PDF文档的数字签名技术

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"iText数字签名是关于在PDF文档中实现数字签名的一种技术，由iText Software的创始人Bruno Lowagie撰写的一份白皮书。本文档主要探讨了PDF技术的初衷，即为了提供可靠的方式来查看和打印文档，并强调了PDF格式在确保一致性输出上的可靠性。随着技术的发展，PDF格式的功能不断丰富，成为了首选的文档格式，特别是在需要数字签名以确保文档完整性和不可篡改性的场景下。" iText 数字签名是PDF文档安全性的关键组成部分，它允许用户对PDF文档进行电子签名，以验证文档的完整性和作者的身份。这一特性对于法律文件、财务报告和其他需要防止欺诈和篡改的文档至关重要。在iText这个开源Java库的帮助下，开发者可以方便地集成数字签名功能到他们的应用程序中，从而为PDF文档提供额外的安全层。 PDF格式自1993年首次发布以来，已经经历了多次更新和扩展，包括支持多媒体、交互表单、嵌入式文件等多种功能。然而，其最核心的价值之一就是保证无论在任何设备上，都能实现一致的显示和打印效果。数字签名在PDF中的应用则进一步增强了这一价值，因为它能够验证文档自签名后未被修改过。 iText软件库提供了丰富的API，用于处理PDF文档的各种操作，包括添加、删除和编辑内容，以及实施数字签名。数字签名过程通常涉及以下步骤： 1. **证书获取**：签名者需要拥有一个有效的数字证书，该证书由受信任的证书颁发机构(CA)签发，包含了签名者的公钥和身份信息。 2. **文档哈希计算**：对原始PDF文档计算哈希值，这个值代表了文档内容的唯一标识。 3. **签名创建**：使用签名者的私钥对哈希值进行加密，生成数字签名。 4. **签名插入**：将签名信息插入到PDF文档中，同时保留原始文档内容。 5. **验证签名**：接收方使用签名者的公钥和证书来验证签名，确保文档未被篡改。通过iText，开发者可以轻松地实现这些步骤，创建符合PDF标准的数字签名。此外，iText还支持时间戳服务，这可以记录文档签名时的时间，增加签名的可信度，即使签名者的证书过期，也能验证签名的有效性。 iText数字签名技术为PDF文档提供了强大的安全保障，确保了文档的完整性和不可篡改性，这对于依赖于电子文件交换的现代业务环境来说，是至关重要的。同时，通过iText库，开发者可以便捷地集成这些高级功能，提升其应用的安全性和专业性。

Digital Signatures for PDF documents

2012

Is the password 'secret'? false

Digest using RIPEMD128: 16

Digest: c9c6d316d6dc4d952a789fd4b8858ed7

Is the password 'password'? true

Is the password 'secret'? false

We’ve already mentioned that the message could be any block of bytes, including the bytes of a

specific byte range of a PDF file. To detect whether or not a PDF file has been altered, we could

create a message digest of those bytes and store it inside the PDF. Then when somebody changes the

PDF, the message digest taken from the changed bytes in the byte range will no longer correspond

with the message digest stored in the PDF. Does this solve our problems of data integrity,

authenticity and non-repudiation?

Not yet. One could easily change the bytes of the PDF file, guess the digest algorithm that was used,

and store a new message digest inside the PDF. To avoid this, we need to introduce the concept of

encryption by means of asymmetric key algorithms.

1.3 Encrypting a message using a public-key encryption

Suppose that Bob wants to send Alice a private message

, but Bob doesn’t trust the channel used to

transmit the message. If it falls in the wrong hands, Bob wants to avoid that the message meant for

Alice’s eyes only, can be read by anyone else.

Bob could use an algorithm to encrypt his message, and then Alice would need to use an algorithm to

decrypt it. If the same key can be used for encryption as well as decryption (or if the key for

decryption can be derived from the key used for encryption), Bob and Alice are using a symmetric

key algorithm. The problem with such an algorithm is that Bob and Alice need to exchange that

algorithm in a safe way first. Anybody with access to the key can decrypt your message.

To avoid this, Bob and Alice can choose an asymmetric key algorithm. In this case two different keys

are used: one key to encrypt, the other one to decrypt the message. One key cannot be derived from

the other. Now if Bob wants to send Alice a private message, Alice can send her public key to Bob.

