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test HPKE encryption/decryption
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The constants PADDED_PLAINTEXT_{A,B}_LENGTH now represent the maximum
payload size for the input, whereas before the message A constant
included the reply key size. This makes the PayloadTooLarge error
represent a maximum and actual size that correspond to the inputs to
encrypt_message_a and encrypt_message_b.
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@nothingmuch @DanGould
nothingmuch authored and DanGould committed Oct 21, 2024
1 parent 69c7800 commit 5d83737
Showing 1 changed file with 176 additions and 5 deletions.
181 changes: 176 additions & 5 deletions payjoin/src/hpke.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,8 +11,11 @@ use hpke::{Deserializable, OpModeR, OpModeS, Serializable};
use serde::{Deserialize, Serialize};

pub const PADDED_MESSAGE_BYTES: usize = 7168;
pub const PADDED_PLAINTEXT_A_LENGTH: usize = PADDED_MESSAGE_BYTES - ELLSWIFT_ENCODING_SIZE;
pub const PADDED_PLAINTEXT_B_LENGTH: usize = PADDED_MESSAGE_BYTES - UNCOMPRESSED_PUBLIC_KEY_SIZE;
pub const PADDED_PLAINTEXT_A_LENGTH: usize = PADDED_MESSAGE_BYTES
- (ELLSWIFT_ENCODING_SIZE + UNCOMPRESSED_PUBLIC_KEY_SIZE + POLY1305_TAG_SIZE);
pub const PADDED_PLAINTEXT_B_LENGTH: usize =
PADDED_MESSAGE_BYTES - (ELLSWIFT_ENCODING_SIZE + POLY1305_TAG_SIZE);
pub const POLY1305_TAG_SIZE: usize = 16; // FIXME there is a U16 defined for poly1305, should bitcoin hpke re-export it?
pub const INFO_A: &[u8; 8] = b"PjV2MsgA";
pub const INFO_B: &[u8; 8] = b"PjV2MsgB";

Expand Down Expand Up @@ -156,10 +159,11 @@ pub fn encrypt_message_a(
INFO_A,
&mut OsRng,
)?;
let mut body = body;
pad_plaintext(&mut body, PADDED_PLAINTEXT_A_LENGTH)?;
let mut plaintext = reply_pk.to_bytes().to_vec();
plaintext.extend(body);
let plaintext = pad_plaintext(&mut plaintext, PADDED_PLAINTEXT_A_LENGTH)?;
let ciphertext = encryption_context.seal(plaintext, &[])?;
let ciphertext = encryption_context.seal(&plaintext, &[])?;
let mut message_a = ellswift_bytes_from_encapped_key(&encapsulated_key)?.to_vec();
message_a.extend(&ciphertext);
Ok(message_a.to_vec())
Expand Down Expand Up @@ -247,7 +251,7 @@ fn pad_plaintext(msg: &mut Vec<u8>, padded_length: usize) -> Result<&[u8], HpkeE
}

/// Error from de/encrypting a v2 Hybrid Public Key Encryption payload.
#[derive(Debug)]
#[derive(Debug, PartialEq)]
pub enum HpkeError {
Secp256k1(bitcoin::secp256k1::Error),
Hpke(hpke::HpkeError),
Expand Down Expand Up @@ -296,3 +300,170 @@ impl error::Error for HpkeError {
}
}
}

#[cfg(test)]
mod test {
use super::*;

#[test]
fn message_a_round_trip() {
let mut plaintext = "foo".as_bytes().to_vec();

let reply_keypair = HpkeKeyPair::gen_keypair();
let receiver_keypair = HpkeKeyPair::gen_keypair();

let message_a = encrypt_message_a(
plaintext.clone(),
reply_keypair.public_key(),
receiver_keypair.public_key(),
)
.expect("encryption should work");
assert_eq!(message_a.len(), PADDED_MESSAGE_BYTES);

let decrypted = decrypt_message_a(&message_a, receiver_keypair.secret_key().clone())
.expect("decryption should work");

assert_eq!(decrypted.0.len(), PADDED_PLAINTEXT_A_LENGTH);

// decrypted plaintext is padded, so pad the expected plaintext
plaintext.resize(PADDED_PLAINTEXT_A_LENGTH, 0);
assert_eq!(decrypted, (plaintext.to_vec(), reply_keypair.public_key().clone()));

