crypto/
aead.rs

1// SPDX-License-Identifier: Apache-2.0
2//! Versioned random-nonce AEAD and X25519 recipient wrapping (ADR 0051).
3//!
4//! AES-256-GCM is the single audited symmetric primitive. Each secret uses a
5//! fresh DEK; the DEK is wrapped to an X25519 recipient. Encryption keys are
6//! never derived from a signing seed.
7
8use aes_gcm::{
9    Aes256Gcm, Key, Nonce,
10    aead::{Aead, KeyInit, Payload},
11};
12use hkdf::Hkdf;
13use sha2::Sha256;
14use x25519_dalek::{PublicKey, SharedSecret, StaticSecret};
15use zeroize::Zeroizing;
16
17/// Domain for HKDF used when wrapping a DEK to an X25519 recipient.
18pub const WRAP_HKDF_INFO: &[u8] = b"heddle-env-wrap-v1";
19
20/// Version tag stored with ciphertext so algorithms can rotate.
21pub const AEAD_AES256_GCM_V1: &str = "aes-256-gcm-v1";
22
23/// Length-prefix + pad buckets. Ciphertext length otherwise tracks plaintext.
24pub const PAD_BUCKETS: &[usize] = &[32, 64, 128, 256, 512, 1024, 2048, 4096];
25
26const NONCE_LEN: usize = 12;
27const X25519_LEN: usize = 32;
28const DEK_LEN: usize = 32;
29const LENGTH_PREFIX: usize = 4;
30
31/// 32-byte data-encryption key. Zeroized on drop.
32#[derive(Clone)]
33pub struct Dek([u8; DEK_LEN]);
34
35impl Drop for Dek {
36    fn drop(&mut self) {
37        // Volatile zeroization the optimizer may not elide (unlike `fill(0)`).
38        zeroize::Zeroize::zeroize(&mut self.0);
39    }
40}
41
42impl Dek {
43    pub fn generate() -> Result<Self, AeadError> {
44        let mut bytes = [0u8; DEK_LEN];
45        fill_random(&mut bytes)?;
46        Ok(Self(bytes))
47    }
48
49    pub fn from_bytes(bytes: [u8; DEK_LEN]) -> Self {
50        Self(bytes)
51    }
52
53    pub fn as_bytes(&self) -> &[u8; DEK_LEN] {
54        &self.0
55    }
56}
57
58/// Exportable X25519 recipient secret. Explicit weaker-custody fallback.
59///
60/// `StaticSecret` zeroizes on drop. Holding this in-process is weaker custody.
61/// The policy broker holds a provider handle over this secret and returns
62/// slot values, never the key bytes.
63#[derive(Clone)]
64pub struct SoftwareRecipientSecret(StaticSecret);
65
66impl SoftwareRecipientSecret {
67    pub fn generate() -> Result<Self, AeadError> {
68        let mut seed = [0u8; X25519_LEN];
69        fill_random(&mut seed)?;
70        Ok(Self(StaticSecret::from(seed)))
71    }
72
73    pub fn from_bytes(bytes: [u8; X25519_LEN]) -> Self {
74        Self(StaticSecret::from(bytes))
75    }
76
77    pub fn to_bytes(&self) -> [u8; X25519_LEN] {
78        self.0.to_bytes()
79    }
80
81    pub fn public_key(&self) -> [u8; X25519_LEN] {
82        PublicKey::from(&self.0).to_bytes()
83    }
84}
85
86/// Random-nonce AES-256-GCM ciphertext plus the pad bucket used.
87#[derive(Clone, Debug, PartialEq, Eq)]
88pub struct AeadCiphertext {
89    pub alg: &'static str,
90    pub nonce: [u8; NONCE_LEN],
91    pub ciphertext: Vec<u8>,
92    pub pad_bucket: u32,
93}
94
95/// DEK wrapped to one X25519 recipient.
