1Tie::EncryptedHash(3) User Contributed Perl DocumentationTie::EncryptedHash(3)
2
6 Tie::EncryptedHash - Hashes (and objects based on hashes) with
7 encrypting fields.
8
10 use Tie::EncryptedHash;
11 my %s = ();
13 tie %s, Tie::EncryptedHash, 'passwd';
14 $s{foo} = "plaintext"; # Normal field, stored in plaintext.
16 print $s{foo}; # (plaintext)
17 $s{_bar} = "signature"; # Fieldnames that begin in single
19 # underscore are encrypted.
20 print $s{_bar}; # (signature) Though, while the password
21 # is set, they behave like normal fields.
22 delete $s{__password}; # Delete password to disable access
23 # to encrypting fields.
24 print $s{_bar}; # (Blowfish NuRVFIr8UCAJu5AWY0w...)
25 $s{__password} = 'passwd'; # Restore password to gain access.
27 print $s{_bar}; # (signature)
28 $s{_baz}{a}{b} = 42; # Refs are fine, we encrypt them too.
30
32 Tie::EncryptedHash augments Perl hash semantics to build secure,
33 encrypting containers of data. Tie::EncryptedHash introduces special
34 hash fields that are coupled with encrypt/decrypt routines to encrypt
35 assignments at STORE() and decrypt retrievals at FETCH(). By design,
36 encrypting fields are associated with keys that begin in single
37 underscore. The remaining keyspace is used for accessing normal hash
38 fields, which are retained without modification.
39 While the password is set, a Tie::EncryptedHash behaves exactly like a
41 standard Perl hash. This is its transparent mode of access.
42 Encrypting and normal fields are identical in this mode. When password
43 is deleted, encrypting fields are accessible only as ciphertext. This
44 is Tie::EncryptedHash's opaque mode of access, optimized for
45 serialization.
46 Encryption is done with Crypt::CBC(3) which encrypts in the cipher
48 block chaining mode with Blowfish, DES or IDEA. Tie::EncryptedHash
49 uses Blowfish by default, but can be instructed to employ any cipher
50 supported by Crypt::CBC(3).
51
53 Tie::EncryptedHash was designed for storage and communication of key
54 material used in public key cryptography algorithms. I abstracted out
55 the mechanism for encrypting selected fields of a structured data
56 record because of the sheer convenience of this data security method.
57 Quite often, applications that require data confidentiality eschew
59 strong cryptography in favor of OS-based access control mechanisms
60 because of the additional costs of cryptography integration. Besides
61 cipher implementations, which are available as ready-to-deploy perl
62 modules, use of cryptography in an application requires code to aid
63 conversion and representation of encrypted data. This code is usually
64 encapsulated in a data access layer that manages encryption,
65 decryption, access control and re-structuring of flat plaintext
66 according to a data model. Tie::EncryptedHash provides these functions
67 under the disguise of a Perl hash so perl applications can use strong
68 cryptography without the cost of implementing a complex data access
69 layer.
70
72 Tied Hash
73 "tie %h, Tie::EncryptedHash, 'Password', 'Cipher';"
74 Ties %h to Tie::EncryptedHash and sets the value of password and cipher
76 to 'Password' and 'Cipher'. Both arguments are optional.
77 Blessed Object
79 "$h = new Tie::EncryptedHash __password =" 'Password',
80 __cipher => 'Cipher';>
81 The new() constructor returns an object that is both tied and blessed
83 into Tie::EncryptedHash. Both arguments are optional. When used in
84 this manner, Tie::EncryptedHash behaves like a class with encrypting
85 data members.
86
88 The attributes __password, __cipher and __hide are reserved for
89 communication with object methods. They are "write-only" from
90 everywhere except the class to which the hash is tied. __scaffolding
91 is inaccessible. Tie::EncryptedHash stores the current encryption
92 password and some transient data structures in these fields and
93 restricts access to them on need-to-know basis.
94 __password
96 "$h{__password} = "new password"; delete $h{__password};"
97 The password is stored under the attribute "__password". In addition
99 to specifying a password at construction, assigning to the __password
100 attribute sets the current encryption password to the assigned value.
101 Deleting the __password unsets it and switches the hash into opaque
102 mode.
103 __cipher
105 "$h{__cipher} = 'DES'; $h{__cipher} = 'Blowfish';"
106 The cipher used for encryption/decryption is stored under the attribute
108 __cipher. The value defaults to 'Blowfish'.
