Negating a value means taking its opposite. The opposite of a ciphertext representing a message in the interval is a ciphertext representing in the interval . This operation does not consume padding nor add noise but it modifies the interval of the encoder.
| |
Operation | |
Type | Noiseless |
Side effects | Modifies encoding |
To compute the opposite of an LWE, simply use the opposite
method, which will return a new ciphertext with the modified value. The method exists in a mutable opposite_inplace
form as well, which modifies the ciphertext itself. All operations in concrete are implemented in immutable and mutable forms, with mutable forms always postfixed with _inplace
.
Here is a complete example for a single LWE:
/// file: main.rsuse concrete::*;fn main() -> Result<(), CryptoAPIError> {// encoderlet encoder = Encoder::new(50., 100., 8, 2)?;// generate a secret keylet secret_key = LWESecretKey::new(&LWE128_1024);// the message to negatelet message: f64 = 95.46;// encode and encryptlet mut ciphertext = LWE::encode_encrypt(&secret_key, message, &encoder)?;// compute the opposite of the ciphertextciphertext.opposite_inplace()?;// decryptionlet output: f64 = ciphertext.decrypt_decode(&secret_key)?;// check the value computedprintln!("opposite({}) = {}", message, output);Ok(())}
As for all operations in Concrete, they can be applied to a vector of messages using the VectorLWE
struct instead of LWE
. The vectorized form has an additional method opposite_nth
that enables negating a single value by passing its index to the function.