What is symmetric encryption?

Using the encryption method of a single-key cryptosystem, the same key can be used for encryption and decryption of information at the same time. This encryption method is called symmetric encryption, also known as single-key encryption.

Table of contents

1 Definition

2 Working process

3 Commonly used algorithms

4 Advantages and disadvantages

Definition

An encryption algorithm that requires the use of the same key for encryption and decryption. Due to its speed, symmetric encryption is often used when the sender of a message needs to encrypt large amounts of data. Symmetric encryption is also called key encryption.

The so-called symmetry means that both parties using this encryption method use the same key to encrypt and decrypt. Keys are instructions that control the encryption and decryption process. An algorithm is a set of rules that dictate how encryption and decryption are performed.

Therefore[1] The security of encryption not only depends on the encryption algorithm itself, but the security of key management is even more important. Because the same key is used for encryption and decryption, how to safely transfer the key to the decryptor becomes a problem that must be solved.

Working process

The following is an example to briefly explain the working process of symmetric encryption. A and B are business partners who live in different cities. Due to business needs, they often mailed important goods to each other. In order to ensure the safety of the goods, they agreed to make a safe box and put the items in it. They created two identical keys and kept them separately so that they could use the key to open the safe when receiving the package and use the key to lock the safe before mailing the goods.

The above is a traditional way to safely deliver important resources to the destination. As long as A and B keep the key carefully, even if someone gets the safe, they will not be able to open it. This idea is used in information encryption for modern computer communications. In symmetric encryption, the data sender processes the plaintext (original data) and the encryption key together with a special encryption algorithm, turning it into a complex encrypted ciphertext and sending it out. After the receiver receives the ciphertext, if it wants to decipher the original text, it needs to use the encryption key and the inverse algorithm of the same algorithm to decrypt the ciphertext in order to restore it to readable plaintext. In a symmetric encryption algorithm, only one key is used, and both the sender and the receiver use this key to encrypt and decrypt data.

openssl enc -ciphername [-in filename] [-out filename] [-pass arg] [-e] [-d] [-a/-base64] [-k password] [- S salt] [-salt] [-md] [-p/-P]

Option description:

-ciphername: Specify the symmetric encryption algorithm (such as des3), which can be directly independent of enc Use, like openssl des3 or openssl enc -des3. It is recommended to use it after enc, so that it does not depend on the hardware.

-in filename: input file, the default is stdin if not specified

-out filename: output file, the default is stdout# if not specified

##-e: Encrypt the input file. If not specified, this option will be used by default.

-d: Decrypt the input file. Decryption will only be performed if this option is explicitly specified.

-pass: Pass the plaintext password for encryption and decryption. If the public key or private key file used when verifying the signature is encrypted, the password needs to be passed for decryption. For the password format, see "

openssl password format"

-k : has been replaced by "-pass" and is still retained for compatibility with older versions of openssl

-base64: Base64 encoding or decryption after encryption and before decryption. If not specified, the default is binary. Note that encoding is not part of encryption and decryption, but the "organizing" of the data format before and after encryption and decryption

-a: Equivalent to -base64

-salt: Use salt for one-way encryption Complicates the results of one-way encryption. This is the default option and uses a random salt value.

-S salt: Do not use a random salt value, but a custom salt value, but it can only be in the hexadecimal range. A combination of characters, that is, any one or more combinations of "0-9a-fA-F"

-p: Print the salt value, key value and IV initialization vector value during encryption and decryption (also complex encryption) A way), the decryption result is also output when decrypting, see the example below

-P: has the same effect as the -p option, but exits the tool directly when printing, without performing encryption or decryption operations

-md: Specify one-way encryption algorithm, default md5. This algorithm is used to encrypt the key part, see the analysis below.

The supported one-way encryption algorithms are:

-md4            to use the md4 message digest algorithm-md5            to use the md5 message digest algorithm-ripemd160      to use the ripemd160 message digest algorithm-sha            to use the sha message digest algorithm-sha1           to use the sha1 message digest algorithm-sha224         to use the sha224 message digest algorithm-sha256         to use the sha256 message digest algorithm-sha384         to use the sha384 message digest algorithm-sha512         to use the sha512 message digest algorithm-whirlpool      to use the whirlpool message digest algorithm

The supported symmetric encryption algorithms are:

