C#从pem格式解码公钥

我写这段代码是为了解码我的公钥，但这段代码无法解码。我转换的base64公钥是

Convert.fromBase64字符串（“MCgCIQCWeC4U3lLF7JCJePjZpK9rd1TOsqbT+/8TT++WL1KUFWIDAKAB）”

这是

DecodeX509PublicKey

函数的输入。密钥由python rsa模块生成

python代码：

pub, prv = rsa.newkeys(256)
pub_pem = pub.save_pkcs1(format='PEM')
pub_pem.replace('-----BEGIN RSA PUBLIC KEY-----\n', '').replace('-----END RSA PUBLIC KEY-----\n', '')

C#代码：

---

为什么

DecodeX509PublicKey

函数无法解码公钥？

您已经创建了一个位大小为256位的测试公钥。那太小了，没用了。现在的问题是BER将长度编码为：

• 如果值字段的大小小于128字节（仅为长度），则为1字节
• 如果值字段的大小小于256字节（

  81 XX

  ，其中

  XX

  仅为无符号数字的长度，此处用十六进制表示）
• 如果值字段的大小小于65536字节（

  82 XX-YY

  ，其中

  XX-YY

  是无符号、大端编码数字的长度，再次以十六进制表示）

当然，如果该值非常小，则长度的第一个字节不以

81

或

82

开头。请注意，例如，

84 00 00 XX YY

有时也具有有效的表示形式

有三种处理方法：

实现模数的一字节长度（快速修复）

仅测试1024位或更高的RSA密钥（始终是一个好主意）

使用BER解码器，甚至是成熟的ASN.1库（也是一个好主意）

---

就我个人而言，我会结合2。和3

“不能解码公钥”是什么意思？会发生什么情况？函数在其第一个

if else

语句中返回null。建议通过库（如Bouncy Castle）进行PEM处理。我测试了第二种方法，但没有成功@Maarten-Bodewe可以是模和指数的符号或endian问题。通常我不会回答“未工作”错误描述。

//------- Parses binary asn.1 X509 SubjectPublicKeyInfo; returns RSACryptoServiceProvider ---
    public static RSACryptoServiceProvider DecodeX509PublicKey(byte[] x509key)
    {
        // encoded OID sequence for  PKCS #1 rsaEncryption szOID_RSA_RSA = "1.2.840.113549.1.1.1"
        byte[] SeqOID = {0x30, 0x0D, 0x06, 0x09, 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x01, 0x01, 0x01, 0x05, 0x00} ;
        byte[] seq = new byte[15];
        // ---------  Set up stream to read the asn.1 encoded SubjectPublicKeyInfo blob  ------
        MemoryStream  mem = new MemoryStream(x509key) ;
        BinaryReader binr = new BinaryReader(mem) ;    //wrap Memory Stream with BinaryReader for easy reading
        byte bt = 0;
        ushort twobytes = 0;

        try{

            twobytes = binr.ReadUInt16();
            if(twobytes == 0x8130)  //data read as little endian order (actual data order for Sequence is 30 81)
                binr.ReadByte();    //advance 1 byte
            else if(twobytes == 0x8230)
                binr.ReadInt16();   //advance 2 bytes
            else
                return null;
            seq = binr.ReadBytes(15);       //read the Sequence OID
            if(!CompareBytearrays(seq, SeqOID)) //make sure Sequence for OID is correct
                return null;

            twobytes = binr.ReadUInt16();
            if(twobytes == 0x8103)  //data read as little endian order (actual data order for Bit String is 03 81)
                binr.ReadByte();    //advance 1 byte
            else if(twobytes == 0x8203)
                binr.ReadInt16();   //advance 2 bytes
            else
                return null;

            bt = binr.ReadByte();
            if(bt != 0x00)      //expect null byte next
                return null;

            twobytes = binr.ReadUInt16();
            if(twobytes == 0x8130)  //data read as little endian order (actual data order for Sequence is 30 81)
                binr.ReadByte();    //advance 1 byte
            else if(twobytes == 0x8230)
                binr.ReadInt16();   //advance 2 bytes
            else
                return null;

            twobytes = binr.ReadUInt16();
            byte lowbyte = 0x00;
            byte highbyte = 0x00;

            if(twobytes == 0x8102)  //data read as little endian order (actual data order for Integer is 02 81)
                lowbyte = binr.ReadByte();  // read next bytes which is bytes in modulus
            else if(twobytes == 0x8202) {
                highbyte = binr.ReadByte(); //advance 2 bytes
                lowbyte = binr.ReadByte();
            }
            else
                return null;
            byte[] modint = {lowbyte, highbyte, 0x00, 0x00} ;   //reverse byte order since asn.1 key uses big endian order
            int modsize = BitConverter.ToInt32(modint, 0) ;

            byte firstbyte = binr.ReadByte();
            binr.BaseStream.Seek(-1, SeekOrigin.Current);

            if(firstbyte == 0x00)   {   //if first byte (highest order) of modulus is zero, don't include it
                binr.ReadByte();    //skip this null byte
                modsize -=1  ;  //reduce modulus buffer size by 1
            }

            byte[] modulus = binr.ReadBytes(modsize);   //read the modulus bytes

            if(binr.ReadByte() != 0x02)         //expect an Integer for the exponent data
                return null;
            int expbytes = (int) binr.ReadByte() ;      // should only need one byte for actual exponent data (for all useful values)
            byte[] exponent = binr.ReadBytes(expbytes);


            //Console.WriteLine("\nExponent", exponent);
            //Console.WriteLine("\nModulus", modulus) ;    

            // ------- create RSACryptoServiceProvider instance and initialize with public key -----
            RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
            RSAParameters RSAKeyInfo = new RSAParameters();
            RSAKeyInfo.Modulus = modulus;
            RSAKeyInfo.Exponent = exponent;
            RSA.ImportParameters(RSAKeyInfo);
            return RSA;
        }
        catch(Exception){
            return null; 
        }

        finally { binr.Close(); }

    }