tiny.reticulum.crypto.pkcs7

1const std = @import("std");23pub const block_length: u8 = 16;4pub const max_data_length: u16 = 65_519;5pub const max_padded_length: u16 = 65_520;67pub const PadError = error{8    InputTooLong,9    OutputTooSmall,10};1112pub const UnpadError = error{13    EmptyInput,14    InvalidPadding,15};1617pub fn paddedLength(data_length: u17) PadError!u16 {18    if (data_length > max_data_length) return error.InputTooLong;19    const remainder: u8 = @intCast(data_length % block_length);20    const padding: u8 = block_length - remainder;21    const result = data_length + padding;22    std.debug.assert(result <= max_padded_length);23    return @intCast(result);24}2526/// Writes the padded data into `out` and returns it, with between one and27/// sixteen copies of the padding count appended, so a caller fills a plaintext28/// out to whole blocks before the cipher runs, following Reticulum@1.5.029/// RNS/Cryptography/PKCS7.py:32-39. Data already a whole number of blocks long30/// gains a full block of padding, so the count is never zero. The call returns31/// `error.InputTooLong` past 65,519 bytes and `error.OutputTooSmall` when `out`32/// is shorter than the padded length.33pub fn pad(data: []const u8, out: []u8) PadError![]u8 {34    if (data.len > max_data_length) return error.InputTooLong;35    const result_length = try paddedLength(@intCast(data.len));36    const result_length_usize: usize = result_length;37    if (out.len < result_length_usize) return error.OutputTooSmall;38    const result = out[0..result_length_usize];39    @memmove(result[0..data.len], data);40    const padding: u8 = @intCast(result.len - data.len);41    @memset(result[data.len..], padding);42    return result;43}4445/// Returns the data with as many trailing bytes removed as its final byte46/// counts, so a caller cuts the padding back off a plaintext the cipher47/// produced, following Reticulum@1.5.0 RNS/Cryptography/PKCS7.py:41-48. A final48/// byte counting more than sixteen returns `error.InvalidPadding`, and empty49/// data returns `error.EmptyInput`. A final byte counting more than the data50/// holds is handled the way the reference's Python slicing handles it: that51/// slicing leaves twice the length less the count, or nothing at all once the52/// count reaches twice the length. A final byte of zero removes nothing.53pub fn unpad(data: []const u8) UnpadError![]const u8 {54    if (data.len == 0) return error.EmptyInput;55    const padding = data[data.len - 1];56    if (padding > block_length) return error.InvalidPadding;57    const stop = pythonSliceStop(data.len, padding);58    return data[0..stop];59}6061fn pythonSliceStop(length: usize, padding: u8) usize {62    if (padding <= length) return length - padding;63    std.debug.assert(length < block_length);64    const doubled = length * 2;65    if (padding >= doubled) return 0;66    return doubled - padding;67}

1pub const pkcs7 = @import("pkcs7.zig");