How to convert between DER and PEM

Before you start

• OpenSSL (Linux, macOS with Homebrew, or Git Bash on Windows). On Windows without Git Bash, certutil handles certificates (step 7); with a JDK, keytool does too (step 8).
• The file to convert. The extension (.cer, .crt, .der) does not tell you the format; the content does.

DER is the binary encoding of a certificate or key. PEM is the same bytes in Base64, between -----BEGIN …----- and -----END …----- lines. Windows, Java and many appliances often hand you DER; Linux servers usually want PEM.

Steps

1. Find out which one you have

head -1 www.cer
head -c 4 www.der | od -An -tx1

-----BEGIN CERTIFICATE-----
 30 82 03 39

• Text that starts with -----BEGIN is PEM, and the word after BEGIN says what it holds.
• Binary that starts with the byte 30 is DER: every certificate, key, CSR and CRL is an ASN.1 SEQUENCE, and that byte opens it.

On Linux and in Git Bash, file -b often names it: Certificate, Version=3 for a DER certificate, PEM certificate for PEM. Do not rely on file for keys; it can mislabel them.

2. Convert a certificate

# DER to PEM
openssl x509 -inform DER -in www.der -out www.pem

# PEM to DER
openssl x509 -in www.pem -outform DER -out www.der

OpenSSL 3 detects DER input on its own for openssl x509, but -inform DER is harmless and needed on OpenSSL 1.1.1. A DER file holds exactly one object; if your PEM file contains several certificates, openssl x509 converts only the first.

3. Convert keys, CSRs and CRLs

Each object type has its own OpenSSL command, and the options are the same:

For a public key, add -pubin to the pkey commands. A DER private key is still a secret: converting it changes nothing about how carefully you must store it. CRLs downloaded from a CA are usually DER; after conversion the PEM file starts with -----BEGIN X509 CRL-----.

4. Convert a PKCS #7 bundle (.p7b)

A .p7b file carries several certificates, often as DER. Unpack it into one PEM file:

openssl pkcs7 -inform DER -in chain.p7b -print_certs -out chain.pem

Our test bundle held two certificates, and chain.pem then had two BEGIN CERTIFICATE blocks. To go the other way, openssl crl2pkcs7 -nocrl -certfile chain.pem -outform DER -out chain.p7b.

5. Convert an issuer certificate you downloaded

The “CA Issuers” address in a certificate’s Authority Information Access extension usually serves DER. For example, letsencrypt.org’s certificate points to its issuer:

openssl s_client -connect letsencrypt.org:443 -servername letsencrypt.org </dev/null 2>/dev/null \
  | openssl x509 -noout -ext authorityInfoAccess
curl -sS -o ye2.der http://ye2.i.lencr.org/
openssl x509 -inform DER -in ye2.der -out ye2.pem
openssl x509 -in ye2.pem -noout -subject -issuer

Authority Information Access: 
    CA Issuers - URI:http://ye2.i.lencr.org/
subject=C=US, O=Let's Encrypt, CN=YE2
issuer=C=US, O=ISRG, CN=Root YE

The server sent the file as application/pkix-cert, which is DER. Plain HTTP is fine here: the certificate is verified by its signature, not by how it travelled. How to build the full chain file shows where such a file goes next.

6. Decode a Base64 block without BEGIN and END lines

Portals and APIs sometimes show a certificate as bare Base64. Decode it straight to DER:

openssl base64 -d -A -in body.txt -out cert.der     # Base64 on one long line
openssl base64 -d -in body.txt -out cert.der        # Base64 wrapped in lines of 64 characters

Choose the right one: on a single long line without -A, OpenSSL can write an empty or cut-off file without printing any error (in our tests, a 1,060-character line gave 0 or 25 bytes). Then convert cert.der as in step 2.

7. On Windows, with certutil

certutil -encode www.der www.pem
certutil -decode www.pem www.der

Input Length = 829
Output Length = 1200
CertUtil: -encode command completed successfully.
Input Length = 1200
Output Length = 829
CertUtil: -decode command completed successfully.

-encode writes a file with -----BEGIN CERTIFICATE----- lines, so OpenSSL and Linux servers read it as PEM. -decode reverses it. Both refuse to overwrite an existing output file; add -f to force it. In the Windows certificate export wizard, “DER encoded binary X.509 (.CER)” gives DER and “Base-64 encoded X.509 (.CER)” gives PEM, with the same extension.

8. In Java, with keytool

keytool -exportcert -alias shop -keystore shop.p12 -file shop.cer        # DER
keytool -exportcert -rfc -alias shop -keystore shop.p12 -file shop.pem   # PEM

keytool -importcert and keytool -printcert accept both formats.

Check it worked

A conversion must not change the certificate. Its SHA-256 fingerprint is computed over the DER bytes, so it is the same for both files, and equal to a plain SHA-256 of the DER file:

openssl x509 -in www.pem -noout -fingerprint -sha256
openssl x509 -inform DER -in www.der -noout -fingerprint -sha256
sha256sum www.der

sha256 Fingerprint=67:E1:E5:8B:37:05:E5:60:D5:61:B0:68:B2:A9:29:BF:87:15:FB:B1:22:F6:C8:01:8E:3A:22:2E:59:34:0C:7D
sha256 Fingerprint=67:E1:E5:8B:37:05:E5:60:D5:61:B0:68:B2:A9:29:BF:87:15:FB:B1:22:F6:C8:01:8E:3A:22:2E:59:34:0C:7D
67e1e58b3705e560d561b068b2a929bf8715fbb122f6c8018e3a222e59340c7d *www.der

You can also paste the PEM certificate into the decoder:

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

If something goes wrong

• Could not read certificate from … (OpenSSL 3.0), Could not find certificate from … (3.2) or unable to load certificate (1.1.1): the file is not a certificate in the format you named. Check step 1: it may be a key, a CSR, a .p7b (step 4) or a PFX (How to convert PFX to PEM).
• Only one certificate came out: openssl x509 reads one object. Use a .p7b (step 4) or keep the bundle in PEM.
• An empty or truncated file after openssl base64 -d: the input is on one line; add -A (step 6).
• CertUtil: -encode command FAILED: 0x80070050 (WIN32: 80 ERROR_FILE_EXISTS): the output file already exists. Add -f, or choose a new name.
• A server still rejects the PEM file: check that it starts with the right BEGIN line and contains what the server expects, such as the full chain: How to build the full chain file.

Why this works

Certificates, keys, CSRs and CRLs are ASN.1 structures, and DER is their one canonical binary encoding. PEM, defined in RFC 7468, is those exact DER bytes in Base64 with a label. Converting therefore changes only the wrapping. A signature covers the DER bytes of the signed data, so it stays valid in either form, and the fingerprint stays the same. The file formats appendix compares DER, PEM, PKCS #7, PKCS #12 and the Java keystores, and Anatomy of an X.509 certificate shows the ASN.1 structure inside.

Go deeper

• RFC 7468: PEM encodings and the labels after BEGIN.
• OpenSSL format options: -inform, -outform and how input formats are detected.
• OpenSSL x509, pkcs7 and crl2pkcs7 manuals.
• RFC 5280, section 4.2.2.1: the Authority Information Access extension and its “CA Issuers” method.
• Microsoft Learn: certutil: -encode and -decode.
• Oracle: keytool: -exportcert and -rfc.