96#[derive(Clone, Debug, PartialEq, Eq)]
97pub struct WrappedDek {
98    pub ephemeral_public: [u8; X25519_LEN],
99    pub nonce: [u8; NONCE_LEN],
100    pub ciphertext: Vec<u8>,
101}
102
103#[derive(Debug, thiserror::Error)]
104pub enum AeadError {
105    #[error("secure random generation failed: {0}")]
106    Random(String),
107    #[error("aead encryption failed")]
108    Encrypt,
109    #[error("aead decryption failed")]
110    Decrypt,
111    #[error("hkdf expansion failed")]
112    Hkdf,
113    #[error("wrapped dek is truncated")]
114    TruncatedWrap,
115    #[error("padded plaintext is truncated or corrupt")]
116    CorruptPadding,
117    #[error("x25519 shared secret is non-contributory (low-order point)")]
118    NonContributory,
119}
120
121/// Choose the pad bucket for a plaintext length (including the 4-byte prefix).
122pub fn pad_bucket_for(plaintext_len: usize) -> usize {
123    let needed = plaintext_len.saturating_add(LENGTH_PREFIX);
124    for &bucket in PAD_BUCKETS {
125        if needed <= bucket {
126            return bucket;
127        }
128    }
129    needed.div_ceil(4096).saturating_mul(4096)
130}
131
132fn fill_random(dest: &mut [u8]) -> Result<(), AeadError> {
133    getrandom::fill(dest).map_err(|err| AeadError::Random(err.to_string()))
134}
135
136/// HKDF-SHA256 extract+expand for a single 32-byte OKM (RFC 5869). The OKM is
137/// wrapping-key material; it zeroizes on drop.
138fn hkdf_sha256(
139    salt: &[u8],
140    ikm: &[u8],
141    info: &[u8],
142) -> Result<Zeroizing<[u8; DEK_LEN]>, AeadError> {
143    let hk = Hkdf::<Sha256>::new(Some(salt), ikm);
144    let mut okm = Zeroizing::new([0u8; DEK_LEN]);
145    hk.expand(info, okm.as_mut_slice())
146        .map_err(|_| AeadError::Hkdf)?;
147    Ok(okm)
148}
149
150fn pad_plaintext(plaintext: &[u8]) -> Result<(Zeroizing<Vec<u8>>, u32), AeadError> {
151    let len = u32::try_from(plaintext.len()).map_err(|_| AeadError::CorruptPadding)?;
152    let bucket = pad_bucket_for(plaintext.len());
153    let bucket_u32 = u32::try_from(bucket).map_err(|_| AeadError::CorruptPadding)?;
154    let mut out = Zeroizing::new(vec![0u8; bucket]);
155    out[..LENGTH_PREFIX].copy_from_slice(&len.to_be_bytes());
156    let end = LENGTH_PREFIX + plaintext.len();
157    if end > bucket {
158        return Err(AeadError::CorruptPadding);
159    }
160    out[LENGTH_PREFIX..end].copy_from_slice(plaintext);
161    Ok((out, bucket_u32))
162}
163
164fn unpad_plaintext(padded: &[u8]) -> Result<Vec<u8>, AeadError> {
165    if padded.len() < LENGTH_PREFIX {
166        return Err(AeadError::CorruptPadding);
167    }
168    let mut len_bytes = [0u8; LENGTH_PREFIX];
169    len_bytes.copy_from_slice(&padded[..LENGTH_PREFIX]);
170    let len = usize::try_from(u32::from_be_bytes(len_bytes)).unwrap_or(usize::MAX);
171    let end = LENGTH_PREFIX.saturating_add(len);
172    if end > padded.len() {
173        return Err(AeadError::CorruptPadding);
174    }
175    if padded[end..].iter().any(|byte| *byte != 0) {
176        return Err(AeadError::CorruptPadding);
177    }
178    Ok(padded[LENGTH_PREFIX..end].to_vec())
179}
180
181/// Encrypt `plaintext` under `dek` with a fresh random nonce. `aad` binds the
182/// ciphertext to a slot/profile so it cannot be replayed onto another record.
