In thistutorial, the author explains the cryptography-related concepts and
packages in JDK,with code examples.Many of the concepts and technical terms thus
learnt will be useful in understanding the Cryptography API in MFCalso.
security-related packages in JDK1.4, as follows:
Architecture & JavaCryptography
iii)JAAS( Java Authentication &
Prior to JDK1.4, many of these packages
were not available within the JDK and had to be separately installed and used.
But, JDK1.4 has incorporated all these within JDK itself).
Understanding the terminology of these
important packages requires that we havesome familiarity with the technical
terms used inthe field of Network
Security. We can begin by saying thatsecure communication ,should ensure the
[ There is also
another requirement (ie)
and it is more to protect resources and programs from users, than with
communicating the data. JAAS deals with that].
These are all standard terms used in
Security. Whena person, say, Sam,wants to send some
information toTom, it must be ensured that
the information thus sent, is not tampered with oraltered on the way. This is known as Data Integrity.
the information is meant only for Tom and so no one else should be able
to understand the message. This is known as Confidentiality. There
should be some indication that the message came from Sam andthere should be some proof for that.
This is Identification.Authentication,that
the message came from Sam is provided byDigitalSignature.There should
preferably be a trusted third party to vouchsafe for the identity and signature
of Sam. This is achieved by Digital Certificate,which
authenticates the signature of Sam. Besides these, sometimesit is equally important thatSam should not be able to say later that he
did not send the message to Tom and the message was actually sent by someone
else ,in his name. This isensuring Non-repudiation.
This purpose also is served byDigital signature. We will now see
step-by-step development of these concepts. Except 'Authorization', other things
can be understood in the context ofeveryday exchange of information.
Confidentiality is achieved byusingCryptography
techniques. For the sake of simple illustration, let us assume that Sam wants
to send amessage
to Tom.( By convention, two persons Alice & Bob are chosen for
illustration, because the original thesis made use of these names. Let us use
Sam and Tom instead).Sam does not want his message to be understood by anybody
else except Tom. So, he encrypts it. When Tom receives the encrypted
message, he 'decrypts' it, so that he can read the original message. The
original message is known as 'plaintext'. After Encryption, it
becomes 'ciphertext'. The process of convertingtheciphertextinto the
original plaintext is known as Decryption. A 'key'is used for controllingEncryption and Decryption.
There are two types of key-based
encryption algorithms, namely, Symmetric algorithm
and Asymmetric Algorithm.
a) SymmetricAlgorithm:This algorithmuses the same key for encryption and
decryption.This is also known as 'Secret key'.
In this scheme, when Sam wants to send a
message to Tom, he encrypts the message by the mutually agreed secret-key and
then sends the ciphertext to Tom. Tom uses the same
secret key and decrypts the message and reads it.
Symmetric key system is faster than the
Asymmetric system but the problem of agreeing on mutual secret keyand preserving the secrecy of thekey while communicating it over the network,
led to the development ofAsymmetric key systems.
the Symmetric key Algorithms are as follows: (Mostof them are implementedin JCA/JCE).
i) DES( Data-Encryption-Standard).. developed in 1970 and recommended by US government. Though
it is not fool-proof, it is considered to be sufficiently safe and is in wide
It has different modes of operation.
Electronic Cook book ( ECB)
Block Chaining ( CBC)
Feedback Mode (OFB)
Feedback Mode ( CFB)
11) TripleDES( also known as DESede) ..
improved and very safe method of DES.
iii) IDEA (International Data Encryption
Algorithm). This is used in PGP ( Pretty-Good-Privacymethod of secure Email).
important advantage of Secret-key algorithm is that a hardware-approach is
possible. This results in very high speed encryption. The hardware
implementation by a VLSI chip can be about 20 times faster than the
corresponding software implementation!IDEA has been implemented in
algorithm was designed by Bruce Schneier. It is not
patented and he has placed the implementation in public domain.
is also a method known as Password-Based Encryption(PBE). We will have a brief description
of this method ,with code example,shortly.
for many of these algorithms are available in SunJCA/JCE
and the programmer just chooses the desired algorithm and uses it. No deep
knowledge of the mathematical theory of the algorithms or how these algorithms
are implemented ,is required .Such topics are highly
mathematical and aredealt with inbooks on Cryptography .
