[Cryptography] End-to-End, One-to-Many, Encryption Question

[Ben Laurie]

On 12 June 2014 07:35, Bill Frantz <frantz at pwpconsult.com> wrote:
> On 6/11/14 at 5:49 PM, kentborg at borg.org (Kent Borg) wrote:
>
>> Is there a way to encrypt once with key A, super-encrypt with key B1 (not
>> knowing any other keys), and finally decrypt with key C1 (not knowing any
>> other keys)?  Or, super-encrypt with key B2, then decrypt with key C2?
>
>
> This problem is similar to the problem which would occur if an encryption
> algorithm was a group. If the algorithm is a group, then there is a key C
> which can decrypt a message which is encrypt(B, encrypt(A, text)). DES was
> proven to not be a group, making triple-DES a viable way to get the security
> of a longer encryption key.

All symmetric crypto algorithms need to have keys that are not a
group, or there is a meet-in-the-middle attack available.