1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559
use crate::{ kdf::{Kdf as KdfTrait, LabeledExpand}, kem::Kem as KemTrait, kex::{Deserializable, Serializable}, setup::ExporterSecret, util::{full_suite_id, FullSuiteId}, HpkeError, }; use core::{marker::PhantomData, u8}; use aead::{AeadInPlace as BaseAead, NewAead as BaseNewAead}; use byteorder::{BigEndian, ByteOrder}; use generic_array::GenericArray; use hkdf::Hkdf; /// Represents authenticated encryption functionality pub trait Aead { /// The underlying AEAD implementation type AeadImpl: BaseAead + BaseNewAead + Clone; /// The algorithm identifier for an AEAD implementation const AEAD_ID: u16; } /// The implementation of AES-GCM-128 pub struct AesGcm128 {} impl Aead for AesGcm128 { type AeadImpl = aes_gcm::Aes128Gcm; // draft02 §8.3: AES-GCM-128 const AEAD_ID: u16 = 0x0001; } /// The implementation of AES-GCM-128 pub struct AesGcm256 {} impl Aead for AesGcm256 { type AeadImpl = aes_gcm::Aes256Gcm; // draft02 §8.3: AES-GCM-256 const AEAD_ID: u16 = 0x0002; } /// The implementation of ChaCha20-Poly1305 pub struct ChaCha20Poly1305 {} impl Aead for ChaCha20Poly1305 { type AeadImpl = chacha20poly1305::ChaCha20Poly1305; // draft02 §8.3: ChaCha20Poly1305 const AEAD_ID: u16 = 0x0003; } // A nonce is the same thing as a sequence counter. But you never increment a nonce. pub(crate) type AeadNonce<A> = GenericArray<u8, <<A as Aead>::AeadImpl as BaseAead>::NonceSize>; pub(crate) type AeadKey<A> = GenericArray<u8, <<A as Aead>::AeadImpl as aead::NewAead>::KeySize>; /// A sequence counter. This is set to `u64` instead of the true nonce size of an AEAD for two /// reasons: /// /// 1. No algorithm that would appear in HPKE would support nonce sizes less than `u64`. /// 2. It is just about physically impossible to encrypt 2^64 messages in sequence. If a computer /// computes 1 encryption every nanosecond, it would take over 584 years to run out of nonces. /// Notably, unlike randomized nonces, counting in sequence doesn't parallelize, so we don't /// have to imagine amortizing this computation across multiple computers. In conclusion, 64 /// bits should be enough for anybody. #[derive(Default, Clone)] struct Seq(u64); // def Context.IncrementSeq(): // if self.seq >= (1 << (8*Nn)) - 1: // raise NonceOverflowError // self.seq += 1 /// Increments the sequence counter. Returns None on overflow. fn increment_seq(seq: &Seq) -> Option<Seq> { // Try to add 1 seq.0.checked_add(1).map(Seq) } // def Context.ComputeNonce(seq): // seq_bytes = I2OSP(seq, Nn) // return xor(self.nonce, seq_bytes) /// Derives a nonce from the given nonce and a "sequence number". The sequence number is treated as /// a big-endian integer with length equal to the nonce length. fn mix_nonce<A: Aead>(base_nonce: &AeadNonce<A>, seq: &Seq) -> AeadNonce<A> { // Write `seq` in big-endian order into a byte buffer that's the size of a nonce let mut seq_buf = AeadNonce::<A>::default(); // We just write to the last seq_size bytes. This is necessary because our AEAD nonces (>= 96 // bits) are always bigger than the sequence buffer (64 bits). We write to the last 64 bits // because this is a big-endian number. let seq_size = core::mem::size_of::<Seq>(); let nonce_size = base_nonce.len(); BigEndian::write_u64(&mut seq_buf[nonce_size - seq_size..], seq.0); // XOR the base nonce bytes with the sequence bytes let new_nonce_iter = base_nonce .iter() .zip(seq_buf.iter()) .map(|(nonce_byte, seq_byte)| nonce_byte ^ seq_byte); // This cannot fail, as the length of Nonce<A> is precisely the length of Seq