pcp/man/details.pod

# -*-perl-*-

=head1 PCP1 KEYS

B<pcp1> keys are stored in a binary file, called B<the vault>.
It's by default located in B<~/.pcpvault> but you can of course
specify another location using the B<-V> option.

There are two kinds of keys: secret and public keys. In reality
a secret key always includes its public key. Both types of keys
can be exported to files and transfered to other people who can
then import them. You should usually only do this with public keys
though.

There is a primary secret key which will always used for operations
when no keyid has been specified. However, you may have as many
secret keys in your vault as you like.

Each key can be identified using its B<keyid> which looks like this:

 0xD49119E85266509F

A public key exported from a secret key will have the same keyid
as the secret key.

If you just want to know details about a key or the vault, use the
B<-t> option.

=head1 ENCRYPTION

There are 2 modes of encryption available in pcp1:

=over

=item B<Standard public key encryption>

In this mode, which is the default, a public key as specified
with B<-i> and a dynamically generated secret key will be used
for encryption. The public part of the generated sender key
will be included with the encrypted file, which the recipient
can use to decrypt it.

Example command:

 pcp1 -e -i 0x2BD734B15CE2722D -I message.txt -O cipher.z85

Here we didn't specify a recipient. Therefore the public
key given with -i will be used directly.

=item B<Self encryption mode>

Pretty Curved Privacy doesn't provide symetric file encryption.
However there are cases when you need to encrypt a file just
for yourself. In such a case the file will be encrypted using
the public key part of your primary secret key and the secret
key itself (thanks to the wonders of ECC this works like a charm).

The file can be decrypted using the primary key pair.

While this works, the security of it totally depends on the
strength of your password, especially if the primary secret 
used for this kind of encryption is stored in a vault on the
same system.

Example command:

 pcp1 -e -I message.txt -O cipher.z85

As you can see we didn't specify -i or -r and therefore pcp1
tries to use the primary keypair for encryption.

=back


=head1 VULNERABILITIES

Currently there are a couple of problems which are not
addressed. These are usually protocol problems, which are
not caused by pcp1.

=over

=item B<No secure native key exchange for store-and-forward systems>

Pretty Curved Privacy is a store-and-forward system, it works
on files and can't use any cool key exchange protocols therefore.
For example there would be B<CurveCP> which guarantees a
secure key exchange. But CurveCP cannot be used offline.

Users have to find other means to exchange keys. That's a pity
since with Curve25519 you can't just publish your public key
to some key server because in order to encrypt a message, both
the recipient AND the sender need to have the public key of
each other. It would be possible to publish public keys,
and attach the senders public key to the encrypted message, but
I'm not sure if such an aproach would be secure enough.

=item B<Curve25519 not widely adopted>

At the time of this writing the ECC algorithm Curve25519
is only rarely used, in most cases by experimental software
(such as Pretty Curved Privacy). As far as I know there haven't
been done the kind of exessive crypto analysis as with other
ECC algorithms.

While I, as the author of pcp1 totally trust D.J.Bernstein, this
may not be the case for you.

In short, I'd suggest not to use it on critical systems yet.

=back

=head1 INTERNALS

=head2 VAULT FORMAT

The vault file contains all public and secret keys. It's a portable
binary file.

The file starts with a header:

 +-------------------------------------------+
 | Field        Size   Description           |
 +-------------------------------------------+
 | File ID    |    1 | Vault Identifier 0xC4 |
 +-------------------------------------------+
 | Version    |    4 | Big endian, version   |
 +-------------------------------------------+
 | Checksum   |   32 | SHA256 Checksum       |
 +-------------------------------------------+

The checksum is a checksum of all keys.

The header is followed by the keys. Each key is preceded by a
key header which looks like this:

 +--------------------------------------------+
 | Field        Size   Description            |
 +--------------------------------------------+
 | Type       |    1 | Key type (S,P,M)       |
 +--------------------------------------------+
 | Size       |    4 | Big endian, keysize    |
 +--------------------------------------------+
 | Version    |    4 | Big endian, keyversion |
 +--------------------------------------------+
 | Checksum   |   32 | SHA256 Key Checksum    |
 +--------------------------------------------+

Type can be one of:

 PCP_KEY_TYPE_MAINSECRET 0x01
 PCP_KEY_TYPE_SECRET     0x02
 PCP_KEY_TYPE_PUBLIC     0x03

The key header is followed by the actual key, see below.

