File formats: PEM, DER, PKCS #12 and friends

A certificate is one fixed structure defined in RFC 5280. The file formats below are only different ways of storing or packing it, sometimes together with a private key or other certificates. Once you know which wrapping you have, converting is one command.

What is the actual data inside?

Certificates, CSRs, CRLs and keys are described in ASN.1 and stored as bytes using DER, the Distinguished Encoding Rules. DER always gives exactly one byte sequence for a given value, which matters because a signature is computed over those exact bytes. A DER certificate usually starts with the bytes 30 82: an ASN.1 SEQUENCE followed by a two-byte length.

od -An -tx1 -N4 cert.der

 30 82 01 b2

What are the main formats?

PEM: DER with a label

RFC 7468 standardises the PEM labels. The label tells you what is inside:

You will also see labels outside that list. RSA PRIVATE KEY and EC PRIVATE KEY are older “traditional” key formats (PKCS #1 for RSA, SEC 1 for elliptic curves) that OpenSSL still reads and can still write:

openssl genrsa -out rsa.key 2048 && head -1 rsa.key
openssl genrsa -traditional -out rsa-old.key 2048 && head -1 rsa-old.key

-----BEGIN PRIVATE KEY-----
-----BEGIN RSA PRIVATE KEY-----

A PEM file can hold several objects one after another. A “full chain” file is the server certificate first, then each intermediate in order. Leave the root out: the client must already have it. RFC 7468 also allows text before the BEGIN line, which is why some tools write “Bag Attributes” lines there; most software ignores them.

PKCS #12 and PFX: the identity in one file

RFC 7292 defines PKCS #12. PFX is the older name for the same format; Windows still uses the .pfx extension. A PKCS #12 file can hold a private key, its certificate, the chain and a friendly name, all protected by a password. That makes it the usual way to move a server identity between systems, and also the file to guard most carefully.

openssl pkcs12 -in bundle.p12 -info -noout -passin pass:changeit

MAC: sha256, Iteration 2048
MAC length: 32, salt length: 8
PKCS7 Encrypted data: PBES2, PBKDF2, AES-256-CBC, Iteration 2048, PRF hmacWithSHA256
Certificate bag
PKCS7 Data
Shrouded Keybag: PBES2, PBKDF2, AES-256-CBC, Iteration 2048, PRF hmacWithSHA256

“Certificate bag” and “Shrouded Keybag” are the certificate and the encrypted private key. OpenSSL 3 protects both with AES-256 by default. Some older systems only open the legacy format, which -legacy produces; it protects the certificates with 40-bit RC2, which is weak, so use it only when you have no choice.

PKCS #7 and P7B: certificates without a key

A .p7b file is a PKCS #7 (or CMS, RFC 5652) SignedData structure that carries certificates but no signature and no key. It is a convenient way to ship a whole chain. To use it with OpenSSL-based software, unpack it into PEM certificates:

openssl pkcs7 -in bundle.p7b -print_certs -noout

subject=CN=www.example.com
issuer=CN=www.example.com

Add -inform DER if the file is binary, and replace -noout with -out certs.pem to write the certificates out.

Java keystores: JKS and PKCS #12

Java stores keys and trusted certificates in a keystore. The old Java-only format is JKS. Since JDK 9 the default keystore type is PKCS #12, the same standard format as above, so a .p12 file from OpenSSL can be used directly as a Java keystore. Each entry has an alias and is either a PrivateKeyEntry (a key with its certificate chain) or a trustedCertEntry (a CA certificate to trust). keytool itself recommends moving away from JKS; the cheat sheet has the conversion command and its warning text.

What do the file extensions mean?

Less than you would hope. An extension is a convention, not a promise:

• .crt and .cer usually hold one certificate, but it can be PEM or DER. Windows’ certificate export offers both “DER encoded binary” and “Base-64 encoded” under the same .cer extension.
• .pem says the file is text with BEGIN lines. It can hold certificates, keys or both.
• .key usually holds a PEM private key, encrypted or not.
• .p12 and .pfx are the same format.
• .p7b and .p7c hold certificate bundles without keys.