GenericArray::from_exact_iter(new_nonce_iter).unwrap() } /// An authenticated encryption tag pub struct AeadTag<A: Aead>(GenericArray<u8, <A::AeadImpl as BaseAead>::TagSize>); impl<A: Aead> Serializable for AeadTag<A> { type OutputSize = <A::AeadImpl as BaseAead>::TagSize; fn to_bytes(&self) -> GenericArray<u8, Self::OutputSize> { self.0.clone() } } impl<A: Aead> Deserializable for AeadTag<A> { fn from_bytes(encoded: &[u8]) -> Result<Self, HpkeError> { if encoded.len() != Self::size() { Err(HpkeError::InvalidEncoding) } else { // Copy to a fixed-size array let mut arr = <GenericArray<u8, Self::OutputSize> as Default>::default(); arr.copy_from_slice(encoded); Ok(AeadTag(arr)) } } } /// The HPKE encryption context. This is what you use to `seal` plaintexts and `open` ciphertexts. pub(crate) struct AeadCtx<A: Aead, Kdf: KdfTrait, Kem: KemTrait> { /// Records whether the nonce sequence counter has overflowed overflowed: bool, /// The underlying AEAD instance. This also does decryption. encryptor: A::AeadImpl, /// The base nonce which we XOR with sequence numbers nonce: AeadNonce<A>, /// The exporter secret, used in the `export()` method exporter_secret: ExporterSecret<Kdf>, /// The running sequence number seq: Seq, /// This binds the `AeadCtx` to the KEM that made it. Used to generate `suite_id`. src_kem: PhantomData<Kem>, /// The full ID of the ciphersuite that created this `AeadCtx`. Used for context binding. suite_id: FullSuiteId, } // Necessary for test_setup_soundness #[cfg(test)] impl<A: Aead, Kdf: KdfTrait, Kem: KemTrait> Clone for AeadCtx<A, Kdf, Kem> { fn clone(&self) -> AeadCtx<A, Kdf, Kem> { AeadCtx { overflowed: self.overflowed, encryptor: self.encryptor.clone(), nonce: self.nonce.clone(), exporter_secret: self.exporter_secret.clone(), seq: self.seq.clone(), src_kem: PhantomData, suite_id: self.suite_id.clone(), } } } impl<A: Aead, Kdf: KdfTrait, Kem: KemTrait> AeadCtx<A, Kdf, Kem> { /// Makes an AeadCtx from a raw key and nonce pub(crate) fn new( key: &AeadKey<A>, nonce: AeadNonce<A>, exporter_secret: ExporterSecret<Kdf>, ) -> AeadCtx<A, Kdf, Kem> { let suite_id = full_suite_id::<A, Kdf, Kem>(); AeadCtx { overflowed: false, encryptor: <A::AeadImpl as aead::NewAead>::new(key), nonce, exporter_secret, seq: <Seq as Default>::default(), src_kem: PhantomData, suite_id, } } // def Context.Export(exporter_context, L): // return LabeledExpand(self.exporter_secret, "sec", exporter_context, L) /// Fills a given buffer with secret bytes derived from this encryption context. This value /// does not depend on sequence number, so it is constant for the lifetime of this context. pub fn export(&self, exporter_ctx: &[u8], out_buf: &mut [u8]) -> Result<(), HpkeError> { // Use our exporter secret as the PRK for an HKDF-Expand op. The only time this fails is // when the length of the PRK is not the the underlying hash function's digest size. But // that's guaranteed by the type system, so we can unwrap(). let hkdf_ctx = Hkdf::<Kdf::HashImpl>::from_prk(self.exporter_secret.as_slice()).unwrap(); // This call either succeeds or returns hkdf::InvalidLength (iff the buffer length is more // than 255x the digest size of the underlying hash function) hkdf_ctx .labeled_expand(&self.suite_id, b"sec", exporter_ctx, out_buf) .map_err(|_| HpkeError::InvalidKdfLength) } } /// The HPKE receiver's context. This is what you use to `open` ciphertexts. pub struct AeadCtxR<A: Aead, Kdf: KdfTrait, Kem: KemTrait>(AeadCtx<A, Kdf, Kem>); // AeadCtx -> AeadCtxR via wrapping impl<A: Aead, Kdf: KdfTrait, Kem: KemTrait> From<AeadCtx<A, Kdf, Kem>> for AeadCtxR<A, Kdf, Kem> { fn