-aes-128-cbc               -aes-128-cbc-hmac-sha1     -aes-128-cfb             
-aes-128-cfb1              -aes-128-cfb8              -aes-128-ctr             
-aes-128-ecb               -aes-128-gcm               -aes-128-ofb             
-aes-128-xts               -aes-192-cbc               -aes-192-cfb             
-aes-192-cfb1              -aes-192-cfb8              -aes-192-ctr             
-aes-192-ecb               -aes-192-gcm               -aes-192-ofb             
-aes-256-cbc               -aes-256-cbc-hmac-sha1     -aes-256-cfb             
-aes-256-cfb1              -aes-256-cfb8              -aes-256-ctr             
-aes-256-ecb               -aes-256-gcm               -aes-256-ofb             
-aes-256-xts               -aes128                    -aes192                  
-aes256                    -bf                        -bf-cbc                  
-bf-cfb                    -bf-ecb                    -bf-ofb                  
-blowfish                  -camellia-128-cbc          -camellia-128-cfb        
-camellia-128-cfb1         -camellia-128-cfb8         -camellia-128-ecb        
-camellia-128-ofb          -camellia-192-cbc          -camellia-192-cfb        
-camellia-192-cfb1         -camellia-192-cfb8         -camellia-192-ecb        
-camellia-192-ofb          -camellia-256-cbc          -camellia-256-cfb        
-camellia-256-cfb1         -camellia-256-cfb8         -camellia-256-ecb        
-camellia-256-ofb          -camellia128               -camellia192             
-camellia256               -cast                      -cast-cbc                
-cast5-cbc                 -cast5-cfb                 -cast5-ecb               
-cast5-ofb                 -des                       -des-cbc                 
-des-cfb                   -des-cfb1                  -des-cfb8                
-des-ecb                   -des-ede                   -des-ede-cbc             
-des-ede-cfb               -des-ede-ofb               -des-ede3                
-des-ede3-cbc              -des-ede3-cfb              -des-ede3-cfb1           
-des-ede3-cfb8             -des-ede3-ofb              -des-ofb                
-des3                      -desx                      -desx-cbc                
-id-aes128-GCM             -id-aes128-wrap            -id-aes128-wrap-pad      
-id-aes192-GCM             -id-aes192-wrap            -id-aes192-wrap-pad      
-id-aes256-GCM             -id-aes256-wrap            -id-aes256-wrap-pad      
-id-smime-alg-CMS3DESwrap  -idea                      -idea-cbc                 
-idea-cfb                  -idea-ecb                  -idea-ofb                
-rc2                       -rc2-40-cbc                -rc2-64-cbc              
-rc2-cbc                   -rc2-cfb                   -rc2-ecb                 
-rc2-ofb                   -rc4                       -rc4-40                  -rc4-hmac-md5              -seed                      -seed-cbc                
-seed-cfb                  -seed-ecb                  -seed-ofb

在给出openssl enc命令用法示例之前，先解释下对称加密和解密的原理和过程。

对称加解密时，它们使用的密码是完全相同的，例如"123456"，但这是密码，且是明文密码，非常不安全，所以应该对此简单密码进行复杂化。最直接的方法是使用单向加密计算出明文密码的hash值，单向加密后新生成的密码已经比较安全(称之为密钥比较好)，可以作为对称加密时的对称密钥。另外，由于同一单向加密算法对相同明文密码的计算结果是完全一致的，这样解密时使用相同的单向加密算法就能计算出完全相同的密钥，也就是解密时的对称密钥。如果想要更安全，还可以在对称加密后对加密文件进行重新编码，如使用"base64"、二进制或hex编码方式进行编码，但对应的在解密前就需要先解码，解码后才能解密。

所以，将对称加、解密的机制简单概括如下：

对称加密机制：根据指定的单向加密算法，对输入的明文密码进行单向加密(默认是md5)，得到固定长度的加密密钥，即对称密钥，再根据指定的对称加密算法，使用对称密钥加密文件，最后重新编码加密后的文件。即单向加密明文密码结果作为对称密钥、使用对称密钥加密文件、对文件重新编码。

对称解密机制：先解码文件，再根据单向加密算法对解密时输入的明文密码计算得到对称密钥，依此对称密钥对称解密解码后的文件。

因此，解密过程中使用的解码方式、单向加密和对称加密算法都必须一致，且输入的密码必须是正确密码。但需要注意的一点是，解密时可以不指定salt，因为加密时使用的salt会记录下来，解密时可以读取该salt。

