When working with Azure Cloud Services' Ubuntu VMs, many developers generate keys using OpenSSL commands like:

openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout myPrivateKey.key -out myCert.pem

This creates an X.509 certificate and private key pair, but the resulting .key file isn't in the format expected by PuTTY-based tools.

The error message unrecognised key type occurs because:

• OpenSSL generates keys in PEM format by default
• PuTTYgen expects either OpenSSH format or its native PPK format
• The X.509 certificate wrapper complicates direct conversion

Method 1: Using OpenSSH Conversion

# First extract the RSA private key
openssl rsa -in myPrivateKey.key -out openssh_format.key

# Then convert to PPK
puttygen openssh_format.key -o myKey.ppk

Method 2: Direct PEM to PPK Conversion

puttygen myPrivateKey.key -O private -o myKey.ppk

For Windows users who need FileZilla compatibility:

1. Install the latest PuTTYgen (0.76+ recommended)
2. Use the conversion command:

   puttygen myPrivateKey.key -t rsa -C "azure-vm-key" -o converted.ppk

For new deployments, generate PuTTY-compatible keys from the start:

ssh-keygen -t rsa -b 2048 -f azure_key
puttygen azure_key -o azure_key.ppk

• Verify key permissions: chmod 600 myPrivateKey.key
• Check for hidden characters: cat -v myPrivateKey.key
• Ensure you're using absolute paths when specifying key files

After successful conversion to PPK format:

1. In FileZilla: Edit > Settings > SFTP
2. Click "Add key file" and select your .ppk file
3. Ensure the username matches your Azure VM's admin account

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When working with Azure Cloud Services' Ubuntu VMs, many developers encounter a frustrating roadblock: the private keys generated through OpenSSL commands aren't recognized by common SSH clients like FileZilla and PuTTYgen. The specific error message:

puttygen: error loading myPrivateKey.key': unrecognised key type

occurs because OpenSSL's default key format isn't directly compatible with PuTTY's PPK format required by these clients.

The command you're using:

openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout myPrivateKey.key -out myCert.pem

generates a key in PKCS#8 format (when using newer OpenSSL versions) or traditional PEM format. PuTTYgen expects OpenSSH's proprietary format, creating the compatibility gap.

Here are three proven approaches to convert your keys properly:

Method 1: Using OpenSSH's ssh-keygen

# First convert to PEM format if needed
openssl rsa -in myPrivateKey.key -out myPrivateKey.pem

# Then convert to OpenSSH format
ssh-keygen -p -f myPrivateKey.pem -m pem

# Finally convert to PPK using PuTTYgen
puttygen myPrivateKey.pem -o myKey.ppk

Method 2: Alternative OpenSSL Command

Generate a compatible key from the start:

openssl genrsa -out myPrivateKey.pem 2048
openssl rsa -in myPrivateKey.pem -pubout -out myPublicKey.pem

Method 3: Using Python's cryptography Library

from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.backends import default_backend

with open("myPrivateKey.key", "rb") as key_file:
    private_key = serialization.load_pem_private_key(
        key_file.read(),
        password=None,
        backend=default_backend()
    )

pem = private_key.private_bytes(
    encoding=serialization.Encoding.PEM,
    format=serialization.PrivateFormat.TraditionalOpenSSL,
    encryption_algorithm=serialization.NoEncryption()
)

with open("converted_key.pem", "wb") as out_file:
    out_file.write(pem)

For Windows users who don't have access to Linux tools:

1. Install the latest PuTTYgen from official site
2. In PuTTYgen, go to Conversions > Import Key
3. Select your .key file and click "Save private key"

When creating new keys for Azure VMs, consider generating them directly in the required format:

ssh-keygen -t rsa -b 2048 -f azure_key

This creates two files: azure_key (private) and azure_key.pub (public) that are immediately compatible with most clients.