from(ctx: AeadCtx<A, Kdf, Kem>) -> AeadCtxR<A, Kdf, Kem> { AeadCtxR(ctx) } } // Necessary for test_setup_soundness #[cfg(test)] impl<A: Aead, Kdf: KdfTrait, Kem: KemTrait> Clone for AeadCtxR<A, Kdf, Kem> { fn clone(&self) -> AeadCtxR<A, Kdf, Kem> { self.0.clone().into() } } impl<A: Aead, Kdf: KdfTrait, Kem: KemTrait> AeadCtxR<A, Kdf, Kem> { // def Context.Open(aad, ct): // pt = Open(self.key, self.ComputeNonce(self.seq), aad, ct) // if pt == OpenError: // raise OpenError // self.IncrementSeq() // return pt /// Does a "detached open in place", meaning it overwrites `ciphertext` with the resulting /// plaintext, and takes the tag as a separate input. /// /// Return Value /// ============ /// Returns `Ok(())` on success. If this context has been used for so many encryptions that /// the sequence number overflowed, returns `Err(HpkeError::SeqOverflow)`. If this happens, /// `plaintext` will be unmodified. If the tag fails to validate, returns /// `Err(HpkeError::InvalidTag)`. If this happens, `plaintext` is in an undefined state. pub fn open( &mut self, ciphertext: &mut [u8], aad: &[u8], tag: &AeadTag<A>, ) -> Result<(), HpkeError> { if self.0.overflowed { // If the sequence counter overflowed, we've been used for far too long. Shut down. Err(HpkeError::SeqOverflow) } else { // Compute the nonce and do the encryption in place let nonce = mix_nonce::<A>(&self.0.nonce, &self.0.seq); let decrypt_res = self .0 .encryptor .decrypt_in_place_detached(&nonce, &aad, ciphertext, &tag.0); if decrypt_res.is_err() { // Opening failed due to a bad tag return Err(HpkeError::InvalidTag); } // Opening was a success // Try to increment the sequence counter. If it fails, this was our last // decryption. match increment_seq(&self.0.seq) { Some(new_seq) => self.0.seq = new_seq, None => self.0.overflowed = true, } Ok(()) } } /// Fills a given buffer with secret bytes derived from this encryption context. This value /// does not depend on sequence number, so it is constant for the lifetime of this context. /// /// Return Value /// ============ /// Returns `Ok(())` on success. If the buffer length is more than about 255x the digest size /// of the underlying hash function, returns an `Err(HpkeError::InvalidKdfLength)`. The exact /// number is given in the "Input Length Restrictions" section of the spec. Just don't use to /// fill massive buffers and you'll be fine. pub fn export(&self, info: &[u8], out_buf: &mut [u8]) -> Result<(), HpkeError> { // Pass to AeadCtx self.0.export(info, out_buf) } } /// The HPKE senders's context. This is what you use to `seal` plaintexts. pub struct AeadCtxS<A: Aead, Kdf: KdfTrait, Kem: KemTrait>(AeadCtx<A, Kdf, Kem>); // AeadCtx -> AeadCtxS via wrapping impl<A: Aead, Kdf: KdfTrait, Kem: KemTrait> From<AeadCtx<A, Kdf, Kem>> for AeadCtxS<A, Kdf, Kem> { fn from(ctx: AeadCtx<A, Kdf, Kem>) -> AeadCtxS<A, Kdf, Kem> { AeadCtxS(ctx) } } // Necessary for test_setup_soundness #[cfg(test)] impl<A: Aead, Kdf: KdfTrait, Kem: KemTrait> Clone for AeadCtxS<A, Kdf, Kem> { fn clone(&self) -> AeadCtxS<A, Kdf, Kem> { self.0.clone().into() } } impl<A: Aead, Kdf: KdfTrait, Kem: KemTrait> AeadCtxS<A, Kdf, Kem> { // def Context.Seal(aad, pt): // ct = Seal(self.key, self.ComputeNonce(self.seq), aad, pt) // self.IncrementSeq() // return ct /// Does a "detached seal in place", meaning it overwrites `plaintext` with the resulting /// ciphertext, and returns the resulting authentication tag /// /// Return Value /// ============ /// Returns `Ok(tag)` on success. If this context has been used for so many encryptions that /// the sequence number