I have a 32 byte octet string ec private key.

And I want to convert this to pem type private key.

I use the secp256r1 curve.

How can I do that?

Is any command or method for that?

  • $\begingroup$ I don't think you can do this with a single command from a command line, you'd have to program it. $\endgroup$
    – Maarten Bodewes
    Jan 7, 2022 at 2:55

1 Answer 1


Meta: this is not really about cryptography, but use of a tool for data processing only partly related to crypto; but since no one voted to close (that I can see) I'll go ahead. This can be deleted if necessary.

Not exactly, but there is a command option to build arbitrary ASN.1 data, which can be adapted for this with a little work, if you have the desired private-value in 'plain' hex: on Unix (if it isn't already hex) you can convert with xxd -p -c32 or od -An -tx1 | tr -d ' \n' or similar, on Windows you're on your own. Given a file with the following contents except substituting your desired private value:


then openssl asn1parse -genconf filename -noout [-out derfile] will create the PKCS8-clear format in DER, and appending | openssl pkey -inform der will convert it to PEM. Or on Unix you can convert to PEM 'manually' with ... | { printf '%s\n' '-----BEGIN PRIVATE KEY-----'; openssl base64; printf '%s\n' '-----END PRIVATE KEY-----'; }

Alternatively and more hackily, the DER encoding of the structure described above is all constant except for the private-value which occurs last, so you can simply concatenate the constant part with the private-value to get PKC8-clear DER, then convert to PEM as above:

# on Unix, given the 32 bytes in binary in file rawfile:
printf '\x30\x41\x02\x01\x00\x30\x13\x06\x07\x2a\x86\x48\xce\x3d\x02\x01\x06\x08\x2a\x86\x48\xce\x3d\x03\x01\x07\x04\x27\x30\x25\x02\x01\x01\x04\x20'; cat rawfile;
# creates DER, and putting that in { } or ( ) and piping the result to
# pkey -inform der or the manual alternative above converts to PEM
  • $\begingroup$ these days 90% of cryptography is formatting bytes and base 64 encoded things in the right ways, and the other 10% is cryptography $\endgroup$ Feb 8, 2023 at 17:27

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