The inspiration for this article came from the fact that I needed a very efficient way to encrypt a sensitive string before sharing it. I was just working on this part of the project in C, so writing this part in C was great as it can be considered efficient in its own right. So I started looking for my options and then I sawthis wonderful little AES implementationof coke. No doubt this is what I was looking for as it supports 128/192/256 bit key lengths and CBC mode. The only thing missing was the pkcs7 padding, but we'll see how that was handled later.
tldr;
You can find the code with links to the rest of the necessary files here!
Some things to note about the code:
- The IV must be 16 bytes long.
- The key and string to be encrypted must be a multiple of 16 bytes. (This is where padding comes in)
- I'm terrible at naming names!!!
Let's start with the basics, the two files needed for this are theaes.cit's ataes.h🇧🇷 The following snippet shows the first part of the code:
#define CBC 1#include "aes.h"//initialization Vectoruint8_t iv[] = { 0x75, 0x52, 0x5f, 0x69, 0x6e, 0x74, 0x65, 0x72, 0x65, 0x73, 0x74, 0x69, 0x6e, 0x67, 0x21, 0x21 }; char* report = "my super secret thing that has to stay like this!";char* key = "thisIstheKey";the first line#define blood count 1, it is mandatory to define the mode we want to use. The rest of the lines are pretty self-explanatory, we set the IV to 16 bytes and then set our confidential report to encrypted with a key. Note here that neither the report nor the key are multiples of 16 bytes, even though the way the algorithm works requires it.
int dlen = strlen(report); # the length of the report klen = strlen(key); # the length of the keyint dlenu = dlen;if (dlen % 16) { dlenu += 16 - (dlen % 16); # make the length a multiple of 16 bytes}int klenu = klen;if (klen % 16) { klenu += 16 - (klen % 16); # make the length a multiple of 16 bytes }In the above code thelenit's atweakKeep the length of the report or key. Now that we want both the report and the key to be multiples of 16 bytes, we need to figure out how long each one takes based on its current length. The formula for this is quite simple, take the modulus of length 16 and subtract it from 16. The result is how many more bytes we have to add to the original length. Then theWhat are you doing?it's atDamn itin the above code you keep the updated length of the report and its key which is now a multiple of 16 bytes.
// uint8_t arraysuint8_t hexaarray[dlenu];uint8_t kexarray[klenu] erstellen;// mit zerosmemset( hexaarray, 0, dlenu );memset( kexarray, 0, klenu );// die uint8_t-Arrays für (int i= 0; i<dlen;i++) { hexaarray[i] = (uint8_t)relatório[i];}for (int i=0;i<klen;i++) { kexarray[i] = (uint8_t)chave[i]; }Since tiny AES takes as parameters the string to be encrypted and the key as character arrays, we need to convert them from strings and fill them accordingly. The above snippet takes care of:
- First, create two character arrays of length corresponding to the updated length we saw earlier (in multiples of 16 bytes).
- Second, initialize both character arrays with zeros
- Third, populate the character arrays with the two strings (report/key)
Now at this point we have two character arrays, one contains the data to be encrypted (report) and the other the key.Both character arrays are padded with zeros as we initialized them with zeros!
The goal now is to have adequate filling and that's what we're looking forpadding pkcs7🇧🇷 Before proceeding with the implementation, let's check if there is already - and it is - a fork of the original project foundherecontains the pkcs7 padding we are looking for. The two files we want are thepkcs7_padding.cit's atpkcs7_padding.h.
int reportPad = pkcs7_padding_pad_buffer (Hexarray, dlen, sizeof (Hexarray), 16); int keyPad = pkcs7_padding_pad_buffer (kexarray, klen, sizeof (Kexarray), 16);The function we want to use first is thispkcs7_padding_pad_bufferwhich takes the report (character array) as input along with the original length of the report, fills the character array and returns the number of paddings added.
Now that the report and key are properly filled in and the right size, we are totally ready to start the encryption process!
The next step is to initialize AES.
//Starte die Verschlüsselungsstruktur AES_ctx ctx;AES_init_ctx_iv(&ctx, kexarray, iv); // encryptAES_CBC_encrypt_buffer(&ctx, hexaarray, dlenu);Two are the main things to be noticed here, the first being thisAES_init_ctx_ivwhich initializes AES with the key and IV and the second is the actual encryption process with theAES_CBC_encrypt_bufferFunction that takes the report's char array as a parameter and also saves the encrypted output there.