overflowed, returns `Err(HpkeError::SeqOverflow)`. If this happens, /// `plaintext` will be unmodified. If an unspecified error happened during encryption, returns /// `Err(HpkeError::Encryption)`. If this happens, the contents of `plaintext` is undefined. pub fn seal(&mut self, plaintext: &mut [u8], aad: &[u8]) -> Result<AeadTag<A>, HpkeError> { if self.0.overflowed { // If the sequence counter overflowed, we've been used for far too long. Shut down. Err(HpkeError::SeqOverflow) } else { // Compute the nonce and do the encryption in place let nonce = mix_nonce::<A>(&self.0.nonce, &self.0.seq); let tag_res = self .0 .encryptor .encrypt_in_place_detached(&nonce, &aad, plaintext); // Check if an error occurred when encrypting let tag = match tag_res { Err(_) => return Err(HpkeError::Encryption), Ok(t) => t, }; // Try to increment the sequence counter. If it fails, this was our last encryption. match increment_seq(&self.0.seq) { Some(new_seq) => self.0.seq = new_seq, None => self.0.overflowed = true, } // Return the tag Ok(AeadTag(tag)) } } // def Context.Export(exporter_context, L): // return LabeledExpand(self.exporter_secret, "sec", exporter_context, L) /// Fills a given buffer with secret bytes derived from this encryption context. This value /// does not depend on sequence number, so it is constant for the lifetime of this context. /// /// Return Value /// ============ /// Returns `Ok(())` on success. If the buffer length is more than 255x the digest size of the /// underlying hash function, returns an `Err(HpkeError::InvalidKdfLength)`. pub fn export(&self, info: &[u8], out_buf: &mut [u8]) -> Result<(), HpkeError> { // Pass to AeadCtx self.0.export(info, out_buf) } } #[cfg(test)] mod test { use super::{AeadTag, AesGcm128, AesGcm256, ChaCha20Poly1305, Seq}; use crate::{kdf::HkdfSha256, kex::Deserializable, test_util::gen_ctx_simple_pair, HpkeError}; /// Tests that encryption context secret export does not change behavior based on the /// underlying sequence number This logic is cipher-agnostic, so we don't make the test generic /// over ciphers. macro_rules! test_export_idempotence { ($test_name:ident, $kem_ty:ty) => { #[test] fn $test_name() { type Kem = $kem_ty; type Kdf = HkdfSha256; // Again, this test is cipher-agnostic type A = ChaCha20Poly1305; // Set up a context. Logic is algorithm-independent, so we don't care about the // types here let (mut sender_ctx, _) = gen_ctx_simple_pair::<A, Kdf, Kem>(); // Get an initial export secret let mut secret1 = [0u8; 16]; sender_ctx .export(b"test_export_idempotence", &mut secret1) .unwrap(); // Modify the context by encrypting something let mut plaintext = *b"back hand"; sender_ctx .seal(&mut plaintext[..], b"") .expect("seal() failed"); // Get a second export secret let mut secret2 = [0u8; 16]; sender_ctx .export(b"test_export_idempotence", &mut secret2) .unwrap(); assert_eq!(secret1, secret2); } }; } /// Tests that sequence overflowing causes an error. This logic is cipher-agnostic, so we don't /// make the test generic over ciphers. macro_rules! test_overflow { ($test_name:ident, $kem_ty:ty) => { #[test] fn $test_name() { type Kem = $kem_ty; type Kdf = HkdfSha256; // Again, this test is cipher-agnostic type A = ChaCha20Poly1305; // Make a sequence number that's at the max let big_seq = { let mut seq = <Seq as Default>::default(); seq.0 = u64::MAX; seq }; let (mut sender_ctx, mut receiver_ctx) = gen_ctx_simple_pair::<A, Kdf, Kem>(); sender_ctx.0.seq = big_seq.clone(); receiver_ctx.0.seq = big_seq.clone(); // These should support precisely one more encryption before it