Different formats for RSA keys

RSA keys can be saved into different formats: PEM, DER, Microsoft PUBLICKEYBLOB and Microsoft PRIVATEKEYBLOB. It might be challenging to convert the keys from one format to another.

Here is some very helpful information from this post http://pumka.net/2009/12/19/reading-writing-and-converting-rsa-keys-in-pem-der-publickeyblob-and-privatekeyblob-formats/

The keys contain those big numbers used for encryption.

·         Private key contains: modulus, private exponent, public exponent, prime 1, prime 2, exponent 1, exponent 2 and coefficient.

·         Public key contains only modulus and public exponent.

• DER format is based on Abstract Syntax Notation One (ASN.1). You can use ASN.1 Editor to view its structure and contents.
• PEM format from OpenSSL is based64 encoded format of DER format
• PRIVATEKEYBLOB and PUBLICKEYBLOB are C-style structure defined by Microsoft.

OpenSSL is a nice tool to convert the keys from one format to another. Here are some examples:

#convert private key from PEM to DER

openssl.exe rsa -inform PEM -in test.pem -outform DER -out test.der

#convert private key from DER to PEM

openssl.exe rsa -inform DER -in test.DER -outform PEM -out test1.pem

#convert public key from PEM to DER

openssl.exe rsa -inform  PEM -pubin  -in testPub.pem -outform DER -out testPub.der

#convert public key from DER to PEM

openssl.exe rsa -inform  DER -pubin  -in testPub.der -outform PEM -out testPub.pem

#convert public key from PEM to PUBLICKEYBLOB

openssl.exe rsa -inform  "PEM" -pubin  -in testPub.pem -outform "MS\ PUBLICKEYBLOB" -out testPub.pblob

#convert private key from PEM to PRIVATEKEYBLOB

openssl.exe rsa -inform  "PEM"   -in test.pem -outform "MS\ PRIVATEKEYBLOB" -out test.pblob

Here is a python script to convert keys from PEM to DER format:

import base64

fileName="c:\\temp\\testPub.pem"

fileContent=""

with open(fileName, 'r') as f:

    fileContent=f.readlines()

fileContent.pop()

fileContent.pop(0)

s=''.join(fileContent)

s=s.replace('\n','')

data =base64.b64decode(s)

print data.encode('hex')