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Crypt-OpenPGP-0.14
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/ Crypt::OpenPGP

NAME

Crypt::OpenPGP - Pure-Perl OpenPGP implementation

SYNOPSIS

my $pgp = Crypt::OpenPGP->new;
my $signature = $pgp->sign(
               Filename   => $file,
               KeyID      => $key_id,
               Passphrase => $pass,
               Detach     => 1,
               Armour     => 1,
         );

my $valid = $pgp->verify(
               Signature  => $signature,
               Files      => [ $file ],
         );

my $ciphertext = $pgp->encrypt(
               Filename   => $file,
               KeyID      => $key_id,
               Armour     => 1,
         );

my $plaintext = $pgp->decrypt(
               Data       => $ciphertext,
               Passphrase => $pass,
         );

DESCRIPTION

Crypt::OpenPGP is a pure-Perl implementation of the OpenPGP standard[1]. In addition to support for the standard itself, Crypt::OpenPGP claims compatibility with many other PGP implementations, both those that support the standard and those that preceded it.

Crypt::OpenPGP provides signing/verification, encryption/decryption, keyring management, and key-pair generation; in short it should provide you with everything you need to PGP-enable yourself. Alternatively it can be used as part of a larger system; for example, perhaps you have a web-form-to-email generator written in Perl, and you'd like to encrypt outgoing messages, because they contain sensitive information. Crypt::OpenPGP can be plugged into such a scenario, given your public key, and told to encrypt all messages; they will then be readable only by you.

This module currently supports RSA and DSA for digital signatures, and RSA and ElGamal for encryption/decryption. It supports the symmetric ciphers 3DES, Blowfish, IDEA, Twofish, and Rijndael (AES). Rijndael is supported for key sizes of 128, 192, and 256 bits. Crypt::OpenPGP supports the digest algorithms MD5, SHA-1, and RIPE-MD/160. And it supports ZIP and Zlib compression.

COMPATIBILITY

One of the highest priorities for Crypt::OpenPGP is compatibility with other PGP implementations, including PGP implementations that existed before the OpenPGP standard.

As a means towards that end, some of the high-level Crypt::OpenPGP methods can be used in compatibility mode; given an argument Compat and a PGP implementation with which they should be compatible, these method will do their best to choose ciphers, digest algorithms, etc. that are compatible with that implementation. For example, PGP2 only supports IDEA encryption, MD5 digests, and version 3 signature formats; if you tell Crypt::OpenPGP that it must be compatible with PGP2, it will only use these algorithms/formats when encrypting and signing data.

To use this feature, supply either sign or encrypt with the Compat parameter, giving it one of the values from the list below. For example:

my $ct = $pgp->encrypt(
              Compat   => 'PGP2',
              Filename => 'foo.pl',
              KeyID    => $key_id,
         );

Because PGP2 was specified, the data will automatically be encrypted using the IDEA cipher, and will be compressed using ZIP.

Here is a list of the current compatibility sets and the algorithms and formats they support.

  • PGP2

    Encryption: symmetric cipher = IDEA, compression = ZIP, modification detection code (MDC) = 0

    Signing: digest = MD5, packet format = version 3

  • PGP5

    Encryption: symmetric cipher = 3DES, compression = ZIP, modification detection code (MDC) = 0

    Signing: digest = SHA-1, packet format = version 3

  • GnuPG

    Encryption: symmetric cipher = Rijndael, compression = Zlib, modification detection code (MDC) = 1

    Signing: digest = RIPE-MD/160, packet format = version 4

If the compatibility setting is unspecified (that is, if no Compat argument is supplied), the settings (ciphers, digests, etc.) fall back to their default settings.

USAGE

Crypt::OpenPGP has the following high-level interface. On failure, all methods will return undef and set the errstr for the object; look below at the ERROR HANDLING section for more information.

Crypt::OpenPGP->new( %args )

Constructs a new Crypt::OpenPGP instance and returns that object. Returns undef on failure.

