Preschool guidelines

Virtual character design

In order to facilitate us to learn the iOS signature mechanism, this paper sets up four virtual characters, respectively

Alice and Bob: Communicate with each other
EVe: Eavesdropper
Mallory: Active attacker

Message communication and eavesdropping

If encryption is not used, both parties communicate in plaintext

But such communications are vulnerable to eavesdropping and theft of confidential information

So how do you prevent information from being monitored? We need to encrypt the communication information.
- The plaintext is first encrypted into ciphertext and sent to the receiver

Encrypt and decrypt?

How to encrypt and decrypt?

The message sender uses the key for encryption

The receiver of the message uses a key for decryption

Type of password

We can divide passwords into two types, depending on how the key is used

Symmetrical password

Symmetric ciphers use the same key for encryption and decryption

There are three commonly used symmetric cryptography algorithms

Data Encryption Standard (DES)

DES is a symmetric cipher algorithm that encrypts 64-bit plaintext into 64-bit ciphertext. The key length is 56 bits. In terms of specifications, the length of the key is actually 64bit, but a bit is set every 7 bits for error checking, so the length of the key is 56bit. DES encrypts only 64bit data at a time. Therefore, you need to iterate DES encryption when encountering large data. And because DES encryption algorithm can be cracked in a short time, it is not recommended to use.

3DES

3DES is a password algorithm obtained by repeating DES three times, also called triple DES. This algorithm is still used by some banking institutions, but the processing speed is not high, and there are security problems.

3DES encrypts plain text into ciphertext by encryption-decryption-encryption

3DES decrypts ciphertext by decrypting, encrypting, and decrypting it into plain text

In the preceding encryption and decryption process, the three keys are different, so they are also called DES-EDE3

If all the keys in the preceding process are the same, the result is equivalent to that of ordinary DES, and the three times encryption and decryption is meaningless

If key 1 and key 3 are the same, but key 2 is different, it is called DES-ede2

Advanced Encryption Standard (AES)

AES is a symmetric cryptographic algorithm that replaces DES as the new standard. The key length of AES can be 128, 192, and 256 bits.

Key distribution problem

What is the key distribution problem?

When symmetric passwords are used, key distribution problems must occur, as shown in the following figure

Suppose Alice sends the message encrypted with symmetric password to Bob, and Bob needs to get Alice’s encrypted key to view the plaintext information, so Alice sends the key to Bob at the same time. In the process of sending the key, Eve may eavesdrop, and Eve gets the stolen key and ciphertext. It can also be parsed to get clear text messages.

How to solve the key distribution problem?

To solve the key distribution problem, you can use the following methods

Share keys in advance. That is, Alice and Bob share a secret key in advance, which cannot be transmitted over the network. However, this method is very troublesome and is not recommended.
Key distribution center
Diffie-hellman key exchange
Public key password. Public key cryptography is now a common way, and it is the main way we learn.

Public-key Cryptography

In public key Cryptography, the key is classified into encryption key and decryption key. They are not the same key, so public key Cryptography is also called Asymmetric Cryptography.

In public key cryptography:

The encryption key is public, so it is called a public key.
The decryption key is kept by the message receiver and cannot be made public. Therefore, it is also called a private key.
The public key and private key are one-to-one and cannot be created separately. A pair of public and private keys is called a key pair.
The ciphertext encrypted by a public key can be decrypted only by using the private key corresponding to the public key.
The ciphertext encrypted by the private key can be decrypted only by using the corresponding public key of the private key.

Public key cryptography solves key distribution problems

As mentioned above, the key distribution problem can be solved by using public key cryptography. The specific solution process is as follows:

First, the message receiver generates a pair of public and private keys.
Sends the public key to the message sender.
The message sender encrypts the message with a public key.
The sender sends the encrypted ciphertext to the receiver.
The receiver decrypts the ciphertext with the private key to obtain the plaintext data.

RSA

RSA is the most widely used public key cryptography algorithm. Its name is a combination of the first letters of the surnames of the three developers — Ron Rivest, Adi Shamir, and Leonard Adleman

Hybrid cryptosystem

Contrast symmetric and asymmetric ciphers

Symmetric cipher can not solve the problem of key distribution well
The encryption and decryption speed of public key passwords is slow. Because the size of the encrypted ciphertext is the same as the size of the message itself, the encryption and decryption speed of a large amount of data is slow.
In order to solve the key distribution problem, and improve the speed of encryption and decryption. Therefore, a combination of symmetric cryptography and public key cryptography was adopted to learn from each other’s strengths. Today, the SSL/TLS used for cryptographic communication on the network are hybrid cryptographic systems.