Anybody can use this key to encrypt messages meant for Alice. When Bob sends Alice such an

encrypted message, Alice will use her private key to decrypt it. Suppose that Chuck intercepts the

message, he won’t be able to read it as long as he doesn’t have access to Alice’s private key.

The most commonly used public-key encryption system is RSA, named after Ron Rivest, Adi Shamir,

and Leonard Adleman, its inventors. When you create a key pair, you can specify a key length in bits.

The higher the number of bits, the stronger the encryption will be. There’s also a downside: with

every doubling of the RSA key length, decryption is 6 to 7 times slower.

Let’s create such a public and private key pair.

1.3.1 Creating a key store

The Java SDK contains a tool named keytool. You can use this tool to create a key store. This key

store will contain your private key, and a digital certificate containing information about you as well

as your public key. See code sample 1.7 to find out how keytool is used.

http://en.wikipedia.org/wiki/Bob_and_Alice

Digital Signatures for PDF documents

2012

Code sample 1.7: creating a key store using keytool

$ keytool -genkey -alias demo -keyalg RSA –keysize 2048 -keystore ks

Enter keystore password:

Re-enter new password:

What is your first and last name?

[Unknown]: Bruno Specimen

What is the name of your organizational unit?

[Unknown]: IT

What is the name of your organization?

[Unknown]: iText Software

What is the name of your City or Locality?

[Unknown]: Ghent

What is the name of your State or Province?

[Unknown]: OVL

What is the two-letter country code for this unit?

[Unknown]: BE

Is CN=Bruno Specimen, OU=IT, O=iText Software, L=Ghent, ST=OVL, C=BE correct?

[no]: yes

Enter key password for <demo>

(RETURN if same as keystore password):

A key store can contain more than one private key, and we could define a password that is different

from the key store password for every key. For the sake of simplicity, we used only one:

“password”. We can use this key store from our Java program to encrypt and decrypt messages.

1.3.2 Encrypting and decrypting messages

Code sample 1.8 shows a simple class containing an encrypt() and a decrypt() method:

Code sample 1.8: A simple class to encrypt and decrypt messages

public class EncryptDecrypt {

protected KeyStore ks;

public EncryptDecrypt(String keystore, String ks_pass)

throws GeneralSecurityException, IOException {

initKeyStore(keystore, ks_pass);

}

public void initKeyStore(String keystore, String ks_pass)

throws GeneralSecurityException, IOException {

ks = KeyStore.getInstance(KeyStore.getDefaultType());

ks.load(new FileInputStream(keystore), ks_pass.toCharArray());

}

public X509Certificate getCertificate(String alias)

throws KeyStoreException {

return (X509Certificate) ks.getCertificate(alias);

}

public Key getPublicKey(String alias)

throws GeneralSecurityException, IOException {

return getCertificate(alias).getPublicKey();

}

public Key getPrivateKey(String alias, String pk_pass)

throws GeneralSecurityException, IOException {

return ks.getKey(alias, pk_pass.toCharArray());

Digital Signatures for PDF documents

2012

}

public byte[] encrypt(Key key, String message)

throws GeneralSecurityException {

Cipher cipher = Cipher.getInstance("RSA");

cipher.init(Cipher.ENCRYPT_MODE, key);

byte[] cipherData = cipher.doFinal(message.getBytes());

return cipherData;

}

public String decrypt(Key key, byte[] message)

throws GeneralSecurityException {

Cipher cipher = Cipher.getInstance("RSA");

cipher.init(Cipher.DECRYPT_MODE, key);

byte[] cipherData = cipher.doFinal(message);

return new String(cipherData);

}

public static void main(String[] args)

throws GeneralSecurityException, IOException {

EncryptDecrypt app =

new EncryptDecrypt("src/main/resources/ks", "password");

Key publicKey = app.getPublicKey("demo");

Key privateKey = app.getPrivateKey("demo", "password");

System.out.println("Let's encrypt 'secret message' with a public key");

byte[] encrypted = app.encrypt(publicKey, "secret message");

System.out.println("Encrypted message: "

+ new BigInteger(1, encrypted).toString(16));

