@@ -10,3 +10,4 @@
 [dev-dependencies]
 rand = "0.10.0"
 aes = "0.8.4"
+aes-gcm = "0.10.3"

@@ -1,2 +1,4 @@
 # plonky2-aes
-Plonky2 circuits for AES encryption system as described at https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.197-upd1.pdf .
+Plonky2 circuits for AES encryption system as described at
+- AES: https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.197-upd1.pdf
+- AES-GaloisCounterMode: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf

@@ -1,3 +1,5 @@
+//! For LICENSE check out https://github.com/0xPARC/plonky2-aes/blob/main/LICENSE
+
 use std::{array, sync::Arc};
 
 use plonky2::{
@@ -12,7 +14,7 @@ };
 
 use crate::{
     constants::{RCON, SBOX},
-    native::{rot_word, shift_rows, State},
+    native_aes::{rot_word, shift_rows, State},
     D,
 };
 
@@ -397,13 +399,11 @@ },
     };
     use rand::RngExt;
 
-    use crate::{
-        circuit::{
-            sbox_lut, state_mix_matrix_bits, ByteArrayTarget, CircuitBuilderAESState,
-            PartialWitnessAESState, PartialWitnessByteArray,
-        },
-        native::{encrypt_block, key_expansion, mix_columns, sub_bytes, State},
+    use super::{
+        sbox_lut, state_mix_matrix_bits, ByteArrayTarget, CircuitBuilderAESState,
+        PartialWitnessAESState, PartialWitnessByteArray,
     };
+    use crate::native_aes::{encrypt_block, key_expansion, mix_columns, sub_bytes, State};
 
     use super::D;
 
@@ -535,7 +535,7 @@
         Ok(())
     }
 
-    /// Analogous tests to `native::tests::test_key_expansion`.
+    /// Analogous tests to `native_aes::tests::test_key_expansion`.
     fn test_key_expansion_op<const NK: usize, const NB: usize, const NR: usize>(
         key: [u8; NK * NB],
     ) -> Result<()>

@@ -3,9 +3,10 @@ #![allow(incomplete_features)]
 #![feature(generic_const_exprs)]
 #![allow(dead_code)] // TMP
 
-mod circuit;
+mod circuit_aes;
 mod constants;
-mod native;
+mod native_aes;
+mod native_gcm;
 
 /// D defines the extension degree of the field used in the Plonky2 proofs (quadratic extension).
 pub const D: usize = 2;

@@ -3,8 +3,8 @@ //!
 //! Rust native implementation of
 //! [AES](https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.197-upd1.pdf) having
 //! in mind the approach that will be done in-circuit; this means that it is not
-//! written in the most rust idiomatic way, but simulating the behavior that we
-//! will do later inside the circuit.
+//! written in the most rust idiomatic way, nor memory-efficient, but simulating
+//! the behavior that we will do later inside the circuit.
 
 use std::array;
 
@@ -12,7 +12,7 @@ use crate::constants::{RCON, SBOX};
 
 pub(crate) type State = [[u8; 4]; 4];
 
-fn flatten_state(s: State) -> [u8; 16] {
+pub(crate) fn flatten_state(s: State) -> [u8; 16] {
     let mut r = [0u8; 16];
     for i in 0..4 {
         for j in 0..4 {
@@ -217,7 +217,7 @@ assert_eq!(flatten_state(c), expected);
     }
 
     use aes::{
-        cipher::{BlockDecrypt, BlockEncrypt, BlockSizeUser, KeyInit, generic_array::GenericArray},
+        cipher::{generic_array::GenericArray, BlockDecrypt, BlockEncrypt, BlockSizeUser, KeyInit},
         {Aes128, Aes192, Aes256},
     };
     use rand::RngExt;

