Could not deserialize key data. The data may be in an incorrect format, it may be encrypted with an unsupported algorithm, or it may be an unsupported key type (e.g. EC curves with explicit parameters).

Checked across releases: cryptography 45.0 and later append "Details: …" (for example "ASN.1 parsing error: short data"). Versions 43–44 raise the text as a tuple with OpenSSL error objects and add "the provided password may be incorrect" to the sentence (and use it for wrong passwords too); 36–42 raise the tuple form without that phrase.

Python cryptography — ValueError from load_pem_private_key / load_der_private_key / load_der_public_key

What it means

cryptography found key data it could not turn into a key object. For PEM input the armour lines were fine (otherwise you would see "Unable to load PEM file"), so the problem is the DER inside: it is truncated, it is a different structure than the loader expects, or it describes a key cryptography refuses to load — EC keys with explicit curve parameters or keys encrypted with an algorithm it does not support. Recent versions append "Details: …" with the low-level ASN.1 reason.

Why it happens

• The base64 body is incomplete or altered

  common

  A line lost while copying a key from a terminal or a secret manager, a key truncated by a size-limited field, or characters changed by an editor. The header and footer survive, so PEM parsing succeeds and DER parsing fails.

• The loader does not match the key structure

  common

  A public key passed to `load_der_private_key`, a certificate passed to a key loader, or an OpenSSH-format key (`-----BEGIN OPENSSH PRIVATE KEY-----`) passed to `load_pem_private_key` instead of `load_ssh_private_key`.

• EC key with explicit curve parameters

  occasional

  Keys produced by some older tools or HSM exports spell out the curve's prime, coefficients and generator instead of naming the curve by OID. cryptography rejects these on purpose (RFC 5480 requires named curves for PKIX).

• Encrypted with an algorithm cryptography does not support

  occasional

  PKCS#8 files encrypted with legacy PBE schemes (for example PBE-SHA1-RC2 or DES variants) cannot be decrypted by current builds. Re-encrypt them with AES (`openssl pkcs8 -topk8 -v2 aes-256-cbc`).

How to fix it

1. 1.

   Check the key with OpenSSL first

   If `openssl asn1parse` already fails, the file is damaged — get a fresh copy. If OpenSSL loads it and cryptography does not, look at the curve encoding or the encryption algorithm.

   openssl asn1parse -in key.pem | head -20     # does the DER parse at all?
   openssl pkey -in key.pem -noout -text | head  # private key: type, curve, size
   openssl pkey -pubin -in pub.pem -noout -text   # public key

2. 2.

   Use the loader that matches the key

   Choose between PEM, DER and OpenSSH loaders from the data itself, and use the public-key loaders for public keys.

   from cryptography.hazmat.primitives import serialization
   
   def load_any_private_key(data: bytes, password: bytes | None = None):
       if data.lstrip().startswith(b"-----BEGIN OPENSSH PRIVATE KEY-----"):
           return serialization.load_ssh_private_key(data, password)
       if data.lstrip().startswith(b"-----BEGIN"):
           return serialization.load_pem_private_key(data, password)
       return serialization.load_der_private_key(data, password)

3. 3.

   Convert explicit parameters to a named curve

   OpenSSL 3 writes EC keys with named curves by default, so re-exporting the key once fixes explicit-parameter keys for every library.

   # Re-encode an EC key with a named curve instead of explicit parameters
   openssl pkey -in explicit.pem -out named.pem
   openssl ec -in named.pem -param_out -noout -text | grep "ASN1 OID"   # e.g. prime256v1

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