Envelope Mail - crypto

Envelope Mail's exact format hasn't been decided on yet. What's needed is an implementation of the following API which performs 'adequately', has a 'reasonable' security margin, isn't encumbered by patents, and can be easily coded using public domain libraries -

String make_random_seed()
(String private_key, String public_key, String key_identifier) make_key(String seed)
String encrypt(String plaintext, String[] public_keys)
(String plaintext, String failure_reason) decrypt(String ciphertext, String private_key)
String[] get_key_identifiers(String ciphertext)

Seeds are 112 bits long, to make key encoding clean.

This API is not parameterized by key length - standardizing on one which will last the next 30 years is fine.

Since no implementation of this API is forthcoming, a dummy one is being used for now.

Below are explanations of all the features in this API, each new one is marked in bold.

Here is an unacceptably minimal encryption/decryption API

(String private_key, String public_key) make_random_key()
String encrypt(String plaintext, String public_key)
(String plaintext, String failure_reason) decrypt(String ciphertext, String private_key)

We want encyption to multiple public keys, so the sender can read a message later as well

(String private_key, String public_key) make_random_key()
String encrypt(String plaintext, String[] public_keys)
(String plaintext, String failure_reason) decrypt(String ciphertext, String private_key)

An extra method to get the public keys encypted avoids having to try every private key available.

(String private_key, String public_key) make_random_key()
String encrypt(String plaintext, String[] public_keys)
(String plaintext, String failure_reason) decrypt(String ciphertext, String private_key)
String[] get_public_keys(String ciphertext)

Since the private keys probably won't be short enough to write down easily (around 128 bits), they should be derived from a seed which is.

(String private_key, String public_key) make_key(String seed)
String encrypt(String plaintext, String[] public_keys)
(String plaintext, String failure_reason) decrypt(String ciphertext, String private_key)
String[] get_public_keys(String ciphertext)

Including the whole public key in the ciphertext is an unnecessary waste of resources, so only an identifier should be used

(String private_key, String public_key, String key_identifier) make_key(String seed)
String encrypt(String plaintext, String[] public_keys)
(String plaintext, String failure_reason) decrypt(String ciphertext, String private_key)
String[] get_key_identifiers(String ciphertext)

It makes sense for random seed generation to be part of the crypto API as well

String make_random_seed()
(String private_key, String public_key, String key_identifier) make_key(String seed)
String encrypt(String plaintext, String[] public_keys)
(String plaintext, String failure_reason) decrypt(String ciphertext, String private_key)
String[] get_key_identifiers(String ciphertext)

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