109 __hide
111 "$h{__hide} = 1;"
112 Setting this attribute hides encrypting fields in opaque mode. 'undef'
114 is returned at FETCH() and EXISTS().
115
117 References
118 A reference stored in an encrypting field is serialized before
119 encryption. The data structure represented by the reference is folded
120 into a single line of ciphertext which is stored under the first level
121 key. In the opaque mode, therefore, only the first level of keys of
122 the hash will be visible.
123 Opaque Mode
125 The opaque mode introduces several other constraints on access of
126 encrypting fields. Encrypting fields return ciphertext on FETCH()
127 unless __hide attribute is set, which forces Tie::EncryptedHash to
128 behave as if encrypting fields don't exist. Irrespective of __hide,
129 however, DELETE() and CLEAR() fail in opaque mode. So does STORE() on
130 an existing encrypting field. Plaintext assignments to encrypting
131 fields are silently ignored, but ciphertext assignments are fine.
132 Ciphertext assignments can be used to move data between different
133 EncryptedHashes.
134 Multiple Passwords and Ciphers
136 Modality of Tie::EncryptedHash's access system breaks down when more
137 than one password is used to with different encrypting fields. This is
138 a feature. Tie::EncryptedHash lets you mix passwords and ciphers in
139 the same hash. Assign new values to __password and __cipher and create
140 a new encrypting field. Transparent mode will be restricted to fields
141 encrypted with the current password.
142 Error Handling
144 Tie::Encrypted silently ignores access errors. It doesn't carp/croak
145 when you perform an illegal operation (like assign plaintext to an
146 encrypting field in opaque mode). This is to prevent data lossage, the
147 kind that results from abnormal termination of applications.
148
150 Autovivification
151 Due to the nature of autovivified references (which spring into
152 existence when an undefined reference is dereferenced), references are
153 stored as plaintext in transparent mode. Analogous ciphertext
154 representations are maintained in parallel and restored to encrypting
155 fields when password is deleted. This process is completely
156 transparent to the user, though it's advisable to delete the password
157 after the final assignment to a Tie::EncryptedHash. This ensures
158 plaintext representations and scaffolding data structures are duly
159 flushed.
160 Data::Dumper
162 Serialization of references is done with Data::Dumper, therefore the
163 nature of data that can be assigned to encrypting fields is limited by
164 what Data::Dumper can grok. We set $Data::Dumper::Purity = 1, so self-
165 referential and recursive structures should be OK.
166 Speed
168 Tie::EncryptedHash'es keep their contents encrypted as much as
169 possible, so there's a rather severe speed penalty. With Blowfish,
170 STORE() on EncryptedHash can be upto 70 times slower than a standard
171 perl hash. Reference STORE()'es will be quicker, but speed gain will
172 be adjusted at FETCH(). FETCH() is about 35 times slower than a
173 standard perl hash. DES affords speed improvements of upto 2x, but is
174 not considered secure for long-term storage of data. These values were
175 computed on a DELL PIII-300 Mhz notebook with 128 Mb RAM running perl
176 5.003 on Linux 2.2.16. Variations in speed might be different on your
177 machine.
178
180 The standard usage for this module would be something along the lines
181 of: populate Tie::EncryptedHash with sensitive data, delete the
182 password, serialize the encrypted hash with Data::Dumper, store the
183 result on disk or send it over the wire to another machine. Later,
184 when the sensitive data is required, procure the EncryptedHash, set the
185 password and accesses the encrypted data fields.
186
188 Data::Dumper(3), Crypt::CBC(3), Crypt::DES(3), Crypt::Blowfish(3),
189 Tie::SecureHash(3)
190
192 The framework of Tie::EncryptedHash derives heavily from Damian
193 Conway's Tie::SecureHash. Objects that are blessed as well as tied are
194 just one of the pleasant side-effects of stealing Damian's code.
195 Thanks to Damian for this brilliant module.
196 PacificNet (http://www.pacificnet.net) loaned me the aforementioned
198 notebook to hack from the comfort of my bed. Thanks folks!
199
201 Vipul Ved Prakash <mail@vipul.net>
202
204 This module is distributed under the same license as Perl itself.
205perl v5.38.0 2023-07-21 Tie::EncryptedHash(3)