Transparent Column Encryption with Empty string

[pafiti]

Hello,

I created a simple table for TCE testing.
I got everything created automatically (function, trigger, decrypted view).
However, when I insert empty string in the column to be encrypted, the decryption fails.

CREATE TABLE users (
id bigserial primary key,
secret text,
key_id uuid not null default 'e3496f2a-787f-45a0-9717-f648496179d1',
nonce bytea default pgsodium.crypto_aead_det_noncegen()
);

SECURITY LABEL FOR pgsodium
ON COLUMN users.secret
IS 'ENCRYPT WITH KEY COLUMN key_id NONCE nonce';

insert into users( secret ) values ( '12345' );
select * from decrypted_users;
insert into users( secret ) values ( '' );
select * from decrypted_users;

SQL Error [22000]: ERROR: pgsodium_crypto_aead_det_decrypt_by_id: invalid message
Where: PL/pgSQL function pgsodium.crypto_aead_det_decrypt(bytea,bytea,uuid,bytea) line 12 at RETURN

Regards,

[Henning Kollenbroich]

I have the same issue when dealing with decrypting empty strings. I have modified the trigger to treat empty strings:

CREATE OR REPLACE FUNCTION public.emails_encrypt_secret_summary()
 RETURNS trigger
 LANGUAGE plpgsql
 SECURITY DEFINER
AS $function$
BEGIN
    -- Encrypt the token if it's not null and key_id is not null
    NEW.summary := CASE 
        WHEN NEW.summary IS NULL THEN 
            NULL
        -- When empty string, save empty string
        WHEN NEW.summary = '' THEN
            ''
        ELSE
            CASE 
                WHEN NEW.key_id IS NULL THEN 
                    NULL 
                ELSE 
                    pg_catalog.encode(
                        pgsodium.crypto_aead_det_encrypt(
                            pg_catalog.convert_to(NEW.summary, 'utf8'),
                            pg_catalog.convert_to(('')::text, 'utf8'),
                            NEW.key_id::uuid,
                            NULL
                        ),
                        'base64'
                    ) 
            END 
    END;
  
    RETURN NEW;
END;
$function$;

Note that you'd also need to handle the decrypt view:

DROP VIEW IF EXISTS public.decrypted_emails;
CREATE OR REPLACE VIEW public.decrypted_emails AS
 SELECT emails.id,
    emails.user_id,
    emails.body,
    emails.summary,
        CASE
            WHEN emails.summary IS NULL THEN NULL::text
            WHEN emails.summary = '' THEN ''
            ELSE
            CASE
                WHEN emails.key_id IS NULL THEN NULL::text
                ELSE convert_from(pgsodium.crypto_aead_det_decrypt(decode(emails.summary, 'base64'::text), convert_to(''::text, 'utf8'::name), emails.key_id, NULL::bytea), 'utf8'::name)
            END
        END AS decrypted_summary,

Or you could just treat empty as null:

CREATE OR REPLACE FUNCTION public.emails_encrypt_secret_summary()
 RETURNS trigger
 LANGUAGE plpgsql
 SECURITY DEFINER
AS $function$
BEGIN
    -- Check if key_id is not already set, else set it to user's key_id
    IF NEW.key_id IS NULL THEN
        NEW.key_id := (
            SELECT key_id 
            FROM user_preferences 
            WHERE user_preferences.user_id = NEW.user_id
        );
    END IF;

    -- Encrypt the token if it's not null and key_id is not null
    NEW.summary := CASE 
        WHEN NEW.summary IS NULL OR NEW.summary = '' THEN 
            NULL
        ELSE
            CASE 
                WHEN NEW.key_id IS NULL THEN 
                    NULL 
                ELSE 
                    pg_catalog.encode(
                        pgsodium.crypto_aead_det_encrypt(
                            pg_catalog.convert_to(NEW.summary, 'utf8'),
                            pg_catalog.convert_to(('')::text, 'utf8'),
                            NEW.key_id::uuid,
                            NULL
                        ),
                        'base64'
                    ) 
            END 
    END;
  
    RETURN NEW;
END;
$function$;

[Jehan-Guillaume (ioguix) de Rorthais]

I really wonder why in the first place decrypting an empty string is failing. Any idea @michelp?