Strings

# Cargo.toml

[dependencies]
tfhe = { version = "0.11.0", 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));
}