System.out.println("Let's decrypt it with the corresponding private key");

String decrypted = app.decrypt(privateKey, encrypted);

System.out.println(decrypted);

System.out.println("You can also encrypt the message with a private key");

encrypted = app.encrypt(privateKey, "secret message");

System.out.println("Encrypted message: "

+ new BigInteger(1, encrypted).toString(16));

System.out.println("Now you need the public key to decrypt it");

decrypted = app.decrypt(publicKey, encrypted);

System.out.println(decrypted);

}

Feel free to experiment with this example, but don’t panic if you get an InvalidKeyException

saying that the key size is invalid. Due to import control restrictions by the governments of a few

countries, the encryption libraries shipped by default with the Java SDK restrict the length, and as a

result the strength, of encryption keys.

If you want to avoid this problem, you need to replace the default security JARs in your Java

installation with the Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files.

These JARs are available for download from http://java.oracle.com/ in eligible countries.

1.3.3 Inspecting our self-signed certificate

As you can see, we create a KeyStore object in the constructor. From this key store, we can obtain

the public certificate that contains the public key to be used to decrypt a message that was

encrypted with the corresponding private key.

Digital Signatures for PDF documents

2012

A public certificate also contains information about the owner of the key, its validity period, and so

on. The information in this certificate is signed by an ‘issuer’. In this case, we’ve created the

certificate ourselves, so we’re owner and issuer at the same time, and we refer to the certificate as

being self-signed. We’ll learn more about other certificates in chapter 3.

Code sample 1.9 shows the result of the toString() method of the Certificate object:

Code sample 1.9: an example of a public certificate

[

Version: V3

Subject: CN=Bruno Specimen, OU=IT, O=iText Software, L=Ghent, ST=OVL, C=BE

Signature Algorithm: SHA1withRSA, OID = 1.2.840.113549.1.1.5

Key: Sun RSA public key, 2048 bits

modulus:

27706646249437583578501322921252037659324960984454438650274096621513733947318221232

90092536075175589409888251417041849614639606544370595805501222639942552792696182924

19557917502293557528812483868420880765808333319067679184013346901221838396913865166

99015383461952441725262486245434952426855074038516834028858534816117097190264270919

71970499616689684012198665415564791592761123642686002605100319784405598279465396131

52730660815729426764990600604032553721917074418187300648866487699179740248069790221

86704383972995455717886346330217224211116969013795163606127880980836981725138593346

185822803712134120722258642329810193

public exponent: 65537

Validity: [From: Sat Aug 04 15:40:30 CEST 2012,

To: Tue Nov 17 14:40:30 CET 2015]

Issuer: CN=Bruno Specimen, OU=IT, O=iText Software, L=Ghent, ST=OVL, C=BE

SerialNumber: [ 501d264e]

]

Algorithm: [SHA1withRSA]

Signature:

0000: 12 ED EA 66 FE 6C 2C FC 0F F4 59 19 44 40 FE BF ...f.l,...Y.D@..

0010: CF 9E 66 D3 DC 62 85 F1 D5 62 76 07 F6 F2 67 04 ..f..b...bv...g.

0020: E8 F6 61 42 02 F9 36 A9 8B 12 6F 8B 4B B6 14 9B ..aB..6...o.K...

0030: 78 2F CA F0 53 76 41 F4 47 B7 5A 2B F7 A1 A9 73 x/..SvA.G.Z+...s

0040: 8E 44 55 31 14 D3 AB 3F 59 6C 53 E7 04 C4 2E 36 .DU1...?YlS....6

0050: DE 2C 1E F5 F9 E3 19 EF 7D 92 67 66 56 73 22 18 .,........gfVs".

0060: EC AF CB 86 22 B8 F0 D0 94 EC 37 97 D1 23 DA 43 ....".....7..#.C

0070: 98 8E 37 34 7E AD 76 78 99 63 21 0D 06 C3 1D 47 ..74..vx.c!....G

0080: 5D 21 0A A6 CD 57 70 C1 A4 23 5E 85 1E B9 80 DC ]!...Wp..#^.....

0090: A1 BF 61 02 12 D1 3D 5F D8 2E C5 5C 16 A2 6D 8D ..a...=_...\..m.