%args can contain:

  • SecRing

    Path to your secret keyring. If unspecified, Crypt::OpenPGP will look for your keyring in a number of default places.

  • PubRing

    Path to your public keyring. If unspecified, Crypt::OpenPGP will look for your keyring in a number of default places.

$pgp->encrypt( %args )

Encrypts a block of data. The encryption is actually done with a symmetric cipher; the key for the symmetric cipher is then encrypted with either the public key of the recipient or using a passphrase that you enter. The former case is using public-key cryptography, the latter, standard symmetric ciphers. In the first case, the session key can only be unlocked by someone with the corresponding secret key; in the second, it can only be unlocked by someone who knows the passphrase.

Given the parameter SignKeyID (see below), encrypt will first sign the message before encrypting it, adding a Signature packet to the encrypted plaintext.

Returns a block of data containing two PGP packets: the encrypted symmetric key and the encrypted data.

On failure returns undef.

%args can contain:

  • Compat

    Specifies the PGP compatibility setting. See COMPATIBILITY, above.

  • Data

    The plaintext to be encrypted. This should be a simple scalar containing an arbitrary amount of data.

    Data is optional; if unspecified, you should specify a filename (see Filename, below).

  • Filename

    The path to a file to encrypt.

    Filename is optional; if unspecified, you should specify the data in Data, above. If both Data and Filename are specified, the data in Data overrides that in Filename.

  • KeyID

    The ID of the public key that should be used to decrypt the symmetric key. In other words, the ID of the key with which the message should be encrypted. The value of the key ID should be specified as either an 8-digit or 16-digit hexadecimal number.

    If you wish to encrypt the message for multiple recipients, the value of KeyID should be a reference to a list of key IDs (as defined above). For each ID in the list, the public key will be looked up in your public keyring, and an encrypted session key packet will be added to the encrypted message.

    This argument is optional; if not provided you should provide the Passphrase option (below) to perform symmetric-key encryption when encrypting the session key.

  • Passphrase

    The mechanism to use symmetric-key, or "conventional", encryption, when encrypting the session key. In other words, this allows you to use Crypt::OpenPGP for encryption/decryption without using public-key cryptography; this can be useful in certain circumstances (for example, when encrypting data locally on disk).

    This argument is optional; if not provided you should provide the KeyID option (above) to perform public-key encryption when encrypting the session key.

  • Cipher

    The name of a symmetric cipher with which the plaintext will be encrypted. Valid arguments are DES3, Blowfish, IDEA, Twofish, Rijndael, Rijndael192, and Rijndael256 (the last two are Rijndael with key sizes of 192 and 256 bits, respectively).

    This argument is optional; Crypt::OpenPGP currently defaults to DES3, but this could change in the future.

  • Compress

    The name of a compression algorithm with which the plaintext will be compressed before it is encrypted. Valid values are ZIP and Zlib.

    By default text is not compressed.

  • Armour

    If true, the data returned from encrypt will be ASCII-armoured. This can be useful when you need to send data through email, for example.

    By default the returned data is not armoured.

  • SignKeyID

    If you wish to sign the plaintext message before encrypting it, provide encrypt with the SignKeyID parameter and give it a key ID with which the message can be signed. This allows recipients of your message to verify its validity.

    By default messages not signed.

  • SignPassphrase

    The passphrase to unlock the secret key to be used when signing the message.

    If you are signing the message--that is, if you have provided the SignKeyID parameter--this argument is required.

  • MDC

    When set to a true value, instructs encrypt to use encrypted MDC (modification detection code) packets instead of standard encrypted data packets. These are a newer form of encrypted data packets that are followed by a SHA-1 hash of the plaintext data. This prevents attacks that modify the encrypted text by using a message digest to detect changes.

    By default MDC is set to 0, and encrypt generates standard encrypted data packets. Set it to a true value to turn on MDC packets. Note that MDC will automatically be turned on if you are using a Compat mode that is known to support it.