如下图所示，分别是加密和解密过程示意图。

示例：

以加密/etc/fstab的备份文件/tmp/test.txt为例。

(1).首先测试openssl enc的编码功能。由于未指定密码选项"-k"或"-pass"，所以仅仅只进行编码而不进行加密，因此也不会提示输入密码。

[root@xuexi tmp]# openssl enc -a -in test.txt -out test_base64.txt

[root@xuexi tmp]# cat test_base64.txt
CiMKIyAvZXRjL2ZzdGFiCiMgQ3JlYXRlZCBieSBhbmFjb25kYSBvbiBUaHUgTWF5
IDExIDA0OjE3OjQ0IDIwMTcKIwojIEFjY2Vzc2libGUgZmlsZXN5c3RlbXMsIGJ5
IHJlZmVyZW5jZSwgYXJlIG1haW50YWluZWQgdW5kZXIgJy9kZXYvZGlzaycKIyBT
ZWUgbWFuIHBhZ2VzIGZzdGFiKDUpLCBmaW5kZnMoOCksIG1vdW50KDgpIGFuZC9v
ciBibGtpZCg4KSBmb3IgbW9yZSBpbmZvCiMKVVVJRD1iMmE3MGZhZi1hZWE0LTRk
OGUtOGJlOC1jNzEwOWFjOWM4YjggLyAgICAgICAgICAgICAgICAgICAgICAgeGZz
ICAgICBkZWZhdWx0cyAgICAgICAgMCAwClVVSUQ9MzY3ZDZhNzctMDMzYi00MDM3
LWJiY2ItNDE2NzA1ZWFkMDk1IC9ib290ICAgICAgICAgICAgICAgICAgIHhmcyAg
ICAgZGVmYXVsdHMgICAgICAgIDAgMApVVUlEPWQ1MDUxMTNjLWRhYTYtNGMxNy04
YjAzLWIzNTUxY2VkMjMwNSBzd2FwICAgICAgICAgICAgICAgICAgICBzd2FwICAg
IGRlZmF1bHRzICAgICAgICAwIDAK

再以base64格式进行解码。

[root@xuexi tmp]# openssl enc -a -d -in test_base64.txt              
 #
# /etc/fstab
# Created by anaconda on Thu May 11 04:17:44 2017#
# Accessible filesystems, by reference, are maintained under '/dev/disk'# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info#
UUID=b2a70faf-aea4-4d8e-8be8-c7109ac9c8b8 /                       xfs     defaults        0 0UUID=367d6a77-033b-4037-bbcb-416705ead095 /boot                   xfs     defaults        0 0UUID=d505113c-daa6-4c17-8b03-b3551ced2305 swap                    swap    defaults        0 0

实际上，上述编码和解码的过程严格地说也是对称加密和解密，因为openssl enc默认会带上加密选项"-e"，只不过因为没有指定输入密码选项，使用的加密密码为空而已，且单向加密算法使用的也是默认值。解密时也一样。

(2).测试使用des3对称加密算法加密test.txt文件。

[root@xuexi tmp]# openssl enc -a -des3 -in test.txt -out test.1 -pass pass:123456 -md md5

加密后，查看加密后文件test.1的结果。

[root@xuexi tmp]# cat test.1U2FsdGVkX1+c/d4NsXnY6Pd7rcZjGSsMRJWQOP0s5sxH6aLE5iCYjKEAbGac//iRwkUUh6a57OpUA3+OOCKB4z+IxBcKo67BUDGR9vYeCfkobH9F+mSfVzZbXBrJmxwf
921tJ+8K+yKB6DjJfufpW+DWXmH8MFyvK60wnYHsfUQOp81EvaUtEfqEKIS8hgg7
4NTOyww+/VMDdc2wmkf08XNQUPlVtLaSx3vuBisxRdu8raiKWGGOB7qCwELCxDqu
NaRCIh0VjjffGohAOMMsAQ2kFCDUKx0Z4Df5fvifhPXoHfsj2lI216BPG5Cy88K2
KV78DoBm4pnMAymo/HRRF95LjvWYZIN88hIVN67u2j9zqSGeuyJakMyDVhYYmrHl
sMr2YTbTwus2DiO6qAzt/0a9nocTVKfGR81Xsh0a0ZudjtrMl5H36YJawpldvUCa
DzXPsbpQrp0VGi2HvJ4EVKKEx2uh8XYWmJ4ytj1s1wtCR6wQhmERtInGwULWTyI+agXStSB5XzsvAJRJvexsaNycj5lAoQ8O6YXEj7B0inB7nBQTFbwkXyvJqXpr1179
i67leYc59OvlhRMA+GLW4g/Mg5dN5SBmgt1ChOJs4887zAUyLYrLvR4zDK6IQN/M
P6F15c9V+m9pw2t32sUQQmYrYqOV/AQf0t0EwvA0Myjmfqtvmp555Q==