Hybrid cryptosystem – encryption

Session Key

A session key is a temporary key that is generated randomly for this communication and is generated using a pseudorandom number generator
As the key of symmetric cipher, the session key is used to encrypt messages to improve the speed

Encryption step – Send a message

First, the message sender needs to get the public key of the message receiver
The second step is to generate the session key, which acts as the key for the symmetric cipher and encrypts the message into ciphertext
Third, use the message receiver’s public key to encrypt the session key
Fourth, the encryption results generated in step 2 and step 3 are sent to the message receiver

The content sent includes

Messages encrypted with session keys (encrypted with symmetric ciphers)
Session key encrypted with public key (encrypted with public key password)

Hybrid cryptosystem – decryption

After receiving the message from == sender ==, you need to decrypt the message as follows:

First, the message receiver decrypts the session key with its own private key
In the second step, decrypt the message with the session key obtained in the first step and get the plaintext data

Hybrid cryptosystem – Summary of complete steps for encryption and decryption

Use Alice as the == message sender == and Bob as the message receiver

Sending a message (encrypted)

Bob first generates a pair of public and private keys
Bob shares the public key with Alice
Alice randomly generates a session key (temporary key)
Alice encrypts the message that needs to be sent with the session key (symmetric cryptography)
Alice encrypts the session key with Bob’s public key (public-key cryptography, also known as asymmetric cryptography)
Alice sends Bob the results of steps 4 and 5

Receive message (decrypt)

Bob decrypts the session key (public-key cryptography, also known as asymmetric cryptography) using his own private key
Bob decrypts the sent message using the session key (symmetric cryptography)

Monomial hash function

A one-way hash function can calculate the hash value based on the message content. The length of the hash value is independent of the length of the message. Whether the message is 1bit, 10M or 100G, the one-way hash function will calculate the fixed length of the hash value.

Characteristics of unidirectional hash functions

A fixed-length hash value is calculated based on a message of arbitrary length.
Fast calculation speed, can quickly calculate the hash value
Different messages, even if there is a difference of 1bit of data, the hash value is different

It’s unidirectional and irreversible

A common one-way hash function

The one-way hash function is also called message digest function and hash function. The output hash value is also known as message digest, fingerprint.

Several common one-way hash functions are as follows

MD4, MD5

Generates a 128bit hash value, MD stands for Message Digest, which is no longer secure

SHA-1

Generates a hash value of 160 bits, which is also currently unsafe

SHA-2

The hash value of SHA-256, SHA-384, and SHA-512 is 256 bits, 384 bits, and 512 bits respectively. The longer the hash value, the more secure it is

SHA-3

Secure Hash algorithm-3 (SHA3) is a unidirectional Hash function Algorithm released as a new standard to replace sha-1, which has been attacked in theory. Companies and cryptographers around the world submitted a number of candidates for SHA-3, and after five years of selection, the Keccak algorithm was officially adopted as the SHA-3 standard in 2012.

Application of unidirectional hash functions

Prevent data from being tampered with

Prevent file tampering by passing the file through a one-way hash function to get the hash value. Store the hash value in a safe place. If the file is tampered with after a period of time, you can get the latest file and compare the latest hash value with the previous hash value to determine whether the file is tampered with.

The software is tampered with. In order to spread the communication load, some software companies will put their software on mirror sites for users to download. So how can users tell if software downloaded from mirror sites has been maliciously tampered with? Generally, software companies will put the hash value obtained through the hash function on the official website for users to compare. As long as the hash value obtained by the downloaded software is consistent with the official website, it indicates that the downloaded software has not been maliciously tampered. For example, you can click the VNC official website to view the software.

Password encryption

When App login, usually need to check account number and password in the database, however, saved passwords are generally SHA – 2 after the hash function of hash value rather than text passwords, so when logging in need for the user to enter the password hashing algorithm, get the hash value, again with a database to store the hash value of comparison, To determine whether the password is correct.

In addition, due to the irreversibility of the hash function, even if others get the hash value of the password stored in the database by improper means, they cannot get the real password of the user. This greatly increases the security of user data.

A digital signature

In fact, it was impossible to verify the authenticity of the message using the previous symmetric, asymmetric, or hybrid cryptography systems. That is, the receiver of the message cannot determine whether the message was sent by the message sender. It is also possible that someone else sent the message disguised as a message sender. So how do you verify the message’s authenticity? Authentication is performed using a digital signature.

Two kinds of behavior of digital signature

Generate a signature. This is done primarily by the message sender and is generated through a “signature key”
Verify the signature. This is done by the message receiver and authenticated by the Authentication key

So how do you guarantee that the message sender signed the signature himself? The answer is to sign the message using the sender’s own private key. Above, we know that the public key is public and available to all, so in public key cryptography, anyone can use the public key to encrypt.