This algorithm is also known as 'Public Key'
algorithm. There are two keys in this scheme. One key is known as 'public
key' and the other key is known as 'private key'.(It should be noted that 'secret key' does
not mean 'private key'.)
basic theory of Public key Cryptography was developed by two research workers
at StanfordUniversityDiffie&Hellman in 1976. TheDH algorithm is known as
Key-Agreement method. RSA algorithm is an implementation ,
named after the initials of the three academics who invented it. ( Rivest,Shamir & Adleman).RSA is the defacto
standard.Another Asymmetric algorithm isDSA
( Digital Signature
Algorithm). Yet another algorithm is known as ECC(Elliptic-Curve
Cryptography). It is reputed to be very efficient and fast.[
However, SunJCA/JCE does not provide ready-made
implementation for ECC.]
The public key and private key are known as 'keypair'.The
public key and private key are mathematically relatedin the sense that if a message is encrypted
by using a particular public key, it can be decrypted by the corresponding
private key and vice-versa(ie) the data can
also be encrypted by using a private key and can be decrypted by the
corresponding public key, and not by any other public key.But
the problem is thatany person who knows
Sam's public keycan decrypt the message.So,RSA
system uses public key of the recipient to encrypt the data.(
But, the private key cannot be derived from public key.Similarly,
the public key cannot be derived from private key).
RSA method is the most widely used scheme.
When Sam wants to send a secret message to Tom, he should know the public
key of Tom to begin with.( Just as we should know
the mail-id of our friend first, if we want to send email to him). Samencrypts the message by using Tom'spublic keyand sends it to Tom.At the receiving end,
Tom uses his (Tom's)private key and decrypts the letter and reads it.The advantage of this scheme is that it ensures that
only Tom will be able to read the message, as only his private-key can decrypt
the message encrypted with his public key.A person's
private key need never be known to anyone else and there is no sharing the key
with another person.Only the public key needs to be
informed to others. ( like the difference in sharing
our mail-id and sharing our password!).Thus key-administration problem is less.
The Asymmetric system has another use as
well. It can be used for creating the Digital Signature, to ensure that
the message came from Sam.Though the message itself can
be signed without creating a digest, the usual method is to sign the messagedigest,so that Integrity of
data also can be ensured.
A 'Message Digest' isa digital fingerprint.It
isoften referred to simply as a digest
( summary) or hash. It is an one-way process ( ie) it is impossible to reconstruct the original
from the hash.
Secure Hash Algorithm) are two examples of such Digesting algorithms.These two are provided in Sun security
package.MD5 accepts some input and produces a 128-bit message digest.SHA-1 is more secure and produces a 160-bit
When Sam wants to send a secure message, he
passes the message through a MessageDigest engine.
The result is a hash.
encrypts thehashby using Sam'sprivate key.( This encryption is done
on the hash and not on the data). Thus we get the Digital Signature.
Finally, Sam encryptsthe original message usingTom's Public key. After this, Sam
sends the package to Tom.
At the receiving end, Tom uses his(Tom's) private key to decrypt the message.By using Sam's public key, hedecrypts the digital signature and so gets
the originalhash( hash1).Using the same oneway
hash algorithm on the text message,Tom
creates another hash( hash2).If hash2exactly matches hash1, it means that the data has not been altered in
transit. Thus, we get assurance of Confidentiality andDataIntegrity.It also ensures the identity of
the sender,becuase the the
hash1 was obtained by using the public key of Sam to decrypt the package.
public key of Sam, used by Tom, has the added assurance from a certificate
authority that it really belongs to Sam,this is aclear-cut
method with no problems except that it is not suitable if the message being
encrypted is of large size. Besides satisfying the requirements of
Authentication, Confidentiality, Integrity and Non-Repudiation, we should also
ensure that the process is fast, in Enterprise
level. The method outlined above is slow and so may not be suitable for large
it is a satisfactory method.
(We will describe a hybrid method used
for large messages, shortly).
Sometimes, it may be enough if there is
Authentication and Non-Repudiation,without
confidentiality. In such cases, it is enough if Sam sends the message-digest
encrypted by Sam's privatekey ,along with the plaintext.
