65 lines
2.9 KiB
Markdown
65 lines
2.9 KiB
Markdown
Cryptography Details
|
|
=====================
|
|
|
|
Here is a high-level description of how this library works. Any discrepancy
|
|
between this documentation and the actual implementation will be considered
|
|
a security bug.
|
|
|
|
Let's start with the following definitions:
|
|
|
|
- HKDF-SHA256(*k*, *n*, *info*, *s*) is the key derivation function specified in
|
|
RFC 5869 (using the SHA256 hash function). The parameters are:
|
|
- *k*: The initial keying material.
|
|
- *n*: The number of output bytes.
|
|
- *info*: The info string.
|
|
- *s*: The salt.
|
|
- AES-256-CTR(*m*, *k*, *iv*) is AES-256 encryption in CTR mode. The parameters
|
|
are:
|
|
- *m*: An arbitrary-length (possibly zero-length) message.
|
|
- *k*: A 32-byte key.
|
|
- *iv*: A 16-byte initialization vector (nonce).
|
|
- PBKDF2-SHA256(*p*, *s*, *i*, *n*) is the password-based key derivation
|
|
function defined in RFC 2898 (using the SHA256 hash function). The parameters
|
|
are:
|
|
- *p*: The password string.
|
|
- *s*: The salt string.
|
|
- *i*: The iteration count.
|
|
- *n*: The output length in bytes.
|
|
- VERSION is the string `"\xDE\xF5\x02\x00"`.
|
|
- AUTHINFO is the string `"DefusePHP|V2|KeyForAuthentication"`.
|
|
- ENCRINFO is the string `"DefusePHP|V2|KeyForEncryption"`.
|
|
|
|
To encrypt a message *m* using a 32-byte key *k*, the following steps are taken:
|
|
|
|
1. Generate a random 32-byte string *salt*.
|
|
2. Derive the 32-byte authentication key *akey* = HKDF-SHA256(*k*, 32, AUTHINFO, *salt*).
|
|
3. Derive the 32-byte encryption key *ekey* = HKDF-SHA256(*k*, 32, ENCRINFO, *salt*).
|
|
4. Generate a random 16-byte initialization vector *iv*.
|
|
5. Compute *c* = AES-256-CTR(*m*, *ekey*, *iv*).
|
|
6. Combine *ctxt* = VERSION || *salt* || *iv* || *c*.
|
|
7. Compute *h* = HMAC-SHA256(*ctxt*, *akey*).
|
|
8. Output *ctxt* || *h*.
|
|
|
|
Decryption is roughly the reverse process (see the code for details, since the
|
|
security of the decryption routine is highly implementation-dependent).
|
|
|
|
For encryption using a password *p*, steps 1-3 above are replaced by:
|
|
|
|
1. Generate a random 32-byte string *salt*.
|
|
2. Compute *k* = PBKDF2-SHA256(SHA256(*p*), *salt*, 100000, 32).
|
|
3. Derive the 32-byte authentication key *akey* = HKDF-SHA256(*k*, 32, AUTHINFO, *salt*)
|
|
4. Derive the 32-byte encryption key *ekey* = HKDF-SHA256(*k*, 32, ENCRINFO, *salt*)
|
|
|
|
The remainder of the process is the same. Notice the reuse of the same *salt*
|
|
for PBKDF2-SHA256 and HKDF-SHA256. The prehashing of the password in step 2 is
|
|
done to prevent a [DoS attack using long
|
|
passwords](https://github.com/defuse/php-encryption/issues/230).
|
|
|
|
For `KeyProtectedByPassword`, the serialized key is encrypted according to the
|
|
password encryption defined above. However, the actual password used for
|
|
encryption is the SHA256 hash of the password the user provided. This is done in
|
|
order to provide domain separation between the message encryption in the user's
|
|
application and the internal key encryption done by this library. It fixes
|
|
a [key replacement chosen-protocol
|
|
attack](https://github.com/defuse/php-encryption/issues/240).
|