SQL solution to http://www.meetup.com/functional-kats-cork/events/234061670/

cipher.sql

-- Tested on Postgres 9.2

create or replace temp view cipher as
select 
    'meetmebythetree'::varchar plain_text, 
    'scones'::varchar secret;

with
    -- Recursively add 'secret' until result is longer than 'plain_text'
    recursive secret_stream(repeated_secret) as (
        select ''::varchar

        union all

        select repeated_secret || (select secret from cipher)
        from secret_stream
        where length(repeated_secret) < (select max(length(plain_text)) from cipher)
    ),

    -- Transform to field-value table
    denormalized_cipher as (
        select 'plain_text' field, plain_text field_value from cipher
        union
        select 'secret', max(repeated_secret) from secret_stream
    ),

    -- For each character in text or secret, create row with its index and value. Example:
    -- 'plain_text', 1, 'm'
    -- 'plain_text', 2, 'e'
    split_cipher as (
        select field, row_number() OVER(partition by field) c_order, c
        from (
            select field, unnest(string_to_array(field_value, NULL)) c
            from denormalized_cipher
        ) t
    ),

    -- Zip plain text and secret together
    zip as (
        select plain_text.c plain_char, secret.c secret_char
        from split_cipher plain_text
        inner join split_cipher secret on secret.c_order = plain_text.c_order
        where plain_text.field = 'plain_text' and secret.field = 'secret'
    ),

    -- Encode each character
    encode as (
        select chr(ascii('a') +  (ascii(plain_char) - ascii('a') + ascii(secret_char) - ascii('a')) % 26) encoded_char
        from zip
    ),

    -- Merge result back into string
    mkstr as (
        select string_agg(encoded_char, '') from encode
    )

SELECT *
FROM mkstr;