So look inside instead. If the first line starts with -----BEGIN, it is PEM, and the label tells you the rest. If not, it is binary: try openssl x509 -inform DER, then openssl pkcs12 -info, then openssl pkcs7 -inform DER. On Linux and in Git Bash, file often recognises certificates:

file -b cert.pem cert.der req.csr

PEM certificate
Certificate, Version=3
PEM certificate request

Do not trust file blindly for keys: it can mislabel a PEM private key. The BEGIN line is more reliable.

How do I convert between them?

Every command in this table was run for this page. -noenc writes an unencrypted key, so protect the result (chmod 600 on Linux) and delete copies you no longer need.

Paste any PEM certificate into the decoder to see the fields inside it:

Try it: Certificate decoderOpen the full tool

Runs in your browser. Nothing you paste is sent anywhere.

Paste a certificate

Try a sampleChoose a sample…A website certificate (www.example.com)An expired certificateA wildcard certificate (*.example.com)A certificate with a Common Name but no SANA self-signed root CAA complete chain: leaf, intermediate and root

-----BEGIN CERTIFICATE----- MIIC1zCCAnygAwIBAgIQWTTOzqWiPshGdQhNb6kmNjAKBggqhkjOPQQDAjB+MQsw CQYDVQQGEwJCRTEZMBcGA1UEChMQUEtJIFplcm8gdG8gSGVybzEnMCUGA1UECxMe U2FtcGxlcyAtIG5vdCBwdWJsaWNseSB0cnVzdGVkMSswKQYDVQQDEyJQS0kgWmVy byB0byBIZXJvIFNhbXBsZSBJc3N1aW5nIENBMB4XDTI2MTAwMTAwMDAwMFoXDTM2 MTAwMTAwMDAwMFowGjEYMBYGA1UEAxMPd3d3LmV4YW1wbGUuY29tMFkwEwYHKoZI zj0CAQYIKoZIzj0DAQcDQgAEw4og6Y/SOr+WrqSv+ng7tOBEsc5N3A3yZRk+9hsR XzPQvB9672aHZHTYFNlSY93RWVcTRieE+8XnIbWIYfwxIqOCAT4wggE6MAwGA1Ud EwEB/wQCMAAwDgYDVR0PAQH/BAQDAgeAMBMGA1UdJQQMMAoGCCsGAQUFBwMBMCcG A1UdEQQgMB6CD3d3dy5leGFtcGxlLmNvbYILZXhhbXBsZS5jb20wHQYDVR0OBBYE FALMipe4vcXJTpwO926S15Y/bobzMB8GA1UdIwQYMBaAFN+ph37v40SN23r6Fg/z OvSNjJqeMEgGCCsGAQUFBwEBBDwwOjA4BggrBgEFBQcwAoYsaHR0cDovL3BraS5l eGFtcGxlLmNvbS9zYW1wbGUtaXNzdWluZy1jYS5jcnQwPQYDVR0fBDYwNDAyoDCg LoYsaHR0cDovL3BraS5leGFtcGxlLmNvbS9zYW1wbGUtaXNzdWluZy1jYS5jcmww EwYDVR0gBAwwCjAIBgZngQwBAgEwCgYIKoZIzj0EAwIDSQAwRgIhAM5dJUPhL+qV artvWtQinfnykhVflJTjMXe8M6PnqQfaAiEAhOLGj1hSEkNxQoMrYNXRaIe69nq6 2unibVFSVw1AC80= -----END CERTIFICATE-----

PEM text, several PEM blocks, or a .pem, .crt, .cer or .der file (drop it on the box). Certificates are public, so they are safe to paste.

DecodeClearChoose a file

Go deeper

• RFC 7468: the PEM textual encodings and their labels.
• RFC 7292: PKCS #12, the personal information exchange format.
• RFC 5958: asymmetric key packages, the successor to the PKCS #8 definition in RFC 5208.
• RFC 5652: the Cryptographic Message Syntax, the modern form of PKCS #7.
• OpenSSL pkcs12 manual and Java keytool documentation: every option for building and reading keystores.
• Chapter 8 explains the fields inside every certificate, whatever its wrapping.