When encrypting strings, you can add padding to hide the actual length of strings. The null character (b'\0') is used as the padding. Here is an example:
# Cargo.toml
[dependencies]
tfhe = { version = "0.11.3", features = ["integer", "strings"] }
use tfhe::{ConfigBuilder, generate_keys, set_server_key, FheAsciiString, FheStringLen, ClearString};
use tfhe::prelude::*;
use tfhe::safe_serialization::safe_serialize;
fn main() {
let config = ConfigBuilder::default().build();
let (cks, sks) = generate_keys(config);
set_server_key(sks);
let r = FheAsciiString::try_encrypt("café is french for coffee", &cks);
// As the input string is not strictly ASCII, it is not compatible
assert!(r.is_err());
let string = FheAsciiString::try_encrypt("tfhe-rs", &cks).unwrap();
// This adds 3 chars of padding to the chars of the input string
let padded_string = FheAsciiString::try_encrypt_with_padding("tfhe-rs", 3, &cks).unwrap();
// This makes it so the string has 10 chars (adds padding or truncates input as necessary)
let other_string = FheAsciiString::try_encrypt_with_fixed_sized("tfhe", 10, &cks).unwrap();
let mut buffer1 = vec![];
safe_serialize(&padded_string, &mut buffer1, 1 << 30).unwrap();
let mut buffer2 = vec![];
safe_serialize(&other_string, &mut buffer2, 1 << 30).unwrap();
// The two strings created with padding, have the same
// memory/disk footprint, even though the lengths are not the same
assert_eq!(buffer1.len(), buffer2.len());
// When a string has no padding, its length is known in clear
let len = string.len();
assert!(matches!(len, FheStringLen::NoPadding(7)));
// When a string has padding, its length is only known as an encrypted value
let FheStringLen::Padding(encrypted_len) = padded_string.len() else {
panic!("Expected len to be encrypted");
};
let padded_string_len: u16 = encrypted_len.decrypt(&cks);
assert_eq!(padded_string_len, 7); // Note padding chars are not counted
// The enum resulting of a len() / is_empty() call can be transformed
// to a FheUint16 using `into_ciphertext`
assert!(string.len().into_ciphertext().is_trivial());
assert!(!padded_string.len().into_ciphertext().is_trivial());
let other_string_len: u16 = other_string.len().into_ciphertext().decrypt(&cks);
assert_eq!(other_string_len, 4);
// Padded and un-padded strings are equal if the content is
assert!(padded_string.eq(&string).decrypt(&cks));
let prefix = ClearString::new("tfhe".to_string());
let (stripped_string, has_been_stripped) = string.strip_prefix(&prefix);
// Notice that stripping, makes the string as being considered as padded
// as it is not possible to homomorphically remove chars
let FheStringLen::Padding(encrypted_len) = stripped_string.len() else {
panic!("Expected len to be encrypted");
};
let stripped_string_len: u16 = encrypted_len.decrypt(&cks);
assert_eq!(stripped_string_len, 3);
let decrypted = stripped_string.decrypt(&cks);
assert_eq!(decrypted, "-rs");
assert!(has_been_stripped.decrypt(&cks));
}
