crypto class design

travis+ml-cryptography at travis+ml-cryptography at
Mon Dec 17 11:38:59 EST 2007

So... supposing I was going to design a crypto library for use within
a financial organization, which mostly deals with credit card numbers
and bank accounts, and wanted to create an API for use by developers,
does anyone have any advice on it?

It doesn't have to be terribly complete, but it does have to be
relatively easy to use correctly (i.e. securely).

I was thinking of something like this:

class crypto_api
    // developers call these based on what they're trying to do
    // these routines simply call crypto_logic layer
    Buffer encrypt_credit_card(Buffer credit_card_number, key_t key);
    Buffer encrypt_bank_account(Buffer bank_account, key_t key);
    Buffer encrypt_other(Buffer buffer, key_t key);

class crypto_logic
    algo_default = ALGO_AES256CBC;
    // encrypt with a given algorithm
    Buffer encrypt(Buffer buffer, key_t key, algo_t aid = algo_default);
    // calls different routines in crypto_implementation layer depending on algorithm used
    Buffer decrypt(Buffer buffer, key_t key);

class crypto_glue
    // calls routines in libraries such as OpenSSL
    // mostly wrappers that translate between our data types and theirs
    Buffer aes256cbc-encrypt(...);
    Buffer aes256cbc-decrypt(...);

The general idea is that crypto_api provides domain-specific APIs that
are easy for anyone to understand, that the logic layer allows for the
selection of different algorithms, and the glue layer is basically a
translation layer to underlying libraries.

It is very important that the API remain stable, because the code
base is large and owned by various groups.

One thing that I'm wondering is how to indicate (e.g.) the overhead in
terms of padding, or whatever, for various algorithms... or if it
matters.  The old code had some really disturbing practices like
assuming that the output buffer was 16 octets bigger, and stuff like
that... scary.

Intend to skim the OpenSSL design and Gutmann's "Design of a
Cryptographic Security Architecture" for ideas.

In God We Trust, All Others Must Provide Source Code
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