$pgp->decrypt( %args )

Decrypts a block of ciphertext. The ciphertext should be of the sort returned from encrypt, in either armoured or non-armoured form. This is compatible with all other implementations of PGP: the output of their encryption should serves as the input to this method.

When called in scalar context, returns the plaintext (that is, the decrypted ciphertext), or undef on failure. When called in list context, returns a two-element list containing the plaintext and the result of signature verification (see next paragraph), or the empty list on failure. Either of the failure conditions listed here indicates that decryption failed.

If decrypt is called in list context, and the encrypted text contains a signature over the plaintext, decrypt will attempt to verify the signature and will return the result of that verification as the second element in the return list. If you call decrypt in list context and the ciphertext does not contain a signature, that second element will be undef, and the errstr will be set to the string No Signature. The second element in the return list can have one of three possible values: undef, meaning that either an error occurred in verifying the signature, or the ciphertext did not contain a signature; 0, meaning that the signature is invalid; or a true value of either the signer's user ID or 1, if the user ID cannot be determined. Note that these are the same values returned from verify (below).

For example, to decrypt a message that may contain a signature that you want verified, you might use code like this:

my($pt, $validity) = $pgp->decrypt( ... );
die "Decryption failed: ", $pgp->errstr unless $pt;
die "Signature verification failed: ", $pgp->errstr
    unless defined $validity || $pgp->errstr ne 'No Signature';

This checks for errors in decryption, as well as errors in signature verification, excluding the error denoting that the plaintext was not signed.

%args can contain:

  • Data

    The ciphertext to be decrypted. This should be a simple scalar containing an arbitrary amount of data.

    Data is optional; if unspecified, you should specify a filename (see Filename, below).

  • Filename

    The path to a file to decrypt.

    Filename is optional; if unspecified, you should specify the data in Data, above. If both Data and Filename are specified, the data in Data overrides that in Filename.

  • Passphrase

    The passphrase to unlock your secret key.

    This argument is mandatory if your secret key is protected.

$pgp->sign( %args )

Creates and returns a digital signature on a block of data.

On failure returns undef.

%args can contain:

  • Compat

    Specifies the PGP compatibility setting. See COMPATIBILITY, above.

  • Data

    The text to be signed. This should be a simple scalar containing an arbitrary amount of data.

    Data is optional; if unspecified, you should specify a filename (see Filename, below).

  • Filename

    The path to a file to sign.

    Filename is optional; if unspecified, you should specify the data in Data, above. If both Data and Filename are specified, the data in Data overrides that in Filename.

  • Detach

    If set to a true value the signature created will be a detached signature; that is, a signature that does not contain the original text. This assumes that the person who will be verifying the signature can somehow obtain the original text (for example, if you sign the text of an email message, the original text is the message).

    By default signatures are not detached.

  • Armour

    If true, the data returned from sign will be ASCII-armoured. This can be useful when you need to send data through email, for example.

    By default the returned signature is not armoured.

  • Clearsign

    If true, the signature created on the data is a clear-text signature. This form of signature displays the clear text of the signed data, followed by the ASCII-armoured signature on that data. Such a format is desirable when sending signed messages to groups of users who may or may not have PGP, because it allows the text of the message to be readable without special software.

    When Clearsign is set to true, Armour and Detach are automatically turned on, because the signature created is a detached, armoured signature.

    By default Clearsign is false.

  • KeyID

    The ID of the secret key that should be used to sign the message. The value of the key ID should be specified as a 16-digit hexadecimal number.

    This argument is mandatory.

  • Passphrase

    The passphrase to unlock your secret key.

    This argument is mandatory if your secret key is protected.

  • Digest

    The digest algorithm to use when creating the signature; the data to be signed is hashed by a message digest algorithm, then signed. Possible values are MD5, SHA1, and RIPEMD160.

