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zig/lib/std/crypto/aes_gcm_siv.zig
Frank Denis b1b2cd7ef8 Parallelize deriveKeys
2025-09-16 23:13:58 +02:00

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const std = @import("std");
const assert = std.debug.assert;
const crypto = std.crypto;
const debug = std.debug;
const mem = std.mem;
const math = std.math;
const modes = @import("modes.zig");
const Polyval = @import("ghash_polyval.zig").Polyval;
const AuthenticationError = crypto.errors.AuthenticationError;
pub const Aes128GcmSiv = AesGcmSiv(crypto.core.aes.Aes128);
pub const Aes256GcmSiv = AesGcmSiv(crypto.core.aes.Aes256);
/// AES-GCM-SIV: Authenticated encryption that remains secure even if you accidentally reuse a nonce.
///
/// What it does: Encrypts data and protects it from tampering. You can also attach
/// unencrypted metadata (like headers) that will be authenticated but not encrypted.
///
/// When to use AES-GCM-SIV:
/// - When you can't guarantee unique nonces (though you should still try to use unique nonces)
///
/// When to use regular AES-GCM instead:
/// - When you can guarantee unique nonces (e.g., using a counter)
/// - When you need slightly better performance
///
/// Security: If you accidentally reuse a nonce with the same key, AES-GCM-SIV only
/// reveals whether two messages are identical. Regular AES-GCM would be catastrophically
/// broken in this scenario, potentially revealing the authentication key.
///
/// Performance: Slightly slower than AES-GCM due to the additional key derivation step.
///
/// Defined in RFC 8452.
fn AesGcmSiv(comptime Aes: anytype) type {
debug.assert(Aes.block.block_length == 16);
return struct {
pub const tag_length = 16;
pub const nonce_length = 12;
pub const key_length = Aes.key_bits / 8;
const zeros: [16]u8 = @splat(0);
/// Derives the authentication and message encryption keys from the master key and nonce.
/// This implements the key derivation as specified in RFC 8452 Section 4.
/// Generates a 128-bit authentication key for POLYVAL and a message encryption key
/// (128 or 256 bits depending on the AES variant).
fn deriveKeys(message_key: *[key_length]u8, auth_key: *[16]u8, key: [key_length]u8, nonce: [nonce_length]u8) void {
const aes = Aes.initEnc(key);
// Derive authentication and message keys per RFC 8452 Section 4
// Each encryption produces 16 bytes, but we only use first 8 bytes of each block
if (key_length == 16) {
// AES-128-GCM-SIV: Process 4 blocks in parallel
var key_blocks: [4 * 16]u8 = undefined;
var cipher_outs: [4 * 16]u8 = undefined;
// Set up all 4 blocks with counters 0-3 and nonce
inline for (0..4) |i| {
mem.writeInt(u32, key_blocks[i * 16 ..][0..4], @intCast(i), .little);
key_blocks[i * 16 + 4 .. i * 16 + 16].* = nonce;
}
// Encrypt all 4 blocks in parallel
aes.encryptWide(4, &cipher_outs, &key_blocks);
// Extract the key material (first 8 bytes of each block)
@memcpy(auth_key[0..8], cipher_outs[0..8]);
@memcpy(auth_key[8..16], cipher_outs[16..24]);
@memcpy(message_key[0..8], cipher_outs[32..40]);
@memcpy(message_key[8..16], cipher_outs[48..56]);
} else {
// AES-256-GCM-SIV: Process 6 blocks in parallel
var key_blocks: [6 * 16]u8 = undefined;
var cipher_outs: [6 * 16]u8 = undefined;
// Set up all 6 blocks with counters 0-5 and nonce
inline for (0..6) |i| {
mem.writeInt(u32, key_blocks[i * 16 ..][0..4], @intCast(i), .little);
key_blocks[i * 16 + 4 .. i * 16 + 16].* = nonce;
}
// Encrypt all 6 blocks in parallel
aes.encryptWide(6, &cipher_outs, &key_blocks);
// Extract the key material (first 8 bytes of each block)
@memcpy(auth_key[0..8], cipher_outs[0..8]);
@memcpy(auth_key[8..16], cipher_outs[16..24]);
@memcpy(message_key[0..8], cipher_outs[32..40]);
@memcpy(message_key[8..16], cipher_outs[48..56]);
@memcpy(message_key[16..24], cipher_outs[64..72]);
@memcpy(message_key[24..32], cipher_outs[80..88]);
}
}
/// Encrypts and authenticates a message using AES-GCM-SIV.
