The decoder will skip any characters that are in the ignore list.
    pub const DecoderWithIgnore = struct {
        /// The characters to ignore.
        ignored_chars: StaticBitSet(256) = undefined,

        /// Decodes a hexadecimal string into a binary buffer.
        /// The output buffer must be half the size of the input buffer.
        pub fn decode(
            self: DecoderWithIgnore,
            bin: []u8,
            encoded: []const u8,
        ) error{ NoSpaceLeft, InvalidCharacter, InvalidPadding }![]const u8 {
            return decodeAny(bin, encoded, self.ignored_chars);
        }

        /// Returns the decoded length of a hexadecimal string, ignoring any characters in the ignore list.
        /// This operation does not run in constant time, but it aims to avoid leaking information about the underlying hexadecimal string.
        pub fn decodedLenForSlice(decoder: DecoderWithIgnore, encoded: []const u8) !usize {
            var hex_len = encoded.len;
            for (encoded) |c| {
                if (decoder.ignored_chars.isSet(c)) hex_len -= 1;
            }
            if (hex_len % 2 != 0) {
                return error.InvalidPadding;
            }
            return hex_len / 2;
        }

        /// Returns the maximum possible decoded size for a given input length after skipping ignored characters.
        pub fn decodedLenUpperBound(hex_len: usize) usize {
            return hex_len / 2;
        }
    };

    /// Creates a new decoder that ignores certain characters.
    /// The decoder will skip any characters that are in the ignore list.
    /// The ignore list must not contain any valid hexadecimal characters.
    pub fn decoderWithIgnore(ignore_chars: []const u8) error{InvalidCharacter}!DecoderWithIgnore {
        var ignored_chars = StaticBitSet(256).initEmpty();
        for (ignore_chars) |c| {
            switch (c) {
                '0'...'9', 'a'...'f', 'A'...'F' => return error.InvalidCharacter,
                else => if (ignored_chars.isSet(c)) return error.InvalidCharacter,
            }
            ignored_chars.set(c);
        }
        return DecoderWithIgnore{ .ignored_chars = ignored_chars };
    }

    fn decodeAny(
        bin: []u8,
        encoded: []const u8,
        ignored_chars: ?StaticBitSet(256),
    ) error{ NoSpaceLeft, InvalidCharacter, InvalidPadding }![]const u8 {
        var bin_pos: usize = 0;
        var state: bool = false;
        var c_acc: u8 = 0;
        for (encoded) |c| {
            const c_num = c ^ 48;
            const c_num0: u8 = @truncate((@as(u16, c_num) -% 10) >> 8);
            const c_alpha: u8 = (c & ~@as(u8, 32)) -% 55;
            const c_alpha0: u8 = @truncate(((@as(u16, c_alpha) -% 10) ^ (@as(u16, c_alpha) -% 16)) >> 8);
            if ((c_num0 | c_alpha0) == 0) {
                if (ignored_chars) |set| {
                    if (set.isSet(c)) {
                        continue;
                    }
                }
                return error.InvalidCharacter;
            }
            const c_val = (c_num0 & c_num) | (c_alpha0 & c_alpha);
            if (bin_pos >= bin.len) {
                return error.NoSpaceLeft;
            }
            if (!state) {
                c_acc = c_val << 4;
            } else {
                bin[bin_pos] = c_acc | c_val;
                bin_pos += 1;
            }
            state = !state;
        }
        if (state) {
            return error.InvalidPadding;
        }
        return bin[0..bin_pos];
    }
}  