NAME¶
Tie::EncryptedHash - Hashes (and objects based on hashes) with encrypting
fields.
SYNOPSIS¶
use Tie::EncryptedHash;
my %s = ();
tie %s, Tie::EncryptedHash, 'passwd';
$s{foo} = "plaintext"; # Normal field, stored in plaintext.
print $s{foo}; # (plaintext)
$s{_bar} = "signature"; # Fieldnames that begin in single
# underscore are encrypted.
print $s{_bar}; # (signature) Though, while the password
# is set, they behave like normal fields.
delete $s{__password}; # Delete password to disable access
# to encrypting fields.
print $s{_bar}; # (Blowfish NuRVFIr8UCAJu5AWY0w...)
$s{__password} = 'passwd'; # Restore password to gain access.
print $s{_bar}; # (signature)
$s{_baz}{a}{b} = 42; # Refs are fine, we encrypt them too.
DESCRIPTION¶
Tie::EncryptedHash augments Perl hash semantics to build secure, encrypting
containers of data. Tie::EncryptedHash introduces special hash fields that are
coupled with encrypt/decrypt routines to encrypt assignments at
STORE()
and decrypt retrievals at
FETCH(). By design,
encrypting fields
are associated with keys that begin in single underscore. The remaining
keyspace is used for accessing normal hash fields, which are retained without
modification.
While the password is set, a Tie::EncryptedHash behaves exactly like a standard
Perl hash. This is its
transparent mode of access. Encrypting and
normal fields are identical in this mode. When password is deleted, encrypting
fields are accessible only as ciphertext. This is Tie::EncryptedHash's
opaque mode of access, optimized for serialization.
Encryption is done with
Crypt::CBC(3) which encrypts in the cipher block
chaining mode with Blowfish, DES or IDEA. Tie::EncryptedHash uses Blowfish by
default, but can be instructed to employ any cipher supported by
Crypt::CBC(3).
MOTIVATION¶
Tie::EncryptedHash was designed for storage and communication of key material
used in public key cryptography algorithms. I abstracted out the mechanism for
encrypting selected fields of a structured data record because of the sheer
convenience of this data security method.
Quite often, applications that require data confidentiality eschew strong
cryptography in favor of OS-based access control mechanisms because of the
additional costs of cryptography integration. Besides cipher implementations,
which are available as ready-to-deploy perl modules, use of cryptography in an
application requires code to aid conversion and representation of encrypted
data. This code is usually encapsulated in a data access layer that manages
encryption, decryption, access control and re-structuring of flat plaintext
according to a data model. Tie::EncryptedHash provides these functions under
the disguise of a Perl hash so perl applications can use strong cryptography
without the cost of implementing a complex data access layer.
CONSTRUCTION¶
Tied Hash¶
"tie %h, Tie::EncryptedHash, 'Password', 'Cipher';"
Ties %h to Tie::EncryptedHash and sets the value of password and cipher to
'Password' and 'Cipher'. Both arguments are optional.
Blessed Object¶
"$h = new Tie::EncryptedHash __password =" 'Password',
__cipher => 'Cipher';>
The
new() constructor returns an object that is both tied and blessed
into Tie::EncryptedHash. Both arguments are optional. When used in this
manner, Tie::EncryptedHash behaves like a class with encrypting data members.
RESERVED ATTRIBUTES¶
The attributes __password, __cipher and __hide are reserved for communication
with object methods. They are "write-only" from everywhere except
the class to which the hash is tied. __scaffolding is inaccessible.
Tie::EncryptedHash stores the current encryption password and some transient
data structures in these fields and restricts access to them on need-to-know
basis.
__password¶
"$h{__password} = "new password"; delete $h{__password};"
The password is stored under the attribute "__password". In addition
to specifying a password at construction, assigning to the __password
attribute sets the current encryption password to the assigned value. Deleting
the __password unsets it and switches the hash into opaque mode.