In digital signatures, anyone can use a public key to verify the signature.

Digital signature versus public key cryptography

A digital signature is simply the reverse of a public key password

The process of digital signature

Common digital signature process

First, the message sender generates a pair of public and private keys.
The message sender sends the public key to the message receiver.
The sender encrypts the message with his private key to obtain the signature information.
The message sender sends the message along with the signature to the message receiver.
The message receiver decrypts the signature information using the public key of the message sender to obtain the message in the signature
The recipient compares the decrypted message with the received message. If the decrypted message is the same with the received message, the signature verification is successful

However, the signature information is obtained by encrypting the original message. If the size of the message is 1 MB, the size of the encrypted signature is also 1 MB, and the message sent to the receiver is 2 MB

Improve the digital signature process

A one-way hash function is used to improve on the previous digital signature process.

First the message sender calculates the hash value of the message using a one-way hash function
The message sender uses its own private key to encrypt the hash value obtained in the first step to generate signature information
The message sender sends the signature information along with the message to the message receiver
The receiver uses the sender’s public key to decrypt the signature information and obtain the decrypted hash value
The message receiver performs a one-way hash of the message to obtain the hash value
The receiver compares the decrypted hash value in the signature with the hash value obtained by the direct hash function. If the hash value is the same, the signature verification succeeds.

Complete signature process

The role of digital signatures

Based on the above points, the functions of digital signature can be summarized:

Verify that the message is complete
Identify whether message content has been tampered with
Prevents the message sender from denying sending the message

Digital signature issues

First of all, what would happen if someone changed the content of the document or the content of the signature? The result is a signature verification failure, proving that the file contents have been tampered with.
Moreover, during the digital signature process, the plaintext of the message is directly sent to the message receiver, which cannot guarantee the security of the message. The function of digital signature is not to guarantee the confidentiality of data, but only to identify whether the message content has been tampered with.

To use digital signatures correctly, it is necessary to verify that the public key of the signature must belong to the real sender. Because a man-in-the-middle attack may occur between the sender and receiver, the specific attack steps are as follows:

The message receiver sends its own public key to the message sender.
The middleman eavesdrops on the contents of the communication and obtains the public key sent out by the receiver of the message.
The middleman intercepts the message receiver’s public key and sends his own public key to the message sender
The sender uses the received public key to encrypt the message and sends the ciphertext to the receiver.
The middleman intercepts the ciphertext, decrypts it with its own private key, and obtains the plaintext message. The message is then encrypted using the public key of the previously intercepted message receiver, and the forged ciphertext is sent to the message receiver.
The message receiver receives the ciphertext, decrypts it with its own private key, and finally gets the plaintext message.

In the above message delivery process, the sender and receiver are unaware of the presence of the middleman, but the message bei has been leaked.

The above communication was attacked by a man-in-the-middle, which would result

The public key was forged by a middleman
Digital signature invalidation

Therefore, the public key must be verified before the signature can be verified. How do you verify the validity of a public key? You need to pass the certificate.

certificate

When we see certificates, we will think of driving licenses, graduation certificates and so on, which are certified by authoritative organizations. In cryptography, the full name of a Certificate is public-key Certificate (PKC). It’s similar to a driver’s license or a student’s card.

The certificate contains personal information such as name, email address, and the person’s public key
And a Certificate Authority (CA) imposes a digital signature

A CA is an individual or organization that can determine that “the public key really belongs to this person” and generate a digital signature

There are international organizations, governmental organizations

There are businesses that make money by providing certification services

Individuals can also set up certification bodies

Use of certificates

There are several steps to use the certificate:

The receiver generates its own key pair. 2. The receiver registers its own public key with the authentication authority
The authentication authority uses its own private key to digitally sign the message receiver’s public key and generate a certificate
The message sender gets the public key (certificate) of the message receiver with the digital signature of the authentication authority from the authentication authority
The sender uses the public key of the authentication authority to verify the digital signature and verify the validity of the public key of the receiver.
The message sender encrypts the message using the message receiver’s public key and sends it to the message receiver.
The message receiver decrypts the ciphertext using its own private key to get the final message

After the authentication process of the authentication authority is added, there is no public key transfer process between the message sender and the message receiver. The message sender obtains the public key of the message receiver from the authentication authority. In this way, the problem of public key forgery caused by the man-in-the-middle attack is eliminated

The procedure for registering and downloading certificates is as follows

The receiver of the message registers the public key with the authentication authority
The authentication authority digitally signs the public key of the message receiver, generates a certificate, and saves the certificate in the repository
The message sender downloads the certificate from the certification authority’s repository
The message sender uses the public key of the authentication authority to verify the certificate and obtain the public key of the message receiver

IOS Reverse Learning eleven (encryption, signature)