    This argument is optional; by default SHA1 will be used.

  • Version

    The format version of the created signature. The two possible values are 3 and 4; version 4 signatures will not be compatible with older PGP implementations.

    The default value is 4, although this could change in the future.

$pgp->verify( %args )

Verifies a digital signature. Returns true for a valid signature, 0 for an invalid signature, and undef if an error occurs (in which case you should call errstr to determine the source of the error). The 'true' value returned for a successful signature will be, if available, the PGP User ID of the person who created the signature. If that value is unavailable, the return value will be 1.

%args can contain:

  • Signature

    The signature data, as returned from sign. This data can be either a detached signature or a non-detached signature. If the former, you will need to specify the list of files comprising the original signed data (see Data or Files, below).

    Either this argument or SigFile is required.

  • SigFile

    The path to a file containing the signature data. This data can be either a detached signature or a non-detached signature. If the former, you will need to specify the list of files comprising the original signed data (see Data or Files, below).

    Either this argument or SigFile is required.

  • Data

    Specifies the original signed data.

    If the signature (in either Signature or SigFile) is a detached signature, either Data or Files is a mandatory argument.

  • Files

    Specifies a list of files comprising the original signed data. The value should be a reference to a list of file paths; if there is only one file, the value can be specified as a scalar string, rather than a reference to a list.

    If the signature (in either Signature or SigFile) is a detached signature, either Data or Files is a mandatory argument.

$pgp->keygen( %args )

NOTE: this interface is alpha and could change in future releases!

Generates a public/secret PGP keypair. Returns two keyblocks (objects of type Crypt::OpenPGP::KeyBlock), a public and a secret keyblock, respectively. A keyblock is essentially a block of keys, subkeys, signatures, and user ID PGP packets.

%args can contain:

  • Type

    The type of key to generate. Currently there are two valid values: RSA and DSA. ElGamal key generation is not supported at the moment.

    This is a required argument.

  • Size

    Bitsize of the key to be generated. This should be an even integer; there is no low end currently implemented in Crypt::OpenPGP, but for the sake of security Size should be at least 1024 bits.

    This is a required argument.

  • Identity

    A string that identifies the owner of the key. Typically this is the combination of the user's name and an email address; for example,

    Foo Bar <foo@bar.com>

    The Identity is used to build a User ID packet that is stored in each of the returned keyblocks.

    This is a required argument.

  • Passphrase

    String with which the secret key will be encrypted. When read in from disk, the key can then only be unlocked using this string.

    This is a required argument.

  • Version

    Specifies the key version; defaults to version 4 keys. You should only set this to version 3 if you know why you are doing so (for backwards compatibility, most likely). Version 3 keys only support RSA.

  • Verbosity

    Set to a true value to enable a status display during key generation; since key generation is a relatively lengthy process, it is helpful to have an indication that some action is occurring.

    Verbosity is 0 by default.

ERROR HANDLING

If an error occurs in any of the above methods, the method will return undef. You should then call the method errstr to determine the source of the error:

$pgp->errstr

In the case that you do not yet have a Crypt::OpenPGP object (that is, if an error occurs while creating a Crypt::OpenPGP object), the error can be obtained as a class method:

Crypt::OpenPGP->errstr

For example, if you try to decrypt some encrypted text, and you do not give a passphrase to unlock your secret key:

my $pt = $pgp->decrypt( Filename => "encrypted_data" )
    or die "Decryption failed: ", $pgp->errstr;

LICENSE

Crypt::OpenPGP is free software; you may redistribute it and/or modify it under the same terms as Perl itself.

AUTHOR & COPYRIGHT

Except where otherwise noted, Crypt::OpenPGP is Copyright 2001 Benjamin Trott, ben@rhumba.pair.com. All rights reserved.

REFERENCES

1 RFC2440 - OpenPGP Message Format (1998). http://www.faqs.org/rfcs/rfc2440.html