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Cross-platform cryptography

RSA string signature with PSS padding

The standard signature algorithm is RSA using a (RSA) Private-/ Public key pair. The signature is generated with the Private key of the signer. For the verification the Public Key of the signer is used - so in a typical environment the Public Key is provided to the recipient(s) of the signed data and the recipient is been able to check = verify the signature.

To get this to work in cross platform systems there are three parameters that have to been equal - that is the hash algorithm that is used. My programs are using SHA256 for this task.

The second important data are the keys itself - it's easy not to recognize that "- Begin Private Key -" isn't the same as "- Begin RSA Private Key -". Using the wrong keys will cause the the verification to fail.

The third parameter is hidden in most implementations, it is the padding. The programs in this example use the PSS-padding (Probabilistic Signature Scheme) that is available platform wide.

Why is the PSS padding more secure than PKCS#1.5 padding?

An important parameter in cryptography is randomness - this prevents from easy textbook attacks. For a digital signature it means: the signature of the the same string and same (private) key results not in the same value. Running my codes will always result in a different (Base64 encoded) string, so an attacker is not been able to "see" that a previous used signature is in use again.

Key generation:

All examples use pre-generated keys that are described on the page RSA sample keys. If you want to see how my keys got generated visit the page RSA key generation.

When comparing the programs you will notice that the keys for C# looking like different as they are not in the "PEM"-format ("---Begin...") but in a XML-format. As it is a little bit tricky to convert the keys between XML- and PEM-format I setup an own page for this point: rsa_key_conversion.md

One note about the key management in my programs: usually the keys are (securely) stored in files or a key store and may have additional password protection. Both scenarios are unhandy in demonstration programs running in an online compiler. That's why I'm using static, hard-coded keys in my programs - please do not do this in production environment! Never ever store a Private Key in the source! The minimum is to load the keys from a secured device.

steps in the program

The program follows the usual sequence:

  1. generate a RSA key pair - here we are using a static, hard-coded key pair in form of a PEM encoded string (Java, PHP, CryptoJs and NodeJs) or a XML-file (C#)
  2. convert the data to sign into a binary format (e.g. a byte array)
  3. start the signature process
  4. load the Private Key
  5. set the signature parameters
  6. sign the "data to sign" and show the result ("signature") in Base64 encoding
  7. start the verification process
  8. load the Public Key
  9. Base64 decoding of the signature
  10. set the verification parameters (same as used for signing)
  11. verify the signature against the "data to sign" and show the result.

As the examples do a full round of signing and verifying I do not provide separate codes for verification only (except Webcrypto examples as they are "single versions" - "signature only" and "verification only").

⚠️ Security warning ⚠️

This is a serious warning regarding the security of the programs shown in these article series. Always keep in mind my disclaimer regarding my programs: All programs are for educational purposes and are not intended to use in production or any other programs where a secure solution is needed. The programs do not have proper exceptional/error handling and in some cases they use insecure key lengths or other methods that are insecure. Never ever use the programs in real life unless checked by a qualified professional cryptographer.

The following links provide the solutions in code and an online compile that runs the code.

Language available Online-compiler
Java ✅ repl.it CpcJavaRsaStringSignaturePssFull
PHP *1) ✅ repl.it CpcPhpRsaStringSignaturePssFull
C# (.net 5) ✅ repl.it CpcCsharpRsaStringSignaturePssFull
Javascript CryptoJs ❌ the signature functionality is not available in CryptoJs
NodeJS Crypto ✅ repl.it CpcNodeJsCryptoRsaSignatureStringFull
NodeJS forge ✅ repl.it CpcNodeJsRsaSignatureStringFull
Webcrypto sign only ✅ your browser WebcryptoRsaPssSignString.html
Webcrypto verify only ✅ your browser WebcryptoRsaPssVerifyString.html
OpenSSL (file signature) ✅ see explanation here: RSA signature PSS OpenSSL
Python *2) ✅ repl.it CpcPythonRsaStringSignaturePssFull
Go ✅ repl.it CpcGoRsaStringSignaturePssFull
Dart *3) ✅ no online compiler available

PHP *1): you need the library phpseclib version 3 to run this program

Python *2) you need the external library pycryptodome, version 3.9.9

Dart *3) you need the external libraries pointycastle version 3.1.1 and basic_utils: version 3.4.0

This is an output:

RSA signature PSS string
dataToSign: The quick brown fox jumps over the lazy dog

* * * sign the plaintext with the RSA private key * * *
used private key:
-----BEGIN PRIVATE KEY-----
MIIEvgIBADANBgkqhkiG9w0BAQEFAASCBKgwggSkAgEAAoIBAQDwSZYlRn86zPi9
...
3PIW4/CddNs8mCSBOqTnoaxh
-----END PRIVATE KEY-----

signature (Base64): fXQk+o3rlHhx/rYbwhdpsQYYRaXMydurGToRaQONfvosBJulom4yvpkncqHx/Yy2ga+U59lHghHCVZKOXxsZdGMgK91oqvDhapBt0VCipE4T1sJiGiFtLPNUJmJkdbpK1puKhmo1ZQniS6KXe8yTOF++V/S3geIFdyicGKJJgSueF0EotkOLFwr5P5UUVH9nowTvVlsFcT0+MFHygKzqZJ/O2R3m8ru9rvZhR3af97k+ZnlCOj+uI3V136Qz2Lta87ydKoK2usSz9NTfisg+7+mS8BZDcmzWZt90ped7Br0HtlaiPL3glcX/njSaR9/Fp1ioOuiO/xbZ27KOaPcPPg==

* * * verify the signature against the plaintext with the RSA public key * * *
used public key:
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA8EmWJUZ/Osz4vXtUU2S+
...
QwIDAQAB
-----END PUBLIC KEY-----

signature (Base64) verified: true

Last update: Aug. 20th 2021

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