__cipher¶
"$h{__cipher} = 'DES'; $h{__cipher} = 'Blowfish';"
The cipher used for encryption/decryption is stored under the attribute
__cipher. The value defaults to 'Blowfish'.
__hide¶
"$h{__hide} = 1;"
Setting this attribute
hides encrypting fields in opaque mode. 'undef' is
returned at
FETCH() and
EXISTS().
BEHAVIOR¶
References¶
A reference stored in an encrypting field is serialized before encryption. The
data structure represented by the reference is folded into a single line of
ciphertext which is stored under the first level key. In the opaque mode,
therefore, only the first level of keys of the hash will be visible.
Opaque Mode¶
The opaque mode introduces several other constraints on access of encrypting
fields. Encrypting fields return ciphertext on
FETCH() unless __hide
attribute is set, which forces Tie::EncryptedHash to behave as if encrypting
fields don't exist. Irrespective of __hide, however,
DELETE() and
CLEAR() fail in opaque mode. So does
STORE() on an existing
encrypting field. Plaintext assignments to encrypting fields are silently
ignored, but ciphertext assignments are fine. Ciphertext assignments can be
used to move data between different EncryptedHashes.
Multiple Passwords and Ciphers¶
Modality of Tie::EncryptedHash's access system breaks down when more than one
password is used to with different encrypting fields. This is a feature.
Tie::EncryptedHash lets you mix passwords and ciphers in the same hash. Assign
new values to __password and __cipher and create a new encrypting field.
Transparent mode will be restricted to fields encrypted with the current
password.
Error Handling¶
Tie::Encrypted silently ignores access errors. It doesn't carp/croak when you
perform an illegal operation (like assign plaintext to an encrypting field in
opaque mode). This is to prevent data lossage, the kind that results from
abnormal termination of applications.
QUIRKS¶
Autovivification¶
Due to the nature of autovivified references (which spring into existence when
an undefined reference is dereferenced), references are stored as plaintext in
transparent mode. Analogous ciphertext representations are maintained in
parallel and restored to encrypting fields when password is deleted. This
process is completely transparent to the user, though it's advisable to delete
the password after the final assignment to a Tie::EncryptedHash. This ensures
plaintext representations and scaffolding data structures are duly flushed.
Data::Dumper¶
Serialization of references is done with Data::Dumper, therefore the nature of
data that can be assigned to encrypting fields is limited by what Data::Dumper
can grok. We set $Data::Dumper::Purity = 1, so self-referential and recursive
structures should be OK.
Speed¶
Tie::EncryptedHash'es keep their contents encrypted as much as possible, so
there's a rather severe speed penalty. With Blowfish,
STORE() on
EncryptedHash can be upto 70 times slower than a standard perl hash. Reference
STORE()'es will be quicker, but speed gain will be adjusted at
FETCH().
FETCH() is about 35 times slower than a standard perl
hash. DES affords speed improvements of upto 2x, but is not considered secure
for long-term storage of data. These values were computed on a DELL PIII-300
Mhz notebook with 128 Mb RAM running perl 5.003 on Linux 2.2.16. Variations in
speed might be different on your machine.
STANDARD USAGE¶
The standard usage for this module would be something along the lines of:
populate Tie::EncryptedHash with sensitive data, delete the password,
serialize the encrypted hash with Data::Dumper, store the result on disk or
send it over the wire to another machine. Later, when the sensitive data is
required, procure the EncryptedHash, set the password and accesses the
encrypted data fields.
SEE ALSO¶
Data::Dumper(3),
Crypt::CBC(3),
Crypt::DES(3),
Crypt::Blowfish(3),
Tie::SecureHash(3)
ACKNOWLEDGEMENTS¶
The framework of Tie::EncryptedHash derives heavily from Damian Conway's
Tie::SecureHash. Objects that are blessed as well as tied are just one of the
pleasant side-effects of stealing Damian's code. Thanks to Damian for this
brilliant module.
PacificNet (
http://www.pacificnet.net) loaned me the aforementioned notebook to
hack from the comfort of my bed. Thanks folks!
AUTHOR¶
Vipul Ved Prakash <mail@vipul.net>
LICENSE¶
This module is distributed under the same license as Perl itself.