What are the differences between .pem, .cer and .der?

Question

What are the differences between .pem, .cer and .der?

As far as I know, .cer contains public key. Are there any open frameworks that I can use to encrypt my data using this public key?

Cupola

I think they're all just file formats which can store any key; there are tools which can convert between them freely. – Grange

@Grange Right you are, but for .cer to .der the cp command may do :) – Thitherto

Answer 1

.pem, .cer and .der are all file extensions for files that may contain a X.509 v3 certificate.

The .der extension

DER is the method of encoding the data that makes up the certificate. DER itself could represent any kind of data, but usually it describes an encoded certificate or a CMS container. CMS is described in PKCS#7 (often stored as .p7) and stands for Cryptographic Message Syntax, which can be used to protect messages and to store certificates to allow a receiver to build a trust path to a certificate in the receivers' trust store.

The structure of a certificate is described using the ASN.1 data representation language. BER and DER are binary encoding methods for data described by ASN.1.

The .pem extension

PEM is a method of encoding binary data as a string (also known as ASCII armor). It contains a header and a footer line (specifying the type of data that is encoded and showing begin/end if the data is chained together) and the data in the middle is the base 64 data. In the case that it encodes a certificate it would simply contain the base 64 encoding of the DER certificate. PEM stands for Privacy Enhanced Mail; mail cannot contain un-encoded binary values such as DER directly.

PEM may also encode / protect other kinds of data that is related to certificates such as public / private keys, certificate requests, etc. If the contents are a common X509v3 certificate then the PEM is encoded as:

-----BEGIN CERTIFICATE-----
... base 64 encoding of the DER encoded certificate
    with line endings and padding with equals signs ...
-----END CERTIFICATE-----

Note that a PEM file may also contain a complete certificate chain, where the chain starts with the leaf / end certificate of the service, followed by the certificate that signed it, usually up to but not including the trusted root certificate. So if you're missing certificates you may want to take a look behind the first one.

The .cer or .crt extension

.cer just stands for certificate. It is normally DER encoded data, but Windows may also accept PEM encoded data. You need to take a look at the content (e.g. using the file utility on posix systems) to see what is within the file to be 100% sure.

Other OpenSSL formats

Take a look at this answer for a more extensive list of what is supported by OpenSSL.

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To use the public key contained in the certificate (and signed by the signature in the certificate) you should use any library that parses X.509 certificates and performs RSA encryption. You could use a tool that detects/handles PEM encoding or you could first convert the certificate to DER by stripping off the PEM encoding.

The OpenSSL command line contains lots of options to convert between PEM and DER, print out high level certificate information or parse the ASN.1 to get a low level view of what is in there.

Details

Like most ASN.1 structures, DER encoded certificate always starts off with a byte 30 which is the tag encoding of an ASN.1 SEQUENCE. If you're seeing a lot of repetition in the file then this is OK; it is just the structure that is strictly defined.

Likewise, the base 64 within a PEM encoded file always starts off with the letter M as an ASN.1 SEQUENCE starts off with a byte 30, so the first 6 bits are 001100, which translates to the number 12, which is the index of the letter M, the thirteenth letter of the alphabet.

Thitherto

could you please elaborate a little bit what "ASCII armor" means in this context? – Soothsay

"...or a CMS container" what does CMS stands here for? – Soothsay

Cryptographic Message Syntax. It's the underlying protocol when you e.g. receive encrypted mail (as integrated into email clients such as Outlook, not PGP) protected using standard PKIX (i.e. X.509 certs & there private keys). – Thitherto

Answer 2

ASN.1, DER, PEM

File extension is immaterial

ASN.1 <-> DER <-> PEM

• Abstract Syntax Notation One(ASN.1) - is interface description language(IDL) for describing data structures. Widely used in telecommunications, networking, cryptography

• Distinguished Encoding Rules(DER) - binary. which is one of main encoding format for ASN.1. It is a subset of Basic Encoding Rules (BER). widely used for cryptography. .der, .cer can be used

• Privacy-Enhanced Mail(PEM) - base64 encoded DER. Block or blocks(e.g. a chain of certificates) of base-64 with plain-text headers and footers to mark the start and end. Main purpose is used for cryptography. It is difficult to transmit binary DER file, for example mail, which supports ASCII. You can find that there are a lot of file extensions with PEM inside like .pem, .crt, .cer .key (for public or private keys), but you should not rely on it. You should open this file and check header/footer

-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----

• Public-Key Cryptography Standards #7: Cryptographic Message Syntax(PKCS#7: CMS). It can be stored as DER or PAM. It is a multi-purpose format for encrypted data, authenticated data or signed data(except private keys)... It can contains attached or detached signature with chain of certificates. .p7b - usually PEM, .p7s - signature file in DER format

• Public-Key Cryptography Standards #12: Cryptographic Message Syntax(PKCS #12: CMS) the same as PKCS#7 but contains private key included

PKCS #7 in PEM format

-----BEGIN PKCS7-----
...
-----END PKCS7-----

Example of detached PKCS#7 signature

data.txt:

Hello, World!