Before wrapping up and presenting the complete code, let's also check out the decryption process.
//reinicia a infusão!! Importante para o trabalho!AES_ctx_set_iv(&ctx,iv);// Start decryptionAES_CBC_decrypt_buffer(&ctx, hexarray, dlenu);size_t actualDataLength = pkcs7_padding_data_length( hexaarray, dlenu, 16);printf("the decrypted STRING = ");for (i= 0 ; i<actualDataLength;i++){ printf("%02x",hexaarray[i]);}printf("\n");Three important things to note about the above snippet:
- The function
AES_ctx_set_ivresets the IV and takes the ctx as a parameterlindohas the key. - The function
AES_CBC_decrypt_bufferwhich takes the encrypted string as an array of characters and returns the decrypted string in that array of characters. - The function
pkcs7_padding_data_lengthwhich in addition to the padding also returns the actual length of the string, so we know the user data in the decrypted string containing the padding.
As noted by Sarah, the pkcs7_padding_data_length function has a small bug for cases where the report string is exactly N times the 16 bytes. This is fixed by changing line 41 of the filepkcs7_padding.cbe the return valuebuffer size🇧🇷 Note that this is a valid solution as we provide printable characters as a report string that always have a value greater than 16, which is our modulus. You will find the updated filehere.
Now it's time to see the final result of the code.
A tree of used files:
.├── aes.c├── aes.h├── Makefile├── pkcs7_padding.c├── pkcs7_padding.h└── test.cBelow is the content oftests. cFile.
#include <stdio.h>#include <string.h>#include <stdint.h>#define CBC 1#include "aes.h"#include "pkcs7_padding.c" static void test_encrypt_cbc(void);int main(void ){ int exit=0;#if defined(AES256) printf("\nTest AES256\n\n");#elif defined(AES192) printf("\nTest AES192\n\n");#elif defined(AES128 ) printf("\nTest AES128\n\n");#else printf("You must specify a symbol between AES128, AES192 or AES256. Exit"); return 0;#endif test_encrypt_cbc(); return exit;}static void test_encrypt_cbc(void){ //initialization vector uint8_t iv[] = {0x75, 0x52, 0x5f, 0x69, 0x6e, 0x74, 0x65, 0x72, 0x65, 0x73, 0x74, 0x69, 0x6e, 0x67, 0x21 , 0x21 }; uint8_t i; char* report = "my super secret thing that has to stay like this!"; char* key = "thisIstheKey"; int dlen = strlen(report); int clen = strlen(key); printf("A PLAIN TEXT STRING = "); for (i=0; i<dlen;i++){ printf("%c", report[i]); } printf("\n"); //correct report length int dlenu = dlen; if (dlen % 16) { dlenu += 16 - (dlen % 16); printf("The original length of the STRING = %d and the length of the filled STRING = %d\n", dlen, dlenu); } //Correct key length int klenu = klen; if (klen % 16) { klenu += 16 - (klen % 16); printf("The original length of the KEY = %d and the length of the filled KEY = %d\n", klen, klenu); } // create arrays uint8_t uint8_t hexaarray[dlenu]; uint8_t kexarray[klenu]; // initialize with zeros memset( hexaarray, 0, dlenu ); memset(kexarray, 0, klenu); // Fill in the uint8_t arrays for (int i=0;i<dlen;i++) { hexaarray[i] = (uint8_t)report[i]; } for (int i=0;i<klen;i++) {kexarray[i] = (uint8_t)key[i]; } int reportPad = pkcs7_padding_pad_buffer(hexarray, dlen, sizeof(hexarray), 16); int keyPad = pkcs7_padding_pad_buffer(kexarray, klen, sizeof(kexarray), 16); printf("The STRING filled in hexadecimal is = "); for (i=0; i<dlenu;i++){ printf("%02x",hexaarray[i]); } printf("\n"); printf("The key entered in hexadecimal is = "); for (i=0; i<klenu;i++){ printf("%02x",kexarray[i]); } printf("\n"); // If you want to check if the padding is valid int valid = pkcs7_padding_valid( hexaarray, dlen, sizeof(hexaarray), 16 ); int valid2 = pkcs7_padding_valid(kexarray, klen, sizeof(kexarray), 16); printf("The pkcs7 padding is valid, report = %d | key = %d\n", valid, valid2); //Start encryption framework AES_ctx ctx; AES_init_ctx_iv(&ctx, kexarray, iv); // encrypt AES_CBC_encrypt_buffer(&ctx, hexaarray, dlenu); printf("the encrypted STRING = "); for (i=0; i<dlenu;i++){ printf("%02x",hexaarray[i]); } printf("\n"); // restart the infusion!! important to work! AES_ctx_set_iv(&ctx,iv); // start decryption AES_CBC_decrypt_buffer(&ctx, hexaarray, dlenu); size_t actualDataLength = pkcs7_padding_data_length(hexaarray, dlenu, 16); printf("The actual data length (no padding) = %ld\n", actualDataLength); printf("the STRING decrypted in hex = "); for (i=0; i<actualDataLength;i++){ printf("%02x",hexaarray[i]); } printf("\n");}An example of the output is shown below