registers an // overflow let msg = b"draxx them sklounst"; let aad = b"with my prayers"; // Do one round trip and ensure it works { let mut plaintext = *msg; // Encrypt the plaintext let tag = sender_ctx .seal(&mut plaintext[..], aad) .expect("seal() failed"); // Rename for clarity let mut ciphertext = plaintext; // Now to decrypt on the other side receiver_ctx .open(&mut ciphertext[..], aad, &tag) .expect("open() failed"); // Rename for clarity let roundtrip_plaintext = ciphertext; // Make sure the output message was the same as the input message assert_eq!(msg, &roundtrip_plaintext); } // Try another round trip and ensure that we've overflowed { let mut plaintext = *msg; // Try to encrypt the plaintext match sender_ctx.seal(&mut plaintext[..], aad) { Err(HpkeError::SeqOverflow) => {} // Good, this should have overflowed Err(e) => panic!("seal() should have overflowed. Instead got {}", e), _ => panic!("seal() should have overflowed. Instead it succeeded"), } // Now try to decrypt something. This isn't a valid ciphertext or tag, but the // overflow should fail before the tag check fails. let mut dummy_ciphertext = [0u8; 32]; let dummy_tag = AeadTag::from_bytes(&[0; 16]).unwrap(); match receiver_ctx.open(&mut dummy_ciphertext[..], aad, &dummy_tag) { Err(HpkeError::SeqOverflow) => {} // Good, this should have overflowed Err(e) => panic!("open() should have overflowed. Instead got {}", e), _ => panic!("open() should have overflowed. Instead it succeeded"), } } } }; } /// Tests that `open()` can decrypt things properly encrypted with `seal()` macro_rules! test_ctx_correctness { ($test_name:ident, $aead_ty:ty, $kem_ty:ty) => { #[test] fn $test_name() { type A = $aead_ty; type Kdf = HkdfSha256; type Kem = $kem_ty; let (mut sender_ctx, mut receiver_ctx) = gen_ctx_simple_pair::<A, Kdf, Kem>(); let msg = b"Love it or leave it, you better gain way"; let aad = b"You better hit bull's eye, the kid don't play"; // Encrypt with the sender context let mut ciphertext = msg.clone(); let tag = sender_ctx .seal(&mut ciphertext[..], aad) .expect("seal() failed"); // Make sure seal() isn't a no-op assert!(&ciphertext[..] != &msg[..]); // Decrypt with the receiver context receiver_ctx .open(&mut ciphertext[..], aad, &tag) .expect("open() failed"); // Change name for clarity let decrypted = ciphertext; assert_eq!(&decrypted[..], &msg[..]); } }; } #[cfg(feature = "x25519-dalek")] test_export_idempotence!(test_export_idempotence_x25519, crate::kem::X25519HkdfSha256); #[cfg(feature = "p256")] test_export_idempotence!(test_export_idempotence_p256, crate::kem::DhP256HkdfSha256); #[cfg(feature = "x25519-dalek")] test_overflow!(test_overflow_x25519, crate::kem::X25519HkdfSha256); #[cfg(feature = "p256")] test_overflow!(test_overflow_p256, crate::kem::DhP256HkdfSha256); #[cfg(feature = "x25519-dalek")] test_ctx_correctness!( test_ctx_correctness_aes128_x25519, AesGcm128, crate::kem::X25519HkdfSha256 ); #[cfg(feature = "p256")] test_ctx_correctness!( test_ctx_correctness_aes128_p256, AesGcm128, crate::kem::DhP256HkdfSha256 ); #[cfg(feature = "x25519-dalek")] test_ctx_correctness!( test_ctx_correctness_aes256_x25519, AesGcm256, crate::kem::X25519HkdfSha256 ); #[cfg(feature = "p256")] test_ctx_correctness!( test_ctx_correctness_aes256_p256, AesGcm256, crate::kem::DhP256HkdfSha256 ); #[cfg(feature = "x25519-dalek")] test_ctx_correctness!( test_ctx_correctness_chacha_x25519, ChaCha20Poly1305, crate::kem::X25519HkdfSha256 ); #[cfg(feature = "p256")] test_ctx_correctness!( test_ctx_correctness_chacha_p256, ChaCha20Poly1305, crate::kem::DhP256HkdfSha256 ); }