Implement PKCS #5 / PKCS #7 padding.

src/primitive/blockcipher/index.zig

@@ -1,6 +1,8 @@
 pub const aes = @import("aes.zig");
 pub const des = @import("des.zig");
 
+pub const padding = @import("padding.zig");
+
 pub const operation_mode = struct {
     pub const gcm = @import("mode_gcm.zig");
 };

src/primitive/blockcipher/padding.zig

@@ -0,0 +1,49 @@
+const std = @import("std");
+const testing = std.testing;
+
+const CryptoError = error{
+    InvalidPadding,
+};
+
+pub fn pkcs5_pad(block: *[8]u8, plaintext_length: usize) void {
+    return pkcs7_pad(8, block, plaintext_length);
+}
+
+pub fn pkcs5_unpad(block: *[8]u8) !u8 {
+    return pkcs7_unpad(8, block);
+}
+
+pub fn pkcs7_pad(
+    comptime block_size: u8,
+    block: *[block_size]u8,
+    plaintext_length: usize,
+) void {
+    const plaintext_block_residue_length: u8 = plaintext_length % block_size;
+    const padding_val: u8 = block_size - plaintext_block_residue_length;
+    @memset(block[plaintext_block_residue_length..], padding_val);
+}
+
+pub fn pkcs7_unpad(
+    comptime block_size: u8,
+    block: *const [block_size]u8,
+) !u8 {
+    const padding_val = block[block_size - 1];
+    if (padding_val == 0 or padding_val > block_size)
+        return CryptoError.InvalidPadding;
+
+    const plaintext_residue_length = block_size - padding_val;
+
+    for (plaintext_residue_length..block_size) |i|
+        if (block[i] != padding_val)
+            return CryptoError.InvalidPadding;
+
+    return plaintext_residue_length;
+}
+
+test "PKCS #7 padding" {
+    // TODO
+}
+
+test "PKCS #7 unpadding" {
+    // TODO
+}