183pub fn encrypt_padded(
184    dek: &Dek,
185    plaintext: &[u8],
186    aad: &[u8],
187) -> Result<AeadCiphertext, AeadError> {
188    let (padded, pad_bucket) = pad_plaintext(plaintext)?;
189    let mut nonce = [0u8; NONCE_LEN];
190    fill_random(&mut nonce)?;
191    let cipher = Aes256Gcm::new(Key::<Aes256Gcm>::from_slice(dek.as_bytes()));
192    let ciphertext = cipher
193        .encrypt(
194            Nonce::from_slice(&nonce),
195            Payload {
196                msg: padded.as_slice(),
197                aad,
198            },
199        )
200        .map_err(|_| AeadError::Encrypt)?;
201    Ok(AeadCiphertext {
202        alg: AEAD_AES256_GCM_V1,
203        nonce,
204        ciphertext,
205        pad_bucket,
206    })
207}
208
209pub fn decrypt_padded(
210    dek: &Dek,
211    sealed: &AeadCiphertext,
212    aad: &[u8],
213) -> Result<Vec<u8>, AeadError> {
214    if sealed.alg != AEAD_AES256_GCM_V1 {
215        return Err(AeadError::Decrypt);
216    }
217    let cipher = Aes256Gcm::new(Key::<Aes256Gcm>::from_slice(dek.as_bytes()));
218    let padded = Zeroizing::new(
219        cipher
220            .decrypt(
221                Nonce::from_slice(&sealed.nonce),
222                Payload {
223                    msg: &sealed.ciphertext,
224                    aad,
225                },
226            )
227            .map_err(|_| AeadError::Decrypt)?,
228    );
229    if padded.len() != sealed.pad_bucket as usize {
230        return Err(AeadError::CorruptPadding);
231    }
232    unpad_plaintext(&padded)
233}
234
235/// Derive the wrapping key from an X25519 shared secret, rejecting a
236/// non-contributory (low-order) shared secret so a forged low-order
237/// `ephemeral_public` cannot coerce an attacker-known all-zero shared secret.
238fn wrap_key_from_shared(
239    shared: &SharedSecret,
240    ephemeral_public: &[u8; X25519_LEN],
241    recipient_public: &[u8; X25519_LEN],
242) -> Result<Zeroizing<[u8; DEK_LEN]>, AeadError> {
243    if !shared.was_contributory() {
244        return Err(AeadError::NonContributory);
245    }
246    let mut salt = [0u8; X25519_LEN * 2];
247    salt[..X25519_LEN].copy_from_slice(ephemeral_public);
248    salt[X25519_LEN..].copy_from_slice(recipient_public);
249    hkdf_sha256(&salt, shared.as_bytes(), WRAP_HKDF_INFO)
250}
251
252/// Wrap `dek` to `recipient_public` with an ephemeral X25519 key. `aad` binds
253/// the wrap to its recipient/profile/slot/version so a wrap cannot be
254/// transplanted onto another record.
255pub fn wrap_dek(
256    dek: &Dek,
257    recipient_public: &[u8; X25519_LEN],
258    aad: &[u8],
259) -> Result<WrappedDek, AeadError> {
260    let ephemeral = SoftwareRecipientSecret::generate()?;
261    let ephemeral_public = ephemeral.public_key();
262    let shared = ephemeral
263        .0
264        .diffie_hellman(&PublicKey::from(*recipient_public));
265    let wrap_key = wrap_key_from_shared(&shared, &ephemeral_public, recipient_public)?;
266    let mut nonce = [0u8; NONCE_LEN];
267    fill_random(&mut nonce)?;
268    let cipher = Aes256Gcm::new(Key::<Aes256Gcm>::from_slice(wrap_key.as_slice()));
269    let ciphertext = cipher
270        .encrypt(
271            Nonce::from_slice(&nonce),
272            Payload {
273                msg: dek.as_bytes().as_slice(),
274                aad,
275            },
276        )
277        .map_err(|_| AeadError::Encrypt)?;
278    Ok(WrappedDek {
279        ephemeral_public,
280        nonce,
281        ciphertext,
282    })
283}
284
285pub fn unwrap_dek(
286    wrapped: &WrappedDek,
287    recipient: &SoftwareRecipientSecret,
288    aad: &[u8],
289) -> Result<Dek, AeadError> {
290    if wrapped.ciphertext.len() < 16 {
291        return Err(AeadError::TruncatedWrap);
292    }
293    let shared = recipient
294        .0
295        .diffie_hellman(&PublicKey::from(wrapped.ephemeral_public));
296    let recipient_public = recipient.public_key();
297    let okm = wrap_key_from_shared(&shared, &wrapped.ephemeral_public, &recipient_public)?;