Teste AES128THE PLAIN TEXT STRING = meine supergeheime Sache, die so bleiben muss! Die Originallänge des STRING = 52 und die Länge des aufgefüllten STRING = 64Die Originallänge des KEY = 12 und die Länge des aufgefüllten KEY = 16The padded STRING in hex is = 6d7920737570657220736563726574207468696e672074686174206e6565647320746f2072656d61696e207468617420776179210c0c0c0c0c0c0c0c0c0c0c0cThe padded key in hex is = 7468697349737468654b657904040404Is the pkcs7 padding valid report = 1 | key = 1the encrypted STRING = cdc4244c4828ed73e78c75a5db94d577d1f69472140204d7a6ce89f6f1d42d2962031470e3dd3d4b99a735504b4b9d8a277ba6bda54a06c2291380fae26f0fd0The actual data length (without the padding) = 52the decrypted STRING in hex = 6d7920737570657220736563726574207468696e672074686174206e6565647320746f2072656d61696e20746861742077617921
The Makefile used to compile was that of the original Github project foundhere🇧🇷 just walkI doand then run./test.elf.
If you want a different key length, you can do this by commenting out the relevant line in the aes.h file.
Conclusion
The result of this AES implementation looked super efficient and works like a charm. In the project I used it on, it serves its purpose perfectly, so I thought it was something to share.
disclaimer: All credit for implementing AES or PKCS7 padding goes to the upstream authors named in the article. The cryptographic security provided by the above must be validated in any case.
Updated to fix a bug in pkcs7_padding.c reported by Sarah - Thanks Sarah!
FAQs
How do you use PKCS 7 Padding? ›
The rules for PKCS padding are very simple: Padding bytes are always added to the clear text before it is encrypted. Each padding byte has a value equal to the total number of padding bytes that are added. For example, if 6 padding bytes must be added, each of those bytes will have the value 0x06.
Does AES-CBC require Padding? ›The AES uses a block size of sixteen octets (128 bits). Padding is required by the AES to maintain a 16-octet (128-bit) blocksize. Padding MUST be added, as specified in [ESP], such that the data to be encrypted (which includes the ESP Pad Length and Next Header fields) has a length that is a multiple of 16 octets.
What is PKCS 7 Padding? ›PKCS7 padding is a generalization of PKCS5 padding (also known as standard padding). PKCS7 padding works by appending N bytes with the value of chr(N) , where N is the number of bytes required to make the final block of data the same size as the block size.
Can AES work in CBC mode? ›Overview. The Cipher Block Chaining (CBC) mode is a typical block cipher mode of operation using block cipher algorithm. In this version, we provide Data Encryption Standard (DES) and Advanced Encryption Standard (AES) processing ability, the cipherkey length for DES should be 64 bits, and 128/192/256 bits for AES.
What is PKCS 7 format? ›The PKCS #7 binary certificate package, based on the Public Key Cryptographic Standards (PKCS) published by RSA Laboratories, is a package used to distribute one or more certificates, or an entire chain of certificates such as the chain depicted in Figure 1.
Does CBC use padding? ›Some block cipher modes (CBC and PCBC essentially) for symmetric-key encryption algorithms require plain text input that is a multiple of the block size, so messages may have to be padded to bring them to this length.
What is AES-CBC padding? ›AES-CBC with PKCS padding, denoted CKM_AES_CBC_PAD, is a mechanism for single- and multiple-part encryption and decryption; key wrapping; and key unwrapping, based on NIST's Advanced Encryption Standard; cipher-block chaining mode; and the block cipher padding method detailed in PKCS #7.