00A0: E1 0B 3C 1F 22 CF 11 18 AA 2D CF 75 C1 F6 C2 E8 ..<."....-.u....

00B0: 40 C2 59 C1 19 8B 86 61 79 12 4F F2 3E EC 61 1B @.Y....ay.O.>.a.

00C0: D3 CF FD 8C 3B F4 6D 1D F2 25 C3 7F 69 B1 B7 D2 ....;.m..%..i...

00D0: 49 96 61 B2 50 2B 70 74 AA 8E DC 18 6A 22 FC 00 I.a.P+pt....j"..

00E0: 96 67 A4 0B 70 62 63 A8 49 D6 E1 36 92 60 FF 0C .g..pbc.I..6.`..

00F0: 0E 72 55 0B A2 EC 3F CE 2E B4 BE E8 42 2C F0 73 .rU...?.....B,.s

]

In code sample 1.8, we obtain the public key from the Certificate object and we get the private

key from the KeyStore object. The public and the private key objects can be used as parameters

for the encrypt() and decrypt() method. These methods are almost identical. The only

difference is the Cipher mode: we’re using either ENCRYPT_MODE or DECRYPT_MODE.

Digital Signatures for PDF documents

2012

1.3.4 Using a public-key algorithm for authentication and non-repudiation

When we run this class, we get the following output:

Let's encrypt 'secret message' with a public key

Encrypted message:

66e7a06a40e5092aa0e70b4d57f2dd139e1902ddc012aa6d75deb6ecc9727fb9219d9aae98c054ebd53

8bfb002d31314c9fa6990d25cb528cf7516bbe3c8923c6670ba1b3673eb40a908a12146369f98fba36c

6beee04479411af7a6226e122a04d119dd9648a5d1be5cb3c52584f42fdffc6719f4d4b1e593a85c377

97d025b60f88d1ad19ad985911768bbb511c732761adace642dc8634925a8e197265ccadf07dd3a6185

50ffaf8dff13b44c91a3f9063d51f33ebd6d896321a795c34c6905f0e227253f3f86da4ca1d5f0da818

010621e1a88d4dc6cc4567d0fbc5461dbd696799f89da26b8eebcb4b4ac72115f021c2519e7036a5902

74f52ddde8d57d

Let's decrypt it with the corresponding private key

secret message

You can also encrypt the message with a private key

Encrypted message:

14d5a453baeb584a5c979ebcc142535684ce9503d1db01aa0bed0a3991dd5c6818ddf6f89bca94e24d8

dd02038564c42168c764d4d1f18fcf5e98f8da92b721add388e1971b63db841c8ddec1ae27b2f100f43

6e1f0a5294a9d2fb641b03a9aed412a4257c46f8b71700255b98a8d406a6daeb65bf64b28f85c786a67

2c56c95fa7dd539c8bfffa960a4d75ec16166088362dc1dbf0cb11ca6e7eb9d2730d885cc28abcd7d9b

56b179350d1975de16ec6c2c9ac2978cf2baa5da4b7d5b650ad5195a7fc5437072c05af55ab106f1231

c75c1a498ed83c113e4d008ee5710f32120641ea9ce7895e9f8c304ef6e8d169a9b0e0b155c91ae5f90

7978cd88e6e0f8

Now you need the public key to decrypt it

secret message

Do you see what happens here? First you take a public key and use it to encrypt a message so that

only somebody with the corresponding key can decrypt it. That’s what encryption is about.

But in the next few lines, you do the inverse: you encrypt a message using your private key. Now

when you share your public key —which is usually the intention— the whole world can decrypt your

message. In this case, you’re not using the encryption mechanism to make sure a third party can’t

read your message, but to make it absolutely clear that you’re the author. If I can decrypt a message

using your public key, I’m 100% sure that it was encrypted using your private key. As only you have

access to your private key, I’m 100% sure that you’re the author. That’s why the concept of

encryption using an asymmetric key algorithm can be used for digital signing.

Figure 1.5 shows the two different concepts. For encryption, the public key (the green key) is used to

encrypt, the private key (the red key) to decrypt. For creating a signature, it’s the other way round.

Figure 1.5: asymmetric key algorithms

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