Invalid ciphertext when performing RSA decryption - c++

Question

I use the following code to encrypt and decrypt a string making use of Crypto++ 5.6.2 library
string to_BER(string spriv,bool b)
{
string HEADER, FOOTER;
if(b)
{
HEADER = "-----BEGIN RSA PRIVATE KEY-----";
FOOTER = "-----END RSA PRIVATE KEY-----";
}
else
{
HEADER = "-----BEGIN PUBLIC KEY-----";
FOOTER = "-----END PUBLIC KEY-----";
}
size_t pos1, pos2;
pos1 = spriv.find(HEADER);
if(pos1 == string::npos)
throw std::runtime_error("PEM header not found");
pos2 = spriv.find(FOOTER, pos1+1);
if(pos2 == string::npos)
throw std::runtime_error("PEM footer not found");
// Start position and length
pos1 = pos1 + HEADER.length();
pos2 = pos2 - pos1;
string keystr = spriv.substr(pos1, pos2);
return keystr;
}
string rsa_encrypt(const string &message, const string &public_key)
{
string keystr=to_BER(public_key,false);
ByteQueue queue;
Base64Decoder decoder;
decoder.Attach(new Redirector(queue));
decoder.Put((const byte*)keystr.data(), keystr.length());
decoder.MessageEnd();
RSAES_PKCS1v15_Encryptor e;
e.AccessKey().Load(queue);
bool key_ok = e.AccessKey().Validate(rng, 3);
if(!key_ok)
{
printf("ERROR IN RSA KEY\n");
return "";
}
string encrypted_data;
StringSource ss1(message, true,
new PK_EncryptorFilter(rng, e,
new StringSink(encrypted_data)
) // PK_EncryptorFilter
); // StringSource
return encrypted_data;
}
string rsa_decrypt(const string &message, const string &private_key,bool b_Base64decode)
{
string keystr=to_BER(private_key,true);
string decoded=message;
if(b_Base64decode)
{
Base64Decoder decoder;
decoder.Put( (byte*)message.data(), message.size() );
decoder.MessageEnd();
word64 size = decoder.MaxRetrievable();
if(size && size <= SIZE_MAX)
{
decoded.resize(size);
decoder.Get((byte*)decoded.data(), decoded.size());
}
}
RSAES_PKCS1v15_Decryptor pri( rng, 1024 );
TransparentFilter privSink( new StringSink(keystr) );
pri.DEREncode( privSink );
privSink.MessageEnd();
string decrypted_data;
try
{
StringSource ss( decoded, true,
new PK_DecryptorFilter( rng, pri, new StringSink( decrypted_data )));
}
catch(Exception *e)
{
printf("ERROR DECRYPTING RSA\n");
return "";
}
return decrypted_data;
}
Then I use this code:
string enc=rsa_encrypt("hola mundo","-----BEGIN PUBLIC KEY----- MIGeMA0GCSqGSIb3DQEBAQUAA4GMADCBiAKBgHIiecdmRAaiTrCbbEOLBPh+fxp2KyGWyMTeWIj56NRk1CFLfBDT6vOWDceFpuTcrAFGR4Np4JLsssqM24F7bZCKv6eQCV8Xjv6GJiGofKEkI4b0zwHHEwq63p+9Rb5jYXlPZ7JIud7Yi96CHbg3foLQzZSSS9oFItGOFF0jDM7lAgMBAAE= -----END PUBLIC KEY-----");
string aaa=rsa_decrypt(
/*"YTbXg1K4OlXGY6eaLuVTFZrN4qi1zg83h0PjeLd9F3Ge3gKUHsJpoE3iLv1+Gj/PepM8ehiilb5kphxCdcELjaYH9wwYHMpUZUQSRLQUTnofOboI6nfHaKnnNV28QMguM39q1hm7X1wNc69D8R+CkWLka2rQof+BXt+41ivnMj8=",*/
enc
,"-----BEGIN RSA PRIVATE KEY----- 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 -----END RSA PRIVATE KEY-----"
,false
);
The problem is that rsa_decrypt fails with a "invalid ciphertext" exception that isn't caught with the try/catch code. The program crashes.
I would like to know what is wrong with my code and how can I avoid the program to crash even if the ciphertext is wrong.
I have found similar threads (crypto++ RSA and "invalid ciphertext") but none of the solutions is valid for me: I don't see any problem with a 0 at the end of the string, I check that the keys are valid, I know that private and public keys are from the same key pair and they work with other RSA libraries.
This is the line were it crashes:
StringSource ss( decoded, true,
new PK_DecryptorFilter( rng, pri, new StringSink( decrypted_data )));