Is AES-CBC PKCS5Padding weak? ›In summary of kelalaka's answer: yes AES/CBC/PKCS5Padding can create a vulnerability to Padding Oracle attack. The modern, safe option is authenticated encryption, e.g. AES/GCM/NoPadding in modern javax.
Does AES use padding? ›AES uses 128-bits (16 bytes), and DES uses 64-bit blocks (8 bytes). The main padding methods are: CMS (Cryptographic Message Syntax). This pads with the same value as the number of padding bytes.
How do I open a PKCS#7 file? ›The PKCS7 File Extension has one primary file type, Cryptographic Message Syntax Standard format, and can be opened with OpenSSL released by Open Source.
Is PKCS 7 the same as p7b? ›
P7B/PKCS#7 Format
p7b or . p7c as the file extension. The thing that separates PKCS#7 formatted certificates is that only certificates can be stored in this format, not private keys. In other words, a P7B file will only consist of certificates and chain certificates.
PKCS#12 (also known as PKCS12 or PFX) is a binary format for storing a certificate chain and private key in a single, encryptable file. PKCS#12 files are commonly used to import and export certificates and private keys on Windows and macOS computers, and usually have the filename extensions .
How do I decrypt AES in CBC? ›- Instantiate the CBC block cipher class with the AES implementation class.
- Initialize it with the key and Initialization Vector (IV) for decryption.
- Process each block of the ciphertext being decrypted.
AES-CBC. First historic block cipher for AES. CBC mode is insecure and must not be used. It's been progressively deprecated and removed from SSL libraries.
How encrypt AES-CBC? ›- Enter text to be Encrypted.
- Select Cipher Mode of Encryption. ECB. CBC.
- Key Size in Bits. 128. 192. 256.
- Enter IV (Optional)
- Enter Secret Key.
Also PKCS#7 format can be used to store one or more certificates without private keys (private keys can be put as a data payload and encrypted this way). PKCS#10 defines format for certificate requests. PKCS#12 provides a container for one or several certificates with private keys.
How to convert PKCS7 to PEM? ›- Obtain OpenSSL.
- Note: In order for OpenSSL software to be successfully installed on a computer system, you must have local system administrator privilege on the computer.
- Convert PKCS #7 (.p7b) to PEM using OpenSSL.
- openssl pkcs7 -print_certs -in certificate.p7b -out certificate.cer.
- Copy your PKCS7.p7b file as PKCS7.crt.
- Open this file with your editor and add these lines. —–BEGIN CERTIFICATE—– ...
- openssl pkcs7 -print_certs -in PKCS7.crt -out certificate.cer.
- openssl pkcs12 -export -in certificate.cer -inkey private.key -out PKCS7.pfx -certfile bundle.cer.
deterministic modes require at least one padding bit, but usually implementations only handle bytes, so the padding would consist of 1.. N/8 bytes. Most other modes (CTR, GCM, OFB, CFB) are streaming modes and do not require padding to be able to encrypt messages of any size.
Why the padding schemes are used in AES 128 CBC? ›Padding is a way to encrypt messages of a size that the block cipher would not be able to decrypt otherwise; it is a convention between whoever encrypts and whoever decrypts.
How padding is done in hashing? ›
For instance, in most hash functions, we are using the following padding rule: appending of a bit 1, followed by a number of 0 such that the total length is a multiple of the block length.
What is AES CBC 256? ›What is 256-bit AES encryption? 256-bit AES encryption refers to the process of concealing plaintext data using the AES algorithm and an AES key length of 256 bits. In addition, 256 bits is the largest AES key length size, as well as its most mathematically complex. It is also the most difficult to crack.
What does AES CBC stand for? ›Share to Facebook Share to Twitter. Abbreviation(s) and Synonym(s): Advanced Encryption Standard-Cipher Block Chaining show sources. NIST SP 800-77 Rev.
What are padding bits? ›Bit padding is the addition of one or more extra bits to a transmission or storage unit to make it conform to a standard size. Some sources identify bit padding as a type of bit stuffing.
Can AES 256 CBC be cracked? ›AES 256 is virtually impenetrable using brute-force methods. While a 56-bit DES key can be cracked in less than a day, AES would take billions of years to break using current computing technology. Hackers would be foolish to even attempt this type of attack. Nevertheless, no encryption system is entirely secure.