@@ -0,0 +1,218 @@
+//! For LICENSE check out https://github.com/0xPARC/plonky2-aes/blob/main/LICENSE
+//!
+//! Rust native implementation of [AES-GCM (Galois Counter
+//! Mode)](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf)
+//! having in mind the approach that will be done in-circuit; this means that it
+//! is not written in the most rust idiomatic way, nor memory-efficient, but
+//! simulating the behavior that we will do later inside the circuit.
+//!
+
+use crate::native_aes::{encrypt_block, flatten_state, key_expansion, State};
+
+// supported tag length
+const TAG_LEN: usize = 128;
+
+/// Section 7.1 from https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf
+pub fn encrypt<const NK: usize, const NB: usize, const NR: usize>(
+    key: &[u8; NK * NB],
+    nonce: &[u8; 12],
+    pt: &[u8], // plaintext
+) -> (Vec<u8>, Vec<u8>)
+where
+    [(); 4 * (NR + 1)]:,
+{
+    let a: &[u8] = &[]; // additional authenticated data // TODO maybe as input
+    let expanded_key: [[u8; 4]; 4 * (NR + 1)] = key_expansion::<NK, NB, NR>(&key);
+
+    // 1. CIPH_K
+    let h = encrypt_block::<NR>(&[0u8; 16], &expanded_key);
+    let h = flatten_state(h);
+
+    // 2. J_0
+    let j0: [u8; 16] = if nonce.len() * 8 == 96 {
+        // J_0 = IV || 0^31 || 1
+        let mut out = [0u8; 16];
+        out[..12].copy_from_slice(&nonce.as_slice());
+        out[12..16].copy_from_slice(&[0x00, 0x00, 0x00, 0x01]);
+        out
+    } else {
+        panic!("unsuported at initial version; nonce.len()=12 (96 bits)");
+    };
+
+    // 3. C=GCTR()
+    let c = gctr(&inc32(j0), &pt);
+
+    // 4. u, v
+    // let u = (16 - ceil(c.len() % 16)) % 16 // TODO WIP
+    let u: usize = 16 * (c.len() as f64 / 16_f64).ceil() as usize - c.len();
+    let v: usize = 16 * (a.len() as f64 / 16_f64).ceil() as usize - a.len();
+
+    // 5. S = GHASH()
+    let a_len: [u8; 8] = (a.len() * 8).to_be_bytes();
+    let c_len: [u8; 8] = (c.len() * 8).to_be_bytes();
+    let ghash_input = &[
+        a.to_vec(),
+        vec![0u8; v],
+        c.clone(),
+        vec![0u8; u],
+        a_len.to_vec(),
+        c_len.to_vec(),
+    ]
+    .concat();
+    let s = ghash(h, ghash_input);
+
+    // 6. T=MSB(GCTR()))
+    let t = msb_t(TAG_LEN, &gctr(&j0, &s));
+
+    (c, t)
+}
+
+/// Section 6.5, Algorithm 3
+fn gctr(icb: &[u8; 16], x: &[u8]) -> Vec<u8> {
+    todo!()
+}
+
+/// Section 6.4, Algorithm 2
+fn ghash(h: [u8; 16], x: &[u8]) -> [u8; 16] {
+    assert!(x.len() % 16 == 0); // multiple of 128 bits
+    let m = x.len() / 16;
+
+    let mut y = [0u8; 16]; // (128 bits)
+    for i in 0..m {
+        let mut xi = [0u8; 16];
+        xi.clone_from_slice(&x[i * 16..i * 16 + 16]);
+        let y_xi = xor_blocks(y, xi);
+        y = gf_2_128_mul(y_xi, h);
+    }
+    y
+}
+fn xor_blocks(b1: [u8; 16], b2: [u8; 16]) -> [u8; 16] {
+    let mut r = [0u8; 16];
+    for i in 0..16 {
+        r[i] = b1[i] ^ b2[i];
+    }
+    r
+}
+
+/// multiplication of blocks (in GF(2^128)), Algorithm 1, Section 6.3
+pub fn gf_2_128_mul(x: [u8; 16], y: [u8; 16]) -> [u8; 16] {
+    // R = 11100001 || 0^120
+    const R: [u8; 16] = [
+        0xE1, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00,
+    ];
+
+    let mut z = [0u8; 16];
+    let mut v = y;
+    for i in 0..128 {