Just now , we saw
that Tommade use of Sam's public key
to verify his Digital Signature. How does Tom getto know the public key of Sam?Sam could have published his public key in
the internet or could have sent it to Tom, personally. A person's public key
can be freely published and shared and for this reason, anybody can use it , not necessarily Sam. A Digital signature of Sam can be
verified only if Sam's public key is available to Tombut as it is a public key,
impersonation is possible. A trusted third-party is required to certify that
the said key is really Sam's public key.This is known
as Digital Certificate and the authorities who issue such certifictes are Certifying
Key Infrastructure( PKI)
When Sam wants
his public key to be certified by a CA, he generates a keypair
and sends the public key to an appropriate CAwith some proof of his identification.
checks the identification and then after satisfying that the key has not been
modified in transit, issues a certificate relating the public key of Sam with
his identity,by signing the
public key of Sam with the private key of the CA. The standard formatof issuing the
certificate is known as X509.
Who is to attest
the CA themselves? The CAare self-attested.
The PKI standard has been developed by RSA
Security Systems in collaboration with Industry leaders like SUN, IBM and Microsoft
and is the industry standard.
becomes invalid after the expiry of validation period. Sometimes, the private
key associated with a public key gets compromised ( ie)
exposed, and in that case also, the cerificate should
be withdrawn( revoked).The owner of the privatekey
also may like to change it.The CA publishes a list of
such defunct certificates and Tom should verify that Sam'scertificate is still valid, before important
Digital Signature makes use of Sam's
private key to sign the hash. An alternative to Digital Signature is to use a secret
key to encrypt the hash. By its very definition, secret key is common to both
Sam & Tom. So Tom can use the secret key at his end and get back the hash.
The code thus generated by mixing the hash and the secret key is known as MAC.
Digital Signature is better than MAC because it does not need any 'secret' key.
In the context of E-Commerce, where there are thousands of parties
, secret key administration is always very difficult.
outlined above is suitable for most purposes. However, for very large amounts
of data, encryption and decryption of databy public-key systems becomestime consuming and requires large resources.
In such cases, it is preferred to use Symmetric Encryption systems with some
modifications. Hybrid Systems make use ofAsymmetric method for agreeing upon a
secret key and the actual encryption and decryption of data is done by
this secretkey.Some such methodis the usual Industrial practice. The Secret keyused here is
valid only for a particular instance of transmission of message and so is
usually called 'session-key'. ( This is not the
'session' as usually understood in servlets, because
this is an one-time operation).
An illustration of Hybrid method is a Digital
Envelope. In this scheme, Sam encrypts the message by a random Secret key, ( known as DEK i.e.Data-Encryption-keyor session key).Next Sam encrypts this
session key with Tom's public key. At this stage, Sam sends both the
encrypted message and the encrypted session key to Tom.
At the receiving end, Tom uses his private
key to get the session key. Using this session key, Tom decrypts the message.As Symmetric method is about 1000 times faster
method , this is a good combination. Though public key method also has been
used here, it is only for encrypting the session key and not the message.This can be further improved by creating a
hash of the message and sigining it. Also, there is
no permanent Secret key betweenSam & Tom,and the required secret key is produced just
as required and then discarded after the job. Thus, the method is fast and secure.
With this background information, let us now
see some simple code examples,specific
a number ofCryptographic Engines in SunJCA & SunJCE. They are listed below.
will be immediately evident that the names will beGreek & Latin to us , unless we
have a background in Crypto terminology. That is why, a broad outline was
given. The function of some of the engines will be evident from the earlier
discussion. A few more of the remaining items will be clear when we deal with
2) KeyPairGenerator( asymmetric)
3) Mac ( message authentication code)
( Blowfish, DES, TripleDES etc)
( SHA1 ..pseudo-random-number-generator)
( JKS, PHCS12)
Let us now
see a series of code-examples to get familiarity with some of the above
engines. For all the examples, we are using JDK1.4.2. Our working
cd to g:securitydemos
set path as :
The easiest to understand is the MessageDigest. "demo1.java"creates the messagedigest of the string s1, by SHA method(Secure Hash Algorithm) . The
given string is first converted into a byte array, because the function md.digest(),
accepts only a bytearray.md.update() simply
adds the array to existing arrays,if any. The digest
object thus created is simply saved as object to the file.
creation of message-digest
storing the string & digest in file