// ensure full plaintext round trips
plaintext[PADDED_PLAINTEXT_A_LENGTH - 1] = 42;
let message_a = encrypt_message_a(
plaintext.clone(),
reply_keypair.public_key(),
receiver_keypair.public_key(),
)
.expect("encryption should work");

let decrypted = decrypt_message_a(&message_a, receiver_keypair.secret_key().clone())
.expect("decryption should work");

assert_eq!(decrypted.0.len(), plaintext.len());
assert_eq!(decrypted, (plaintext.to_vec(), reply_keypair.public_key().clone()));

let unrelated_keypair = HpkeKeyPair::gen_keypair();
assert_eq!(
decrypt_message_a(&message_a, unrelated_keypair.secret_key().clone()),
Err(HpkeError::Hpke(hpke::HpkeError::OpenError))
);

let mut corrupted_message_a = message_a.clone();
corrupted_message_a[3] ^= 1; // corrupt dhkem
assert_eq!(
decrypt_message_a(&corrupted_message_a, receiver_keypair.secret_key().clone()),
Err(HpkeError::Hpke(hpke::HpkeError::OpenError))
);
let mut corrupted_message_a = message_a.clone();
corrupted_message_a[PADDED_MESSAGE_BYTES - 3] ^= 1; // corrupt aead ciphertext
assert_eq!(
decrypt_message_a(&corrupted_message_a, receiver_keypair.secret_key().clone()),
Err(HpkeError::Hpke(hpke::HpkeError::OpenError))
);

plaintext.resize(PADDED_PLAINTEXT_A_LENGTH + 1, 0);
assert_eq!(
encrypt_message_a(
plaintext.clone(),
reply_keypair.public_key(),
receiver_keypair.public_key(),
),
Err(HpkeError::PayloadTooLarge {
actual: PADDED_PLAINTEXT_A_LENGTH + 1,
max: PADDED_PLAINTEXT_A_LENGTH,
})
);
}

#[test]
fn message_b_round_trip() {
let mut plaintext = "foo".as_bytes().to_vec();

let reply_keypair = HpkeKeyPair::gen_keypair();
let receiver_keypair = HpkeKeyPair::gen_keypair();

let message_b =
encrypt_message_b(plaintext.clone(), &receiver_keypair, reply_keypair.public_key())
.expect("encryption should work");

assert_eq!(message_b.len(), PADDED_MESSAGE_BYTES);

let decrypted = decrypt_message_b(
&message_b,
receiver_keypair.public_key().clone(),
reply_keypair.secret_key().clone(),
)
.expect("decryption should work");

assert_eq!(decrypted.len(), PADDED_PLAINTEXT_B_LENGTH);
// decrypted plaintext is padded, so pad the expected plaintext
plaintext.resize(PADDED_PLAINTEXT_B_LENGTH, 0);
assert_eq!(decrypted, plaintext.to_vec());

plaintext[PADDED_PLAINTEXT_B_LENGTH - 1] = 42;
let message_b =
encrypt_message_b(plaintext.clone(), &receiver_keypair, reply_keypair.public_key())
.expect("encryption should work");

assert_eq!(message_b.len(), PADDED_MESSAGE_BYTES);

let decrypted = decrypt_message_b(
&message_b,
receiver_keypair.public_key().clone(),
reply_keypair.secret_key().clone(),
)
.expect("decryption should work");
assert_eq!(decrypted.len(), plaintext.len());
assert_eq!(decrypted, plaintext.to_vec());

let unrelated_keypair = HpkeKeyPair::gen_keypair();
assert_eq!(
decrypt_message_b(
&message_b,
receiver_keypair.public_key().clone(),
unrelated_keypair.secret_key().clone() // wrong decryption key
),
Err(HpkeError::Hpke(hpke::HpkeError::OpenError))
);
assert_eq!(
decrypt_message_b(
&message_b,
unrelated_keypair.public_key().clone(), // wrong auth key
reply_keypair.secret_key().clone()
),
Err(HpkeError::Hpke(hpke::HpkeError::OpenError))
);

let mut corrupted_message_b = message_b.clone();
corrupted_message_b[3] ^= 1; // corrupt dhkem
assert_eq!(
decrypt_message_b(
&corrupted_message_b,
receiver_keypair.public_key().clone(),
reply_keypair.secret_key().clone()
),
Err(HpkeError::Hpke(hpke::HpkeError::OpenError))
);
let mut corrupted_message_b = message_b.clone();
corrupted_message_b[PADDED_MESSAGE_BYTES - 3] ^= 1; // corrupt aead ciphertext
assert_eq!(
decrypt_message_b(
&corrupted_message_b,
receiver_keypair.public_key().clone(),
reply_keypair.secret_key().clone()
),
Err(HpkeError::Hpke(hpke::HpkeError::OpenError))
);

plaintext.resize(PADDED_PLAINTEXT_B_LENGTH + 1, 0);
assert_eq!(
encrypt_message_b(plaintext.clone(), &receiver_keypair, reply_keypair.public_key()),
Err(HpkeError::PayloadTooLarge {
actual: PADDED_PLAINTEXT_B_LENGTH + 1,
max: PADDED_PLAINTEXT_B_LENGTH
})
);
}
}

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