=head2 SECRET KEY FORMAT

A secret key is a binary structure with the following format:


 +---------------------------------------------------------+
 | Field         Size      Description                     |
 +-------------+--------+----------------------------------+
 | Public      |     32 | Curve25519 Public Key Part       |
 +-------------|--------|----------------------------------+
 | Secret      |     32 | Curve25519 Secret Key Unencrypted|
 +-------------|--------|----------------------------------+
 | ED25519 Pub |     32 | ED25519 Public Key Part          |
 +-------------|--------|----------------------------------+
 | ED25519 Sec |     64 | ED25519 Secret Key Unencrypted   |
 +-------------|--------|----------------------------------+
 | Nonce       |     24 | Nonce for secret key encryption  |
 +-------------|--------|----------------------------------+
 | Encrypted   |     48 | Encrypted Curve25519 Secret Key  |
 +-------------|--------|----------------------------------+
 | Owner       |    255 | String, Name of Owner            |
 +-------------|--------|----------------------------------+
 | Mail        |    255 | String, Email Address            |
 +-------------|--------|----------------------------------+
 | ID          |     17 | String, Key ID                   |
 +-------------|--------|----------------------------------+
 | Ctime       |      4 | Creation time, sec since epoch   |
 +-------------|--------|----------------------------------+
 | Version     |      4 | Key version                      |
 +-------------|--------|----------------------------------+
 | Serial      |      4 | Serial Number                    |
 +-------------|--------|----------------------------------+
 | Type        |      1 | Key Type                         |
 +-------------+--------+----------------------------------+

Some notes:

The secret key fields will be filled with random data if the
key is encrypted. The first byte of it will be set to 0 in that
case.

The key id is a computed JEN Hash of the secret and public
key concatenated, put into hex, as a string.

The key version is a static value, currently 0x2. If the key
format changes in the future, this version number will be
increased to distinguish old from new keys.

Exported keys will be encoded in Z85 encoding. When such an
exported key is imported, only the actual Z85 encoded data
will be used. Header lines and lines starting with whitespace
will be ignored. They are only there for convenience.

Key generation works like this:

=over

=item *

Generate a random seed (32 bytes).

=item *

Generate a ED25519 keypair from that seed.

=item *

Take the first 32 bytes of the generated ED25519 secret
and generate a SHA512 hash from it.

=item *

Clamp bytes 0 and 31 which turns it into a Curve25519 secret.

=item *

Do scalar multiplication from that secret to retrieve
the matching public key.

=back

Take a look at the function B<pcp_keypairs()> for details. 

=head2 ENCRYPTED OUTPUT FORMAT

Encrypted output will always be Z85 encoded and has the following
format:

 +---------------------------------------------------------+
 | Field         Size      Description                     |
 +-------------+--------+----------------------------------+
 | Pubkey      |     32 | Publix key of the sender         |
 +-------------|--------|----------------------------------+
 | Nonce       |     24 | Random Nonce                     |
 +-------------|--------|----------------------------------+
 | Encrypted   |      ~ | The actual encrypted data        |
 +-------------|--------|----------------------------------+

=head2 SIGNATURE FORMAT

Signatures will always be Z85 encoded and have the following
format:

 +---------------------------------------------------------+
 | Field         Size      Description                     |
 +-------------+--------+----------------------------------+
 | Key ID      |     17 | Signers key id
 +-------------|--------|----------------------------------+
 | Ctime       |      4 | Creation time, sec since epoch   |
 +-------------|--------|----------------------------------+
 | Version     |      4 | Signature version                |
 +-------------|--------|----------------------------------+
 | Signature   |     96 | ED25519 signature of SHA256 Hash |
 +-------------|--------|----------------------------------+

The actual signature is not a signature over the whole content
of an input file but of a SHA256 hash of the content.

=head2 Z85 ENCODING

B<pcp1> uses Z85 to encode exported keys and encrypted messages.
Therefore it includes a Z85 utility mode:

B<pcp1> can be used to encode and decode strings to Z85 encoding.

The option B<-z> encodes B<to> Z85, the option B<-Z> does the opposite
and decodes B<from> Z85.

If no input file have been specified using B<-I>, B<pcp1> expects the
input to come from B<STDIN>, otherwise it reads the contents
of B<file>.

Encoded or decoded output will be written to B<STDOUT> unless an
output file has been specified using the option B<-O>.

=head3 Z85 EXAMPLES

To encode a given file to Z85 and write the output to another:

 pcp1 -z myfile.bin > myfile.z85

To decode the file created above and restore the original:

 pcp1 -Z -d myfile.z85 > myfile.bin

To encode something from stdin to Z85:

 ps axuw | pcp1 -z > pslist.z85

To decode the above and print to stdout:

 pcp1 -Z -d pslist.z85

=head3 Z85 BACKGROUND

The Z85 encoding format is described here: B<http://rfc.zeromq.org/spec:32>.
It's part of ZeroMQ (B<http://zeromq.org>). Z85 is based on ASCII85 with
a couple of modifications (portability, readability etc).

To fulfil the requirements of the ZeroMQ Z85 functions, B<pcp1>
does some additional preparations of raw input before actually doing the 
encoding, since the input for zmq_z85_encode() must be divisible by 4:

Expand the input so that the resulting size is divisible by 4.

Fill the added bytes with zeroes.

Prepend the input with a one byte value which holds the number of zeroes
added in the previous step.

Example:

Raw input:

 hello\0

Here, the input size is 6, which is insufficient, therefore it has to be expanded
to be 8. After the process the input looks like this:

 1hello\0\0

So, we padded the input with 1 zero (makes 7 bytes) and preprended it with the
value 1 (the number of zeros added): makes 8 bytes total.

After decoding Z85 input the process will be reversed.

B<Trying to use another tool to decode an Z85 encoded string produced
by z85, might not work therefore, unless the tool takes the padding scheme
outlined above into account>.