ECDSA signature verification, how to do it correctly?

jaclas · 2025-09-11 11:19:09

The other day I'm sitting on ECDSA cryptography, the hash signature I know how to do, but verification I can't get through. I couldn't find any example either on the forum or in the tests.
By the way of “guessing” I came to the code, which unfortunately returns False on data that was also generated by mORMot cryptography.
Verification receives on input the PEM certificate, signature (in Base64Uri) and input data, it is only supposed to check if the signature is correct.
Here is my function:

function VerifyECDSASignature_(const aInputString: string; const aPEMCertificate: string; const aBase64UriSignature: RawUtf8): boolean;
var
  lECDSACrypter: TCryptAsym;
  lCertificate: ICryptCert;
  lDerEncodedSignatureRBS: RawByteString;
  lPublicKeyRaw: RawByteString;
  lInputDataRBS: RawByteString;
  lInputUTF8: RawUtf8;
  lRawSignature64RBS: RawByteString;
begin
  Result := False;
  lECDSACrypter := Asym('ES256');
  if lECDSACrypter = nil then
  begin
    Exit;
  end;
  lInputUTF8 := StringToUtf8(aPEMCertificate);
  lCertificate := X509Load(lInputUTF8);
  if lCertificate = nil then
  begin
    Exit;
  end;
  lPublicKeyRaw := lCertificate.GetPublicKey;
  if lPublicKeyRaw = '' then
  begin
    Exit;
  end;

  lRawSignature64RBS := Base64UriToBin(aBase64UriSignature, nil);
  if Length(lRawSignature64RBS) <> SizeOf(TEccSignature) then
  begin
    Exit;
  end;
  lDerEncodedSignatureRBS := SetSignatureSecurityRaw(caaES256, lRawSignature64RBS);
  if lDerEncodedSignatureRBS = '' then
  begin
    Exit;
  end;

  lInputDataRBS := StringToUtf8(aInputString);

  Result := lECDSACrypter.Verify(lInputDataRBS, lPublicKeyRaw, lDerEncodedSignatureRBS, 'sha256');
end;

Here certificate:

What did I do wrong?

flydev · 2025-09-11 14:09:01

Why your function returns False
• Assumes signature is always raw 64 bytes (R||S)
• Your check Length(...) <> SizeOf(TEccSignature) exits early for any DER signature. OpenSSL (and many toolchains) output DER. So you never reach Verify for DER input.
• Public key format mismatch
• lCertificate.GetPublicKey returns the X.509 SubjectPublicKey (SPKI) in ASN.1. TCryptAsym.Verify expects EC public key in one of a few raw/PEM/DER encodings it can parse; SPKI isn’t always accepted directly by its low-level PemDerRawToEcc(). You either need to rewrap the SPKI to the EC compressed key DER, or use ICryptCert.Verify (which does that internally).
• Data bytes/encoding

---

read code in gist

credit: gpt

ps: I tested the code

@ab: do you think the ai-model hallucinated?

jaclas · 2025-09-11 16:30:04

flydev wrote:

Why your function returns False
• lCertificate.GetPublicKey returns the X.509 SubjectPublicKey (SPKI) in ASN.1. TCryptAsym.Verify expects EC public key in one of a few raw/PEM/DER encodings it can parse; SPKI isn’t always accepted directly by its low-level PemDerRawToEcc(). You either need to rewrap the SPKI to the EC compressed key DER, or use ICryptCert.Verify (which does that internally).

Thanks a lot! That was a spot-on shot. Unfortunately cryptography is not my strongest point, I do it a bit on instinct, and the lack of documentation in this area does not help.
Thanks again, now it's ok.

ab · 2025-09-11 19:41:55

The more I use the AI tools, the more they hallucinate.

Sometimes, they help a little.
But most of the time, I loose time because they provide wrong answers.

mORMot Open Source

#1 2025-09-11 11:19:09

ECDSA signature verification, how to do it correctly?

#2 2025-09-11 14:09:01

Re: ECDSA signature verification, how to do it correctly?

#3 2025-09-11 16:30:04

Re: ECDSA signature verification, how to do it correctly?

#4 2025-09-11 19:41:55

Re: ECDSA signature verification, how to do it correctly?

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