+        // xi: i-th bit of x
+        let byte_index = i / 8;
+        let bit_index = 7 - (i % 8);
+        let xi = (x[byte_index] >> bit_index) & 1;
+
+        if xi == 1 {
+            z = xor_blocks(z, v);
+        }
+        let lsb = v[15] & 1;
+        right_shift_one(&mut v);
+        if lsb == 1 {
+            v = xor_blocks(v, R);
+        }
+    }
+    z
+}
+fn right_shift_one(block: &mut [u8; 16]) {
+    let mut carry = 0u8;
+
+    for byte in block.iter_mut() {
+        let new_carry = *byte & 1;
+        *byte = (*byte >> 1) | (carry << 7);
+        carry = new_carry;
+    }
+}
+
+/// increment the right-most 32 bits of the given block (128 bits). Section 2.
+pub fn inc32(b: [u8; 16]) -> [u8; 16] {
+    let mut r = b.clone();
+    // counter = last 32bits
+    let counter = u32::from_be_bytes([r[12], r[13], r[14], r[15]]);
+    let incremented = counter.wrapping_add(1);
+    let bytes = incremented.to_be_bytes();
+    r[12..16].copy_from_slice(&bytes);
+    r
+}
+
+/// returns the t left-most (most-significant in BE) bits of the block
+pub fn msb_t(t: usize, block: &[u8]) -> Vec<u8> {
+    assert!(t <= 128, "MSB_t err: max 128 bits");
+
+    let mut out = Vec::with_capacity((t + 7) / 8); // ceiling bytes
+    let mut bits_left = t;
+    for byte in block.iter() {
+        if bits_left == 0 {
+            break;
+        }
+        if bits_left >= 8 {
+            out.push(*byte);
+            bits_left -= 8;
+        } else {
+            // take the top `bits_left` bits of the byte
+            let mask = 0xFFu8 << (8 - bits_left);
+            out.push(byte & mask);
+            break;
+        }
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use aes_gcm::{
+        aead::{Aead, AeadCore, Key, KeyInit, OsRng},
+        Aes128Gcm, Nonce,
+    };
+
+    /// test checking against NIST test vector
+    #[test]
+    fn test_aes_gcm_encrypt_nist_vector() {
+        // https://csrc.nist.gov/CSRC/media/Projects/Cryptographic-Standards-and-Guidelines/documents/examples/AES_GCM.pdf
+        let key: [u8; 16] = [
+            0xFE, 0xFF, 0xE9, 0x92, 0x86, 0x65, 0x73, 0x1C, 0x6D, 0x6A, 0x8F, 0x94, 0x67, 0x30,
+            0x83, 0x08,
+        ];
+        let nonce: [u8; 12] = [
+            0xCA, 0xFE, 0xBA, 0xBE, 0xFA, 0xCE, 0xDB, 0xAD, 0xDE, 0xCA, 0xF8, 0x88,
+        ];
+        let plaintext = [
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            0x00, 0x00,
+        ];
+        let aad: &[u8] = &[];
+        let expected_ciphertext: &[u8] = &[
+            0x32, 0x47, 0x18, 0x4B, 0x3C, 0x4F, 0x69, 0xA4, 0x4D, 0xBC, 0xD2, 0x28, 0x87, 0xBB,
+            0xB4, 0x18,
+        ];
+
+        todo!();
+    }
+
+    /// test checking against external aes-gcm lib
+    #[test]
+    fn test_with_external_lib() -> anyhow::Result<()> {
+        let key: &[u8; 16] = &[42; 16];
+        let nonce: &[u8; 12] = &[111; 12];
+        let pt = b"plaintext test message";
+
+        let key: &Key<Aes128Gcm> = key.into();
+        let cipher = Aes128Gcm::new(&key);
+        let nonce = Nonce::from_slice(nonce);
+
+        let ciphertext = cipher.encrypt(&nonce, pt.as_ref()).unwrap();
+
+        let plaintext = cipher.decrypt(&nonce, ciphertext.as_ref()).unwrap();
+        assert_eq!(&plaintext, pt);
+        Ok(())
+    }
+}