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@fmoliveira
Last active June 17, 2021 10:49
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Exports RSA key pair to PEM format.
using System;
using System.Security.Cryptography;
namespace SomeProgram
{
static class Program
{
static void Main()
{
using (RSACryptoServiceProvider rsa = new RSACryptoServiceProvider(KEY_SIZE))
{
string publicKey = RSAKeysToPEM.GetPublicPEM(rsa);
string privateKey = RSAKeysToPEM.GetPrivatePEM(rsa);
}
}
}
}
using System;
using System.IO;
using System.Security.Cryptography;
namespace SomeClassLibrary
{
/// <summary>
/// Exports RSA private and public key pair to PEM format.
/// Source: http://stackoverflow.com/questions/23734792/c-sharp-export-private-public-rsa-key-from-rsacryptoserviceprovider-to-pem-strin
/// </summary>
public static class RSAKeysToPEM
{
public static String GetPrivatePEM(RSACryptoServiceProvider csp)
{
TextWriter outputStream = new StringWriter();
if (csp.PublicOnly) throw new ArgumentException("CSP does not contain a private key", "csp");
var parameters = csp.ExportParameters(true);
using (var stream = new MemoryStream())
{
var writer = new BinaryWriter(stream);
writer.Write((byte)0x30); // SEQUENCE
using (var innerStream = new MemoryStream())
{
var innerWriter = new BinaryWriter(innerStream);
EncodeIntegerBigEndian(innerWriter, new byte[] { 0x00 }); // Version
EncodeIntegerBigEndian(innerWriter, parameters.Modulus);
EncodeIntegerBigEndian(innerWriter, parameters.Exponent);
EncodeIntegerBigEndian(innerWriter, parameters.D);
EncodeIntegerBigEndian(innerWriter, parameters.P);
EncodeIntegerBigEndian(innerWriter, parameters.Q);
EncodeIntegerBigEndian(innerWriter, parameters.DP);
EncodeIntegerBigEndian(innerWriter, parameters.DQ);
EncodeIntegerBigEndian(innerWriter, parameters.InverseQ);
var length = (int)innerStream.Length;
EncodeLength(writer, length);
writer.Write(innerStream.GetBuffer(), 0, length);
}
var base64 = Convert.ToBase64String(stream.GetBuffer(), 0, (int)stream.Length).ToCharArray();
outputStream.WriteLine("-----BEGIN RSA PRIVATE KEY-----");
// Output as Base64 with lines chopped at 64 characters
for (var i = 0; i < base64.Length; i += 64)
{
outputStream.WriteLine(base64, i, Math.Min(64, base64.Length - i));
}
outputStream.WriteLine("-----END RSA PRIVATE KEY-----");
}
return outputStream.ToString();
}
public static String GetPublicPEM(RSACryptoServiceProvider csp)
{
TextWriter outputStream = new StringWriter();
var parameters = csp.ExportParameters(false);
using (var stream = new MemoryStream())
{
var writer = new BinaryWriter(stream);
writer.Write((byte)0x30); // SEQUENCE
using (var innerStream = new MemoryStream())
{
var innerWriter = new BinaryWriter(innerStream);
EncodeIntegerBigEndian(innerWriter, new byte[] { 0x00 }); // Version
EncodeIntegerBigEndian(innerWriter, parameters.Modulus);
EncodeIntegerBigEndian(innerWriter, parameters.Exponent);
//All Parameter Must Have Value so Set Other Parameter Value Whit Invalid Data (for keeping Key Structure use "parameters.Exponent" value for invalid data)
EncodeIntegerBigEndian(innerWriter, parameters.Exponent); // instead of parameters.D
EncodeIntegerBigEndian(innerWriter, parameters.Exponent); // instead of parameters.P
EncodeIntegerBigEndian(innerWriter, parameters.Exponent); // instead of parameters.Q
EncodeIntegerBigEndian(innerWriter, parameters.Exponent); // instead of parameters.DP
EncodeIntegerBigEndian(innerWriter, parameters.Exponent); // instead of parameters.DQ
EncodeIntegerBigEndian(innerWriter, parameters.Exponent); // instead of parameters.InverseQ
var length = (int)innerStream.Length;
EncodeLength(writer, length);
writer.Write(innerStream.GetBuffer(), 0, length);
}
var base64 = Convert.ToBase64String(stream.GetBuffer(), 0, (int)stream.Length).ToCharArray();
outputStream.WriteLine("-----BEGIN PUBLIC KEY-----");
// Output as Base64 with lines chopped at 64 characters
for (var i = 0; i < base64.Length; i += 64)
{
outputStream.WriteLine(base64, i, Math.Min(64, base64.Length - i));
}
outputStream.WriteLine("-----END PUBLIC KEY-----");
return outputStream.ToString();
}
}
private static void EncodeLength(BinaryWriter stream, int length)
{
if (length < 0) throw new ArgumentOutOfRangeException("length", "Length must be non-negative");
if (length < 0x80)
{
// Short form
stream.Write((byte)length);
}
else
{
// Long form
var temp = length;
var bytesRequired = 0;
while (temp > 0)
{
temp >>= 8;
bytesRequired++;
}
stream.Write((byte)(bytesRequired | 0x80));
for (var i = bytesRequired - 1; i >= 0; i--)
{
stream.Write((byte)(length >> (8 * i) & 0xff));
}
}
}
private static void EncodeIntegerBigEndian(BinaryWriter stream, byte[] value, bool forceUnsigned = true)
{
stream.Write((byte)0x02); // INTEGER
var prefixZeros = 0;
for (var i = 0; i < value.Length; i++)
{
if (value[i] != 0) break;
prefixZeros++;
}
if (value.Length - prefixZeros == 0)
{
EncodeLength(stream, 1);
stream.Write((byte)0);
}
else
{
if (forceUnsigned && value[prefixZeros] > 0x7f)
{
// Add a prefix zero to force unsigned if the MSB is 1
EncodeLength(stream, value.Length - prefixZeros + 1);
stream.Write((byte)0);
}
else
{
EncodeLength(stream, value.Length - prefixZeros);
}
for (var i = prefixZeros; i < value.Length; i++)
{
stream.Write(value[i]);
}
}
}
}
}
@lbehm
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lbehm commented Oct 24, 2017

To everyone reading this: This does not export a certificate!

GetPublicPEM() returns the public part of the RSA key in the RSA key format. No common application uses that.
GetPrivatePEM() works great!

@lbehm
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lbehm commented Oct 24, 2017

@mockpit I dont think jwt uses that format, and no this can't be the way to get the public pem

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