1. create private key and self-signed certificate, fill fields like: pass phrase for your private key and fields for your certificate

//openssl req -x509 -newkey rsa:4096 -keyout <name_for_new_private_key> -out <name_for_new_certificate> -days 365
openssl req -x509 -newkey rsa:4096 -keyout myPrivateKey -out myCert -days 365

myPrivateKey has PEM format:

-----BEGIN ENCRYPTED PRIVATE KEY-----
MIIJnzBJBgkqhkiG9w0BBQ0wPDAbBgkqhkiG9w0BBQwwDgQI9Qxgmq0oK+ICAggA
...
-----END ENCRYPTED PRIVATE KEY-----

myCert has PEM format:

-----BEGIN CERTIFICATE-----
MIIFMjCCAxoCCQD3+UllOGhfkTANBgkqhkiG9w0BAQsFADBbMQswCQYDVQQGEwJV
...
-----END CERTIFICATE-----

2. sign data.txt using phrase for your private key

//openssl cms -sign -signer <cert_file> -inkey <private_key_file> -binary -in <data_file> -outform [DER, PEM] -out <name_for_new_signature>

generating dataSignature in PEM format

openssl cms -sign -signer myCert -inkey myPrivateKey -binary -in data.txt -outform PEM -out dataSignature

-----BEGIN CMS-----
MIII/gYJKoZIhvcNAQcCoIII7zCCCOsCAQExDTALBglghkgBZQMEAgEwCwYJKoZI
...
-----END CMS-----

generating dataSignature in DER format

openssl cms -sign -signer myCert -inkey myPrivateKey -binary -in data.txt -outform DER -out dataSignature

3082 08fe 0609 2a86 4886 f70d 0107 02a0
...

3. Verify signature

//openssl cms -verify -binary -inform <PEM_or_DER_format_of_signature> -in <signature_file> -content <data_file> -noverify > /dev/null

//let's say that we generated signature in PEM format on previous step
openssl cms -verify -binary -inform PEM -in dataSignature -content data.txt -noverify > /dev/null

//Verification successful

//-noverify - Do not verify the signers certificate of a signed message
//-nointern - By default searching signing certificate inside. With this option only the certificates specified in the -certfile option are used.

You are able to review PKCS7

//openssl cms -cmsout -in <signature_file> -inform [PEM, DER] -noout -print
openssl cms -cmsout -in dataSignature -inform PEM -noout -print

CMS_ContentInfo: 
  contentType: pkcs7-signedData (1.2.840.113549.1.7.2)
  d.signedData: 
    version: 1
    digestAlgorithms:
        algorithm: sha256 (2.16.840.1.101.3.4.2.1)
        parameter: <ABSENT>
    encapContentInfo: 
      eContentType: pkcs7-data (1.2.840.113549.1.7.1)
      eContent: <ABSENT>
    certificates:
      d.certificate: 
        cert_info: 
          version: <ABSENT>
          serialNumber: 17868393695656042385
...

ASN.1 looks like or use lapo.it

//openssl asn1parse -inform [PEM, DER] -i -in <(fold -w 64 <signature_file>)
openssl asn1parse -inform PEM -i -in dataSignature

//for PEM sometimes helps next command: openssl asn1parse -i -in dataSignature <(fold -w 64 dataSignature)

  0:d=0  hl=4 l=2302 cons: SEQUENCE          
    4:d=1  hl=2 l=   9 prim:  OBJECT            :pkcs7-signedData
   15:d=1  hl=4 l=2287 cons:  cont [ 0 ]        
   19:d=2  hl=4 l=2283 cons:   SEQUENCE          
   23:d=3  hl=2 l=   1 prim:    INTEGER           :01
   26:d=3  hl=2 l=  13 cons:    SET               
   28:d=4  hl=2 l=  11 cons:     SEQUENCE          
   30:d=5  hl=2 l=   9 prim:      OBJECT            :sha256
   41:d=3  hl=2 l=  11 cons:    SEQUENCE          
   43:d=4  hl=2 l=   9 prim:     OBJECT            :pkcs7-data
   54:d=3  hl=4 l=1334 cons:    cont [ 0 ]        
   58:d=4  hl=4 l=1330 cons:     SEQUENCE          
   62:d=5  hl=4 l= 794 cons:      SEQUENCE          
   66:d=6  hl=2 l=   9 prim:       INTEGER           :F7F9496538685F91
   77:d=6  hl=2 l=  13 cons:       SEQUENCE          
   79:d=7  hl=2 l=   9 prim:        OBJECT            :sha256WithRSAEncryption
...

[Signature, Certificate]

Airedale