In this tutorial, 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.
There are three security-related packages in JDK1.4, as follows:
- JCA/JCE(Java Cryptography
Architecture & JavaCryptography
- JSSE( Java
- JAAS( Java Authentication &AuhorizationService)
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 following.
[There is also another requirement (ie) Authorization 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.
Secondly, 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 a message 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.
Some of the Symmetric key Algorithms are as follows: (Mostof them are implementedin JCA/JCE).
- 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 use. It has different modes of operation.
- Electronic Cook book ( ECB)
- Cipher Block Chaining ( CBC)
- Output Feedback Mode (OFB)
- Cipher Feedback Mode ( CFB)
- TripleDES( also known as DESede). An improved and very safe method of DES.
- IDEA (International Data Encryption Algorithm). This is used in PGP ( Pretty-Good-Privacymethod of secure Email).
An 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 hardware.
- Blowfish…This algorithm was designed by Bruce Schneier. It is not patented and he has placed the implementation in public domain.
- v) There is also a method known as Password-Based Encryption(PBE). We will have a brief description of this method ,with code example,shortly. Ready-madeimplementations 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’.)
The 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 related in 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.
MD5(MessageDigest-5) andSHA-1( 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.
He then 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 and DataIntegrity.It also ensures the identity of the sender,because the the hash1 was obtained by using the public key of Sam to decrypt the package.
If the 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 messages.Otherwise, 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 Authority.
Public 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. The CA 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.
Acertificate 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 transactions.
Message-Authentication-Code ( MAC)
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.
The scheme 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 thanAsymmetric 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 to JDK.
There are a number ofCryptographic Engines in SunJCA & SunJCE. They are listed below. It 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 code examples.
- KeyGenerator( symmetric) (Blowfish, DES,TripleDES,HmacMD5, HmacSHA1,RC5)
- KeyPairGenerator( asymmetric) (DiffieHellman, DSA, RSA)
- Mac ( message authentication code) HmacMD5, HmacSHA1)
- MessageDigest (MD5,SHA1)
- Signature (MD5withRSA, SHA1withRSA, SHA1withDSA)
- Cipher ( Blowfish, DES, TripleDES etc)
- CertificateFactory ( X509)
- KeyAgreement( DiffieHellman)
- SecureRandom ( SHA1PRNG) (ie) ( SHA1 ..pseudo-random-number-generator)
- KeySore ( 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 directory is
g:securitydemos. cd to g:securitydemos We should set <b>path </b>as : c:windowscommand;d:<b>jdk1.4.2</b>bin
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.
******************************************* // demo1.java // creation of message-digest // storing the string & digest in file importjava.io.*;