///
/// `c`: The ciphertext buffer to write the encrypted data to.
/// `tag`: The authentication tag buffer to write the computed tag to.
/// `m`: The plaintext message to encrypt.
/// `ad`: The associated data to authenticate.
/// `npub`: The nonce to use for encryption.
/// `key`: The encryption key.
pub fn encrypt(c: []u8, tag: *[tag_length]u8, m: []const u8, ad: []const u8, npub: [nonce_length]u8, key: [key_length]u8) void {
debug.assert(c.len == m.len);
debug.assert(m.len <= (1 << 36));
debug.assert(ad.len <= (1 << 36));
var auth_key: [16]u8 = undefined;
var message_key: [key_length]u8 = undefined;
deriveKeys(&message_key, &auth_key, key, npub);
// Calculate POLYVAL over additional data and plaintext
const block_count = (math.divCeil(usize, ad.len, Polyval.block_length) catch unreachable) +
(math.divCeil(usize, m.len, Polyval.block_length) catch unreachable) + 1;
var mac = Polyval.initForBlockCount(&auth_key, block_count);
// Process additional data
mac.update(ad);
mac.pad();
// Process plaintext
mac.update(m);
mac.pad();
// Length block
var length_block: [16]u8 = undefined;
mem.writeInt(u64, length_block[0..8], @as(u64, ad.len) * 8, .little);
mem.writeInt(u64, length_block[8..16], @as(u64, m.len) * 8, .little);
mac.update(&length_block);
// Get POLYVAL result
var s: [16]u8 = undefined;
mac.final(&s);
// XOR with nonce to get pre-tag
for (npub, 0..) |b, i| {
s[i] ^= b;
}
// Clear most significant bit of last byte
s[15] &= 0x7f;
// Encrypt to get tag
const tag_aes = Aes.initEnc(message_key);
tag_aes.encrypt(tag, &s);
// Use tag as initial counter for CTR mode
var counter: [16]u8 = tag.*;
counter[15] |= 0x80; // Set most significant bit
// Encrypt message using CTR mode with 32-bit little-endian counter
const aes_ctx = Aes.initEnc(message_key);
modes.ctrSlice(@TypeOf(aes_ctx), aes_ctx, c, m, counter, .little, 0, 4);
}
/// Decrypts and authenticates a message using AES-GCM-SIV.
///
/// `m`: Message buffer to write the decrypted data to.
/// `c`: The ciphertext to decrypt.
/// `tag`: The authentication tag.
/// `ad`: The associated data.
/// `npub`: The nonce.
/// `key`: The decryption key.
/// Asserts `c.len == m.len`.
pub fn decrypt(m: []u8, c: []const u8, tag: [tag_length]u8, ad: []const u8, npub: [nonce_length]u8, key: [key_length]u8) AuthenticationError!void {
assert(c.len == m.len);
assert(c.len <= (1 << 36));
assert(ad.len <= (1 << 36));
var auth_key: [16]u8 = undefined;
var message_key: [key_length]u8 = undefined;
deriveKeys(&message_key, &auth_key, key, npub);
// Decrypt message using CTR mode with 32-bit little-endian counter
var counter: [16]u8 = tag;
counter[15] |= 0x80; // Set most significant bit
const aes_ctx = Aes.initEnc(message_key);
modes.ctrSlice(@TypeOf(aes_ctx), aes_ctx, m, c, counter, .little, 0, 4);
// Verify tag by recalculating POLYVAL
const block_count = (math.divCeil(usize, ad.len, Polyval.block_length) catch unreachable) +
(math.divCeil(usize, m.len, Polyval.block_length) catch unreachable) + 1;
var mac = Polyval.initForBlockCount(&auth_key, block_count);
// Process additional data
mac.update(ad);
mac.pad();
// Process decrypted plaintext
mac.update(m);
mac.pad();
// Length block
var length_block: [16]u8 = undefined;
mem.writeInt(u64, length_block[0..8], @as(u64, ad.len) * 8, .little);
mem.writeInt(u64, length_block[8..16], @as(u64, m.len) * 8, .little);
mac.update(&length_block);
// Get POLYVAL result
var s: [16]u8 = undefined;
mac.final(&s);
// XOR with nonce to get pre-tag
for (npub, 0..) |b, i| {
s[i] ^= b;
}