Why is it better to use CBC encryption mode rather than ECB encryption mode? ›ECB mode's issues arise from the fact that each block of the plaintext is encrypted completely independently. CBC mode eliminates this problem by carrying information from the encryption or decryption of one block to the next.
Why is CBC mode insecure? ›The problem with CBC mode is that the decryption of blocks is dependant on the previous ciphertext block. This means attackers can manipulate the decryption of a block by tampering with the previous block using the commutative property of XOR.
What is the best mode for AES? ›AES-GCM instead of AES-CBC
Both the AES-CBC and AES-GCM are able to secure your valuable data with a good implementation. but to prevent complex CBC attacks such as Chosen Plaintext Attack(CPA) and Chosen Ciphertext Attack(CCA) it is necessary to use Authenticated Encryption. So the best option is for that is GCM.
AES CBC decryption will be much faster using AES-NI pipelining.
Where is padding applied to an element? ›An element's padding area is the space between its content and its border. Note: Padding creates extra space within an element. In contrast, margin creates extra space around an element.
What is PKCS 7 detached signature? ›
Detached Signatures
PKCS#7: Includes the signature and certificate without the signed data. RNIF1. 1: Uses PKCS#7 and a detached format. S/MIME2: May include a MIME multipart message consisting of the original data in one segment and a binary format signature or a base64-encoded signature in a second segment.
- Open the openssl command line to create and initialize a new PKCS12 key store.
- Create a new self-signed certificate: openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 -nodes -subj /CN=MyHost.com.
- Create a PKCS12 file :
- Step 1 - Open Certificate Pick Up Email on Android Device. ...
- Step 2 - Enter Certificate Pick-Up Password. ...
- Step 3 - Create a PKCS#12 Passphrase. ...
- Step 4 - Download the Certificate onto Your Device. ...
- Step 5 – Name Your Certificate.
...
DER system certificate by following these steps:
- Install the exported . ...
- Click on the Details tab and choose Copy to File...
- A Windows Certificate Export Wizard will come up. ...
- Select Cryptographic Message Syntax Standard - PKCS #7 Certificates (.
- Save your PKCS#7 file onto your computer in a location you will be able to locate easily later.
- Navigate into IIS7. ...
- Select Complete Certificate Request... from the right-hand menu.
- Browse to the location of your . ...
- Enter the common name or the domain of the certificate under Friendly Name.
- Click OK.
The difference between the PKCS#5 and PKCS#7 padding mechanisms is the block size; PKCS#5 padding is defined for 8-byte block sizes, PKCS#7 padding would work for any block size from 1 to 255 bytes. So fundamentally PKCS#5 padding is a subset of PKCS#7 padding for 8 byte block sizes.
Does pkcs12 contain private key? ›A PKCS#12 or . pfx file is a file which contains both private key and X. 509 certificate, ready to be installed by the customer into servers such as IIS, Tomkat or Exchange.
What is PKCS explain with an example? ›Public-Key Cryptography Standards (PKCS) are a set of standard protocols, numbered from 1 to 15. These standards were developed to enable secure information exchange on the internet by using a public key infrastructure (PKI).
How to convert PFX certificate to pkcs12? ›- Copy the CRT and KEY files to the OpenSSL installation directory. ...
- Open a Windows command prompt and, if necessary, navigate to the OpenSSL installation directory.
- Generate a PKCS#12 (PFX) keystore file from the certificate file and your private key. ...
- Type an export password to protect the PKCS#12 (PFX) file.
- No, you cannot decrypt without knowing the key. What would the point of encryption be if anyone could decrypt the message without even having the key? ...
- This is not possible, without knowing the key and iv.
How do I decrypt an AES encrypted file? ›
- Locate the file that needs to be decrypted. The encrypted file will have an “. ...
- Double click on the file, or right click on the file and select AES Decrypt.
- You will be prompted to enter a password. This is the password that was set when the file was encrypted. ...
- Enter the password and click OK.
- For 128-bit key: openssl enc -aes-128-cbc -k secret -P -md sha1.
- For 192-bit key: openssl enc -aes-192-cbc -k secret -P -md sha1.