298    let cipher = Aes256Gcm::new(Key::<Aes256Gcm>::from_slice(okm.as_slice()));
299    let dek_bytes = Zeroizing::new(
300        cipher
301            .decrypt(
302                Nonce::from_slice(&wrapped.nonce),
303                Payload {
304                    msg: wrapped.ciphertext.as_slice(),
305                    aad,
306                },
307            )
308            .map_err(|_| AeadError::Decrypt)?,
309    );
310    let dek_arr: [u8; DEK_LEN] = dek_bytes
311        .as_slice()
312        .try_into()
313        .map_err(|_| AeadError::TruncatedWrap)?;
314    Ok(Dek::from_bytes(dek_arr))
315}
316
317#[cfg(test)]
318mod tests {
319    use super::*;
320
321    #[test]
322    fn padded_round_trip_hides_exact_length_inside_a_bucket() {
323        let dek = Dek::generate().expect("dek");
324        let sealed = encrypt_padded(&dek, b"secret-value", b"aad-v1").expect("encrypt");
325        assert_eq!(sealed.alg, AEAD_AES256_GCM_V1);
326        assert_eq!(sealed.pad_bucket, 32);
327        assert_ne!(&sealed.ciphertext, b"secret-value");
328        let plain = decrypt_padded(&dek, &sealed, b"aad-v1").expect("decrypt");
329        assert_eq!(plain, b"secret-value");
330    }
331
332    #[test]
333    fn wrong_aad_cannot_decrypt() {
334        let dek = Dek::generate().expect("dek");
335        let sealed = encrypt_padded(&dek, b"secret-value", b"slot-a").expect("encrypt");
336        decrypt_padded(&dek, &sealed, b"slot-b").expect_err("aad mismatch");
337    }
338
339    #[test]
340    fn wrap_round_trip_and_wrong_recipient_fails() {
341        let dek = Dek::generate().expect("dek");
342        let alice = SoftwareRecipientSecret::generate().expect("alice");
343        let bob = SoftwareRecipientSecret::generate().expect("bob");
344        let wrapped = wrap_dek(&dek, &alice.public_key(), b"wrap-aad-v1").expect("wrap");
345        let opened = unwrap_dek(&wrapped, &alice, b"wrap-aad-v1").expect("alice unwraps");
346        assert_eq!(opened.as_bytes(), dek.as_bytes());
347        assert!(
348            unwrap_dek(&wrapped, &bob, b"wrap-aad-v1").is_err(),
349            "bob cannot unwrap alice's wrap"
350        );
351    }
352
353    #[test]
354    fn wrap_aad_mismatch_cannot_unwrap() {
355        let dek = Dek::generate().expect("dek");
356        let alice = SoftwareRecipientSecret::generate().expect("alice");
357        let wrapped = wrap_dek(&dek, &alice.public_key(), b"recip|profile|SLOT|v1").expect("wrap");
358        assert!(
359            unwrap_dek(&wrapped, &alice, b"recip|profile|SLOT|v2").is_err(),
360            "a wrap must not unwrap under a different binding (transplant/rollback)"
361        );
362    }
363
364    #[test]
365    fn low_order_ephemeral_public_is_rejected() {
366        // A forged wrap whose ephemeral_public is a low-order point yields an
367        // all-zero shared secret on the recipient side; the contributory check
368        // must reject it rather than derive an attacker-known wrap key.
369        let dek = Dek::generate().expect("dek");
370        let alice = SoftwareRecipientSecret::generate().expect("alice");
371        let good = wrap_dek(&dek, &alice.public_key(), b"aad").expect("wrap");
372        // The canonical all-zero point is low order.
373        let forged = WrappedDek {
374            ephemeral_public: [0u8; X25519_LEN],
375            nonce: good.nonce,
376            ciphertext: good.ciphertext,
377        };
378        assert!(
379            matches!(
380                unwrap_dek(&forged, &alice, b"aad"),
381                Err(AeadError::NonContributory)
382            ),
383            "a low-order ephemeral_public must be rejected as non-contributory"
384        );
385    }
386
387    #[test]
388    fn pad_buckets_jump_to_4k_increments_after_4k() {
389        assert_eq!(pad_bucket_for(0), 32);
390        assert_eq!(pad_bucket_for(28), 32);
391        assert_eq!(pad_bucket_for(29), 64);
392        assert_eq!(pad_bucket_for(4093), 8192);
393    }
394}
crypto/aead.rs

crypto/
aead.rs