// Clear most significant bit of last byte
s[15] &= 0x7f;
// Encrypt to get expected tag
const tag_aes = Aes.initEnc(message_key);
var computed_tag: [tag_length]u8 = undefined;
tag_aes.encrypt(&computed_tag, &s);
// Verify tag
const verify = crypto.timing_safe.eql([tag_length]u8, computed_tag, tag);
if (!verify) {
crypto.secureZero(u8, &computed_tag);
@memset(m, undefined);
return error.AuthenticationFailed;
}
}
};
}
const htest = @import("test.zig");
const testing = std.testing;
test "Aes128GcmSiv - RFC 8452 Test Vector 1" {
// Test vector from RFC 8452 Appendix C.1
const key = [_]u8{
0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
};
const nonce = [_]u8{
0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00,
};
const ad = "";
const m = "";
var c: [m.len]u8 = undefined;
var tag: [Aes128GcmSiv.tag_length]u8 = undefined;
Aes128GcmSiv.encrypt(&c, &tag, m, ad, nonce, key);
try htest.assertEqual("dc20e2d83f25705bb49e439eca56de25", &tag);
}
test "Aes128GcmSiv - RFC 8452 Test Vector 2" {
// Test vector from RFC 8452 Appendix C.1
const key = [_]u8{
0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
};
const nonce = [_]u8{
0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00,
};
const plaintext = [_]u8{
0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
};
const ad = "";
var c: [plaintext.len]u8 = undefined;
var tag: [Aes128GcmSiv.tag_length]u8 = undefined;
Aes128GcmSiv.encrypt(&c, &tag, &plaintext, ad, nonce, key);
try htest.assertEqual("b5d839330ac7b786", &c);
try htest.assertEqual("578782fff6013b815b287c22493a364c", &tag);
var m2: [plaintext.len]u8 = undefined;
try Aes128GcmSiv.decrypt(&m2, &c, tag, ad, nonce, key);
try testing.expectEqualSlices(u8, &plaintext, &m2);
}
test "Aes128GcmSiv - RFC 8452 Test Vector 3" {
// Test vector from RFC 8452 Appendix C.1
const key = [_]u8{
0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
};
const nonce = [_]u8{
0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00,
};
const plaintext = [_]u8{
0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00,
};
const ad = "";
var c: [plaintext.len]u8 = undefined;
var tag: [Aes128GcmSiv.tag_length]u8 = undefined;
Aes128GcmSiv.encrypt(&c, &tag, &plaintext, ad, nonce, key);
try htest.assertEqual("7323ea61d05932260047d942", &c);
try htest.assertEqual("a4978db357391a0bc4fdec8b0d106639", &tag);
var m2: [plaintext.len]u8 = undefined;
try Aes128GcmSiv.decrypt(&m2, &c, tag, ad, nonce, key);
try testing.expectEqualSlices(u8, &plaintext, &m2);
}
test "Aes256GcmSiv - RFC 8452 Test Vector" {
// Test vector from RFC 8452 Appendix C.2
const key = [_]u8{
0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
};
const nonce = [_]u8{
0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00,
};
const ad = "";
const m = "";
var c: [m.len]u8 = undefined;
var tag: [Aes256GcmSiv.tag_length]u8 = undefined;
Aes256GcmSiv.encrypt(&c, &tag, m, ad, nonce, key);
try htest.assertEqual("07f5f4169bbf55a8400cd47ea6fd400f", &tag);
}
test "Aes128GcmSiv - Decrypt with wrong tag" {
const key: [Aes128GcmSiv.key_length]u8 = @splat(0x69);
const nonce: [Aes128GcmSiv.nonce_length]u8 = @splat(0x42);
const m = "Test message";
const ad = "";
var c: [m.len]u8 = undefined;
var tag: [Aes128GcmSiv.tag_length]u8 = undefined;
Aes128GcmSiv.encrypt(&c, &tag, m, ad, nonce, key);
// Corrupt the tag
tag[0] ^= 0x01;
var m2: [m.len]u8 = undefined;
try testing.expectError(error.AuthenticationFailed, Aes128GcmSiv.decrypt(&m2, &c, tag, ad, nonce, key));
}
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