解密文件test.1。

[root@xuexi tmp]# openssl enc -a -des3 -d -in test.1 -out test.2 -pass pass:123456 -md md5 [root@xuexi tmp]# cat test.2
 #
# /etc/fstab
# Created by anaconda on Thu May 11 04:17:44 2017#
# Accessible filesystems, by reference, are maintained under '/dev/disk'# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info#
UUID=b2a70faf-aea4-4d8e-8be8-c7109ac9c8b8 /                       xfs     defaults        0 0UUID=367d6a77-033b-4037-bbcb-416705ead095 /boot                   xfs     defaults        0 0UUID=d505113c-daa6-4c17-8b03-b3551ced2305 swap                    swap    defaults        0 0

(3).加密时带上点盐salt。其实不写时默认就已经加入了，只不过是加入随机盐值。使用-S可以指定明确要使用的盐的值。但是盐的值只能是16进制范围内字符的组合，即"0-9a-fA-F"的任意一个或多个组合。

[root@xuexi tmp]# openssl enc -a -des3 -S 'Fabc' -in test.txt -out test.1 -pass pass:123456 -md md5

解密。解密时不用指定salt值，即使指定了也不会影响解密结果。      

[root@xuexi tmp]# openssl enc -a -des3 -d -in test.1 -pass pass:123456 -md md5               
 #
# /etc/fstab
# Created by anaconda on Thu May 11 04:17:44 2017#
# Accessible filesystems, by reference, are maintained under '/dev/disk'# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info#
UUID=b2a70faf-aea4-4d8e-8be8-c7109ac9c8b8 /                       xfs     defaults        0 0UUID=367d6a77-033b-4037-bbcb-416705ead095 /boot                   xfs     defaults        0 0UUID=d505113c-daa6-4c17-8b03-b3551ced2305 swap                    swap    defaults        0 0

[root@xuexi tmp]# openssl enc -a -des3 -d -S 'Fabcxdasd' -in test.1 -pass pass:123456 -md md5
 #
# /etc/fstab
# Created by anaconda on Thu May 11 04:17:44 2017#
# Accessible filesystems, by reference, are maintained under '/dev/disk'# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info#
UUID=b2a70faf-aea4-4d8e-8be8-c7109ac9c8b8 /                       xfs     defaults        0 0UUID=367d6a77-033b-4037-bbcb-416705ead095 /boot                   xfs     defaults        0 0UUID=d505113c-daa6-4c17-8b03-b3551ced2305 swap                    swap    defaults        0 0

(4).在测试下"-p"和"-P"选项的输出功能。小写字母p不仅输出密钥算法结果，还输出加解密的内容，而大写字母P则只输出密钥算法结果。

加密时的情况。

[root@xuexi tmp]# openssl enc -a -des3 -S 'Fabc' -in test.txt -out test.1 -pass pass:123456 -md md5 -psalt=FABC000000000000
key=885FC58E6C822AEFC8032B4B98FA0355F8482BD654739F3D
iv =5128FDED01EE1499

其中key就是单向加密明文密码后得到的对称密钥，iv是密码运算时使用的向量值。

再看解密时的情况，此处加上了salt。

[root@xuexi tmp]# openssl enc -a -des3 -d -S 'Fabc' -in test.1 -pass pass:123456 -md md5 -Psalt=FABC000000000000
key=885FC58E6C822AEFC8032B4B98FA0355F8482BD654739F3D
iv =5128FDED01EE1499

若解密时不指定salt，或者随意指定salt，结果如下。

[root@xuexi tmp]# openssl enc -a -des3 -d -in test.1 -pass pass:123456 -md md5 -P         salt=FABC000000000000
key=885FC58E6C822AEFC8032B4B98FA0355F8482BD654739F3D
iv =5128FDED01EE1499

[root@xuexi tmp]# openssl enc -a -des3 -S 'FabM' -d -in test.1 -pass pass:123456 -md md5 -Psalt=FABC000000000000
key=885FC58E6C822AEFC8032B4B98FA0355F8482BD654739F3D
iv =5128FDED01EE1499

可见，解密时，只要指定和加密时相同编码格式和单向加密算法，密钥的结果就是一样的，且解密时明确指定salt是无意义的，因为它可以读取到加密时使用的salt。

甚至，解密时指定不同的对称加密算法，密钥结果也是一样的。

[root@xuexi tmp]# openssl enc -a -desx -d -in test.1 -pass pass:123456 -md md5 -p salt=FABC000000000000
key=885FC58E6C822AEFC8032B4B98FA0355F8482BD654739F3D
iv =5128FDED01EE1499

由此，能推理出对称加密时使用的对称密钥和对称算法是毫无关系的。

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