- For 256-bit key: openssl enc -aes-256-cbc -k secret -P -md sha1. “secret” is a passphrase for generating the key. The output from the command is similar to:
Websites that support SSLv3 and CBC-mode ciphers are potentially vulnerable to an active MITM (Man-in-the-middle) attack. This attack, called POODLE, is similar to the BEAST attack and also allows a network attacker to extract the plaintext of targeted parts of an SSL connection, usually cookie data.
Why are CBC ciphers weak? ›"Due to the difficulties in implementing CBC cipher suites, and the numerous known exploits against bugs in specific implementations, Qualys SSL Labs began marking all CBC cipher suites as WEAK in May 2019.
Why is TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 considered weak? ›TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 and TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 may show up as weak when you performed a SSL report test. This is due to known attacks toward OpenSSL implementation. Dataverse uses Windows implementation that is not based on OpenSSL and therefore is not vulnerable.
Is AES in CBC mode secure? ›Although AES-CBC with HMAC authentication is generally considered secure, CBC is potentially vulnerable to padding attacks, such as POODLE.
Does AES support CBC mode? ›CBC (short for cipher-block chaining) is a AES block cipher mode that trumps the ECB mode in hiding away patterns in the plaintext. CBC mode achieves this by XOR-ing the first plaintext block (B1) with an initialization vector before encrypting it.
How do I enable AES encryption? ›- Start the wsadmin scripting tool.
- Generate the properties file and the AES key file that are needed for AES encryption, and save the configuration. Generate the properties file and, if the AES key file was not generated by the aesKeystore parameter, the AES key file. ...
- Exit the wsadmin tool.
- Restart the server.
Also PKCS#7 format can be used to store one or more certificates without private keys (private keys can be put as a data payload and encrypted this way). PKCS#10 defines format for certificate requests. PKCS#12 provides a container for one or several certificates with private keys.
What is padding scheme in RSA? ›For example RSA Encryption padding is randomized, ensuring that the same message encrypted multiple times looks different each time. It also avoids other weaknesses, such as encrypting the same message using different RSA keys leaking the message, or an attacker creating messages derived from some other ciphertexts.
What is the difference between pkcs7 and pkcs5? ›
The difference between the PKCS#5 and PKCS#7 padding mechanisms is the block size; PKCS#5 padding is defined for 8-byte block sizes, PKCS#7 padding would work for any block size from 1 to 255 bytes.
How do I open PKCS7 file in Windows? ›- Open Windows Explorer.
- Locate the PKCS #7 file that contains the certificates you want to view.
- In the details pane, double-click the PKCS #7 file.
- In the console tree, double-click the folder containing the PKCS #7 file, and then click Certificates.
...
DER system certificate by following these steps:
- Install the exported . ...
- Click on the Details tab and choose Copy to File...
- A Windows Certificate Export Wizard will come up. ...
- Select Cryptographic Message Syntax Standard - PKCS #7 Certificates (.
CSS Demo: padding
An element's padding area is the space between its content and its border. Note: Padding creates extra space within an element. In contrast, margin creates extra space around an element.
Definition and Usage. An element's padding is the space between its content and its border. The padding property is a shorthand property for: padding-top. padding-right.
Which is better RSA or AES? ›The Advance Encryption Standard (AES) cipher text method is a more accurate and elegant cryptographic method. According to testing results and the text files used, it has been concluded that the AES algorithm outperforms the Data Encryption Standard (DES) and RSA algorithms [6,7].
How do I verify my PKCS7 signature? ›- Connect to the instance.
- Create a new file named certificate. ...
- Extract the certificate from the certificate file and store it in a variable named $Store . ...
- Verify the signature.
After you receive the certificate from the CA, double-click on the certificate to open it. Locate the path of the certificate on your computer and double-click on the certificate again to open it.
Is PKCS 7 the same as P7B? ›P7B/PKCS#7 Format
p7b or . p7c as the file extension. The thing that separates PKCS#7 formatted certificates is that only certificates can be stored in this format, not private keys. In other words, a P7B file will only consist of certificates and chain certificates.
Padding is used in a block cipher where we fill up the blocks with padding bytes. AES uses 128-bits (16 bytes), and DES uses 64-bit blocks (8 bytes). The main padding methods are: CMS (Cryptographic Message Syntax).
How does PKCS work? ›
PKCS #5: Password-based Cryptography Standard.
This standard applies pseudo random functions -- hash-based message authentication code, cipher or hash -- to the input password, along with a salt value, to produce a derived key that can be used as a cryptographic key.