Lavabit technology

How does asymmetric encryption protect your privacy? The short description is that for users of this feature, incoming e-mail messages are encrypted before they’re saved onto our servers. Once a message has been encrypted, only someone who has the account password can decrypt the message. Like all safety measures, encryption is only effective if it’s used. To ensure privacy, Lavabit has developed a complex system that makes the entire encryption and decryption process transparent to the end user.

This process works by combining three different encryption schemes with Elliptical Curve Cryptography (ECC) as the cornerstone. When a user activates the asymmetric encryption feature, two ECC keys are generated with 521 bits of strength. The first key, or the public key, is stored in plain text on the server. This public key is used to encrypt incoming messages. Because of how ECC works, only someone with the second “private” key can decipher messages encrypted with the public key. To protect the private key from attackers, it is encrypted using the Advanced Encryption Standard (AES) with a 256 bit key. AES is a synchronous encryption scheme that uses a secret passphrase to encrypt/decrypt a ciphered message. In the case of Lavabit’s secure e-mail system, the ciphered message is a user’s private key and the secret passphrase is a hashed version of the user’s password.

To ensure maximum security, passwords are hashed using the Secure Hash Algorithm (SHA). SHA takes the plaintext password as its input and produces a random 512 bit string as the output. With only the SHA output, it is cryptographically impossible to determine the original input. Effectively, hashing is a repeatable one-way process.

To increase the randomness of our hash outputs and the difficulty of reversing the process, Lavabit combines the password with the account name and a cryptographic salt. This combined string is then hashed three consecutive times, with the former iteration’s output being used as the input value of the next iteration. The output of the first hash iteration is used as the secret passphrase for AES mentioned above. The third iteration is stored in our password database and is used to verify that users entered their password correctly.

The product of this encryption process is a message that is cryptographically impossible to read without the password. We say cryptographically impossible because, in theory, an attacker with unlimited computing resources could use brute force to decipher the original message. However in practice, the key lengths Lavabit has chosen equal enough possible inputs that a brute-force attack shouldn’t be feasible for a long time to come.