Answer 1

The problem is that rsa_decrypt fails with a "invalid ciphertext" exception
You create a new private key, and then encode it into keystr:
RSAES_PKCS1v15_Decryptor pri( rng, 1024 );
TransparentFilter privSink( new StringSink(keystr) );
pri.DEREncode( privSink );
Instead, you should use private_key (and not the new key) and do something like you did in the encryptor:
string keystr=to_BER(private_key,true);
ByteQueue queue;
Base64Decoder decoder;
decoder.Attach(new Redirector(queue));
decoder.Put((const byte*)keystr.data(), keystr.length());
decoder.MessageEnd();
RSAES_PKCS1v15_Decryptor d;
d.AccessKey().Load(queue);
... that isn't caught...
Wrap everything in a try/catch block, and not just the PK_DecryptorFilter call.
You can see the difference (or lackof) between the public and private keys with something like:
try {
string s1("-----BEGIN PUBLIC KEY-----\n"
"MIGeMA0GCSqGSIb3DQEBAQUAA4GMADCBiAKBgHIiecdmRAai"
"TrCbbEOLBPh+fxp2KyGWyMTeWIj56NRk1CFLfBDT6vOWDceF"
"puTcrAFGR4Np4JLsssqM24F7bZCKv6eQCV8Xjv6GJiGofKEk"
"I4b0zwHHEwq63p+9Rb5jYXlPZ7JIud7Yi96CHbg3foLQzZSS"
"S9oFItGOFF0jDM7lAgMBAAE="
"\n-----END PUBLIC KEY-----");
string s2("-----BEGIN RSA PRIVATE KEY-----\n"
"MIICWwIBAAKBgHIiecdmRAaiTrCbbEOLBPh+fxp2KyGWyMTeWIj"
"56NRk1CFLfBDT6vOWDceFpuTcrAFGR4Np4JLsssqM24F7bZCKv6"
"eQCV8Xjv6GJiGofKEkI4b0zwHHEwq63p+9Rb5jYXlPZ7JIud7Yi"
"96CHbg3foLQzZSSS9oFItGOFF0jDM7lAgMBAAECgYBxFL9bVHNp"
"rz4PtK3bbc2K9qmv6gxpxx88Dp/hdtm8NfoG0uclNRHALZeRa1Y"
"jwo+Y46zPAwPCDkpGbLC+5S9zfBjtrx/+8zjTyMVb2CcGLfR0H2"
"E/hcCjADXNxs+fmpB3+jyPhgH5ANaTmAXqGXOP56I0Fqo8xCfU/"
"zQELCtzMQJBAN2Kq+9bQW3nKAAJEZqWQlAEjuBQfe1lrvLxc/Ag"
"Vl9XLWrHre7HSlkyqcFemvqhzlZy1wz0Nv5VpOIGcAKefEMCQQC"
"D4xSbkF1kzZyj8k6P3iUW6ezaK4krOZnpq/wDyOtj0DBAtLt3ap"
"yv3BUbe7AH1e8llJ5a8UYVHlsOdRUio+m3AkAB0LYR8wR5OxCn1"
"2sughavAyqMifxOKqwhT3sst4cdpuA3ZMV3FGj2jCS58eWBMjw3"
"lx9N+t5MfTUpqPXX/6ZzAkBx9eTXqv3YXYZtb7GMxQI9c3Jy7k1"
"/aS1iaXbA+nrTa5BWSRT30cqEduJSNiVcD/KuAWZ35KWPGATMUE"
"UsAoCvAkEAuVg0OA6L3xJphKEbVYXvTwXrXcoVjjLDnNYjUJuNW"
"rFFjBuNE4whWvdg76Panw3vMhgFc6yVr+VE5XOc7rXPvA=="
"\n-----END RSA PRIVATE KEY-----");
ArraySource as1(s1, true), as2(s2, true);
RSA::PublicKey k1;
RSA::PrivateKey k2;
PEM_Load(as1, k1);
PEM_Load(as2, k2);
AutoSeededRandomPool prng;
k1.ThrowIfInvalid(prng, 3);
k2.ThrowIfInvalid(prng, 3);
Integer i1 = k1.GetModulus() - k2.GetModulus();
Integer i2 = k1.GetPublicExponent() - k2.GetPublicExponent();
cout << i1 << " " << i2 << endl;
} catch (const Exception& ex) {
cerr << ex.what() << endl;
}
Related, the encapsulation boundaries (-----BEGIN PUBLIC KEY----- and -----END PUBLIC KEY-----) should be on their own lines. Instead of a space, you should use a new line (\n). So something like:
string enc=rsa_encrypt(
"hola mundo",
"-----BEGIN PUBLIC KEY-----\nMIGeMA0G...gMBAAE=\n-----END PUBLIC KEY-----"
);
And:
string aaa=rsa_decrypt(
enc,
"-----BEGIN RSA PRIVATE KEY-----\nMIICWwIB...7rXPvA==\n-----END RSA PRIVATE KEY-----",
false
);
Related, you should probably use RSAES_OAEP_SHA_Encryptor and RSAES_OAEP_SHA_Decryptor. An aaproachable treatment is available at A bad couple of years for the cryptographic token industry.